Anaesthesia, 1992, Volume 47, pages 1088- 1089

Intermittent positive pressure ventilation through a laryngeal mask airway Is a nasogastric tube useful?

P.J.Graziotti, MB, BS, FFARCS, Provisional Fellow in Anaesthetics, Anaesthetics Department, Sir Charles Gairdner Hospital, Verdun Street, Nedlands, 6009, Western Australia.

Summary A nasogastric tube was used to aspirate air insuffated into the stomach during intermittent positive pressure ventilation through a

laryngeal mask airway and a tracheal tube. No difference was found in the amount aspirated between patients with a tracheal tube, a laryngeal mask airway with the nasogastric tube closed or a laryngeal mask airway with the nasogastric tube open. when the nasogastric tube was aspirated at 15 min intervals for thefirst hour of anaesthesia. Key words Equipment; laryngeal mask airway, nasogastric tube. Complications; gastric insufflation. Laryngeal mask airways (LMA) are now firmly established in anaesthetic practice. Acceptance has been rapid because of the advantages they have over a facemask. It is easier to maintain an airway with a LMA, they free the anaesthetist's hands, and allow capnography. They can be used either alone or with other aids to overcome a difficult airway. They also allow artificial ventilation of the lungs (IPPV) if it becomes necessary. There are a number of questions which surround the use of the LMA for IPPV. These concern the isolation of the gastrointestinal tract from the respiratory tract. Payne [I] has demonstrated that in a number of patients with a LMA in place, the oesophagus could be seen through the end of the LMA with a fibreoptic bronchoscope. Brain [2] suggested that this was due to malpositioning of the mask, and that if the mask is properly inserted the oesophagus should not be seen. One possible consequence of inadequate isolation is gastric insufflation. Gastric insufflation has been implicated as a potential factor in reducing diaphragmatic function after operation [3]. It may also be expected to increase the incidence of nausea and have an adverse effect on the gastro-oesophageal sphincter. The aim of the study was to determine if significantly more air was aspirated through a nasogastric tube in patients who underwent IPPV through a LMA than control groups, and to see if an open nasogastric tube would relieve air insufflated into the stomach. Methods The experiment was divided into two parts. Part one compared the amount of air aspirated through a nasogastric tube in patients undergoing IPPV via a tracheal tube with those undergoing IPPV via a LMA. Part two compared two groups, both undergoing IPPV through a LMA with a nasograstric tube in place. One group had the nasogastric tube left open and the other closed. Approval was obtained from the Hospital Ethics Committee. After giving informed, written consent, 50 fasted patients, who had been classified as ASA 1 or 2 undergoing peripheral surgery, were randomly allocated to one of three groups. Accepted 5 May 1992.

Ten patients were allocated to group I , 20 to group 2 and 20 to group 3. All patients were premedicated with temazepam and anaesthesia was induced with propofol 2.5 mg.kg-' and atracurium 0.3 mg.kg-l. The patients' lungs were gently ventilated by facemask with N 2 0 / 0 2and enflurane 1% for 3 min. A 16 FR single lumen nasogastric tube was inserted and its position checked by injecting 2 ml of air and listening over the stomach. Initial gastric contents were then aspirated. A three-way tap was placed on the end of the nasogastric tube of the patients in groups 1 and 2 and closed off t o the patient. The tracheas of group 1 patients were then intubated, groups 2 and 3 had a LMA inserted. The lungs of all patients were ventilated to normocapnia using an Ohmeda 7000 ventilator, with the tidal volume adjusted to keep the airway pressure less than 20 cmH,O. The nasogastric tube in all groups was aspirated with a 20 ml syringe at 15 min intervals for the first hour. In groups 1 and 2 , the three-way tap on the end of the nasogastric tube was closed off to the patient after each aspiration. If no air or fluid was aspirated at an interval, 2 ml of air was injected through the nasogastric tube to clear any possible blockage, and aspiration was attempted again. Parametric data were analysed using Student's [-test, nonparametric data using the Mann-Whitney U test. Results Forty-five patients were studied. Three patients were withdrawn because the operation was unexpectedly shortened and in two patients the nasogastric tube could not be placed in the stomach with confidence. Patients were similar in age and weight (Table l), and there was no significant difference in the respiratory variables between the three groups (Table 2). All patients were ventilated with airway pressures below 20 cmH20, and a high proportion of patients in groups 2 and 3 had an audible leak around the LMA (Table 2). There was no significant difference in the amounts of air aspirated at each interval between groups 1 (tracheal tube) and 2 (LMA with nasogastric tube closed) o r between groups 2 and 3 (LMA with nasogastric tube open). The

Forum Table 1. Demographic data. 1.0 (7.0) 40.0 (21.0) 75.0 (3.5)

Group (n) Age; years (SD) Weight; kg (SD)

2.0 (20.0) 47.0 (20.0) 67.0 (12.0)

3.0 (18.0) 37.0 (20.0) 71.0 (14.0)

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volumes aspirated in all groups were small and decreased with time (Tables 3 and 4). The total volume aspirated did not exceed 40 ml in any group. One patient in group 2 and two in group 3 aspirated a total volume of more than 20 ml, but less than 40 ml (Tables 5 and 6 ) .

