Acta Anaesthesiol Scand 1992: 36: 775-778
A device for measuring the lateral wall cuff pressure of endotracheal tubes N. LOMHOLT Department of Pharmacology, University of Copenhagen, Denmark
A new method for measuring the lateral wall pressure on the trachea from cuffs of endotracheal tubes is presented. The method is based on measuring the force necessary to force a small, constant flow of air through a Teflon or silicone rubber envelope, placed between the cuff and the tracheal wall. The pressures recorded are 0.1 to 0.3 kPa higher than the theoretically correct values. The response time is 0.28 s for 90% of full deflection. Dynamic recordings of the lateral wall pressure of high and low residual volume cuffs can be obtained for analysis of the interaction between the c u 5 and the tracheal compliance. No method for accurate, dynamic recording of the lateral wall cuff pressure has previously been published. Received I4 September 1991, accepted for publication 15 March 1992
Key words: Anesthesia technique; cuff pressure measurement; tracheal tubes.
The cuffs of endotracheal and tracheostomy tubes are supposed to seal the trachea and protect the patient against aspiration of gastric contents. The sealing function is also necessary for satisfactory artificial ventilation, to secure the intended ventilation. During recent decades, the development of cuff technology has aimed at performing these functions without damaging the trachea, including protection of its mucosa from ischaemia. These demands have been met by the development of thin-walled, high-residual-volume cuffs. Still, low-residual-volume cuffs are used, and not always just for short procedures (1), where they can be used with a reasonably high safety margin. The output of a major American manufacturer consists of about 1/3 low-residual-volume cuffed tubes and about 1/3 medium-volume-cuff tubes (personal communication). As both the sealing function and the damaging effect of the cuff are effects of the lateral wall pressure (LWP), considerable interest has been devoted to measuring the LWP. Apparently, none of the suggested methods has been able to give an accurate continuous recording of the LWP (2). The aim of the present investigation was to design a monitor for measuring LWP, with a sufficiently short response time for use as a transducer for continuous recording, thus enabling an analysis of the interaction between the cuff and the tracheal wall. Low residual volume cuffs, which depend on stretching the cuff membrane for sealing the trachea, transmit only part of the intracuff pressure (ICP) to the tracheal wall. This pressure can only be measured with an LWP monitor. In contrast, the LWP of an ideal high residual
volume cuff is identical to the ICP, because no part of the cuff pressure is used for stretching the cuff membrane, which lies in folds on the tracheal wall.
MATERIAL AND METHODS Two different LWP monitors were constructed, one made from Teflon@film and one made from silicone rubber sheet. Two layers of Teflon FEP film, (from Dupont, U.S.A.) 0.025 mm thick, and gluable on one side, were glued together along the edges of two opposite sides, while the two layers were separated by a 6-mm strip of aluminum foil, 0.013 mm thick. E 41 silicone adhesive from Wacker Chemie (Germany), was used tojoin the two layers together. After polymerization of the silicone adhesive, the aluminum strip was removed with hydrochloric acid. The Teflon envelope was cut to a length of 80 mm and closed at one end with E 41 glue. Near the closed end, the envelope was connected to a thin-walled silicone tube, i.d. 2.4 mm, 0.d. 3.0 mm, 25 cm long, through a 2.5-mm sidehole in the envelope and a corresponding sidehole in the silicone tube (Fig. 1). The silicone monitor was constructed exactly like the Teflon monitor, except that silicone rubber sheet, 0.250 mm thick, from Dow Corning (U.S.A.), was used instead. The airflow from a reduction valve, supplied from a high-pressure gas source, was adjusted with precision flowmeten. The air supply was connected to the silicone rubber tube, and the pressure, necessary to force the air through the envelope, was measured with a Uniflow pressure transducer from Baxter Healthcare Div. (U.S.A.). The pressure was recorded with a Clevite-Gould recorder (Gould Inc., U.S.A.). The basic flow resistance of the two free-standing LWP monitors was measured in the range 100 to 1000 ml/min. The two monitors were calibrated by simultaneously recording the ICP in a high residual volume cuff placed in a model trachea, and the pressure necessary to force 300 ml/min through the monitor, placed between the cuff and the wall of the model trachea. The cuff was inflated to 9 kPa in steps of 1 kPa. Transparent copies of the two recordings were superimposed upon each other, to facilitate the detection of minor differences between the curves.
