Anaesthesia 2014
doi:10.1111/anae.12677
Original Article Evaluation of changes in tidal volume during mask ventilation following administration of neuromuscular blocking drugs* R. Sachdeva,1 T. R. Kannan,2 C. Mendonca3 and M. Patteril3 1 Consultant Anaesthetist, University Hospital Birmingham, Birmingham, UK 2 Consultant Anaesthetist, Luton and Dunstable Hospital, Luton, UK 3 Consultant Anaesthetist, University Hospital, Coventry, UK
Summary The practice of checking the ability to mask ventilate before administering neuromuscular blocking drugs remains controversial. We prospectively evaluated the changes in the expired tidal volume during pressure-controlled ventilation (two-handed mask ventilation technique) as a surrogate marker to assess the ease of mask ventilation following administration of rocuronium. After informed consent, 125 patients were anaesthetised using a standard induction technique consisting of fentanyl, propofol and rocuronium, with anaesthesia then maintained with isoflurane in oxygen. The mean (SD) expired tidal volume before administration of rocuronium increased by 61 (13) ml at 2 min following onset of neuromuscular block (p < 0.001). This supports the concept that neuromuscular blockade induced by rocuronium facilitates mask ventilation. .................................................................................................................................................................
Correspondence to: R. Sachdeva Email: [email protected] *Presented in part to the Society for Airway Management, Toronto, Ontario, Canada, September 2012; and at the Difficult Airway Society Annual Meeting, Cardiff, UK, November 2012. Accepted: 11 March 2014
Introduction Despite ongoing debate, the clinical practice of administering a neuromuscular blocking drug (NBD) at induction of anaesthesia varies. Some authors argue that successful mask ventilation should be established before administration of NBDs [1, 2], while others argue against the practice [3]. The main concern is that inability to mask ventilate following administration of a NBD can compromise patients’ safety, if tracheal intubation proves to be impossible. In addition, neuromuscular blockade-induced upper airway collapse may cause complete airway obstruction [4]. On the other hand, difficult mask ventilation secondary to laryngospasm and opioid-induced muscle rigidity can be © 2014 The Association of Anaesthetists of Great Britain and Ireland
improved following administration of NBD, making mask ventilation easier [5]. Furthermore, in cases where mask ventilation is difficult or impossible after NBD tracheal intubation can be achieved successfully [6]. The ability to reverse the rocuronium-induced neuromuscular blockade using sugammadex [7, 8] may be a further reassuring factor in promoting the use of a NBD before checking the ability to mask ventilate. Previous studies measuring the effect of NBDs on mask ventilation have either failed to show any improvement in mask ventilation or been subjective in nature [6, 9–12]. Goodwin et al. [9] measured peak airway pressures and ratio of expired and inspired tidal volume to assess the adequacy of mask ventilation 1
Anaesthesia 2014
Sachdeva et al. | Neuromuscular blockade and mask ventilation
following administration of a NBD. However, they used hand ventilation which may not have ensured constant inflation pressures. Therefore, we wished to evaluate any changes in expired tidal volume before and after administering NBD during pressure-controlled ventilation. A secondary aim was to assess the effect of body mass index (BMI) on tidal volume and ease of mask ventilation.
Methods After obtaining approval from the regional ethics committee, adult patients aged > 18 years, of ASA physical status 1–3, scheduled to undergo surgery under general anaesthesia and receiving a NBD to facilitate tracheal intubation were consented and recruited to the study. Patients in whom a NBD was not indicated or mask ventilation was inappropriate (reflux, rapid sequence induction, difficult airway), any patient with severe respiratory disease and those with super obesity (BMI > 50 kg.m 2) were not studied. The anaesthetic machine (Datex Ohmeda S5 Avance; GE Healthcare, Madison, WI, USA) with an integral pressure gauge and spirometer was checked and calibrated before use on each patient. When the patient arrived in the anaesthetic room, blood pressure, heart rate and ECG monitoring were commenced. Following pre-oxygenation, general anaesthesia was induced using fentanyl 1.5 lg.kg 1 and propofol 2–3 mg.kg 1. A standardised head and neck position was used for all patients. The head end of the patient trolley was elevated by 15° and the head and neck maintained in the ‘sniffing the morning air’ position with one pillow under the head, ensuring that the lower level of the pillow did not extend beyond the shoulders. The patients’ lungs were ventilated using pressure-controlled ventilation at 15 cmH2O pressure, Table 1 Han scale for mask ventilation [13].
