The Journal of Laryngology and Otology April 1979. Vol. 93. pp. 349-356
The effect of nitrous oxide on middle ear pressure in children during anaesthesia By CHANDRA BHAN SINGH and RICHARD KIRK (Sheffield) IT is well known that there is an increase in middle ear pressure, which can be a nuisance during reconstructive ear surgery, when the anaesthetic technique includes the use of nitrous oxide by inhalation or ventilation. Thomsen et al. (1965) studied variations in middle ear pressure during nitrous oxide anaesthesia in two volunteers. Rasmussen (1967) studied middle ear and maxillary sinus pressure variations during nitrous oxide anaesthesia in older children and adults. Waun et al. (1967) studied the effect of nitrous oxide on middle ear mechanics and hearing acuity. They found a decrease in compliance, and some of their patients had a mild conductive deafness lasting for two to three days. It is possible that there are some children undergoing surgery for disorders of middle ear function in whom such mechanical effects play a significant therapeutic role. In order to investigate this possibility it is necessary to know the normal sequence of events, and the aim of the present paper is to record the events in the middle ears of children with no middle ear disorder during the course of general anaesthesia involving the use of nitrous oxide. Method Selection of cases Twenty children between the ages of 4 and 11 years were selected from cases undergoing routine tonsillectomy and adenoidectomy. None of the children had any recent history of ear disease and their tympanic membranes were normal pre-operatively on otoscopy. They all had pre-operative and post-operative pure tone audiograms, and middle ear compliance and pressure measurements (using a Peters AP 61 impedence bridge), on the day before and the day after the operation. Anaesthesia One of two techniques was employed, the choice in individual cases being made by the child :(a) Inhalation of 33 per cent oxygen, 66 per cent nitrous oxide and 1 per cent halothane, delivered by face mask, until the level of anaesthesia was deep enough to allow intubation. 349
350
C. B. SINGH AND R. KIRK
(b) Induction by intravenous pentothal and scoline, followed by intubation and maintenance of anaesthesia by a gas mixture containing 32 per cent oxygen, 66 per cent nitrous oxide and 2 per cent halothane. The younger children generally chose the first technique and the older children preferred the latter. Middle ear pressure measurement during anaesthesia Prior to the commencement of anaesthesia, an airtight seal was obtained with a soft plastic insert around the tip of the probe of the acoustic impedence meter. The meter was adjusted so that there was a zero reading on the compliance scale when the adjusted pressure in the ear canal was equal to the pressure in the middle ear. As pressure changed in the middle ear, the pressure in the ear canal was constantly monitored and adjusted in order to maintain a steady reading of zero on the compliance scale. Changes in pressure were plotted against time on an X-Y-T plotter and it was assumed that these external pressure changes were equal to the pressure changes taking place within the middle ear space. Various factors limited the duration of the measurement. Leakage of pressure from the ear canal was a common problem, following both active and passive movements of the head. It became more likely as the pressure in the ear canal increased above atmospheric pressure and, in some cases, the probe was spontaneously blown out of the ear. It was not possible in practice to continue the readings in the operating theatre. The end-point of the pressure measurements, therefore, was determined by the time the child spent in the anaesthetic room. Results The findings for all cases during anaesthesia are summarized in Table I. The range of middle ear pressures immediately prior to anaesthetic induction was +50 to —190 mm H 2 O. The lower limit of normal middle ear pressure in children is taken as —200 mm H 2 O. All cases showed a rise in pressure during anaesthesia. The absolute pressure ranged from +215 to +500 mm H 2 O and the total rise in pressure ranged from +195 mm to 560 mm H 2 O. In all cases, the pressure, having reached a maximum, fell abruptly, presumably due to opening of the Eustachian tube. The time interval between the onset of nitrous oxide administration and the first fall in pressure varied between 1 minute 24 seconds and 17 minutes; and in those cases which could be followed up for a long enough time, the subsequent intervals lay between 30 seconds and 6 minutes 20 seconds. The rise of
EFFECT OF NITROUS OXIDE ON MIDDLE EAR PRESSURE IN CHILDREN
351
TABLE I Age 4 4 5 6 6 6 6 7 7 8 8 8 8 9 9 9 10 10 11 11 MEPO MEPM TPRX T/Cycle GradientM
MEPO
+ 20 -30
-110 -60
+ 20 -50 -160 0 -60 -80
+ 50 + 10 -190
-70 -130
+ 10 -50
+40 -60 -40
MEP M
+400 + 350 +230 + 500 + 215 +390 + 570 + 380 + 270 + 270 +460 +460 + 250 + 280 + 290 + 230 +230 +240 + 300 + 350
TPR t 7-25 14-6 3-8 6-8 5-3 10-6 18-2 13-6 6-2 3-7 9-7 13-6 5-2 17-0 7-75 5-5 5-8 1-4 6-3 110
T/Cycle . —. 2-5 1-9 —. 10 —. 10 0-5 20 6-3 1-2 30 — 1-5 10 1-2 1-25 —
GradientM 60 40 100 90 50 80 50 40 80 120 60 50 120 30 60 60 . 70 120 100 60
— Initial middle ear pressure, in mm H 2 O. — Maximum middle ear pressure, in mm H 2 O. — Time taken for initial peak pressure, in minutes. —• Time interval between the subsequent peak pressure, in minutes. — Pressure rise per minute.