776
N. LOMHOLT
o3 Cuff: P
Fig. 1. A sketch of the lateral wall cuff pressure monitor. Dimensions: 80 mm long, 8 mm broad, 0.050 mm thick.
0
The response time of the two monitors was measured with the same setup as just described. The cuff pressure was suddenly released, while rhr LWP and ICP were recorded at high speed, 125 mm/s.
RESULTS The results are presented in Fig. 2, 3 and 4. The basic resistance to flow was 5 to 6 times higher in the silicone rubber monitor than in the Teflon monitor. At 300 ml/min, the Teflon monitor had a resistance of 0.05 kPa, while the silicone monitor had a resistance of 0.3 kPa. The calibration of the Teflon monitor showed that the recorded LWP was 0.1 to 0.3 kPa higher than the ICP of the high volume cuff. With the silicone monitor, the difference between the ICP and the LWP was slightly larger. The response time for 90% of full deflection was 0.28 s for the Teflon monitor and 0.38 s for the silicone monitor. DISCUSSION The recordings showed that the measured LWP was somewhat higher than the ICP of a high residual residual volume cuff, while the pressures in theory should be identical. In spite of this, it was possible to obtain satisfactory dynamic recordings of the function of endotracheal tube cuffs during controlled ventilation (Fig. 5). During the construction, four conditions proved to be important for a reliable performance:
kPa
0 70 0 60 0 50 0 40 0 30
000100
200
300
400
500
600
700
800
000
1000
rnl/rnin
Fig. 2. The flow resistance of the two freestanding lateral wall cuff pressure monitors. Silicone rubber monitor: 0, Teflon film monitor:
A.
01
02
03
04
7
O5sek
LWP
0 I
0
I
1
I
I
0.1
02
03
04
0 5 sek
Fig. 3. Measurement of the response time of the Teflon film monitor by a modified step-test. The cuff pressure in a high residual volume cuff in a model trachea and the lateral wall cuff pressure were recorded simultaneously during a sudden release of the cuff pressure.
The monitor should be as thin as possible. Incorrect results were obtained if the monitor was elevated above the tracheal wall. The airflow to the monitor should be delivered through a sidehole and not at the end. The constant flow rate to the monitor was shown to be critical. With a 300 ml * min- flow, consistent results were obtained, but if the flow was reduced, the response time increased and the peaks of the pressure recordings were reduced. The deadspace of the tubes from the reduction valve to the monitor should be as small as possible. With too large a deadspace, the LWP curves became distorted due to damping. Several attempts have been made to measure LWP, but apparently none has been quite satisfactory, and some of the results are rather controversial. Calibration of the LWP-measuring devices against the ICP of high residual volume cuffs was not done in any of the publications. Nevertheless, some information can be deduced from the published figures, as to whether the devices were recording the LWP correctly. Under two conditions, information about the function of the LWP devices can be obtained: 1) With a rigid, cylindrical model trachea and a high residual volume cuff, the LWP and the ICP should be nearly identical. 2) With the same type of trachea, the LWP and the airway pressure at peak inflation should also be nearly identical, if high and low residual volume cuffs are inflated to “just seal”. The monitors presented here comply with both these demands. Knowlson & Bassett (3) used an envelope similar to the present one for measuring LWP. The
’
LATERAL WALL CUFF PRESSURE MONITOR
777
Fig. 4. Calibration of the Teflon film monitor by simultaneous recording of the lateral wall cuff pressure and the intracuff pressure, which was increased stepwise to 9 kPa. The two curves were superimposed upon each other and show the displacement of the upper lateral wall pressure curve.
major difference between the principle used in their method and our method is that theirs could only be used for static and not for dynamic measurements. Their envelope was connected to a balloon reservoir, air was rapidly forced into the balloon and the air escaped through the open envelope. When the envelope collapsed, due to the pressure from the cuff, the remaining pressure in the balloon was considered to be the LWP. Knowlson & Bassett tested their LWP monitor in patients with controlled ventilation, intubated with low residual volume cuffs. Dynamic recordings with the present LWP method have shown that the LWP is high during the expiratory phase, and decreases during inspiration to give “just seal”, because the trachea is dilated by the pressure inflating the lungs. This is probably why Knowlson & Bassett found values 2 to 3 times higher than airway pressure.
kPa
Fig. 5. Recording of the airway pressure (lower curve), the cuff pressure (middle curve), and the lateral wall cuff pressure (upper curve) during ventilation of a model lung and a model trachea to an airway pressure of 5 and 6 kPa, through an endotracheal tube with a high residual volume cuff.