Grade 1 Grade 2 Grade 3
Grade 4
2
at a rate of 12 breaths.min 1. The initial grade of mask ventilation was assessed as described by Han et al. [13] (Table 1). If required, an appropriate sized oral airway was inserted, an optimum jaw thrust was performed and the mask was held with both hands as tightly as possible to achieve an adequate tidal volume. Once an oropharyngeal airway had been inserted, it was left in situ until laryngoscopy. Anaesthesia was maintained with isoflurane (minimum alveolar concentration, MAC of 1.0) with a fresh gas flow of 6 l.min 1. After the end-tidal concentration of isoflurane reached 1 MAC, the expired tidal volumes with each breath were recorded for 2 min. At this time, rocuronium 0.6 mg.kg 1 was administered and ventilation was continued, whilst the onset of neuromuscular blockade was monitored using train-of-four by stimulation of the ulnar nerve. Following the absence of twitches on train-of-four, the expired tidal volumes with each breath were recorded for another 2 min. The individual patient values for expired tidal volume before administering the NBD, 1 min after the onset of block and 2 min after the onset of block were each averaged to get a mean tidal volume (Fig. 1). All recordings were performed by one of the investigators (RS). Data were collected using numbers software on the iPad 1 (2010; Apple, Cupertino, CA, USA). The primary endpoint in this study was a change in the expired tidal volume following administration of NBD. An improvement in the expired tidal volume by 20% following administration of NBD was considered to be significant. Based on the pilot data from 25 patients, the SD for expired tidal volume was 171 ml. In total, 125 patients were required for a power of 0.9 with a p value of 0.05. The mean expired tidal volumes were compared using paired t-tests. To evaluate the effect of BMI, patients were classified into two groups: those with BMI < 30 kg.m 2 (low-BMI group) and those with BMI of ≥ 30 kg.m 2 (high-BMI group).
Description
Results
Ventilated by mask single-handed Difficult to ventilate, oropharyngeal airway inserted Ventilated by mask with jaw thrust, airway manoeuvers and two-handed technique Unable to ventilate
All 125 patients enrolled completed the study. The baseline characteristics, known predictors of difficult airway and grades of mask ventilation before administration of NBD are presented in Table 2. The lungs of all patients in the study were successfully ventilated by mask using pressure-controlled © 2014 The Association of Anaesthetists of Great Britain and Ireland
Sachdeva et al. | Neuromuscular blockade and mask ventilation
Anaesthesia 2014
Table 2 Patients’ baseline and peri-operative characteristics. Values are mean (SD) or numbers. The laryngoscopy grade is a modified Cormack–Lehane grading [14].
Age; years Weight; kg BMI; kg.m 2 Male:female Mallampati score; 1/2/3/4 Edentulous; yes/no Receding jaw/small neck/limited extension at neck/nil Beard; yes/no Grade of mask ventilation before NBD; 1/2/3/4 History of snoring; yes/no Laryngoscopic grade 1/2a/2b/3/4
50.7 (17.6) 75.9 (15.7) 27.4 (4.7) 48:77 40/76/9/0 25/100 3/7/3/112 8/117 45/72/8/0 56/69 77/37/7/4/0
NBD, neuromuscular blocking drug.
Figure 1 Flowchart for methodology. NBD, neuromuscular blocking drug.
ventilation with a pressure of 15 cmH2O. Breath-bybreath expired tidal volumes of three random patients is presented in Fig. 2. There was a significant increase in the mean (SD) tidal volume following administration of NBD from 525 (116) ml to 586 (129) ml (p < 0.001; Fig. 3). There was no deterioration in tidal volume in any of the patients in whom NBD was administered (Fig. 4). There were 40 (32%) patients with BMI ≥ 30 kg.m 2, a proportionately higher number of © 2014 The Association of Anaesthetists of Great Britain and Ireland
Figure 2 Time course of breath-by-breath changes in tidal volume during mask ventilation in three representative patients. The time period A-B is the 24 breaths before administration of neuromuscular blocking drug at time B; time C denotes the time of disappearance of the fourth twitch of the train-of-four. Time period C-D represents the 24 breaths after administration of neuromuscular blocking drug. whom were classified as having grade-2 mask ventilation (Table 3). The increase in mean expired tidal volumes was significantly higher in the low-BMI group compared with the high-BMI group at 1 and 2 min (Fig. 5).