pressure tended to reach a constant rate and this rate is also recorded in the table. The maximum rate of rise of pressure varied from 30 mm H 2 O to 120 mm H2O per minute. Table II shows the results of middle ear pressure and compliance measurements made during the afternoon before, and 24 hours after, the operation. Figure 1 shows a normal recording during anaesthesia, pre-operative and post-operative pure-tone audiograms and middle ear pressure recordings. Review of literature and discussion
Eger and Saidman (1965) studied the pressure and volume changes in hollow gas-filled body spaces during N 2 O anaesthesia. They found that the spaces with expandable walls (i.e., intestines, pneumoperitoneum and pneumothorax) increased in volume and that pressure increased in nonexpandable body spaces. Diffusion of N 2 O depends mainly on its pressure gradient between middle ear spaces and blood. The tympano-mastoid cavity is a closed space with non-expandable walls, except for the tympanic membrane, as the Eustachian tube is normally closed and only opens during swallowing or yawning.
352
C. B. SINGH AND R. KIRK
TABLE II PRESSURE/COMPLIANCE STUDIES
Pressure (in mm H2O) Age 4 4 5
6 6 6 6 7
7 8
8 8 8 9 9 9 10 10
11 11
Compliance (in cm.)
a)Pre b) Post
Neg/ Pos
a) Pre b) Post
Total shift
a) - 40 b) -270 a) -140 b)-300 a) -180 b) -280 a) - 30 b ) - 70 a) 0 b)-440 a) - 20 b) -320 a) -300 b) -600 a) - 20 b) -280 a ) + 50 b)-140 a)-100 b)-300 a) + 60 b) -380 a) 0 b) -170 a) -150 b)-400 a) 0 b) - 3 5 0 a) - 40 b) -300 a) - 20 b) -450 a) 0 b) -330 a) -140 b) -250 a) - 70 b)-400 a) - 40 b ) - 70
Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg
a) 0-5 b)0-7 a) 0-8 b)0-6 a) 0-4 b)0-4 a) 0-3 b)0-4 a) 0-6 b)0-6 a) 0-7 b)0-9 a) 0-3 b)01 a) 0-4 b)0-4 a) 0-3 b)0-4 a) 0-6 b)0-2 a) 0-3 b)0-8 a) 1-7 b)l-7 a) 0-8 b)0-2 a) 0-4 b)0-5 a) 0-4 b)0-4 a) 0-2 b)0-3 a) 0-3 b)0-5 a) 0-4 b)0-5 a) 1-2 b) 11 a) 0-3 b)0-4
+0-2 -0-2 0
+01 0
+0-2 -0-2 0
+01 -0-4 -0-4 0
-0-6 +01 0
+01 +0-2 +01 -01 +01
EFFECT OF NITROUS OXIDE ON MIDDLE EAR PRESSURE IN CHILDREN
353
MIDDLE EAR PRESSURERISE IN A NORMAL PATIENT UNDER N-0 ANAESTHESIA +400
+200
V y •
—
A V
A >
—
--200
4
6
10
Time (mins) 10.2.76-pm. PatlentEM
-20
-
0
-
S ?o
-
S 40
IM f •
Ear
Pressure
Compliance
R
-70
1.2
L
-80
1.5
-
I 100
Curve
A
A
-
120 125
500
2000
8000
Frequency (Hz I
12.2.77-am. PatlentEM -20 0
Ear
20 40
Pressure
Compliance
R
-400
1.1
L
-340
1.3
Curve
60 80 100 120 125
500
2000
8000
Frequency ( H z )
FIG. 1 Shows a normal recording during anaesthesia, pre-operative and post-operative pure-tone audiograms and middle ear pressure recordings.
354
C. B. SINGH AND R. KIRK
N 2 O is carried in the blood in large quantities. Its quantity in the blood depends on its partition co-efficient and partial tension in the lungs. The partition co-efficient of a gas is denned as the amount of gas which is dissolved in 1 ml of fluid at 0°C, when the pressure of the gas over the fluid is 760 mm of mercury. The partition co-efficient of N2O in blood is 0-430 and for nitrogen is 0-012. N 2 O is 34 times more soluble in blood than nitrogen. Thomsen et al. (1965) first reported the middle ear pressure changes under N 2 O anaesthesia. The middle ear pressure before N2O inhalation was 0 mm H 2 O in the first patient and —100 mm H2O in the second. The second patient had some impairment of tubal function. The middle ear pressure rose to +200 mm H2O in the first patient, with 60 per cent N2O inhalation. In the second volunteer, who inhaled 50 per cent N2O and 30 per cent O2, the middle ear pressure rose to +190 mm of water, at which level there was an abrupt fall of pressure to 0 mm H 2 O. The experiment was continued and this was followed by a further rise of middle ear pressure to 240 mm H 2 O before another abrupt fall in pressure. Thomsen et al. thought this was due to opening of the Eustachian tube during yawning. Rasmussen (1967) studied pressure changes in eight subjects with normal ears. He recorded pressure gradients between 20 mm and 100 mm H2O/minute during 30 per cent N 2 O+O 2 +Halothane anaesthesia. He noticed that the middle ear pressure rose to between +200 mm and +400 mm of H2O when a sudden drop in pressure occurred. This, he attributed to the passive opening of the Eustachian tube, the time for which varied from less than 5 minutes to 20 minutes. He also noted that if anaesthesia was continuous, the pressure, after an initial passive Eustachian tube opening, rose again to the previous peak level, when a further sudden drop in pressure occurred. This cycle continued at regular intervals. Rasmussen also demonstrated that the middle ear pressure increases rapidly only when the subject is inhaling N 2 O. This did not occur when Halothane was used, as a separate gas or combined with oxygen. Patterson and Bartlett (1976) studied middle ear pressures during 70 per cent N 2 O anaesthesia on six normal subjects. The middle ear pressure rose to +375 mm H 2 O in 25 minutes before a sudden drop in pressure occurred. They attributed this to passive venting of the Eustachian tube due to a rise in intratympanic pressure. In our group of normal children (Table I), the middle ear pressure rose between +200 mm H 2 O and +500 mm H2O in 1 minute 30 seconds to 17 minutes, before a sudden drop occurred, due to passive opening of the Eustachian tube. The drop in pressure almost reached the pre-anaesthesia level. It was followed by a further rise in the middle ear pressure, which took less time to reach its peak. This was followed by another sudden drop in pressure to the base level. This cycle continued as long as the child was under observation during N 2 O anaesthesia.
