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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 fo r th efir st hour of anaesthesia.

Key words

Equipment;

laryngeal mask airway, nasogastric tube.

Complications;

gastric insufflation.

Laryngeal mask airways (LM A) are now firmly established

in anaesthetic practice. Acceptance has been rapid because

of the advantages they have over a facem ask. It is easier to

maintain an airway with a LMA, they free the anaesthe-

tist'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 num ber 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 L M A

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 m ask,

and that if the mask is properly inserted the oesophagus

should not be seen. One possible consequence of inade-

quate isolation is gastric insufflation. Gastric insufflation

has been implicated as a potential factor in reducing dia-

phragmatic 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 nasogas-

tric 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 grou p 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.

Ten patients were allocated to grou p

I 20

t o g roup

2

and

20

t o g roup 3. All patients were premedicated with tema-

zepam and anaesthesia was induced with propofol

2.5

mg.kg-' and atracurium 0.3 mg.kg-l. The patients '

lungs were gently ventilated by facema sk with N20 02 nd

enflurane

1

for 3 min. A 16FR single lumen nasogastric

tube w as 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 to the pat ient .

The t racheas 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 an y possible blockage, and aspiration was attem pted

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 with-

drawn 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 vari-

ables between t he three groups (Tab le 2). All patients were

ventilated with airway pressures below 20 c m H 2 0 , a n d a

high proportion of patients in groups

2

and

3

had an

audible leak aroun d the LM A (Table 2).

The re was no significant difference in the am oun ts of air

aspirated at each interval between groups 1 (tracheal tube)

and

2

(LMA with nasogastric tube closed) or between

groups

2

and

3

(LMA with nasogastric tube open). The

Accepted 5 May 1992.

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Table

1.

Demographic data.

G r o u p n)

1.0

(7.0)

2.0 (20.0) 3.0 (18.0)

Age; years (SD)

40.0 (21.0)

47.0

(20.0)

37.0 (20.0)

Weight; kg (SD)

75.0 (3.5)

67.0

(12.0)

71.0 (14.0)

Table

2. Respiratory parameters.

G r o u p n) (7) 2 (20) 3 (18)

Tidal volume; ml

714.0 (124.0)

662.0 (93.0) 691.0 (121.0)

Minute volume

I

6.7 (1.7) 6.8 (1.8)

7.1 (2.1)

Airway pressure;

18.0 (2.0)

19.0 (2.1) 17.0 (4.2)

Leak

n X)

0

9.0 (45.0) 8.0 (48.0)

(SD)

SD)

cmH,O (SD)

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.

G r o u p I Group

2

Time (min) TT; ml (SD) LMA; ml (SD) p value

15 2.57 (3.3) 2.85 (5.9) 0.88

30 0.3 (0.8) 1.65 (3.0) 0.07

45 0.29 (0.76) 0.85 (2.0) 0.3

60 0.29 (0.8) 0.2 (0.89) 0.8

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.

G r oup 2 G r oup

LMA with LMA with

nasogastric tube nasogastric tube

closed; open;

Time (min ) ml

(SD)

ml (SD) p value

I5 2.85 (5.84) 3.00 (6.65) 0.94

30 .65 (3.0) 1.33 (3.55) 0.77

45 0.85 (2.0) 2. (8.22) 0.53

60

0.2 (0.89) 0.39 (0.85) 0.51

Table 5. Frequency distribution of total volumes aspirated in

groups and 2.

Total volume (ml) 0-20 2 0 4 0 40

G r o u p

(TT)

n 7( ) 7 (100) 0 0

G r oup 2 ( L MA ) n 20( ) 18 (90) 2(10)

0

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

Gr ou p 2 18 (90) 2 10)

0

(LMA with

nasogastric tube

closed)

n 20

YO)

G r oup 3

(LM A with

nasogastric tube

n 18 (X)

open)

0

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 gro up

2

and two in g roup

3

aspirated

a total volume of more than 20 ml, but less than 40 ml

(Tables 5 and 6 ) .

Discussion

There is no readily applicable gold standard for me asuring

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 amou nt aspirated should be the same in all groups. The

aim

of

the study was to d emonstrate a difference between

groups if one existed, not to document the amount of

gastric insufflation which may have occurred.

If gastric insufflation was occ urrin g in the patie nts under-

going IPPV through a

L M A ,

insufflated air should have

accumulated in the stom achs of patients in gro up 2. Also,

the air insufflated in the patients in group

3

should have

escaped through the nasogastic tube, which was o pen to the

atm osph ere. Th e absence of a significant difference between

these two groups suggests that either the nasogastric tube

was not allowing insufflated air to escape, or gastric insuf-

flation was not occurring. The small volumes of air

aspirated and th e 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 t o become blocked. In this study, if no a ir 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 differ-

ence in the amount of air aspirated between patients in

groups 1 and

2

mitigates against this. The incidence of leak

around the

L M A

in groups 2 an d 3 was higher than

reported in other studies [4] his was most likely due to

the nasogastric tube in s i tu 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 naso-

gastric tube.

In adults, less than 40 ml of air in the stomach is

considered physiologically insignificant. Intragastric

pressure doe s not rise until at least

50

ml is injected in to the

stomach of small animals. [5]. None of the patients in this

s tudy had more than 40 ml aspirated in total. Gastric

volumes of air ar e therefore likely t o be a small, although

the accuracy of this technique in determining the am oun t 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, an d the increased incidence of leak

aro un d the LM A 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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