Discussion Our results showed a significant increase in the expired tidal volume following administration of NBD. These results are consistent with the previous study published by Warters et al. [15], but differ from those of Goodwin et al. [9] who concluded that 3
Anaesthesia 2014
Sachdeva et al. | Neuromuscular blockade and mask ventilation
Table 3 Grade of minute ventilation (MV) (see Table 1) before administering neuromuscular blocking drug. Values are number (proportion).
Figure 3 Expired tidal volumes before and after administering neuromuscular blocking drug (NBD). A = before administering NBD, B = 1 min after disappearance of the fourth twitch and C = 2 min after disappearance of fourth twitch of the train-offour. p < 0.001 for A vs B and C. The thick horizontal line within the box is the median; the edges of the box the IQR and the whiskers the range.
Figure 4 Mean expired tidal volumes achieved before (A) and after (B) administration of the neuromuscular blocking drug (NBD) in each of the 125 patients. The two points with error bars denote the mean value (SD) before and after administration of NBD. neuromuscular blockade did not affect efficiency of mask ventilation in patients with normal airways. Notably, our study is much larger than previous studies, and includes patients with a high BMI. We 4
Grade of MV
BMI < 30 kg.m (n = 85)
Grade 1 Grade 2 Grade 3
37 (43.5%) 45 (52.9%) 3 (3.5%)
2
BMI ≥ 30 kg.m (n = 40)
2
9 (22.5%) 26 (65.0%) 5 (12.5%)
Figure 5 Mean expired tidal volumes before and after administering neuromuscular blocking drug (NBD) in low- and high-BMI groups. A = before administering NBD, B = 1 min after disappearance of the fourth twitch and C = 2 min after disappearance of the fourth twitch of the train-of-four. ■ BMI < 30 kg.m 2 and □ BMI ≥ 30 kg.m 2. p = 0.054 for A vs B and p = 0.001 for A vs C for BMI < 30 kg.m 2; p < 0.001 for A vs B and C for BMI ≥ 30 kg.m 2. were careful to ensure that a constant mean and plateau pressure were maintained throughout; therefore the change in tidal volume was directly related to the effect of the neuromuscular blockade. However, we did employ a two-handed technique, which is not routine in all patients. Goodwin et al.’s study showed no significant improvement in ventilation after administering NBD [9]. In their study, patients’ lungs were hand ventilated, which might have led to variation in the tidal volumes administered with each breath. They used the ratio between expired and inspired tidal volume (VTE/ VTI) to assess the efficiency of mask ventilation. This measure can be affected by a leak in the breathing system and its ability accurately to reflect the efficiency of ventilation has been questioned [16]. A relatively small study (31 patients) by Ikeda et al. [17] compared the effect of rocuronium and suxamethonium on mask ventilation in patients with © 2014 The Association of Anaesthetists of Great Britain and Ireland
Sachdeva et al. | Neuromuscular blockade and mask ventilation
normal upper airway anatomy. They concluded that rocuronium did not worsen mask ventilation but that suxamethonium improved it. They suggested that pharyngeal fasciculations during administration of suxamethonium were associated with the airway dilatation. Unusually, however, they maintained the head in a neutral, rather than optimal position, which may have influenced the results, and they did not perform any additional manoeuvres to maximise ventilation. Their target tidal volume was 2 ml.kg 1 which is surprisingly small, and therefore they had to exclude four of the 18 patients in their rocuronium group due to inadequate ventilation [17]. In a subgroup of six patients, they performed endoscopic examination of the airway during suxamethonium-induced neuromuscular blockade and suggested that pharyngeal fasciculation induced by suxamethonium leads to contraction of both pharyngeal dilators and contractors, resulting in re-opening of the airway. However, they omitted to perform this investigation of the behaviour of the pharyngeal muscles during rocuronium-induced neuromuscular blockade. We graded the ease of mask ventilation before administration of NBD using the Han scale [13]. Being a subjective measure, it was difficult to quantify the change in Han scale following administration of NBD. Warters et al. developed an alternative scale for evaluating mask ventilation, but our data collection had already been completed when their study was published [15], so we could not employ it in this study. However, we find that Warters scale is complex and rather difficult to use in practice. Our result of a mean increase in tidal volume of 61 ml is potentially clinically significant. Average oxygen consumption in a healthy subject is ~250 ml.min 1 [18], so an increase in minute volume of 732 ml.min 1 (i.e, 61 9 12 breaths), represents a value of about three times the oxygen consumption. The improvement in tidal volume following NBD can be attributed to anatomical and physiological factors, along with physical properties of gas flow. In a conscious patient, the tone of the upper airway muscles and natural reflexes provide a clear passage to airflow. It is known that following induction of general anaesthesia, the tongue falls back, and depending on the volume of the oropharynx, may rest against the © 2014 The Association of Anaesthetists of Great Britain and Ireland
Anaesthesia 2014
hard/soft palate and posterior pharyngeal wall [19]. Increasing depths of anaesthesia are associated with profound inhibition of genioglossus muscle activity, which is a dilator to the airway, resulting in increased airway collapse, thereby hindering passage of gas flow [20]. We postulate that, unlike induction agents, NBDs not only reduce the tone of muscles but also reduce their cross-sectional area. To conclude, mask ventilation became easier after administration of NBD, as evidenced by improvement of expired tidal volume. Hence, administration of NBD should be considered to facilitate mask ventilation in patients with normal airway anatomy. However, as our study did not involve patients with difficult airways, further research is needed to assess if this conclusion also applies to this important subgroup.
Acknowledgements We would like to thank Helen Parsons and Richard Crossman, Research Fellows at the Division of Health Sciences, Warwick Medical School, for their help with statistical analysis, and to Dr Ian Calder for suggestions in improving the final manuscript.
Competing interests No funding or competing interests declared.
References 1. Richardson MG, Litman RS. Ventilation before paralysis: crossing the Rubicon, slowly. Anesthesiology 2012; 117: 456–8. 2. Xue FS, Liao X, Wang Q, Yuan YJ, Xiong J, Liu JH. Is it unnecessary to confirm successful face mask ventilation before administration of a neuromuscular blocking agent? Anaesthesia 2011; 66: 519–20. 3. Calder I, Yentis SM. Could ‘‘safe practice’’ be compromising safe practice? Should anaesthetists have to demonstrate that face mask ventilation is possible before giving a neuromuscular blocker? Anaesthesia 2008; 63: 113–5. 4. Odeh M, Schnall R, Gavriely N, Oliven A. Effect of upper airway muscle contraction on supraglottic resistance and stability. Respiration Physiology 1993; 92: 139–50. 5. Bennet JA, Abrams JT, Van Riper DF, Horrow JC. Difficult or impossible ventilation after sufentanil-induced anesthesia is caused primarily by vocal cord closure. Anesthesiology 1997; 87: 1070–4. 6. Kheterpal S, Martin L, Shanks AM, Tremper KK. Prediction and outcomes of impossible mask ventilation: a review of 50,000 anesthetics. Anesthesiology 2009; 110: 891–7. 7. Sorensen MK, Bretlau C, Gatke MR, Sorensen AM, Rasmussen LS. Rapid sequence induction and intubation with rocuroniumsugammadex compared with succinylcholine: a randomized trial. British Journal of Anaesthesia 2012; 108: 682–9. 5
Anaesthesia 2014 8. Lee C, Jahr JS, Candiotti KA, Warriner B, Zornow MH, Naguib M. Reversal of profound neuromuscular block by sugammadex administered three minutes after rocuronium: a comparison with spontaneous recovery from succinylcholine. Anesthesiology 2009; 110: 1020–5. 9. Goodwin MW, Pandit JJ, Hames K, Popat MT, Yentis SM. The effect of neuromuscular blockade on the efficiency of mask ventilation of the lungs. Anaesthesia 2003; 58: 60–3. 10. Kheterpal S, Han R, Tremper KK, et al. Incidence and predictors of difficult and impossible mask ventilation. Anesthesiology 2006; 105: 885–91. 11. Combes X, Andriamifidy L, Dufresne E. Comparison of two induction regimens using or not using muscle relaxant: impact on postoperative upper airway discomfort. British Journal of Anaesthesia 2007; 99: 276–81. 12. Lieutaud T, Billard V, Khalaf H, Debaene B. Muscle relaxation and increasing doses of propofol improve intubating conditions. Canadian Journal of Anesthesia 2003; 50: 121–6. 13. Han R, Tremper KK, Kheterpal S, O’Reilly M. Grading scale for mask ventilation. Anesthesiology 2004; 101: 267.