EFFECT OF NITROUS OXIDE ON MIDDLE EAR PRESSURE IN CHILDREN
355
Our findings are in agreement with other workers (Rasmussen, 1967; Grimaldi, 1976; Patterson and Bartlett, 1976), except that the peak pressure was reached in a shorter period in most cases. This may be due to the quick saturation of blood with N 2 O in children. Differences in time taken for the middle ear pressure to reach its peak may also be related to the volume of tympanomastoid segments in children and the vascularity of the middle ear mucosa. Pneumatization of the temporal bone varies a great deal in all age groups. It is possible that children in whom the middle ear pressure rose quickly to a peak may have less pneumatization of the tympano-mastoid segment than children who had a slower rise in pressure. The interval between successive openings of the Eustachian tubes seems to be more or less constant in each case. This rhythmic opening of the Eustachian tube could not be attributed either to repeated swallowing or to yawning, as the children were kept in a stage of anaesthesia which abolishes these movements. It seems reasonable to assume that the observed drop in middle ear pressure was caused by passive opening of the Eustachian tube due to a rise in intratympanic pressure. We found that, on average, the time taken for the middle ear pressure to return to its normal pre-anaesthetic level, after turning off the N 2 O, was directly related to the time taken to reach the initial peak pressure. Summary
We have studied pressure changes in children with normal middle ears under nitrous oxide anaesthesia. The pre-anaesthetic middle ear pressures ranged between + 5 0 and -190mmH2O. We found that the middle ear pressure increased in all cases to a maximum pressure peak, at which a sudden drop in pressure occurred due to passive opening of the Eustachian tube. This pressure varied between +210 and +500 mm H2O. The pressure did not drop to the pre-anaesthetic level. In all cases, in comparison with pre-anaesthetic pressure values we found, 24 hours after anaesthesia, a definitive decrease in pressure with a reduction ranging from —30 to —400 mm H 2 O. Audiometric correlations between pre- and post-operative findings showed a decreased acuity in nine patients, increased acuity in six patients, and no significant change in five patients. Compliance results also showed no significant change between pre- and post-operative measurements. Our results show the effect that N 2 O has on a group of normal children, namely to raise the middle ear pressure until a level is attained which causes passive opening of the Eustachian tube. They also show that, 24 hours after the operation, even though the N2O is responsible for changes in middle ear mechanics which cause a mild conductive deafness in some patients, there is no significant change in acuity overall.
356
C. B. SINGH AND R. KIRK
Acknowledgements We would like to thank Mr. J. T. Buffin, Consultant Medical Audiolo gist, The Children's Hospital, Sheffield, and all theatre staff, anaesthetist; and members of the Audiology Department of the Children's Hospital Sheffield, for their co-operation and assistance. REFERENCES EGER, E. I., and SAEDMAN, L. J. (1965) Anaesthesiology, 26, 61.
GRIMALDI, P. M. (1976) Journal of Laryngology and Otology, 90(2), 141. PATTERSON, M. E., and BARTLETT, P. C. (1976) Laryngoscope, 86(3), 399. RASMUSSEN, P. E. (1967) Ada Otolaryngologica, 63, 7. THOMSEN, K. A., TERKUDSEN, K., and ARNFRED, I. (1965) Archives of Otolaryngology, 82,609 WAUN, J., SWETTZER, R. S., and HAMILTON, W. K. (1967) Anaesthesiology, 28, 846.
The effect of nitrous oxide on middle ear pressure in children during anaesthesia By CHANDRA BHAN SINGH and RICHARD KIRK (Sheffield) IT is well known that there is an increase in middle ear pressure, which can be a nuisance during reconstructive ear surgery, when the anaesthetic technique includes the use of nitrous oxide by inhalation or ventilation. Thomsen et al. (1965) studied variations in middle ear pressure during nitrous oxide anaesthesia in two volunteers. Rasmussen (1967) studied middle ear and maxillary sinus pressure variations during nitrous oxide anaesthesia in older children and adults. Waun et al. (1967) studied the effect of nitrous oxide on middle ear mechanics and hearing acuity. They found a decrease in compliance, and some of their patients had a mild conductive deafness lasting for two to three days. It is possible that there are some children undergoing surgery for disorders of middle ear function in whom such mechanical effects play a significant therapeutic role. In order to investigate this possibility it is necessary to know the normal sequence of events, and the aim of the present paper is to record the events in the middle ears of children with no middle ear disorder during the course of general anaesthesia involving the use of nitrous oxide. Method Selection of cases Twenty children between the ages of 4 and 11 years were selected from cases undergoing routine tonsillectomy and adenoidectomy. None of the children had any recent history of ear disease and their tympanic membranes were normal pre-operatively on otoscopy. They all had pre-operative and post-operative pure tone audiograms, and middle ear compliance and pressure measurements (using a Peters AP 61 impedence bridge), on the day before and the day after the operation. Anaesthesia One of two techniques was employed, the choice in individual cases being made by the child :(a) Inhalation of 33 per cent oxygen, 66 per cent nitrous oxide and 1 per cent halothane, delivered by face mask, until the level of anaesthesia was deep enough to allow intubation. 349
