Original Study
Journal of Veterinary Emergency and Critical Care 24(5) 2014, pp 578–585 doi: 10.1111/vec.12210
Noninvasive continuous positive airway pressure delivered using a pediatric helmet in dogs recovering from general anesthesia Francesco Staffieri, DVM, PhD; Antonio Crovace, DVM; Valentina De Monte, DVM, PhD; Paola Centonze, DVM; Giulio Gigante, DVM and Salvatore Grasso, MD Abstract
Objective – To evaluate the feasibility and efficacy of noninvasive continuous positive airway pressure (CPAP) administered with a pediatric helmet in healthy dogs recovering from general anesthesia. Design – Randomized, cross-over, clinical study. Setting – University teaching hospital. Animals – Fifteen healthy female, client-owned dogs recovering from general anesthesia following elective ovariohysterectomy. Interventions – All dogs received the same standardized anesthetic protocol (acepromazine, morphine, propofol, and isoflurane in oxygen). After extubation, a pediatric helmet was placed on all dogs and connected to a venturi valve supplied with medical air. In all patients, the gas flow was set to 50 L/minute and the FiO2 to 0.21. Dogs received the following sequence of treatments, each lasting 20 minutes: 0 CPAP (pre-CPAP), CPAP of 5 cm H2 O (CPAP), and again 0 CPAP (post-CPAP). Measurements and Main Results – During the entire study, the following data were collected: pressure and FiO2 inside the helmet, mean arterial pressure, respiratory rate, heart rate, sedation score (0 = awake, 10 = deep sedation), and tolerance to the helmet (0 = excellent, 4 = poor). At the end of each phase, an arterial blood sample was sampled. As compared with the pre-CPAP and the post-CPAP periods, during the CPAP period, the PaCO2 , alveolar-arterial oxygen gradient (P[A−a]O2 ), and respiratory rate significantly decreased. The PaO2 was higher at CPAP (105.6 ± 4.0 mm Hg) compared with pre-CPAP (80.6 ± 6.9 mm Hg) and post-CPAP (86.7 ± 5.8 mm Hg). Tolerance and sedation scores during the CPAP period were not different from those in the pre-CPAP and post-CPAP periods. Conclusions – Noninvasive CPAP applied through a helmet is a feasible and effective supportive technique in dogs recovering from general anesthesia. (J Vet Emerg Crit Care 2014; 24(5): 578–585) doi: 10.1111/vec.12210 Keywords: atelectasis, CPAP, dogs, respiratory distress
Introduction Noninvasive ventilation is a type of ventilatory support that does not require an artificial airway (eg, From the Dipartimento dell’Emergenza e dei Trapianti d’Organo, Sezione di Cliniche Veterinarie e Produzioni Animali, (Staffieri, Crovace, De Monte, Centonze, Gigante); and Dipartimento dell’Emergenza e dei Trapianti d’Organo, Sezione di Anestesiologia e Rianimazione (Grasso), SP per Casamassima km 3, 70010 Valenzano, Bari, Italy. Supported by the intramural fundings of the Dipartimento dell’Emergenza e dei Trapianti d’Organo, Sezione di Cliniche Veterinarie e Produzioni Animali. Presented in part at the Association of Veterinary Anaesthetists, Spring Meeting in Davos, 21–23 March 2012.
Abbreviations
CPAP FiO2 Helmet HR MAP pHelmet P(A−a)O2 PPCs RR WOB
continuous positive airway pressure FiO2 inside the helmet heart rate mean arterial pressure pressure inside the helmet alveolar to arterial partial pressure gradient of oxygen postoperative pulmonary complications respiratory rate work of breathing
The authors declare no conflict of interests. Address correspondence and reprint requests to Dr. Francesco Staffieri, Dipartimento dell’Emergenza e dei Trapianti d’Organo, Sezione di Cliniche Veterinarie e Produzioni Animali, SP per Casamassima km 3, 70010 Valenzano, Bari, Italy. Email: francesco.staffieri@ uniba.it Submitted January 15, 2013; Accepted June 27, 2014.
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endotracheal tube, tracheostomy) and compared with invasive ventilation, requires minimal sedation, improves C Veterinary Emergency and Critical Care Society 2014
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patient comfort, and reduces nosocomial infection rates in people.1 Noninvasive ventilation may be applied as continuous positive airway pressure (CPAP) and intermittent positive pressure ventilation.2 CPAP is a breathing mode by which the patient spontaneously breathes through a pressurized circuit against a threshold resistor that maintains a preset positive airway pressure during both inspiration and expiration.3 Several studies in people have demonstrated the efficacy of CPAP to reduce the work of breathing and atelectasis, improve oxygenation, and reduce the need for intubation in patients who develop hypoxemia after surgery or as a consequence of lung diseases.3–10 Different clinical situations could benefit from CPAP in veterinary medicine and these include postoperative respiratory assistance, exacerbation of the brachycephalic syndrome, and acute respiratory failure. Recently, invasive CPAP has been demonstrated to improve oxygenation in horses under general anesthesia.11,12 The interface by which CPAP is administered plays a critical role in the efficacy of treatment.13 A recent study demonstrated an improvement in arterial oxygenation when CPAP was administered through a conical face mask equipped with a Boussignac valve in dogs.14 In this study, CPAP was applied for a short time in sedated patients and continuously assisted by an operator to guarantee mask sealing. Clinical experience suggests that it is sometimes difficult to guarantee mask sealing because of the wide variety of facial conformation among different dog breeds.15 Comfort of the patient is also an issue, with sedation and continuous assistance by an experienced operator often required.15,16 The head helmet as an interface for the administration of CPAP has been recently introduced in human medicine with interesting results.8,17,18 The helmet is designed to contain the head while providing a seal all around the patient’s neck. With the helmet, positive pressure is applied around the head as it would be in a pressurized chamber.19 The main advantage of the helmet is the comfort of the patient. The helmet has also been used in neonates who, like veterinary patients, often do not tolerate the mask without deep sedation.9,20, 21 The aim of this study was to evaluate the feasibility of administering noninvasive CPAP through a pediatric helmet in dogs recovering from general anesthesia. Our hypothesis was that, similarly to its use in children, the helmet could be effective and well tolerated for delivering CPAP in dogs.
Materials and Methods The study was conducted in compliance with the Italian Animal Welfare and statutes of the University of Bari, C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
relating to the use of client-owned animals in clinical investigations. Animals Fifteen healthy, female, client-owned dogs of mixed breed, with a body weight >10 kg, scheduled for elective ovariohysterectomy, were enrolled in the study. Written owner informed consent was obtained. Preoperative screening included a CBC, serum biochemical analyses, and thoracic radiographs. Dogs with abnormal clinicopathological findings or physical examination evidence of pulmonary disease were excluded from the study. Anesthesia and surgery All dogs were premedicated with intramuscular administration of acepromazinea (0.03 mg/kg) followed by morphineb (0.4 mg/kg). When an adequate level of sedation was achieved, an IV catheter was aseptically placed at the level of the right cephalic vein. General anesthesia was induced with IV propofolc (5–6 mg/kg) and, after orotracheal intubation, maintained with inhalation of isofluraned in oxygen (100 mL/kg/min) through a rebreathing circuit. All patients were spontaneously breathing throughout the procedure. The depth of anesthesia was assessed from evaluation of the palpebral reflex, position of the eyes, and jaw tone. The main cardiovascular and respiratory parameters were continuously monitored throughout anesthesia.e All patients underwent ovariohysterectomy through a midline laparotomy. At the end of surgery, isoflurane administration was discontinued, and patients were extubated when the swallowing reflex recovered. In all patients, the duration of anesthesia (from induction to discontinuation of isoflurane administration) and extubation time (from discontinuation of isoflurane administration to extubation) were recorded. Equipment The helmetf is made of transparent, latex-free polyvinyl chloride (Figure 1A). At its base, it is equipped with a rigid plastic collar that joins the helmet to a soft elastic collar, which adheres to the neck while avoiding air leakage. The device is also equipped with 2 armpit braces that can be secured to a pair of hooks on the plastic ring. The braces are used in human patients to improve the stability of the helmet. The pediatric helmet is available in 2 sizes: small, with an internal gas volume of 7 L (for infants 10 kg). The helmet is equipped with 2 gas ports that act as the inlet and outlet for inspiratory and expiratory gas flow, respectively (Figure 1A). The inlet opening is connected to the fresh gas source 579
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Figure 1: Equipment used to provide noninvasive continuous positive airway pressure (CPAP) in dogs recovering from general anesthesia. The helmet (A) is equipped with an inlet (1) and outlet (2) gas port. The CPAP valve (3) is located on the outlet gas port. The helmet also has an antiasphyxia bidirectional safety valve (4) and an accessory port (5). The flow delivery system (B) consists of an oxygen flowmeter (7) connected to a venturi valve (6) and a second oxygen flowmeter (8) that bypasses the venturi valve (9).