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Sachdeva et al. | Neuromuscular blockade and mask ventilation 14. Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anaesthesia 1984; 3: 1105–11. 15. Warters RD, Szabo TA, Spinale FG, DeSantis SM, Reves JG. The effect of neuromuscular blockade on mask ventilation. Anaesthesia 2011; 66: 163–7. 16. El-Orbany M, Woehlck HJ. Difficult mask ventilation. Anesthesia and Analgesia 2009; 109: 1870–80. 17. Ikeda A, Isono S, Sato Y, et al. Effects of muscle relaxants on mask ventilation in anesthetized persons with normal upper airway anatomy. Anesthesiology 2012; 117: 487–93. 18. Pandit JJ, Duncan T, Robbins PA. Total oxygen uptake with two maximal breathing techniques and the tidal volume breathing technique: a physiologic study of preoxygenation. Anesthesiology 2003; 99: 841–6. 19. Hillman DR, Platt PR, Eastwood PR. The upper airway during anaesthesia. British Journal of Anaesthesia 2003; 91: 31–9. 20. Eastwood PR, Platt PR, Shepherd K, Maddison K, Hillman DR. Collapsibility of the upper airway at different concentrations of propofol anesthesia. Anesthesiology 2005; 103: 470–7.
© 2014 The Association of Anaesthetists of Great Britain and Ireland
doi:10.1111/anae.12677
Original Article Evaluation of changes in tidal volume during mask ventilation following administration of neuromuscular blocking drugs* R. Sachdeva,1 T. R. Kannan,2 C. Mendonca3 and M. Patteril3 1 Consultant Anaesthetist, University Hospital Birmingham, Birmingham, UK 2 Consultant Anaesthetist, Luton and Dunstable Hospital, Luton, UK 3 Consultant Anaesthetist, University Hospital, Coventry, UK
Summary The practice of checking the ability to mask ventilate before administering neuromuscular blocking drugs remains controversial. We prospectively evaluated the changes in the expired tidal volume during pressure-controlled ventilation (two-handed mask ventilation technique) as a surrogate marker to assess the ease of mask ventilation following administration of rocuronium. After informed consent, 125 patients were anaesthetised using a standard induction technique consisting of fentanyl, propofol and rocuronium, with anaesthesia then maintained with isoflurane in oxygen. The mean (SD) expired tidal volume before administration of rocuronium increased by 61 (13) ml at 2 min following onset of neuromuscular block (p < 0.001). This supports the concept that neuromuscular blockade induced by rocuronium facilitates mask ventilation. .................................................................................................................................................................