350
C. B. SINGH AND R. KIRK
(b) Induction by intravenous pentothal and scoline, followed by intubation and maintenance of anaesthesia by a gas mixture containing 32 per cent oxygen, 66 per cent nitrous oxide and 2 per cent halothane. The younger children generally chose the first technique and the older children preferred the latter. Middle ear pressure measurement during anaesthesia Prior to the commencement of anaesthesia, an airtight seal was obtained with a soft plastic insert around the tip of the probe of the acoustic impedence meter. The meter was adjusted so that there was a zero reading on the compliance scale when the adjusted pressure in the ear canal was equal to the pressure in the middle ear. As pressure changed in the middle ear, the pressure in the ear canal was constantly monitored and adjusted in order to maintain a steady reading of zero on the compliance scale. Changes in pressure were plotted against time on an X-Y-T plotter and it was assumed that these external pressure changes were equal to the pressure changes taking place within the middle ear space. Various factors limited the duration of the measurement. Leakage of pressure from the ear canal was a common problem, following both active and passive movements of the head. It became more likely as the pressure in the ear canal increased above atmospheric pressure and, in some cases, the probe was spontaneously blown out of the ear. It was not possible in practice to continue the readings in the operating theatre. The end-point of the pressure measurements, therefore, was determined by the time the child spent in the anaesthetic room. Results The findings for all cases during anaesthesia are summarized in Table I. The range of middle ear pressures immediately prior to anaesthetic induction was +50 to —190 mm H 2 O. The lower limit of normal middle ear pressure in children is taken as —200 mm H 2 O. All cases showed a rise in pressure during anaesthesia. The absolute pressure ranged from +215 to +500 mm H 2 O and the total rise in pressure ranged from +195 mm to 560 mm H 2 O. In all cases, the pressure, having reached a maximum, fell abruptly, presumably due to opening of the Eustachian tube. The time interval between the onset of nitrous oxide administration and the first fall in pressure varied between 1 minute 24 seconds and 17 minutes; and in those cases which could be followed up for a long enough time, the subsequent intervals lay between 30 seconds and 6 minutes 20 seconds. The rise of
EFFECT OF NITROUS OXIDE ON MIDDLE EAR PRESSURE IN CHILDREN
351
TABLE I Age 4 4 5 6 6 6 6 7 7 8 8 8 8 9 9 9 10 10 11 11 MEPO MEPM TPRX T/Cycle GradientM
MEPO
+ 20 -30
-110 -60
+ 20 -50 -160 0 -60 -80
+ 50 + 10 -190
-70 -130
+ 10 -50
+40 -60 -40
MEP M
+400 + 350 +230 + 500 + 215 +390 + 570 + 380 + 270 + 270 +460 +460 + 250 + 280 + 290 + 230 +230 +240 + 300 + 350
TPR t 7-25 14-6 3-8 6-8 5-3 10-6 18-2 13-6 6-2 3-7 9-7 13-6 5-2 17-0 7-75 5-5 5-8 1-4 6-3 110
T/Cycle . —. 2-5 1-9 —. 10 —. 10 0-5 20 6-3 1-2 30 — 1-5 10 1-2 1-25 —
GradientM 60 40 100 90 50 80 50 40 80 120 60 50 120 30 60 60 . 70 120 100 60
— Initial middle ear pressure, in mm H 2 O. — Maximum middle ear pressure, in mm H 2 O. — Time taken for initial peak pressure, in minutes. —• Time interval between the subsequent peak pressure, in minutes. — Pressure rise per minute.
pressure tended to reach a constant rate and this rate is also recorded in the table. The maximum rate of rise of pressure varied from 30 mm H 2 O to 120 mm H2O per minute. Table II shows the results of middle ear pressure and compliance measurements made during the afternoon before, and 24 hours after, the operation. Figure 1 shows a normal recording during anaesthesia, pre-operative and post-operative pure-tone audiograms and middle ear pressure recordings. Review of literature and discussion
Eger and Saidman (1965) studied the pressure and volume changes in hollow gas-filled body spaces during N 2 O anaesthesia. They found that the spaces with expandable walls (i.e., intestines, pneumoperitoneum and pneumothorax) increased in volume and that pressure increased in nonexpandable body spaces. Diffusion of N 2 O depends mainly on its pressure gradient between middle ear spaces and blood. The tympano-mastoid cavity is a closed space with non-expandable walls, except for the tympanic membrane, as the Eustachian tube is normally closed and only opens during swallowing or yawning.