Table 2. Respiratory parameters. Group (n) Tidal volume; ml (SD) Minute volume I (SD) Airway pressure; cmH,O (SD) Leak n (X)

Discussion

1 (7)

2 (20)

3 (18)

714.0 (124.0)

662.0 (93.0)

691.0 (121.0)

6.7

(1.7)

6.8 (1.8)

7.1

(2.1)

18.0

(2.0)

19.0 (2.1)

17.0

(4.2)

9.0 (45.0)

0

8.0 (48.0)

Table 3. Volume of air aspirated from the nasogastric tube in groups I and 2. Volumes are mean ml (SD) aspirated at each I5 min interval. Time (min) 15 30 45 60

Group I TT; ml (SD)

Group 2 LMA; ml (SD)

p value

2.57 (3.3) 0.3 (0.8) 0.29 (0.76) 0.29 (0.8)

2.85 (5.9) 1.65 (3.0) 0.85 (2.0) 0.2 (0.89)

0.88 0.07 0.3 I 0.8 1

TT, tracheal tube: LMA, laryngeal mask airway. Table 4. Volumes of air aspirated from the nasogastric tube in groups 2 and 3. Volumes are mean ml (SD) aspirated at each 15 min interval.

Time (min)

Group 2 LMA with nasogastric tube closed; ml (SD)

Group 3 LMA with nasogastric tube open; ml (SD)

p value

2.85 (5.84) I .65 (3.0) 0.85 (2.0) 0.2 (0.89)

3.00 (6.65) 1.33 (3.55) 2. I I (8.22) 0.39 (0.85)

0.94 0.77 0.53 0.51

I5 30 45 60

Table 5. Frequency distribution of total volumes aspirated in groups I and 2. Total volume (ml)

0-20

Group I (TT) n = 7(%) Group 2 (LMA) n = 20(%)

7 (100)

2040

> 40

0

0 0

18 (90) 2(10)

For abbreviations, see Table 3. Table 6. Frequency distribution of total volume aspirated from groups 2 and 3. Total volume (ml)

0-20

2040

> 40

Group 2 (LMA with nasogastric tube closed) n = 20 (YO)

18 (90)

2 (10)

0

Group 3 (LMA with nasogastric tube open) n = 18 (X)

0

There is no readily applicable gold standard for measuring air insufflated into the stomach during IPPV. This study assumes that if a nasogastric tube were to be useful in preventing gastric insufflation, it could also be used to measure the amount of air in the stomach. Inaccuracy in the amount aspirated should be the same in all groups. The aim of the study was to demonstrate a difference between groups if one existed, not to document the amount of gastric insufflation which may have occurred. If gastric insufflation was occurring in the patients undergoing IPPV through a LMA, insufflated air should have accumulated in the stomachs of patients in group 2. Also, the air insufflated in the patients in group 3 should have escaped through the nasogastic tube, which was open to the atmosphere. The absence of a significant difference between these two groups suggests that either the nasogastric tube was not allowing insufflated air to escape, or gastric insufflation was not occurring. The small volumes of air aspirated and the decreasing volumes with time support the latter. In either case, the nasogastric tube was performing no useful function. It is still possible, however, that air was insufflated into the stomach of these patients, but aspiration was not possible through the nasogastric tube. It is possible, and even common in some circumstances, for a nasogastric tube to become blocked. In this study, if no air or fluid was aspirated at any interval, 2 ml of air was injected into the nasogastric tube to unblock it and further aspiration attempted. It was assumed that if a significant amount of air was in the stomach, this manoeuvre would have allowed its aspiration. It is possible that a nasogastric tube would increase the likelihood of gastric insufflation. The absence of a difference in the amount of air aspirated between patients in groups 1 and 2 mitigates against this. The incidence of leak around the LMA in groups 2 and 3 was higher than reported in other studies [4].This was most likely due to the nasogastric tube in situ, which did not allow a tight fit between the LMA and the posterior pharyngeal wall. Ventilator settings were adjusted to minimise the leak, but it should be noted that if airway pressures are maintained below 20 cm, leaks are unusual in patients without a nasogastric tube. In adults, less than 40 ml of air in the stomach is considered physiologically insignificant. Intragastric pressure does not rise until at least 50 ml is injected into the stomach of small animals. [5]. None of the patients in this study had more than 40 ml aspirated in total. Gastric volumes of air are therefore likely to be a small, although the accuracy of this technique in determining the amount of air insufflated is not established. In summary, this study suggests that a nasogastric tube is of no value when placed prophylactically in normal patients undergoing IPPV through a LMA. Because of the known morbidity associated with placing a nasogastric tube, the possibility that it may adversely affect the lower oesophageal sphincter, and the increased incidence of leak around the LMA when the nasogastric tube is in place, use of a nasogastric tube is not recommended in this group of patients. Because of the small numbers involved in this study, it is possible that significant gastric insufflation may

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occur in a small proportion of patients and vigilance by the anaesthetist is therefore always necessary. References [I]PAYNEJ. The use of the fibreoptic laryngoscope to confirm the position of the laryngeal mask. Anaesthesia 1989;44: 865.