The first demand, that LWP and ICP should be nearly identical with high residual volume cuffs, if the LWP measuring device records correctly, is met by Leigh & Maynard (4),who used a 3-mm-thick bladder as the LWP measuring device in an Imatrach model trachea from Mallinckrodt (U.S.A.). They found nearly identical LWPs (2-2.9 kPa) and ICPs (2.4-3.3 kPa). Carroll and colleagues (5) also found identical LWPs (0.7-1.9 kPa) and ICPs (0.7-1.9 kPa) with high residual volume cuffs. They measured LWP with a pressure transducer implanted in the tracheal wall of an anaesthetized dog. From Table 1, it can be seen that the results of Ludemann & Witte ( 6 ) met the second demand for correct recording of the LWP, as they found nearly identical LWP and airway pressure at the “just seal” condition with a high residual volume cuff in tracheas from human cadavers. A model trachea, even with a correct compliance, is very different from a normal trachea. How could the present LWP monitors be expected to perform in a patient’s trachea? It might be expected that the measured LWP with high volume cuffs would deviate from the ICP, and that the measurements at “just seal” conditions might be different. The reason for this is that the tracheal cartilages probably carry a larger part of the pressure from the cuff, because even thinwalled, high-residual-volume cuffs cannot be expected to distribute their pressure evenly between the cartilage and the softer parts of the trachea. With lowresidual-volume cuffs, the variations in cross-section configuration of the trachea will certainly influence the results, and show areas of high and low pressure (7, 8). In three papers (%11), the LWP was calculated from the difference between ICPs in low-residual-vol-
778
N. LOMHOLT
Table I Summary of information from 8 papers on measurement of the lateral wall cuff pressure of high and low residual volume cuffs in relation to the airway pressure, when the cuff seals the trachea airtight, i.e. the “just seal” condition. LWP device
Trachea
Airway
H M D, DC, HC
15 mm constant ventilation 12.5 mm - 16.5 mm 18 mm 22 mm 10 mm constant ventilation 10 mm 25 mm constant ventilation
Knowlson (3) Leigh (4.) Carroll (2)
Envelope Bladder Pressure transducer
Liidemann (6) Kamen (12)
Bladder Bladder
Wu (13) Dobrin (14)
Pressure transducer Force transducer
HC M HC M DC
Wu (15)
Pressure transducer
M
High volume LWP
Low volume LWP
15-22 mm 5-70 mm
24-45 mm 105 155 mm 13&147 mm
19 mm
36-184 mm 110 mm
2645 2&50 40-80 15-27
mm mm mm mm
180 mm 74-240 mm 80-325 mm 105-350 mm 70-75 mm
H: patient trachea, HC: human cadaver trachea, D: dog trachea, DC: dog cadaver trachea, M: model trachea. All figures are mmHg. 10 mmHg is equal to 1.33 kPa.
ume cuffs, whether they were inflated freestanding or with the same volume in model or normal tracheas. Due to the pronounced time-dependent hysteresis of low-residual-volume cuffs, such calculations are not valid (unpublished results). It is difficult to explain why LWP, 50 to 300 mmHg (6.7 to 40 kPa) higher than airway pressure are given for the “just seal” condition in most investigations with low-residual-volume cuffs (3-6, 12-15), (Table 1). It is hoped that further experience with the method presented here will solve this controversy between theory and practice.
REFERENCES 1. Pippin L K, Short D H, Bowes J B. Long-term tracheal intubation practice in the United Kingdom. Anaeslhesia 1983: 38:
791-795.
2. Carroll R G, McGinnis G E, Grenvik A. Performance characteristics of tracheal cuffs. Znt Anaesth Clin 1974 12: I 11-141. 3. Knowlson G T G, Bassett H F M. The pressure exerted on the trachea by endotracheal inflatable cuffs. Br J Anaesth 1970 42: 834-837. 4. Leigh J M, Maynard J P. Pressure on the tracheal mucosa from cuffed tubes. Br Med J 1979: i: 1173-1 174. 5. Carroll R G, Hedden M, Safar P. Intratracheal cuffs: performance characteristics. Anesthesiology 1969: 31: 275-281. 6. Liidemann C, Witte U. Messungen des auf die Trachealwand ausgeiibten Druckes bei Beatmungskaniilen mit herkommlichen und einer neuartigen Blockungsmanchette. Prakt Aniisth 1972: 7 : 212-217.