while the outlet opening is connected to the CPAP valve, which allows adjustment of the CPAP level (Figure 1A). The helmet also has an anti-asphyxia bidirectional safety valve (Figure 1A) made with a precalibrated spring in a membrane system; in the event that fresh gas delivery stops, this valve opens immediately, allowing the patient to breathe ambient air. The valve is positioned on the cover of an opening that when removed allows the clinician immediate access to the patient’s face. The expiratory port is equipped with a disposable manometer that measures the pressure inside the helmet (Figure 1A). Two small accessory ports are available on the rigid collar for the introduction of small catheters (eg, nasogastric tube, aspiration tube). The fresh gas flow is delivered with a dedicated flow delivery systemg (Figure 1B), which has been designed for delivery of gas (air and O2 ) in CPAP ventilation to patients during spontaneous breathing, allowing high variable gas flows with different FiO2 . The main parts of the StarVent 2 system are 2 oxygen flowmeters and a venturi valve (Figure 1B). One flowmeter supplies the venturi valve, while the second one bypasses the valve and is used as an accessory flowmeter. The O2 flow passes through the venturi valve, increasing total flow by aspirating ambient air, and generates a high fresh gas flow that is a mixture of O2 and air. The second flowmeter bypasses the venturi valve and is activated by an operator when a higher FiO2 is required (Figure 1B). The oxygen flow rate of the 2 flowmeters can be selected, and in combination with CPAP, administered to the patient to allow regulation of the flow and FiO2 of the fresh gas. The manufacturer provides a table that indicates the flow to set on the flowmeters on the basis of the desired level of FiO2 580
and CPAP. The device is connected to the O2 source with a dedicated connector and to the helmet with a 250-cm adult smoothbore circuit. In order to deliver fresh gas flow with a constant FiO2 of 0.21 in this study, we connected the StarVENT to a pressurized medical air source. The adequacy of the flow delivered to the patient was continuously monitored with a calibrated heated Fliesch pneumotachograph that was positioned at the outlet of the device.
Study protocol A pediatric small (7 L) helmet was positioned on all dogs immediately after extubation (recovering of the swallowing reflex). The head of the patient was positioned inside the helmet, with the elastic collar placed at the level of the proximal third of the neck (Figure 1A). If the helmet was too small, the 9-L helmet was applied. The venturi valve was supplied with 10 L/min of medical air, which generated 50 L/min of fresh gas flow after passage through the venturi valve (based on the calibration data provided by the manufacturer). All patients underwent 3 consecutive ventilatory phases, each characterized by a different level of CPAP. For the first 20 minutes, 0 cm H2 O of CPAP was administered (pre-CPAP phase), after which the CPAP level was adjusted to 5 cm H2 O for 20 minutes (CPAP phase), and finally the CPAP level was returned to 0 cm H2 O for another 20 minutes (post-CPAP). All phases were in the same sequence for each patient. Dogs were positioned in sternal recumbency during the entire study. The following physiological parameters were monitored during the study: heart rate (HR) and rhythm C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
CPAP administration by means a helmet in dogs
Table 1: Helmet tolerance score 1 2 3 4
The patient is comfortable; no agitation; no attempts to remove the helmet. The patient tolerates the helmet, but looks stressed and afraid; no attempts to remove the helmet. The patient tries to remove the helmet and is agitated. It is still possible to keep the helmet on by gently restraining the patient. The patient does not tolerate the helmet, is restless, attempts to move and to pull the helmet, and needs extra sedation to tolerate the helmet.
(using an ECG), respiratory rate (RR) by the observation of chest wall excursions, and noninvasive mean arterial pressure (MAP) at the level of the left metatarsal artery. The pressure inside the helmet (pHelmet) was continuously monitored by the use of the manometer that was equipped with the helmet. An operator (MC) continuously monitored the pressure during the CPAP phase to detect any variation of the pHelmet from the predetermined level of CPAP (5 cm H2 O). The FiO2 of the gas inside the helmet (FiO2 Helmet) was continuously monitored by an oximeterh with the sampling line inserted inside the helmet. The minimum and maximum alarm limits of the oximeter were adjusted to 20.5% and 22.0%, respectively, in order to detect any variation in FiO2 from the predetermined value (21%). At the end of each ventilation phase and before the beginning of the next, the sedation level of the patient and tolerance to the helmet were evaluated, an arterial blood sample was collected (1 mL) from the right metatarsal artery, and the following parameters were recorded: HR, RR, MAP, pHelmet, and FiO2 Helmet. Assessment of sedation and tolerance to the helmet The level of sedation shown by the dogs was assessed by an experienced observer (PC) on a 10-cm visual analog scale, with a score of 0 indicating no sedation and 10 indicating deep sedation. Sedation was assessed by observing the dog’s resting state, its response to clapping and calling its name, its reaction to touch, and its tendency to stand and ability to walk. Sedation progressed from ability to walk without ataxia, to standing with ataxia, to maintaining sternal recumbency, to being able to lift the head only.22 The tolerance of the patient to the helmet was assessed by the same experienced observer, using a simple descriptive scale (Table 1) ranging from 0 (the patient is comfortable, shows no agitation, makes no attempts to remove the helmet) to 4 (the patient does not tolerate the helmet, is restless, attempts to move and to pull the helmet, and needs extra sedation to tolerate the helmet). For the specific aim of the study, if the score was 4, the helmet was removed and the dog was excluded from the study. Arterial blood gas analysis Immediately after collection, arterial blood samples were sealed and analyzed with an automated, daily calibrated C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
analyzer.i All values were corrected for the body temperature of the animal measured at the time of sampling. Arterial blood pH (pH) and blood gas partial pressures of O2 and CO2 (PaO2 , PaCO2 ; mm Hg) were measured. The alveolar-arterial oxygen gradient (P[A−a]O2 ) was calculated for each patient by using the alveolar gas equation: P(A − a)O2 = ([PB − PH2O ] × FiO2 − PaCO2 /R) − PaO2 where PB is the barometric pressure, PH2O the water vapor pressure, and R the respiratory quotient, assumed to be 0.8. The PB was recorded by the analyzer during each analysis, and the PH2O was corrected for the rectal temperature of the patient recorded at the time of arterial blood collection. Statistical analysis Data were testedj for normal distribution with the Kolmogorov–Smirnov test. For all collected data, the mean ± SD (parametric data) or the median and range (nonparametric data) were calculated. Nonparametric data (sedation and tolerance scores) were tested with the Kruskal–Wallis test, followed by the Dunn test. Parametric data, obtained at pre-CPAP, CPAP, and post-CPAP, were compared with a 2-way ANOVA for repeated measures (time and treatment) followed by a StudentNewman-Keuls test (P < 0.05).