Correspondence to: R. Sachdeva Email: [email protected] *Presented in part to the Society for Airway Management, Toronto, Ontario, Canada, September 2012; and at the Difficult Airway Society Annual Meeting, Cardiff, UK, November 2012. Accepted: 11 March 2014
Introduction Despite ongoing debate, the clinical practice of administering a neuromuscular blocking drug (NBD) at induction of anaesthesia varies. Some authors argue that successful mask ventilation should be established before administration of NBDs [1, 2], while others argue against the practice [3]. The main concern is that inability to mask ventilate following administration of a NBD can compromise patients’ safety, if tracheal intubation proves to be impossible. In addition, neuromuscular blockade-induced upper airway collapse may cause complete airway obstruction [4]. On the other hand, difficult mask ventilation secondary to laryngospasm and opioid-induced muscle rigidity can be © 2014 The Association of Anaesthetists of Great Britain and Ireland
improved following administration of NBD, making mask ventilation easier [5]. Furthermore, in cases where mask ventilation is difficult or impossible after NBD tracheal intubation can be achieved successfully [6]. The ability to reverse the rocuronium-induced neuromuscular blockade using sugammadex [7, 8] may be a further reassuring factor in promoting the use of a NBD before checking the ability to mask ventilate. Previous studies measuring the effect of NBDs on mask ventilation have either failed to show any improvement in mask ventilation or been subjective in nature [6, 9–12]. Goodwin et al. [9] measured peak airway pressures and ratio of expired and inspired tidal volume to assess the adequacy of mask ventilation 1
Anaesthesia 2014
Sachdeva et al. | Neuromuscular blockade and mask ventilation
following administration of a NBD. However, they used hand ventilation which may not have ensured constant inflation pressures. Therefore, we wished to evaluate any changes in expired tidal volume before and after administering NBD during pressure-controlled ventilation. A secondary aim was to assess the effect of body mass index (BMI) on tidal volume and ease of mask ventilation.
Methods After obtaining approval from the regional ethics committee, adult patients aged > 18 years, of ASA physical status 1–3, scheduled to undergo surgery under general anaesthesia and receiving a NBD to facilitate tracheal intubation were consented and recruited to the study. Patients in whom a NBD was not indicated or mask ventilation was inappropriate (reflux, rapid sequence induction, difficult airway), any patient with severe respiratory disease and those with super obesity (BMI > 50 kg.m 2) were not studied. The anaesthetic machine (Datex Ohmeda S5 Avance; GE Healthcare, Madison, WI, USA) with an integral pressure gauge and spirometer was checked and calibrated before use on each patient. When the patient arrived in the anaesthetic room, blood pressure, heart rate and ECG monitoring were commenced. Following pre-oxygenation, general anaesthesia was induced using fentanyl 1.5 lg.kg 1 and propofol 2–3 mg.kg 1. A standardised head and neck position was used for all patients. The head end of the patient trolley was elevated by 15° and the head and neck maintained in the ‘sniffing the morning air’ position with one pillow under the head, ensuring that the lower level of the pillow did not extend beyond the shoulders. The patients’ lungs were ventilated using pressure-controlled ventilation at 15 cmH2O pressure, Table 1 Han scale for mask ventilation [13].
Grade 1 Grade 2 Grade 3
Grade 4
2
at a rate of 12 breaths.min 1. The initial grade of mask ventilation was assessed as described by Han et al. [13] (Table 1). If required, an appropriate sized oral airway was inserted, an optimum jaw thrust was performed and the mask was held with both hands as tightly as possible to achieve an adequate tidal volume. Once an oropharyngeal airway had been inserted, it was left in situ until laryngoscopy. Anaesthesia was maintained with isoflurane (minimum alveolar concentration, MAC of 1.0) with a fresh gas flow of 6 l.min 1. After the end-tidal concentration of isoflurane reached 1 MAC, the expired tidal volumes with each breath were recorded for 2 min. At this time, rocuronium 0.6 mg.kg 1 was administered and ventilation was continued, whilst the onset of neuromuscular blockade was monitored using train-of-four by stimulation of the ulnar nerve. Following the absence of twitches on train-of-four, the expired tidal volumes with each breath were recorded for another 2 min. The individual patient values for expired tidal volume before administering the NBD, 1 min after the onset of block and 2 min after the onset of block were each averaged to get a mean tidal volume (Fig. 1). All recordings were performed by one of the investigators (RS). Data were collected using numbers software on the iPad 1 (2010; Apple, Cupertino, CA, USA). The primary endpoint in this study was a change in the expired tidal volume following administration of NBD. An improvement in the expired tidal volume by 20% following administration of NBD was considered to be significant. Based on the pilot data from 25 patients, the SD for expired tidal volume was 171 ml. In total, 125 patients were required for a power of 0.9 with a p value of 0.05. The mean expired tidal volumes were compared using paired t-tests. To evaluate the effect of BMI, patients were classified into two groups: those with BMI < 30 kg.m 2 (low-BMI group) and those with BMI of ≥ 30 kg.m 2 (high-BMI group).