A device for measuring the lateral wall cuff pressure of endotracheal tubes N. LOMHOLT Department of Pharmacology, University of Copenhagen, Denmark
A new method for measuring the lateral wall pressure on the trachea from cuffs of endotracheal tubes is presented. The method is based on measuring the force necessary to force a small, constant flow of air through a Teflon or silicone rubber envelope, placed between the cuff and the tracheal wall. The pressures recorded are 0.1 to 0.3 kPa higher than the theoretically correct values. The response time is 0.28 s for 90% of full deflection. Dynamic recordings of the lateral wall pressure of high and low residual volume cuffs can be obtained for analysis of the interaction between the c u 5 and the tracheal compliance. No method for accurate, dynamic recording of the lateral wall cuff pressure has previously been published. Received I4 September 1991, accepted for publication 15 March 1992
Key words: Anesthesia technique; cuff pressure measurement; tracheal tubes.
The cuffs of endotracheal and tracheostomy tubes are supposed to seal the trachea and protect the patient against aspiration of gastric contents. The sealing function is also necessary for satisfactory artificial ventilation, to secure the intended ventilation. During recent decades, the development of cuff technology has aimed at performing these functions without damaging the trachea, including protection of its mucosa from ischaemia. These demands have been met by the development of thin-walled, high-residual-volume cuffs. Still, low-residual-volume cuffs are used, and not always just for short procedures (1), where they can be used with a reasonably high safety margin. The output of a major American manufacturer consists of about 1/3 low-residual-volume cuffed tubes and about 1/3 medium-volume-cuff tubes (personal communication). As both the sealing function and the damaging effect of the cuff are effects of the lateral wall pressure (LWP), considerable interest has been devoted to measuring the LWP. Apparently, none of the suggested methods has been able to give an accurate continuous recording of the LWP (2). The aim of the present investigation was to design a monitor for measuring LWP, with a sufficiently short response time for use as a transducer for continuous recording, thus enabling an analysis of the interaction between the cuff and the tracheal wall. Low residual volume cuffs, which depend on stretching the cuff membrane for sealing the trachea, transmit only part of the intracuff pressure (ICP) to the tracheal wall. This pressure can only be measured with an LWP monitor. In contrast, the LWP of an ideal high residual
volume cuff is identical to the ICP, because no part of the cuff pressure is used for stretching the cuff membrane, which lies in folds on the tracheal wall.
MATERIAL AND METHODS Two different LWP monitors were constructed, one made from Teflon@film and one made from silicone rubber sheet. Two layers of Teflon FEP film, (from Dupont, U.S.A.) 0.025 mm thick, and gluable on one side, were glued together along the edges of two opposite sides, while the two layers were separated by a 6-mm strip of aluminum foil, 0.013 mm thick. E 41 silicone adhesive from Wacker Chemie (Germany), was used tojoin the two layers together. After polymerization of the silicone adhesive, the aluminum strip was removed with hydrochloric acid. The Teflon envelope was cut to a length of 80 mm and closed at one end with E 41 glue. Near the closed end, the envelope was connected to a thin-walled silicone tube, i.d. 2.4 mm, 0.d. 3.0 mm, 25 cm long, through a 2.5-mm sidehole in the envelope and a corresponding sidehole in the silicone tube (Fig. 1). The silicone monitor was constructed exactly like the Teflon monitor, except that silicone rubber sheet, 0.250 mm thick, from Dow Corning (U.S.A.), was used instead. The airflow from a reduction valve, supplied from a high-pressure gas source, was adjusted with precision flowmeten. The air supply was connected to the silicone rubber tube, and the pressure, necessary to force the air through the envelope, was measured with a Uniflow pressure transducer from Baxter Healthcare Div. (U.S.A.). The pressure was recorded with a Clevite-Gould recorder (Gould Inc., U.S.A.). The basic flow resistance of the two free-standing LWP monitors was measured in the range 100 to 1000 ml/min. The two monitors were calibrated by simultaneously recording the ICP in a high residual volume cuff placed in a model trachea, and the pressure necessary to force 300 ml/min through the monitor, placed between the cuff and the wall of the model trachea. The cuff was inflated to 9 kPa in steps of 1 kPa. Transparent copies of the two recordings were superimposed upon each other, to facilitate the detection of minor differences between the curves.