Results The body weight and age of the dogs included in the study (mean ± SD) were 23.3 ± 8.5 kg and 18.8 ± 4.9 months, respectively. The anesthesia and extubation times were 47.3 ± 4.5 and 6.2 ± 3.2 minutes, respectively. It was possible to complete the study in all patients, and none of the dogs developed severe intolerance (score 4) to the helmet (Table 2). The FiO2 Helmet and the pHelmet during the CPAP phase were always at the predetermined levels (0.21 and 5 cm H2 O, respectively) (Table 2). All dogs properly fit into the small helmet (7 L). The mean values of HR and MAP were similar at each study time (Table 2). Arrhythmias were not observed during the study. The RR and the PaCO2 (Figure 2) were significantly lower at CPAP (10.5 ± 1.4 breaths/minute and 39.2 ± 581
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Figure 2: Mean ± SD values of respiratory rate (RR) and PaCO2 in 15 dogs during recovery from general anesthesia. Each dog received 3 different consecutive levels of continuous positive airway pressure (CPAP) through a pediatric helmet: pre-CPAP = 0 cm H2 O, CPAP = 5 cm H2 O, post-CPAP = 0 cm H2 O. ∗ P < 0.05 compared with pre- and post-CPAP. Table 2: Mean ± SD values of heart rate (HR), mean arterial pressure (MAP), arterial blood pH (pH), pressure inside the helmet (pHelmet), and FiO2 inside the helmet (FiO2 Helmet), and median (range) values of the tolerance and sedation scores in 15 dogs during recovery from general anesthesia. Each dog received 3 different consecutive levels of continuous positive airway pressure (CPAP) through a pediatric helmet: pre-CPAP = 0 cm H2 O, CPAP = 5 cm H2 O, post-CPAP = 0 cm H2 O
Parameters HR (beats/minute) MAP (mm Hg) pH pHelmet (cm H2 O) FiO2 Helmet Tolerance score Sedation score
Pre-CPAP
CPAP
Post-CPAP
113.1 ± 11.4 84.5 ± 11.4 7.30 ± 0.01 0.5 ± 0.0 0.21 ± 0.01 1.5 (1–2) 6.5 (5–8)
104.5 ± 6.8 85.5 ± 9.4 7.40 ± 0.02 5.3 ± 0.1 0.21 ± 0.01 1.5 (1–2) 5 (3–8)
110.2 ± 8.2 84.2 ± 7.5 7.35 ± 0.03 0.3 ± 0.0 0.21 ± 0.01 1.5 (1–3) 4.5 (3–6)
3.0 mm Hg) than at pre-CPAP (16.0 ± 2.3 breaths/minute and 55.2 ± 4.4 mm Hg) and post-CPAP (15.0 ± 1.6 breaths/minute and 47.5 ± 4.0 mm Hg). PaO2 (Figure 3) was significantly higher at CPAP (105.6 ± 4.0 mm Hg) than at pre-CPAP (80.6 ± 6.9 mm Hg) and post-CPAP (86.7 ± 5.8 mm Hg). P(A-a)O2 (Figure 3) was significantly lower at CPAP (4.5 ± 1.9 mm Hg) than at pre-CPAP (14.0 ± 7.0 mm Hg) and post-CPAP (14.0 ± 7.7 mm Hg). The mean value of pHelmet was 5.3 ± 0.1 cm H2 O at CPAP, 0.5 ± 0.0 cm H2 O at pre-CPAP, and 0.3 ± 0.0 cm 582
H2 O at post-CPAP (Table 2). The median (range) values of the tolerance and sedation scores were similar at each study time (Table 2).
Discussion Our data show that a pediatric helmet is an adequate and well-tolerated interface for administering CPAP in dogs with a body weight >10 kg. It is associated with improvement in lung function and adequate oxygenation during recovery from general anesthesia. The key factors that influence the interface performance in delivering CPAP are sealing of the interface, amount of air leakage, CO2 washout, and patient comfort.23 Unlike the traditional face mask,17 the helmet seals the neck surface with a soft elastic silicone collar that is easily tolerated. Theoretically, unlike in people, the dogs’ haircoats could have affected the sealing of the helmet. However, our data showed that it was possible to keep the desired CPAP level (5 cm H2 O) in all dogs. On the other hand, complete sealing is impossible with any kind of interface, and the unavoidable leaks should be compensated by the high gas flow.13 An ideal CPAP system should maintain a constant airway pressure throughout the respiratory cycle.24 The pressure inside the helmet results from the combination C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
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Figure 3: Mean ± SD values of PaO2 and alveolar-arterial oxygen gradient (P[A−a]O2 ) in 15 dogs during recovery from general anesthesia. Each dog received 3 different consecutive levels of continuous positive airway pressure (CPAP) through a pediatric helmet: pre-CPAP = 0 cm H2 O, CPAP = 5 cm H2 O, post-CPAP = 0 cm H2 O. ∗ P < 0.05 compared with pre- and post-CPAP.
of a constant gas flow delivered to the inlet port and resistance (CPAP valve) to the expiratory gas flow positioned at the outlet port.8 The delivered gas flow also plays a critical role in CO2 washout.23 The helmet is a semiclosed environment, where CO2 concentration depends on 2 main factors: the amount of CO2 produced by the subject and the flow of the fresh gas that flushes the helmet.23 Therefore, a relatively high gas flow (2–3 times the patient’s estimated minute volume) is required to avoid CO2 rebreathing.23 In our study, independent of patient size, the inspiratory flow was kept constant at 50 L/minute, a value far above the estimated minute volume in dogs (ࣅ 205 mL/kg/min).25 The decrease of PaCO2 and respiratory rate during CPAP in our dogs indirectly confirms the absence of rebreathing. Further studies are required in order to evaluate the minimum fresh gas flow threshold necessary to avoid rebreathing in different sized dogs under different clinical conditions. The physiological data collected in this study indicate that CPAP is effective in improving lung function in dogs recovering from general anesthesia. Postoperative pulmonary dysfunction after abdominal surgery may have an impact in terms of incidence of hypoxemia, hypercapnia, postoperative pulmonary infections, and C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
prolongation of hospital stay in people.26,27 The advantages of postoperative CPAP administration have been well established in people, especially for patients undergoing abdominal procedures at higher risk of postoperative pulmonary complications (PPCs) (eg, obese patients, patients with preexisting pulmonary disease).2,3, 28 In the veterinary literature, few studies have investigated postoperative pulmonary function in healthy dogs after abdominal surgery. Campbell et al29 suggested that opioid administration in dogs undergoing ovariohysterectomy may be associated with mild postoperative hypoxemia and hypercapnia. Pulmonary atelectasis has been recognized as one of the most important factors that can affect lung function in the postoperative period in people.26,27 Collapsed (atelectasis) and poorly aerated alveoli develop in the most dependent area of the lungs and contribute to the ventilation-to-perfusion mismatch in terms of venous admixture.26,30 Unfortunately, data on the persistence of atelectasis in animals in the postoperative period are scanty. The low PaO2 and the high P(A-a)O2 during the pre- and post-CPAP phases in our patients suggest the persistence of pulmonary atelectasis during recovery that was likely re-expanded (recruited) by CPAP, as witnessed by the fact that PaO2 and P(Aa)O2 returned to within normal limits for awake healthy 583
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dogs.31 Since our dogs were breathing at 0.21 FiO2 during recovery, we can state that the improvement in oxygenation was the pure effect of the application of CPAP. In agreement with the findings of the Campbell study,31 the results of our study showed that dogs also presented moderate/severe hypercapnia at pre-CPAP and postCPAP compared with the CPAP phase when the PaCO2 was within normal limits.31 Moreover, dogs in the CPAP phase also had a significantly lower RR during CPAP administration than dogs in the other 2 phases. The concomitant decrease in PaCO2 and RR with CPAP suggests an increase in tidal volume with an overall increase in minute volume. Our data agree with the physiological studies in people that demonstrated that noninvasive CPAP is able to increase functional residual capacity, improve alveolar ventilation, maintain airway patency, and reduce atelectasis formation and venous admixture.32 These beneficial effects of CPAP are the result of 2 mechanisms: the direct application of positive pressure to the respiratory system and the indirect effect on the work of breathing (WOB).33 The diaphragmatic WOB can be divided into elastic (lung elastic recoil, chest wall recoil, and surface tension) and resistive (airway resistances) components. The application of noninvasive CPAP reduces WOB primarily by the reduction of upper airway resistance, preventing airway collapse with the inspiratory negative pressure.33 Thus most of the diaphragmatic strength can be used to expand the chest wall and the lungs, promoting an increase in lung volumes (tidal volume and functional residual capacity) and the re-expansion of atelectasis. The application of CPAP may affect hemodynamics through several mechanisms: decreased venous return, increased right ventricular afterload, decreased ventricular compliance, and decreased ventricular contractility.34 Overall, the hemodynamic consequences of the application of CPAP depend on prior ventricular loading conditions and ventricular function.34 The cardiovascular variables measured in this study (HR and blood pressure) were not different in CPAP than in no CPAP. However, future studies using more invasive cardiovascular monitoring need to measure cardiac output in order to evaluate the effect of CPAP on oxygen delivery, before it can be used in compromised animals. The administration of a FiO2 of 0.21 was related to the specific aim of the study to investigate the pure effect of CPAP on lung function. Indeed, in clinical practice, the FiO2 would be adjusted to the clinically required level by regulating the oxygen flowmeters, following the predetermined setting provided by the manufacturer. In this regard, the table produced for the flowmeter settings for the different FiO2 s needs to be assessed for validation for veterinary use. The patient’s comfort strongly influ584
ences the efficacy of CPAP therapy.13 Our data demonstrate that the helmet is well tolerated in dogs recovering from general anesthesia (average tolerance score of 1.5). Our patients were, however, sedated and the helmet was applied for a short period. Further studies are required to evaluate the degree of helmet tolerance in awake patients and for longer periods. This study was performed in healthy patients recovering from general anesthesia, and although the results are promising for future applications, the efficacy of this system in different pathological conditions should be supported by further studies. Because of the clinical nature of the study, it was not possible to use more invasive respiratory monitoring that may have given us more useful information about the effects of CPAP on respiratory system mechanics. The results of this study confirmed the feasibility and safety of the helmet system in delivering CPAP in healthy dogs with a body weight >10 kg recovering from a short period of anesthesia. If these data can be confirmed in a larger population of patients and in different pathological conditions, noninvasive CPAP therapy through the helmet could become an important therapeutic option in small animal practice. More studies are also required to evaluate the feasibility of the helmet in a wider range of body sizes. The design and development of specific veterinary helmets could further improve the efficacy and therapeutic impact of CPAP administered through a helmet in dogs.