Description
Results
Ventilated by mask single-handed Difficult to ventilate, oropharyngeal airway inserted Ventilated by mask with jaw thrust, airway manoeuvers and two-handed technique Unable to ventilate
All 125 patients enrolled completed the study. The baseline characteristics, known predictors of difficult airway and grades of mask ventilation before administration of NBD are presented in Table 2. The lungs of all patients in the study were successfully ventilated by mask using pressure-controlled © 2014 The Association of Anaesthetists of Great Britain and Ireland
Sachdeva et al. | Neuromuscular blockade and mask ventilation
Anaesthesia 2014
Table 2 Patients’ baseline and peri-operative characteristics. Values are mean (SD) or numbers. The laryngoscopy grade is a modified Cormack–Lehane grading [14].
Age; years Weight; kg BMI; kg.m 2 Male:female Mallampati score; 1/2/3/4 Edentulous; yes/no Receding jaw/small neck/limited extension at neck/nil Beard; yes/no Grade of mask ventilation before NBD; 1/2/3/4 History of snoring; yes/no Laryngoscopic grade 1/2a/2b/3/4
50.7 (17.6) 75.9 (15.7) 27.4 (4.7) 48:77 40/76/9/0 25/100 3/7/3/112 8/117 45/72/8/0 56/69 77/37/7/4/0
NBD, neuromuscular blocking drug.
Figure 1 Flowchart for methodology. NBD, neuromuscular blocking drug.
ventilation with a pressure of 15 cmH2O. Breath-bybreath expired tidal volumes of three random patients is presented in Fig. 2. There was a significant increase in the mean (SD) tidal volume following administration of NBD from 525 (116) ml to 586 (129) ml (p < 0.001; Fig. 3). There was no deterioration in tidal volume in any of the patients in whom NBD was administered (Fig. 4). There were 40 (32%) patients with BMI ≥ 30 kg.m 2, a proportionately higher number of © 2014 The Association of Anaesthetists of Great Britain and Ireland
Figure 2 Time course of breath-by-breath changes in tidal volume during mask ventilation in three representative patients. The time period A-B is the 24 breaths before administration of neuromuscular blocking drug at time B; time C denotes the time of disappearance of the fourth twitch of the train-of-four. Time period C-D represents the 24 breaths after administration of neuromuscular blocking drug. whom were classified as having grade-2 mask ventilation (Table 3). The increase in mean expired tidal volumes was significantly higher in the low-BMI group compared with the high-BMI group at 1 and 2 min (Fig. 5).
Discussion Our results showed a significant increase in the expired tidal volume following administration of NBD. These results are consistent with the previous study published by Warters et al. [15], but differ from those of Goodwin et al. [9] who concluded that 3
Anaesthesia 2014
Sachdeva et al. | Neuromuscular blockade and mask ventilation
Table 3 Grade of minute ventilation (MV) (see Table 1) before administering neuromuscular blocking drug. Values are number (proportion).
Figure 3 Expired tidal volumes before and after administering neuromuscular blocking drug (NBD). A = before administering NBD, B = 1 min after disappearance of the fourth twitch and C = 2 min after disappearance of fourth twitch of the train-offour. p < 0.001 for A vs B and C. The thick horizontal line within the box is the median; the edges of the box the IQR and the whiskers the range.