776
N. LOMHOLT
o3 Cuff: P
Fig. 1. A sketch of the lateral wall cuff pressure monitor. Dimensions: 80 mm long, 8 mm broad, 0.050 mm thick.
0
The response time of the two monitors was measured with the same setup as just described. The cuff pressure was suddenly released, while rhr LWP and ICP were recorded at high speed, 125 mm/s.
RESULTS The results are presented in Fig. 2, 3 and 4. The basic resistance to flow was 5 to 6 times higher in the silicone rubber monitor than in the Teflon monitor. At 300 ml/min, the Teflon monitor had a resistance of 0.05 kPa, while the silicone monitor had a resistance of 0.3 kPa. The calibration of the Teflon monitor showed that the recorded LWP was 0.1 to 0.3 kPa higher than the ICP of the high volume cuff. With the silicone monitor, the difference between the ICP and the LWP was slightly larger. The response time for 90% of full deflection was 0.28 s for the Teflon monitor and 0.38 s for the silicone monitor. DISCUSSION The recordings showed that the measured LWP was somewhat higher than the ICP of a high residual residual volume cuff, while the pressures in theory should be identical. In spite of this, it was possible to obtain satisfactory dynamic recordings of the function of endotracheal tube cuffs during controlled ventilation (Fig. 5). During the construction, four conditions proved to be important for a reliable performance:
kPa
0 70 0 60 0 50 0 40 0 30
000100
200
300
400
500
600
700
800
000
1000
rnl/rnin
Fig. 2. The flow resistance of the two freestanding lateral wall cuff pressure monitors. Silicone rubber monitor: 0, Teflon film monitor:
A.
01
02
03
04
7
O5sek
LWP
0 I
0
I
1
I
I
0.1
02
03
04
0 5 sek
Fig. 3. Measurement of the response time of the Teflon film monitor by a modified step-test. The cuff pressure in a high residual volume cuff in a model trachea and the lateral wall cuff pressure were recorded simultaneously during a sudden release of the cuff pressure.
The monitor should be as thin as possible. Incorrect results were obtained if the monitor was elevated above the tracheal wall. The airflow to the monitor should be delivered through a sidehole and not at the end. The constant flow rate to the monitor was shown to be critical. With a 300 ml * min- flow, consistent results were obtained, but if the flow was reduced, the response time increased and the peaks of the pressure recordings were reduced. The deadspace of the tubes from the reduction valve to the monitor should be as small as possible. With too large a deadspace, the LWP curves became distorted due to damping. Several attempts have been made to measure LWP, but apparently none has been quite satisfactory, and some of the results are rather controversial. Calibration of the LWP-measuring devices against the ICP of high residual volume cuffs was not done in any of the publications. Nevertheless, some information can be deduced from the published figures, as to whether the devices were recording the LWP correctly. Under two conditions, information about the function of the LWP devices can be obtained: 1) With a rigid, cylindrical model trachea and a high residual volume cuff, the LWP and the ICP should be nearly identical. 2) With the same type of trachea, the LWP and the airway pressure at peak inflation should also be nearly identical, if high and low residual volume cuffs are inflated to “just seal”. The monitors presented here comply with both these demands. Knowlson & Bassett (3) used an envelope similar to the present one for measuring LWP. The
’
LATERAL WALL CUFF PRESSURE MONITOR
777
Fig. 4. Calibration of the Teflon film monitor by simultaneous recording of the lateral wall cuff pressure and the intracuff pressure, which was increased stepwise to 9 kPa. The two curves were superimposed upon each other and show the displacement of the upper lateral wall pressure curve.
major difference between the principle used in their method and our method is that theirs could only be used for static and not for dynamic measurements. Their envelope was connected to a balloon reservoir, air was rapidly forced into the balloon and the air escaped through the open envelope. When the envelope collapsed, due to the pressure from the cuff, the remaining pressure in the balloon was considered to be the LWP. Knowlson & Bassett tested their LWP monitor in patients with controlled ventilation, intubated with low residual volume cuffs. Dynamic recordings with the present LWP method have shown that the LWP is high during the expiratory phase, and decreases during inspiration to give “just seal”, because the trachea is dilated by the pressure inflating the lungs. This is probably why Knowlson & Bassett found values 2 to 3 times higher than airway pressure.
kPa
Fig. 5. Recording of the airway pressure (lower curve), the cuff pressure (middle curve), and the lateral wall cuff pressure (upper curve) during ventilation of a model lung and a model trachea to an airway pressure of 5 and 6 kPa, through an endotracheal tube with a high residual volume cuff.