Footnotes a b c d e f g h i j
Prequillant 1%; Fatro SpA, Bologna, Italy. Morfina Cloridrato 1%; Molteni SpA, Firenze, Italy. PropVet 1%; Esteve Hospira Inc, Chicago, IL. Isoba; Schering-Plough SpA, Milano, Italy. SC 6002XL; Siemens, Danvers, MA. CaStar; StarMed, Mirandola, Italy. StarVENT 2; StarMed. MiniOX; MSA, Gurnee, IL. VetStat; IDEXX Laboratories Inc., Westbrook, ME. MedCalc; version 9.2.0.1, MedCalc Software bvba, Mariakerke, Belgium.
References 1. Chiumello D, Chevallard G, Gregoretti C. Non-invasive ventilation in postoperative patients: a systematic review. Intensive Care Med 2011; 37:918–929. 2. Jaber S, Michelet P, Chanques G. Role of non-invasive ventilation (NIV) in the perioperative period. Best Pract Res Clin Anaesthesiol 2010; 24:253–265. 3. Squadrone V, Coha M, Cerutti E, et al. Continuous positive airway pressure for treatment of postoperative hypoxemia: a randomized controlled trial. JAMA 2005; 293:589–595. 4. Stock MC, Downs JB, Gauer PK, et al. Prevention of postoperative pulmonary complications with CPAP, incentive spirometry, and conservative therapy. Chest 1985; 87:151–157. 5. Dehaven CB, Jr., Hurst JM, Branson RD. Postextubation hypoxemia treated with a continuous positive airway pressure mask. Crit Care Med 1985; 13:46–48. C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
CPAP administration by means a helmet in dogs 6. Lindner KH, Lotz P, Ahnefeld FW. Continuous positive airway pressure effect on functional residual capacity, vital capacity and its subdivisions. Chest 1987; 92:66–70. 7. Delclaux C, L’Her E, Alberti C, et al. Treatment of acute hypoxemic nonhypercapnic respiratory insufficiency with continuous positive airway pressure delivered by a face mask: a randomized controlled trial. JAMA 2000; 284:2352–2360. 8. Rocco M, Dell’Utri D, Morelli A, et al. Noninvasive ventilation by helmet or face mask in immunocompromised patients: a casecontrol study. Chest 2004; 126:1508–1515. 9. Piastra M, Antonelli M, Chiaretti A, et al. Treatment of acute respiratory failure by helmet-delivered non-invasive pressure support ventilation in children with acute leukemia: a pilot study. Intensive Care Med 2004; 30:472–476. 10. Antonelli M, Pennisi MA, Pelosi P, et al. Noninvasive positive pressure ventilation using a helmet in patients with acute exacerbation of chronic obstructive pulmonary disease: a feasibility study. Anesthesiology 2004; 100:16–24. 11. Mosing M, Rysnik M, Bardell D, et al. Use of continuous positive airway pressure (CPAP) to optimise oxygenation in anaesthetised horses—a clinical study. Equine Vet J 2013; 45:414-418. 12. MacFarlane PD, Mosing M. Early experience with continuous positive airway pressure (CPAP) in 5 horses—a case series. Can Vet J 2012; 53:426–429. 13. Courtney SE, Barrington KJ. Continuous positive airway pressure and noninvasive ventilation. Clin Perinatol 2007; 34:73–92. 14. Briganti A, Melanie P, Portela D, et al. Continuous positive airway pressure administered via face mask in tranquilized dogs. J Vet Emerg Crit Care (San Antonio) 2010; 20:503–508. 15. Manning AM. Oxygen therapy and toxicity. Vet Clin North Am Small Anim Pract 2002; 32:1005–1020. 16. Marks SL. Cardiopulmonary resuscitation and oxygen therapy. Vet Clin North Am Small Anim Pract 1999; 29:959–969. 17. Chiumello D, Pelosi P, Carlesso E, et al. Noninvasive positive pressure ventilation delivered by helmet vs. standard face mask. Intensive Care Med 2003; 29:1671–1679. 18. Pelosi P, Severgnini P, Aspesi M, et al. Non-invasive ventilation delivered by conventional interfaces and helmet in the emergency department. Eur J Emerg Med 2003; 10:79–86. 19. Patroniti N, Foti G, Manfio A, et al. Head helmet versus face mask for non-invasive continuous positive airway pressure: a physiological study. Intensive Care Med 2003; 29:1680–1687. 20. Milesi C, Ferragu F, Jaber S, et al. Continuous positive airway pressure ventilation with helmet in infants under 1 year. Intensive Care Med 2010; 36:1592–1596.
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21. Codazzi D, Nacoti M, Passoni M, et al. Continuous positive airway pressure with modified helmet for treatment of hypoxemic acute respiratory failure in infants and a preschool population: a feasibility study. Pediatr Crit Care Med 2006; 7:455–460. 22. Brodbelt DC, Taylor PM, Stanway GW. A comparison of preoperative morphine and buprenorphine for postoperative analgesia for arthrotomy in dogs. J Vet Pharmacol Ther 1997; 20:284– 289. 23. Taccone P, Hess D, Caironi P, et al. Continuous positive airway pressure delivered with a “helmet”: effects on carbon dioxide rebreathing. Crit Care Med 2004; 32:2090–2096. 24. Katz JA, Kraemer RW, Gjerde GE. Inspiratory work and airway pressure with continuous positive airway pressure delivery systems. Chest 1985; 88:519–526. 25. Gillespie DJ, Hyatt RE. Respiratory mechanics in the unanesthetized dog. J Appl Physiol 1974; 36:98–102. 26. Duggan M, Kavanagh BP. Perioperative modifications of respiratory function. Best Pract Res Clin Anaesthesiol 2010; 24:145–155. 27. Warner DO. Preventing postoperative pulmonary complications: the role of the anesthesiologist. Anesthesiology 2000; 92:1467–1472. 28. Ferreyra GP, Baussano I, Squadrone V, et al. Continuous positive airway pressure for treatment of respiratory complications after abdominal surgery: a systematic review and meta-analysis. Ann Surg 2008; 247:617–626. 29. Campbell VL, Drobatz KJ, Perkowski SZ. Postoperative hypoxemia and hypercarbia in healthy dogs undergoing routine ovariohysterectomy or castration and receiving butorphanol or hydromorphone for analgesia. J Am Vet Med Assoc 2003; 222:330– 336. 30. Staffieri F, Franchini D, Carella GL, et al. Computed tomographic analysis of the effects of two inspired oxygen concentrations on pulmonary aeration in anesthetized and mechanically ventilated dogs. Am J Vet Res 2007; 68:925–931. 31. Haskins S, Pascoe PJ, Ilkiw JE, et al. Reference cardiopulmonary values in normal dogs. Comp Med 2005; 55:156–161. 32. Abboud N, Rehder K, Rodarte JR, et al. Lung volumes and closing capacity with continuous positive airway pressure. Anesthesiology 1975; 42:138–142. 33. Keidan I, Fine GF, Kagawa T, et al. Work of breathing during spontaneous ventilation in anesthetized children: a comparative study among the face mask, laryngeal mask airway and endotracheal tube. Anesth Analg 2000; 91:1381–1388. 34. Montner PK, Greene ER, Murata GH, et al. Hemodynamic effects of nasal and face mask continuous positive airway pressure. Am J Respir Crit Care Med 1994; 149:1614–1618.