352
C. B. SINGH AND R. KIRK
TABLE II PRESSURE/COMPLIANCE STUDIES
Pressure (in mm H2O) Age 4 4 5
6 6 6 6 7
7 8
8 8 8 9 9 9 10 10
11 11
Compliance (in cm.)
a)Pre b) Post
Neg/ Pos
a) Pre b) Post
Total shift
a) - 40 b) -270 a) -140 b)-300 a) -180 b) -280 a) - 30 b ) - 70 a) 0 b)-440 a) - 20 b) -320 a) -300 b) -600 a) - 20 b) -280 a ) + 50 b)-140 a)-100 b)-300 a) + 60 b) -380 a) 0 b) -170 a) -150 b)-400 a) 0 b) - 3 5 0 a) - 40 b) -300 a) - 20 b) -450 a) 0 b) -330 a) -140 b) -250 a) - 70 b)-400 a) - 40 b ) - 70
Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg Neg
a) 0-5 b)0-7 a) 0-8 b)0-6 a) 0-4 b)0-4 a) 0-3 b)0-4 a) 0-6 b)0-6 a) 0-7 b)0-9 a) 0-3 b)01 a) 0-4 b)0-4 a) 0-3 b)0-4 a) 0-6 b)0-2 a) 0-3 b)0-8 a) 1-7 b)l-7 a) 0-8 b)0-2 a) 0-4 b)0-5 a) 0-4 b)0-4 a) 0-2 b)0-3 a) 0-3 b)0-5 a) 0-4 b)0-5 a) 1-2 b) 11 a) 0-3 b)0-4
+0-2 -0-2 0
+01 0
+0-2 -0-2 0
+01 -0-4 -0-4 0
-0-6 +01 0
+01 +0-2 +01 -01 +01
EFFECT OF NITROUS OXIDE ON MIDDLE EAR PRESSURE IN CHILDREN
353
MIDDLE EAR PRESSURERISE IN A NORMAL PATIENT UNDER N-0 ANAESTHESIA +400
+200
V y •
—
A V
A >
—
--200
4
6
10
Time (mins) 10.2.76-pm. PatlentEM
-20
-
0
-
S ?o
-
S 40
IM f •
Ear
Pressure
Compliance
R
-70
1.2
L
-80
1.5
-
I 100
Curve
A
A
-
120 125
500
2000
8000
Frequency (Hz I
12.2.77-am. PatlentEM -20 0
Ear
20 40
Pressure
Compliance
R
-400
1.1
L
-340
1.3
Curve
60 80 100 120 125
500
2000
8000
Frequency ( H z )
FIG. 1 Shows a normal recording during anaesthesia, pre-operative and post-operative pure-tone audiograms and middle ear pressure recordings.
354
C. B. SINGH AND R. KIRK
N 2 O is carried in the blood in large quantities. Its quantity in the blood depends on its partition co-efficient and partial tension in the lungs. The partition co-efficient of a gas is denned as the amount of gas which is dissolved in 1 ml of fluid at 0°C, when the pressure of the gas over the fluid is 760 mm of mercury. The partition co-efficient of N2O in blood is 0-430 and for nitrogen is 0-012. N 2 O is 34 times more soluble in blood than nitrogen. Thomsen et al. (1965) first reported the middle ear pressure changes under N 2 O anaesthesia. The middle ear pressure before N2O inhalation was 0 mm H 2 O in the first patient and —100 mm H2O in the second. The second patient had some impairment of tubal function. The middle ear pressure rose to +200 mm H2O in the first patient, with 60 per cent N2O inhalation. In the second volunteer, who inhaled 50 per cent N2O and 30 per cent O2, the middle ear pressure rose to +190 mm of water, at which level there was an abrupt fall of pressure to 0 mm H 2 O. The experiment was continued and this was followed by a further rise of middle ear pressure to 240 mm H 2 O before another abrupt fall in pressure. Thomsen et al. thought this was due to opening of the Eustachian tube during yawning. Rasmussen (1967) studied pressure changes in eight subjects with normal ears. He recorded pressure gradients between 20 mm and 100 mm H2O/minute during 30 per cent N 2 O+O 2 +Halothane anaesthesia. He noticed that the middle ear pressure rose to between +200 mm and +400 mm of H2O when a sudden drop in pressure occurred. This, he attributed to the passive opening of the Eustachian tube, the time for which varied from less than 5 minutes to 20 minutes. He also noted that if anaesthesia was continuous, the pressure, after an initial passive Eustachian tube opening, rose again to the previous peak level, when a further sudden drop in pressure occurred. This cycle continued at regular intervals. Rasmussen also demonstrated that the middle ear pressure increases rapidly only when the subject is inhaling N 2 O. This did not occur when Halothane was used, as a separate gas or combined with oxygen. Patterson and Bartlett (1976) studied middle ear pressures during 70 per cent N 2 O anaesthesia on six normal subjects. The middle ear pressure rose to +375 mm H 2 O in 25 minutes before a sudden drop in pressure occurred. They attributed this to passive venting of the Eustachian tube due to a rise in intratympanic pressure. In our group of normal children (Table I), the middle ear pressure rose between +200 mm H 2 O and +500 mm H2O in 1 minute 30 seconds to 17 minutes, before a sudden drop occurred, due to passive opening of the Eustachian tube. The drop in pressure almost reached the pre-anaesthesia level. It was followed by a further rise in the middle ear pressure, which took less time to reach its peak. This was followed by another sudden drop in pressure to the base level. This cycle continued as long as the child was under observation during N 2 O anaesthesia.