Figure 4 Mean expired tidal volumes achieved before (A) and after (B) administration of the neuromuscular blocking drug (NBD) in each of the 125 patients. The two points with error bars denote the mean value (SD) before and after administration of NBD. neuromuscular blockade did not affect efficiency of mask ventilation in patients with normal airways. Notably, our study is much larger than previous studies, and includes patients with a high BMI. We 4
Grade of MV
BMI < 30 kg.m (n = 85)
Grade 1 Grade 2 Grade 3
37 (43.5%) 45 (52.9%) 3 (3.5%)
2
BMI ≥ 30 kg.m (n = 40)
2
9 (22.5%) 26 (65.0%) 5 (12.5%)
Figure 5 Mean expired tidal volumes before and after administering neuromuscular blocking drug (NBD) in low- and high-BMI groups. A = before administering NBD, B = 1 min after disappearance of the fourth twitch and C = 2 min after disappearance of the fourth twitch of the train-of-four. ■ BMI < 30 kg.m 2 and □ BMI ≥ 30 kg.m 2. p = 0.054 for A vs B and p = 0.001 for A vs C for BMI < 30 kg.m 2; p < 0.001 for A vs B and C for BMI ≥ 30 kg.m 2. were careful to ensure that a constant mean and plateau pressure were maintained throughout; therefore the change in tidal volume was directly related to the effect of the neuromuscular blockade. However, we did employ a two-handed technique, which is not routine in all patients. Goodwin et al.’s study showed no significant improvement in ventilation after administering NBD [9]. In their study, patients’ lungs were hand ventilated, which might have led to variation in the tidal volumes administered with each breath. They used the ratio between expired and inspired tidal volume (VTE/ VTI) to assess the efficiency of mask ventilation. This measure can be affected by a leak in the breathing system and its ability accurately to reflect the efficiency of ventilation has been questioned [16]. A relatively small study (31 patients) by Ikeda et al. [17] compared the effect of rocuronium and suxamethonium on mask ventilation in patients with © 2014 The Association of Anaesthetists of Great Britain and Ireland
Sachdeva et al. | Neuromuscular blockade and mask ventilation
normal upper airway anatomy. They concluded that rocuronium did not worsen mask ventilation but that suxamethonium improved it. They suggested that pharyngeal fasciculations during administration of suxamethonium were associated with the airway dilatation. Unusually, however, they maintained the head in a neutral, rather than optimal position, which may have influenced the results, and they did not perform any additional manoeuvres to maximise ventilation. Their target tidal volume was 2 ml.kg 1 which is surprisingly small, and therefore they had to exclude four of the 18 patients in their rocuronium group due to inadequate ventilation [17]. In a subgroup of six patients, they performed endoscopic examination of the airway during suxamethonium-induced neuromuscular blockade and suggested that pharyngeal fasciculation induced by suxamethonium leads to contraction of both pharyngeal dilators and contractors, resulting in re-opening of the airway. However, they omitted to perform this investigation of the behaviour of the pharyngeal muscles during rocuronium-induced neuromuscular blockade. We graded the ease of mask ventilation before administration of NBD using the Han scale [13]. Being a subjective measure, it was difficult to quantify the change in Han scale following administration of NBD. Warters et al. developed an alternative scale for evaluating mask ventilation, but our data collection had already been completed when their study was published [15], so we could not employ it in this study. However, we find that Warters scale is complex and rather difficult to use in practice. Our result of a mean increase in tidal volume of 61 ml is potentially clinically significant. Average oxygen consumption in a healthy subject is ~250 ml.min 1 [18], so an increase in minute volume of 732 ml.min 1 (i.e, 61 9 12 breaths), represents a value of about three times the oxygen consumption. The improvement in tidal volume following NBD can be attributed to anatomical and physiological factors, along with physical properties of gas flow. In a conscious patient, the tone of the upper airway muscles and natural reflexes provide a clear passage to airflow. It is known that following induction of general anaesthesia, the tongue falls back, and depending on the volume of the oropharynx, may rest against the © 2014 The Association of Anaesthetists of Great Britain and Ireland
Anaesthesia 2014
hard/soft palate and posterior pharyngeal wall [19]. Increasing depths of anaesthesia are associated with profound inhibition of genioglossus muscle activity, which is a dilator to the airway, resulting in increased airway collapse, thereby hindering passage of gas flow [20]. We postulate that, unlike induction agents, NBDs not only reduce the tone of muscles but also reduce their cross-sectional area. To conclude, mask ventilation became easier after administration of NBD, as evidenced by improvement of expired tidal volume. Hence, administration of NBD should be considered to facilitate mask ventilation in patients with normal airway anatomy. However, as our study did not involve patients with difficult airways, further research is needed to assess if this conclusion also applies to this important subgroup.
Acknowledgements We would like to thank Helen Parsons and Richard Crossman, Research Fellows at the Division of Health Sciences, Warwick Medical School, for their help with statistical analysis, and to Dr Ian Calder for suggestions in improving the final manuscript.
Competing interests No funding or competing interests declared.
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