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Journal of Veterinary Emergency and Critical Care 24(5) 2014, pp 578–585 doi: 10.1111/vec.12210
Noninvasive continuous positive airway pressure delivered using a pediatric helmet in dogs recovering from general anesthesia Francesco Staffieri, DVM, PhD; Antonio Crovace, DVM; Valentina De Monte, DVM, PhD; Paola Centonze, DVM; Giulio Gigante, DVM and Salvatore Grasso, MD Abstract
Objective – To evaluate the feasibility and efficacy of noninvasive continuous positive airway pressure (CPAP) administered with a pediatric helmet in healthy dogs recovering from general anesthesia. Design – Randomized, cross-over, clinical study. Setting – University teaching hospital. Animals – Fifteen healthy female, client-owned dogs recovering from general anesthesia following elective ovariohysterectomy. Interventions – All dogs received the same standardized anesthetic protocol (acepromazine, morphine, propofol, and isoflurane in oxygen). After extubation, a pediatric helmet was placed on all dogs and connected to a venturi valve supplied with medical air. In all patients, the gas flow was set to 50 L/minute and the FiO2 to 0.21. Dogs received the following sequence of treatments, each lasting 20 minutes: 0 CPAP (pre-CPAP), CPAP of 5 cm H2 O (CPAP), and again 0 CPAP (post-CPAP). Measurements and Main Results – During the entire study, the following data were collected: pressure and FiO2 inside the helmet, mean arterial pressure, respiratory rate, heart rate, sedation score (0 = awake, 10 = deep sedation), and tolerance to the helmet (0 = excellent, 4 = poor). At the end of each phase, an arterial blood sample was sampled. As compared with the pre-CPAP and the post-CPAP periods, during the CPAP period, the PaCO2 , alveolar-arterial oxygen gradient (P[A−a]O2 ), and respiratory rate significantly decreased. The PaO2 was higher at CPAP (105.6 ± 4.0 mm Hg) compared with pre-CPAP (80.6 ± 6.9 mm Hg) and post-CPAP (86.7 ± 5.8 mm Hg). Tolerance and sedation scores during the CPAP period were not different from those in the pre-CPAP and post-CPAP periods. Conclusions – Noninvasive CPAP applied through a helmet is a feasible and effective supportive technique in dogs recovering from general anesthesia. (J Vet Emerg Crit Care 2014; 24(5): 578–585) doi: 10.1111/vec.12210 Keywords: atelectasis, CPAP, dogs, respiratory distress
Introduction Noninvasive ventilation is a type of ventilatory support that does not require an artificial airway (eg, From the Dipartimento dell’Emergenza e dei Trapianti d’Organo, Sezione di Cliniche Veterinarie e Produzioni Animali, (Staffieri, Crovace, De Monte, Centonze, Gigante); and Dipartimento dell’Emergenza e dei Trapianti d’Organo, Sezione di Anestesiologia e Rianimazione (Grasso), SP per Casamassima km 3, 70010 Valenzano, Bari, Italy. Supported by the intramural fundings of the Dipartimento dell’Emergenza e dei Trapianti d’Organo, Sezione di Cliniche Veterinarie e Produzioni Animali. Presented in part at the Association of Veterinary Anaesthetists, Spring Meeting in Davos, 21–23 March 2012.
Abbreviations
CPAP FiO2 Helmet HR MAP pHelmet P(A−a)O2 PPCs RR WOB
continuous positive airway pressure FiO2 inside the helmet heart rate mean arterial pressure pressure inside the helmet alveolar to arterial partial pressure gradient of oxygen postoperative pulmonary complications respiratory rate work of breathing
The authors declare no conflict of interests. Address correspondence and reprint requests to Dr. Francesco Staffieri, Dipartimento dell’Emergenza e dei Trapianti d’Organo, Sezione di Cliniche Veterinarie e Produzioni Animali, SP per Casamassima km 3, 70010 Valenzano, Bari, Italy. Email: francesco.staffieri@ uniba.it Submitted January 15, 2013; Accepted June 27, 2014.
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endotracheal tube, tracheostomy) and compared with invasive ventilation, requires minimal sedation, improves C Veterinary Emergency and Critical Care Society 2014
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patient comfort, and reduces nosocomial infection rates in people.1 Noninvasive ventilation may be applied as continuous positive airway pressure (CPAP) and intermittent positive pressure ventilation.2 CPAP is a breathing mode by which the patient spontaneously breathes through a pressurized circuit against a threshold resistor that maintains a preset positive airway pressure during both inspiration and expiration.3 Several studies in people have demonstrated the efficacy of CPAP to reduce the work of breathing and atelectasis, improve oxygenation, and reduce the need for intubation in patients who develop hypoxemia after surgery or as a consequence of lung diseases.3–10 Different clinical situations could benefit from CPAP in veterinary medicine and these include postoperative respiratory assistance, exacerbation of the brachycephalic syndrome, and acute respiratory failure. Recently, invasive CPAP has been demonstrated to improve oxygenation in horses under general anesthesia.11,12 The interface by which CPAP is administered plays a critical role in the efficacy of treatment.13 A recent study demonstrated an improvement in arterial oxygenation when CPAP was administered through a conical face mask equipped with a Boussignac valve in dogs.14 In this study, CPAP was applied for a short time in sedated patients and continuously assisted by an operator to guarantee mask sealing. Clinical experience suggests that it is sometimes difficult to guarantee mask sealing because of the wide variety of facial conformation among different dog breeds.15 Comfort of the patient is also an issue, with sedation and continuous assistance by an experienced operator often required.15,16 The head helmet as an interface for the administration of CPAP has been recently introduced in human medicine with interesting results.8,17,18 The helmet is designed to contain the head while providing a seal all around the patient’s neck. With the helmet, positive pressure is applied around the head as it would be in a pressurized chamber.19 The main advantage of the helmet is the comfort of the patient. The helmet has also been used in neonates who, like veterinary patients, often do not tolerate the mask without deep sedation.9,20, 21 The aim of this study was to evaluate the feasibility of administering noninvasive CPAP through a pediatric helmet in dogs recovering from general anesthesia. Our hypothesis was that, similarly to its use in children, the helmet could be effective and well tolerated for delivering CPAP in dogs.
Materials and Methods The study was conducted in compliance with the Italian Animal Welfare and statutes of the University of Bari, C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
relating to the use of client-owned animals in clinical investigations. Animals Fifteen healthy, female, client-owned dogs of mixed breed, with a body weight >10 kg, scheduled for elective ovariohysterectomy, were enrolled in the study. Written owner informed consent was obtained. Preoperative screening included a CBC, serum biochemical analyses, and thoracic radiographs. Dogs with abnormal clinicopathological findings or physical examination evidence of pulmonary disease were excluded from the study. Anesthesia and surgery All dogs were premedicated with intramuscular administration of acepromazinea (0.03 mg/kg) followed by morphineb (0.4 mg/kg). When an adequate level of sedation was achieved, an IV catheter was aseptically placed at the level of the right cephalic vein. General anesthesia was induced with IV propofolc (5–6 mg/kg) and, after orotracheal intubation, maintained with inhalation of isofluraned in oxygen (100 mL/kg/min) through a rebreathing circuit. All patients were spontaneously breathing throughout the procedure. The depth of anesthesia was assessed from evaluation of the palpebral reflex, position of the eyes, and jaw tone. The main cardiovascular and respiratory parameters were continuously monitored throughout anesthesia.e All patients underwent ovariohysterectomy through a midline laparotomy. At the end of surgery, isoflurane administration was discontinued, and patients were extubated when the swallowing reflex recovered. In all patients, the duration of anesthesia (from induction to discontinuation of isoflurane administration) and extubation time (from discontinuation of isoflurane administration to extubation) were recorded. Equipment The helmetf is made of transparent, latex-free polyvinyl chloride (Figure 1A). At its base, it is equipped with a rigid plastic collar that joins the helmet to a soft elastic collar, which adheres to the neck while avoiding air leakage. The device is also equipped with 2 armpit braces that can be secured to a pair of hooks on the plastic ring. The braces are used in human patients to improve the stability of the helmet. The pediatric helmet is available in 2 sizes: small, with an internal gas volume of 7 L (for infants 10 kg). The helmet is equipped with 2 gas ports that act as the inlet and outlet for inspiratory and expiratory gas flow, respectively (Figure 1A). The inlet opening is connected to the fresh gas source 579
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Figure 1: Equipment used to provide noninvasive continuous positive airway pressure (CPAP) in dogs recovering from general anesthesia. The helmet (A) is equipped with an inlet (1) and outlet (2) gas port. The CPAP valve (3) is located on the outlet gas port. The helmet also has an antiasphyxia bidirectional safety valve (4) and an accessory port (5). The flow delivery system (B) consists of an oxygen flowmeter (7) connected to a venturi valve (6) and a second oxygen flowmeter (8) that bypasses the venturi valve (9).