EFFECT OF NITROUS OXIDE ON MIDDLE EAR PRESSURE IN CHILDREN
355
Our findings are in agreement with other workers (Rasmussen, 1967; Grimaldi, 1976; Patterson and Bartlett, 1976), except that the peak pressure was reached in a shorter period in most cases. This may be due to the quick saturation of blood with N 2 O in children. Differences in time taken for the middle ear pressure to reach its peak may also be related to the volume of tympanomastoid segments in children and the vascularity of the middle ear mucosa. Pneumatization of the temporal bone varies a great deal in all age groups. It is possible that children in whom the middle ear pressure rose quickly to a peak may have less pneumatization of the tympano-mastoid segment than children who had a slower rise in pressure. The interval between successive openings of the Eustachian tubes seems to be more or less constant in each case. This rhythmic opening of the Eustachian tube could not be attributed either to repeated swallowing or to yawning, as the children were kept in a stage of anaesthesia which abolishes these movements. It seems reasonable to assume that the observed drop in middle ear pressure was caused by passive opening of the Eustachian tube due to a rise in intratympanic pressure. We found that, on average, the time taken for the middle ear pressure to return to its normal pre-anaesthetic level, after turning off the N 2 O, was directly related to the time taken to reach the initial peak pressure. Summary
We have studied pressure changes in children with normal middle ears under nitrous oxide anaesthesia. The pre-anaesthetic middle ear pressures ranged between + 5 0 and -190mmH2O. We found that the middle ear pressure increased in all cases to a maximum pressure peak, at which a sudden drop in pressure occurred due to passive opening of the Eustachian tube. This pressure varied between +210 and +500 mm H2O. The pressure did not drop to the pre-anaesthetic level. In all cases, in comparison with pre-anaesthetic pressure values we found, 24 hours after anaesthesia, a definitive decrease in pressure with a reduction ranging from —30 to —400 mm H 2 O. Audiometric correlations between pre- and post-operative findings showed a decreased acuity in nine patients, increased acuity in six patients, and no significant change in five patients. Compliance results also showed no significant change between pre- and post-operative measurements. Our results show the effect that N 2 O has on a group of normal children, namely to raise the middle ear pressure until a level is attained which causes passive opening of the Eustachian tube. They also show that, 24 hours after the operation, even though the N2O is responsible for changes in middle ear mechanics which cause a mild conductive deafness in some patients, there is no significant change in acuity overall.
356
C. B. SINGH AND R. KIRK
Acknowledgements We would like to thank Mr. J. T. Buffin, Consultant Medical Audiolo gist, The Children's Hospital, Sheffield, and all theatre staff, anaesthetist; and members of the Audiology Department of the Children's Hospital Sheffield, for their co-operation and assistance. REFERENCES EGER, E. I., and SAEDMAN, L. J. (1965) Anaesthesiology, 26, 61.
GRIMALDI, P. M. (1976) Journal of Laryngology and Otology, 90(2), 141. PATTERSON, M. E., and BARTLETT, P. C. (1976) Laryngoscope, 86(3), 399. RASMUSSEN, P. E. (1967) Ada Otolaryngologica, 63, 7. THOMSEN, K. A., TERKUDSEN, K., and ARNFRED, I. (1965) Archives of Otolaryngology, 82,609 WAUN, J., SWETTZER, R. S., and HAMILTON, W. K. (1967) Anaesthesiology, 28, 846.