[2]BRAINAIJ. Studies on the laryngeal mask: first, learn the art. Anaesthesia 1 9 9 1 ; 46: 417-18. [3]BRAINAIJ. The Intavent laryngeal mask instruction manual. Henley-on-Thames; lntavent International SA. 1990:23. [4]BRAINAIJ. The laryngeal mask-a new concept in airway management. British Journal of Anaesthesia 1983;55: 801-5.

Anaesthesia, 1992, Volume 47, pages 1090- 1092

Postoperative oxygenation in the elderly following general or local anaesthesia for ophthalmic surgery G. J. McCarthy, FFARCSI, FRCAnaes, R. K. Mirakhur, MD, PhD, FRCAnaes, Department of Anaesthetics, The Queen's University of Belfast, P. Elliott, MD, FRCAnaes, Department of Anaesthetics, Royal Victoria Hospital, Belfast, Northern Ireland.

Summary Peripheral oxygen saturation was recorded on the pre-operative night (between 2400 and 0600 h ) , the immediate postoperative period (Jirst 60 min) and thejrst postoperative night (2400 to 0600 h ) in 18 elderly patients aged 70 years or over presentingfiw elective ophthalmic surgery. Nine patients had surgery performed under general anaesthesia employing muscle relaxants and controlled ventilation and nine under [ocal anaesthesia using a peribulbar block. The median (interquartile range) percentage of time during which the patients had an oxygen saturation of less than 90% was 0 (0-0.2) and 0.04 (0-0.4) on the pre-operative night, 0.7 (0-1.4) and 0.3 (0-1.2) in the immediate postoperative period, and 0.05 (0-0.16) and 0 (0-0.3) on the postoperative night in the general and local anaesthesia patients respectively. There were no significant differences between general and local anaesthesia in respect of these data and the overall incidence of signijicant desaturation was low. The present study could not demonstrate any adverse effect of general anaesthesia on oxygen saturation in patients undergoing minimally invasive surgery.

Key words Anaesthesia; geriatric. Anaesthetic techniques, regional; peribulbar block. Hypoxia.

Various studies have demonstrated a decrease in peripheral oxygen saturation (Spo,)in the period following anaesthesia and surgery [I+]. The important factors in its causation may be general anaesthesia, the type of surgery, ability to breathe effectively and the method of postoperative analgesia. It is also thought that age has an important influence [2, 31. There is, however, little information available about the effects of anaesthesia itself on elderly patients undergoing minimally invasive surgical procedures which do not impede breathing or require strong analgesics. The present study was designed to assess postoperative oxygenation in this population using ophthalmic surgery under local or general anaesthesia as a model. Methods

Eighteen ASA grade 1 or 2 patients (aged 70 years or over) scheduled to undergo lens extraction were included in the study with their informed consent and ethics committee approval. Patients with obesity, renal or hepatic impairment, chronic bronchitis, asthma, or who were receiving any sedative medication or acetazolamide were not studied. Patients were randomly allocated to receive either a local (LA) or general (GA) anaesthetic. Continuous recordings Accepted 18 May 1992.

of oxygen saturation were made on the night before surgery on all patients using a Datex Satlite oximeter employing a finger probe linked to a microcomputer for data storage. Recordings of oxygen saturation were made every 10 s and stored so that 360 readings were collected for every hour. Measurements took place between 2200 and 0600 h to suit ward routine. The period up to 2400 h was used to allow the patients to become accustomed to the probe, and data were analysed only between 2400 and 0600 h. One of the investigators (GMcC) was present during the recording of all the Spo, data. No night sedation was allowed during the study period and premedication consisted of paracetamol (1 .O g) 90 min before operation. The general anaesthesia group received thiopentone 2-4 mg.kg-', fentanyl 1 pg.kg-l, and vecuronium 0.08 mg.kg- ', followed by ventilation to normocarbia with 66% nitrous oxide in oxygen and 1.5 MAC halothane (age adjusted). Residual neuromuscular block was antagonised with neostigmine 40 pg.kg-l and glycopyrronium 10 pg.kg-l and antagonism confirmed by the absence of apparent fade to a tetanic stimulus. Local anaesthesia was instituted by peribulbar block using 7 ml of 2% lignocaine, 3 ml of 0.5%' bupivacaine with 1 :200000 adrenaline and 150 units of hyaluronidase. No sedation was given to

Intermittent positive pressure ventilation through a laryngeal mask airway. Is a nasogastric tube useful?

A nasogastric tube was used to aspirate air insufflated into the stomach during intermittent positive pressure ventilation through a laryngeal mask ai...
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