while the outlet opening is connected to the CPAP valve, which allows adjustment of the CPAP level (Figure 1A). The helmet also has an anti-asphyxia bidirectional safety valve (Figure 1A) made with a precalibrated spring in a membrane system; in the event that fresh gas delivery stops, this valve opens immediately, allowing the patient to breathe ambient air. The valve is positioned on the cover of an opening that when removed allows the clinician immediate access to the patient’s face. The expiratory port is equipped with a disposable manometer that measures the pressure inside the helmet (Figure 1A). Two small accessory ports are available on the rigid collar for the introduction of small catheters (eg, nasogastric tube, aspiration tube). The fresh gas flow is delivered with a dedicated flow delivery systemg (Figure 1B), which has been designed for delivery of gas (air and O2 ) in CPAP ventilation to patients during spontaneous breathing, allowing high variable gas flows with different FiO2 . The main parts of the StarVent 2 system are 2 oxygen flowmeters and a venturi valve (Figure 1B). One flowmeter supplies the venturi valve, while the second one bypasses the valve and is used as an accessory flowmeter. The O2 flow passes through the venturi valve, increasing total flow by aspirating ambient air, and generates a high fresh gas flow that is a mixture of O2 and air. The second flowmeter bypasses the venturi valve and is activated by an operator when a higher FiO2 is required (Figure 1B). The oxygen flow rate of the 2 flowmeters can be selected, and in combination with CPAP, administered to the patient to allow regulation of the flow and FiO2 of the fresh gas. The manufacturer provides a table that indicates the flow to set on the flowmeters on the basis of the desired level of FiO2 580
and CPAP. The device is connected to the O2 source with a dedicated connector and to the helmet with a 250-cm adult smoothbore circuit. In order to deliver fresh gas flow with a constant FiO2 of 0.21 in this study, we connected the StarVENT to a pressurized medical air source. The adequacy of the flow delivered to the patient was continuously monitored with a calibrated heated Fliesch pneumotachograph that was positioned at the outlet of the device.
Study protocol A pediatric small (7 L) helmet was positioned on all dogs immediately after extubation (recovering of the swallowing reflex). The head of the patient was positioned inside the helmet, with the elastic collar placed at the level of the proximal third of the neck (Figure 1A). If the helmet was too small, the 9-L helmet was applied. The venturi valve was supplied with 10 L/min of medical air, which generated 50 L/min of fresh gas flow after passage through the venturi valve (based on the calibration data provided by the manufacturer). All patients underwent 3 consecutive ventilatory phases, each characterized by a different level of CPAP. For the first 20 minutes, 0 cm H2 O of CPAP was administered (pre-CPAP phase), after which the CPAP level was adjusted to 5 cm H2 O for 20 minutes (CPAP phase), and finally the CPAP level was returned to 0 cm H2 O for another 20 minutes (post-CPAP). All phases were in the same sequence for each patient. Dogs were positioned in sternal recumbency during the entire study. The following physiological parameters were monitored during the study: heart rate (HR) and rhythm C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
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Table 1: Helmet tolerance score 1 2 3 4
The patient is comfortable; no agitation; no attempts to remove the helmet. The patient tolerates the helmet, but looks stressed and afraid; no attempts to remove the helmet. The patient tries to remove the helmet and is agitated. It is still possible to keep the helmet on by gently restraining the patient. The patient does not tolerate the helmet, is restless, attempts to move and to pull the helmet, and needs extra sedation to tolerate the helmet.
(using an ECG), respiratory rate (RR) by the observation of chest wall excursions, and noninvasive mean arterial pressure (MAP) at the level of the left metatarsal artery. The pressure inside the helmet (pHelmet) was continuously monitored by the use of the manometer that was equipped with the helmet. An operator (MC) continuously monitored the pressure during the CPAP phase to detect any variation of the pHelmet from the predetermined level of CPAP (5 cm H2 O). The FiO2 of the gas inside the helmet (FiO2 Helmet) was continuously monitored by an oximeterh with the sampling line inserted inside the helmet. The minimum and maximum alarm limits of the oximeter were adjusted to 20.5% and 22.0%, respectively, in order to detect any variation in FiO2 from the predetermined value (21%). At the end of each ventilation phase and before the beginning of the next, the sedation level of the patient and tolerance to the helmet were evaluated, an arterial blood sample was collected (1 mL) from the right metatarsal artery, and the following parameters were recorded: HR, RR, MAP, pHelmet, and FiO2 Helmet. Assessment of sedation and tolerance to the helmet The level of sedation shown by the dogs was assessed by an experienced observer (PC) on a 10-cm visual analog scale, with a score of 0 indicating no sedation and 10 indicating deep sedation. Sedation was assessed by observing the dog’s resting state, its response to clapping and calling its name, its reaction to touch, and its tendency to stand and ability to walk. Sedation progressed from ability to walk without ataxia, to standing with ataxia, to maintaining sternal recumbency, to being able to lift the head only.22 The tolerance of the patient to the helmet was assessed by the same experienced observer, using a simple descriptive scale (Table 1) ranging from 0 (the patient is comfortable, shows no agitation, makes no attempts to remove the helmet) to 4 (the patient does not tolerate the helmet, is restless, attempts to move and to pull the helmet, and needs extra sedation to tolerate the helmet). For the specific aim of the study, if the score was 4, the helmet was removed and the dog was excluded from the study. Arterial blood gas analysis Immediately after collection, arterial blood samples were sealed and analyzed with an automated, daily calibrated C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
analyzer.i All values were corrected for the body temperature of the animal measured at the time of sampling. Arterial blood pH (pH) and blood gas partial pressures of O2 and CO2 (PaO2 , PaCO2 ; mm Hg) were measured. The alveolar-arterial oxygen gradient (P[A−a]O2 ) was calculated for each patient by using the alveolar gas equation: P(A − a)O2 = ([PB − PH2O ] × FiO2 − PaCO2 /R) − PaO2 where PB is the barometric pressure, PH2O the water vapor pressure, and R the respiratory quotient, assumed to be 0.8. The PB was recorded by the analyzer during each analysis, and the PH2O was corrected for the rectal temperature of the patient recorded at the time of arterial blood collection. Statistical analysis Data were testedj for normal distribution with the Kolmogorov–Smirnov test. For all collected data, the mean ± SD (parametric data) or the median and range (nonparametric data) were calculated. Nonparametric data (sedation and tolerance scores) were tested with the Kruskal–Wallis test, followed by the Dunn test. Parametric data, obtained at pre-CPAP, CPAP, and post-CPAP, were compared with a 2-way ANOVA for repeated measures (time and treatment) followed by a StudentNewman-Keuls test (P < 0.05).