The first demand, that LWP and ICP should be nearly identical with high residual volume cuffs, if the LWP measuring device records correctly, is met by Leigh & Maynard (4),who used a 3-mm-thick bladder as the LWP measuring device in an Imatrach model trachea from Mallinckrodt (U.S.A.). They found nearly identical LWPs (2-2.9 kPa) and ICPs (2.4-3.3 kPa). Carroll and colleagues (5) also found identical LWPs (0.7-1.9 kPa) and ICPs (0.7-1.9 kPa) with high residual volume cuffs. They measured LWP with a pressure transducer implanted in the tracheal wall of an anaesthetized dog. From Table 1, it can be seen that the results of Ludemann & Witte ( 6 ) met the second demand for correct recording of the LWP, as they found nearly identical LWP and airway pressure at the “just seal” condition with a high residual volume cuff in tracheas from human cadavers. A model trachea, even with a correct compliance, is very different from a normal trachea. How could the present LWP monitors be expected to perform in a patient’s trachea? It might be expected that the measured LWP with high volume cuffs would deviate from the ICP, and that the measurements at “just seal” conditions might be different. The reason for this is that the tracheal cartilages probably carry a larger part of the pressure from the cuff, because even thinwalled, high-residual-volume cuffs cannot be expected to distribute their pressure evenly between the cartilage and the softer parts of the trachea. With lowresidual-volume cuffs, the variations in cross-section configuration of the trachea will certainly influence the results, and show areas of high and low pressure (7, 8). In three papers (%11), the LWP was calculated from the difference between ICPs in low-residual-vol-
778
N. LOMHOLT
Table I Summary of information from 8 papers on measurement of the lateral wall cuff pressure of high and low residual volume cuffs in relation to the airway pressure, when the cuff seals the trachea airtight, i.e. the “just seal” condition. LWP device
Trachea
Airway
H M D, DC, HC
15 mm constant ventilation 12.5 mm - 16.5 mm 18 mm 22 mm 10 mm constant ventilation 10 mm 25 mm constant ventilation
Knowlson (3) Leigh (4.) Carroll (2)
Envelope Bladder Pressure transducer
Liidemann (6) Kamen (12)
Bladder Bladder
Wu (13) Dobrin (14)
Pressure transducer Force transducer
HC M HC M DC
Wu (15)
Pressure transducer
M
High volume LWP
Low volume LWP
15-22 mm 5-70 mm
24-45 mm 105 155 mm 13&147 mm
19 mm
36-184 mm 110 mm
2645 2&50 40-80 15-27
mm mm mm mm
180 mm 74-240 mm 80-325 mm 105-350 mm 70-75 mm
H: patient trachea, HC: human cadaver trachea, D: dog trachea, DC: dog cadaver trachea, M: model trachea. All figures are mmHg. 10 mmHg is equal to 1.33 kPa.
ume cuffs, whether they were inflated freestanding or with the same volume in model or normal tracheas. Due to the pronounced time-dependent hysteresis of low-residual-volume cuffs, such calculations are not valid (unpublished results). It is difficult to explain why LWP, 50 to 300 mmHg (6.7 to 40 kPa) higher than airway pressure are given for the “just seal” condition in most investigations with low-residual-volume cuffs (3-6, 12-15), (Table 1). It is hoped that further experience with the method presented here will solve this controversy between theory and practice.
REFERENCES 1. Pippin L K, Short D H, Bowes J B. Long-term tracheal intubation practice in the United Kingdom. Anaeslhesia 1983: 38:
791-795.
2. Carroll R G, McGinnis G E, Grenvik A. Performance characteristics of tracheal cuffs. Znt Anaesth Clin 1974 12: I 11-141. 3. Knowlson G T G, Bassett H F M. The pressure exerted on the trachea by endotracheal inflatable cuffs. Br J Anaesth 1970 42: 834-837. 4. Leigh J M, Maynard J P. Pressure on the tracheal mucosa from cuffed tubes. Br Med J 1979: i: 1173-1 174. 5. Carroll R G, Hedden M, Safar P. Intratracheal cuffs: performance characteristics. Anesthesiology 1969: 31: 275-281. 6. Liidemann C, Witte U. Messungen des auf die Trachealwand ausgeiibten Druckes bei Beatmungskaniilen mit herkommlichen und einer neuartigen Blockungsmanchette. Prakt Aniisth 1972: 7 : 212-217.