Results The body weight and age of the dogs included in the study (mean ± SD) were 23.3 ± 8.5 kg and 18.8 ± 4.9 months, respectively. The anesthesia and extubation times were 47.3 ± 4.5 and 6.2 ± 3.2 minutes, respectively. It was possible to complete the study in all patients, and none of the dogs developed severe intolerance (score 4) to the helmet (Table 2). The FiO2 Helmet and the pHelmet during the CPAP phase were always at the predetermined levels (0.21 and 5 cm H2 O, respectively) (Table 2). All dogs properly fit into the small helmet (7 L). The mean values of HR and MAP were similar at each study time (Table 2). Arrhythmias were not observed during the study. The RR and the PaCO2 (Figure 2) were significantly lower at CPAP (10.5 ± 1.4 breaths/minute and 39.2 ± 581
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Figure 2: Mean ± SD values of respiratory rate (RR) and PaCO2 in 15 dogs during recovery from general anesthesia. Each dog received 3 different consecutive levels of continuous positive airway pressure (CPAP) through a pediatric helmet: pre-CPAP = 0 cm H2 O, CPAP = 5 cm H2 O, post-CPAP = 0 cm H2 O. ∗ P < 0.05 compared with pre- and post-CPAP. Table 2: Mean ± SD values of heart rate (HR), mean arterial pressure (MAP), arterial blood pH (pH), pressure inside the helmet (pHelmet), and FiO2 inside the helmet (FiO2 Helmet), and median (range) values of the tolerance and sedation scores in 15 dogs during recovery from general anesthesia. Each dog received 3 different consecutive levels of continuous positive airway pressure (CPAP) through a pediatric helmet: pre-CPAP = 0 cm H2 O, CPAP = 5 cm H2 O, post-CPAP = 0 cm H2 O
Parameters HR (beats/minute) MAP (mm Hg) pH pHelmet (cm H2 O) FiO2 Helmet Tolerance score Sedation score
Pre-CPAP
CPAP
Post-CPAP
113.1 ± 11.4 84.5 ± 11.4 7.30 ± 0.01 0.5 ± 0.0 0.21 ± 0.01 1.5 (1–2) 6.5 (5–8)
104.5 ± 6.8 85.5 ± 9.4 7.40 ± 0.02 5.3 ± 0.1 0.21 ± 0.01 1.5 (1–2) 5 (3–8)
110.2 ± 8.2 84.2 ± 7.5 7.35 ± 0.03 0.3 ± 0.0 0.21 ± 0.01 1.5 (1–3) 4.5 (3–6)
3.0 mm Hg) than at pre-CPAP (16.0 ± 2.3 breaths/minute and 55.2 ± 4.4 mm Hg) and post-CPAP (15.0 ± 1.6 breaths/minute and 47.5 ± 4.0 mm Hg). PaO2 (Figure 3) was significantly higher at CPAP (105.6 ± 4.0 mm Hg) than at pre-CPAP (80.6 ± 6.9 mm Hg) and post-CPAP (86.7 ± 5.8 mm Hg). P(A-a)O2 (Figure 3) was significantly lower at CPAP (4.5 ± 1.9 mm Hg) than at pre-CPAP (14.0 ± 7.0 mm Hg) and post-CPAP (14.0 ± 7.7 mm Hg). The mean value of pHelmet was 5.3 ± 0.1 cm H2 O at CPAP, 0.5 ± 0.0 cm H2 O at pre-CPAP, and 0.3 ± 0.0 cm 582
H2 O at post-CPAP (Table 2). The median (range) values of the tolerance and sedation scores were similar at each study time (Table 2).
Discussion Our data show that a pediatric helmet is an adequate and well-tolerated interface for administering CPAP in dogs with a body weight >10 kg. It is associated with improvement in lung function and adequate oxygenation during recovery from general anesthesia. The key factors that influence the interface performance in delivering CPAP are sealing of the interface, amount of air leakage, CO2 washout, and patient comfort.23 Unlike the traditional face mask,17 the helmet seals the neck surface with a soft elastic silicone collar that is easily tolerated. Theoretically, unlike in people, the dogs’ haircoats could have affected the sealing of the helmet. However, our data showed that it was possible to keep the desired CPAP level (5 cm H2 O) in all dogs. On the other hand, complete sealing is impossible with any kind of interface, and the unavoidable leaks should be compensated by the high gas flow.13 An ideal CPAP system should maintain a constant airway pressure throughout the respiratory cycle.24 The pressure inside the helmet results from the combination C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
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Figure 3: Mean ± SD values of PaO2 and alveolar-arterial oxygen gradient (P[A−a]O2 ) in 15 dogs during recovery from general anesthesia. Each dog received 3 different consecutive levels of continuous positive airway pressure (CPAP) through a pediatric helmet: pre-CPAP = 0 cm H2 O, CPAP = 5 cm H2 O, post-CPAP = 0 cm H2 O. ∗ P < 0.05 compared with pre- and post-CPAP.
of a constant gas flow delivered to the inlet port and resistance (CPAP valve) to the expiratory gas flow positioned at the outlet port.8 The delivered gas flow also plays a critical role in CO2 washout.23 The helmet is a semiclosed environment, where CO2 concentration depends on 2 main factors: the amount of CO2 produced by the subject and the flow of the fresh gas that flushes the helmet.23 Therefore, a relatively high gas flow (2–3 times the patient’s estimated minute volume) is required to avoid CO2 rebreathing.23 In our study, independent of patient size, the inspiratory flow was kept constant at 50 L/minute, a value far above the estimated minute volume in dogs (ࣅ 205 mL/kg/min).25 The decrease of PaCO2 and respiratory rate during CPAP in our dogs indirectly confirms the absence of rebreathing. Further studies are required in order to evaluate the minimum fresh gas flow threshold necessary to avoid rebreathing in different sized dogs under different clinical conditions. The physiological data collected in this study indicate that CPAP is effective in improving lung function in dogs recovering from general anesthesia. Postoperative pulmonary dysfunction after abdominal surgery may have an impact in terms of incidence of hypoxemia, hypercapnia, postoperative pulmonary infections, and C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
prolongation of hospital stay in people.26,27 The advantages of postoperative CPAP administration have been well established in people, especially for patients undergoing abdominal procedures at higher risk of postoperative pulmonary complications (PPCs) (eg, obese patients, patients with preexisting pulmonary disease).2,3, 28 In the veterinary literature, few studies have investigated postoperative pulmonary function in healthy dogs after abdominal surgery. Campbell et al29 suggested that opioid administration in dogs undergoing ovariohysterectomy may be associated with mild postoperative hypoxemia and hypercapnia. Pulmonary atelectasis has been recognized as one of the most important factors that can affect lung function in the postoperative period in people.26,27 Collapsed (atelectasis) and poorly aerated alveoli develop in the most dependent area of the lungs and contribute to the ventilation-to-perfusion mismatch in terms of venous admixture.26,30 Unfortunately, data on the persistence of atelectasis in animals in the postoperative period are scanty. The low PaO2 and the high P(A-a)O2 during the pre- and post-CPAP phases in our patients suggest the persistence of pulmonary atelectasis during recovery that was likely re-expanded (recruited) by CPAP, as witnessed by the fact that PaO2 and P(Aa)O2 returned to within normal limits for awake healthy 583
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dogs.31 Since our dogs were breathing at 0.21 FiO2 during recovery, we can state that the improvement in oxygenation was the pure effect of the application of CPAP. In agreement with the findings of the Campbell study,31 the results of our study showed that dogs also presented moderate/severe hypercapnia at pre-CPAP and postCPAP compared with the CPAP phase when the PaCO2 was within normal limits.31 Moreover, dogs in the CPAP phase also had a significantly lower RR during CPAP administration than dogs in the other 2 phases. The concomitant decrease in PaCO2 and RR with CPAP suggests an increase in tidal volume with an overall increase in minute volume. Our data agree with the physiological studies in people that demonstrated that noninvasive CPAP is able to increase functional residual capacity, improve alveolar ventilation, maintain airway patency, and reduce atelectasis formation and venous admixture.32 These beneficial effects of CPAP are the result of 2 mechanisms: the direct application of positive pressure to the respiratory system and the indirect effect on the work of breathing (WOB).33 The diaphragmatic WOB can be divided into elastic (lung elastic recoil, chest wall recoil, and surface tension) and resistive (airway resistances) components. The application of noninvasive CPAP reduces WOB primarily by the reduction of upper airway resistance, preventing airway collapse with the inspiratory negative pressure.33 Thus most of the diaphragmatic strength can be used to expand the chest wall and the lungs, promoting an increase in lung volumes (tidal volume and functional residual capacity) and the re-expansion of atelectasis. The application of CPAP may affect hemodynamics through several mechanisms: decreased venous return, increased right ventricular afterload, decreased ventricular compliance, and decreased ventricular contractility.34 Overall, the hemodynamic consequences of the application of CPAP depend on prior ventricular loading conditions and ventricular function.34 The cardiovascular variables measured in this study (HR and blood pressure) were not different in CPAP than in no CPAP. However, future studies using more invasive cardiovascular monitoring need to measure cardiac output in order to evaluate the effect of CPAP on oxygen delivery, before it can be used in compromised animals. The administration of a FiO2 of 0.21 was related to the specific aim of the study to investigate the pure effect of CPAP on lung function. Indeed, in clinical practice, the FiO2 would be adjusted to the clinically required level by regulating the oxygen flowmeters, following the predetermined setting provided by the manufacturer. In this regard, the table produced for the flowmeter settings for the different FiO2 s needs to be assessed for validation for veterinary use. The patient’s comfort strongly influ584
ences the efficacy of CPAP therapy.13 Our data demonstrate that the helmet is well tolerated in dogs recovering from general anesthesia (average tolerance score of 1.5). Our patients were, however, sedated and the helmet was applied for a short period. Further studies are required to evaluate the degree of helmet tolerance in awake patients and for longer periods. This study was performed in healthy patients recovering from general anesthesia, and although the results are promising for future applications, the efficacy of this system in different pathological conditions should be supported by further studies. Because of the clinical nature of the study, it was not possible to use more invasive respiratory monitoring that may have given us more useful information about the effects of CPAP on respiratory system mechanics. The results of this study confirmed the feasibility and safety of the helmet system in delivering CPAP in healthy dogs with a body weight >10 kg recovering from a short period of anesthesia. If these data can be confirmed in a larger population of patients and in different pathological conditions, noninvasive CPAP therapy through the helmet could become an important therapeutic option in small animal practice. More studies are also required to evaluate the feasibility of the helmet in a wider range of body sizes. The design and development of specific veterinary helmets could further improve the efficacy and therapeutic impact of CPAP administered through a helmet in dogs.
Footnotes a b c d e f g h i j
Prequillant 1%; Fatro SpA, Bologna, Italy. Morfina Cloridrato 1%; Molteni SpA, Firenze, Italy. PropVet 1%; Esteve Hospira Inc, Chicago, IL. Isoba; Schering-Plough SpA, Milano, Italy. SC 6002XL; Siemens, Danvers, MA. CaStar; StarMed, Mirandola, Italy. StarVENT 2; StarMed. MiniOX; MSA, Gurnee, IL. VetStat; IDEXX Laboratories Inc., Westbrook, ME. MedCalc; version 9.2.0.1, MedCalc Software bvba, Mariakerke, Belgium.
References 1. Chiumello D, Chevallard G, Gregoretti C. Non-invasive ventilation in postoperative patients: a systematic review. Intensive Care Med 2011; 37:918–929. 2. Jaber S, Michelet P, Chanques G. Role of non-invasive ventilation (NIV) in the perioperative period. Best Pract Res Clin Anaesthesiol 2010; 24:253–265. 3. Squadrone V, Coha M, Cerutti E, et al. Continuous positive airway pressure for treatment of postoperative hypoxemia: a randomized controlled trial. JAMA 2005; 293:589–595. 4. Stock MC, Downs JB, Gauer PK, et al. Prevention of postoperative pulmonary complications with CPAP, incentive spirometry, and conservative therapy. Chest 1985; 87:151–157. 5. Dehaven CB, Jr., Hurst JM, Branson RD. Postextubation hypoxemia treated with a continuous positive airway pressure mask. Crit Care Med 1985; 13:46–48. C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
CPAP administration by means a helmet in dogs 6. Lindner KH, Lotz P, Ahnefeld FW. Continuous positive airway pressure effect on functional residual capacity, vital capacity and its subdivisions. Chest 1987; 92:66–70. 7. Delclaux C, L’Her E, Alberti C, et al. Treatment of acute hypoxemic nonhypercapnic respiratory insufficiency with continuous positive airway pressure delivered by a face mask: a randomized controlled trial. JAMA 2000; 284:2352–2360. 8. Rocco M, Dell’Utri D, Morelli A, et al. Noninvasive ventilation by helmet or face mask in immunocompromised patients: a casecontrol study. Chest 2004; 126:1508–1515. 9. Piastra M, Antonelli M, Chiaretti A, et al. Treatment of acute respiratory failure by helmet-delivered non-invasive pressure support ventilation in children with acute leukemia: a pilot study. Intensive Care Med 2004; 30:472–476. 10. Antonelli M, Pennisi MA, Pelosi P, et al. Noninvasive positive pressure ventilation using a helmet in patients with acute exacerbation of chronic obstructive pulmonary disease: a feasibility study. Anesthesiology 2004; 100:16–24. 11. Mosing M, Rysnik M, Bardell D, et al. Use of continuous positive airway pressure (CPAP) to optimise oxygenation in anaesthetised horses—a clinical study. Equine Vet J 2013; 45:414-418. 12. MacFarlane PD, Mosing M. Early experience with continuous positive airway pressure (CPAP) in 5 horses—a case series. Can Vet J 2012; 53:426–429. 13. Courtney SE, Barrington KJ. Continuous positive airway pressure and noninvasive ventilation. Clin Perinatol 2007; 34:73–92. 14. Briganti A, Melanie P, Portela D, et al. Continuous positive airway pressure administered via face mask in tranquilized dogs. J Vet Emerg Crit Care (San Antonio) 2010; 20:503–508. 15. Manning AM. Oxygen therapy and toxicity. Vet Clin North Am Small Anim Pract 2002; 32:1005–1020. 16. Marks SL. Cardiopulmonary resuscitation and oxygen therapy. Vet Clin North Am Small Anim Pract 1999; 29:959–969. 17. Chiumello D, Pelosi P, Carlesso E, et al. Noninvasive positive pressure ventilation delivered by helmet vs. standard face mask. Intensive Care Med 2003; 29:1671–1679. 18. Pelosi P, Severgnini P, Aspesi M, et al. Non-invasive ventilation delivered by conventional interfaces and helmet in the emergency department. Eur J Emerg Med 2003; 10:79–86. 19. Patroniti N, Foti G, Manfio A, et al. Head helmet versus face mask for non-invasive continuous positive airway pressure: a physiological study. Intensive Care Med 2003; 29:1680–1687. 20. Milesi C, Ferragu F, Jaber S, et al. Continuous positive airway pressure ventilation with helmet in infants under 1 year. Intensive Care Med 2010; 36:1592–1596.
C Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12210
21. Codazzi D, Nacoti M, Passoni M, et al. Continuous positive airway pressure with modified helmet for treatment of hypoxemic acute respiratory failure in infants and a preschool population: a feasibility study. Pediatr Crit Care Med 2006; 7:455–460. 22. Brodbelt DC, Taylor PM, Stanway GW. A comparison of preoperative morphine and buprenorphine for postoperative analgesia for arthrotomy in dogs. J Vet Pharmacol Ther 1997; 20:284– 289. 23. Taccone P, Hess D, Caironi P, et al. Continuous positive airway pressure delivered with a “helmet”: effects on carbon dioxide rebreathing. Crit Care Med 2004; 32:2090–2096. 24. Katz JA, Kraemer RW, Gjerde GE. Inspiratory work and airway pressure with continuous positive airway pressure delivery systems. Chest 1985; 88:519–526. 25. Gillespie DJ, Hyatt RE. Respiratory mechanics in the unanesthetized dog. J Appl Physiol 1974; 36:98–102. 26. Duggan M, Kavanagh BP. Perioperative modifications of respiratory function. Best Pract Res Clin Anaesthesiol 2010; 24:145–155. 27. Warner DO. Preventing postoperative pulmonary complications: the role of the anesthesiologist. Anesthesiology 2000; 92:1467–1472. 28. Ferreyra GP, Baussano I, Squadrone V, et al. Continuous positive airway pressure for treatment of respiratory complications after abdominal surgery: a systematic review and meta-analysis. Ann Surg 2008; 247:617–626. 29. Campbell VL, Drobatz KJ, Perkowski SZ. Postoperative hypoxemia and hypercarbia in healthy dogs undergoing routine ovariohysterectomy or castration and receiving butorphanol or hydromorphone for analgesia. J Am Vet Med Assoc 2003; 222:330– 336. 30. Staffieri F, Franchini D, Carella GL, et al. Computed tomographic analysis of the effects of two inspired oxygen concentrations on pulmonary aeration in anesthetized and mechanically ventilated dogs. Am J Vet Res 2007; 68:925–931. 31. Haskins S, Pascoe PJ, Ilkiw JE, et al. Reference cardiopulmonary values in normal dogs. Comp Med 2005; 55:156–161. 32. Abboud N, Rehder K, Rodarte JR, et al. Lung volumes and closing capacity with continuous positive airway pressure. Anesthesiology 1975; 42:138–142. 33. Keidan I, Fine GF, Kagawa T, et al. Work of breathing during spontaneous ventilation in anesthetized children: a comparative study among the face mask, laryngeal mask airway and endotracheal tube. Anesth Analg 2000; 91:1381–1388. 34. Montner PK, Greene ER, Murata GH, et al. Hemodynamic effects of nasal and face mask continuous positive airway pressure. Am J Respir Crit Care Med 1994; 149:1614–1618.
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