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AUTOMATIC FLOW INTERRUPTION BRONCHOSCOPE: A LABORATORY STUDY
Br. J. Anaesth. (1975), 47, 385
AUTOMATIC FLOW INTERRUPTION BRONCHOSCOPE: A LABORATORY STUDY A. D. JARDINE, M. J. HARRISON AND T. E. J. HEALY SUMMARY
A laboratory assessment of a ventilating bronchoscope incorporating automatic flow interruption suggests that it will safely provide adequate ventilation. There are techniques designed to avoid inadequate pulmonary ventilation during bronchoscopy, but none is free from the risk of hypoxia and hypercarbia. The application of the venturi system by Sanders in 1967 provided a new approach, allowing simultaneous ventilation and bronchoscopy. Several authors (Pender et al., 1968; Spoerel, 1969; Grant, 1971; Giesecke et al., 1973) have shown that adequate ventilation and oxygenation can be achieved in clinical practice, using instruments incorporating this principle. However, despite Spoerel's suggestion (1969) in favour of automatic interruption of the ventilating gas flow, manual operation is still used invariably. This study reports the assessment of a bronchoscope incorporating the venturi principle and time cycled automatic flow interruption of the ventilating gases. Tests have been performed to determine the effects of the time cycled interruption on the tidal volume, the oxygen concentration and the volume of air entrained. EQUIPMENT
The bronchoscope incorporates distal side vents and has been modified proximally to accept a detachable venturi pattern injector (fig. 1) with a needle diameter 0.8/1.0 mm. Unlike most venturi systems (fig. 2), the injector does not pass to a point distal from its point of entry into the bronchoscope. Furthermore, encroachment by the injector into the bronchoscope lumen is minimal. The injector is connected by a plastic pressure hose to the outlet of a ventilator. The ventilator allows gas pressure, flow and cycling time to be varied. The gas supply pressure used (4.2 kg/cm1, 60 Lb./sq. in.) corresponds with current United Kingdom piped gas practice although the ventilator will function with pressures up to 6.0 kg/cm1, 85 Lb./sq. in.). A. D . JARDINE, M.B., CH.B.; M . J. HARRISON, M.B., B.S., F.F.AJLC.S.; T. E. J. HEALY, M.B., B.S., B.SC., D.A.,
F.F.A.RX.S., Department of Anaesthetic Studies, General Hospital, Nottingham.
• Stage I Entrapment Gas flow
Venturi Stage I
Driving gas FIG. 1. Venturi bronchoscope.
25-30% Oxygen
Air entrainment 20-40% (Stage!)
Air
Air entrainment 45-50% (Stage I )
Gas supply Oxygen 60 (Lb./sq.in.) FIG. 2. Venturi and injector bronchoscope. METHOD
An artificial lung (Manley lung ventilator performance analyser with improved calibration features) was connected to the distal (patient) end of the bronchoscope and the side vents were occluded to prevent gas loss. In clinical use, gas leakage between bronchoscope and trachea would be expected and therefore the variable leak incorporated in the artificial lung was set at its itiavimiim value (fig. 3). The total delivered tidal volume and its component volumes were measured under different conditions. All measurements were repeated five times and the mean of the readings used to construct the graphs. The component volumes were driving gas, air or oxygen entrained by the venturi and air entrained at the proximal (operator) end of the bronchoscope. During measurements of
386
BRITISH JOURNAL OF ANAESTHESIA
Bronchoscope
Variable compliance Variable air leak,'
700
Inspiratory time 1.0 sec Compliance 0.05 itrd/cm.
1
600
500
Rubber seals Variable airway resistance
~~
Pressure
9au9e
§ 1
m
o>
FIG. 3. The experimental model.
injector gas volume an eye piece was used to occlude the operator end and prevent air entrainment at stage II (fig. 1) and in addition a rubber seal was used to occlude the venturi at stage I (fig. 1), during measurement of driving gas volume. All volumes were assessed at a driving gas pressure of 4, 3 and 2 kg/cm 1 for both good (0.05 litre/cm HaO) and poor (0.02 litre/cm H,O) simulated compliances, and with varying (5, 20, 50 and 200 cm H,O/litre/sec) airway resistance. Inspiratory/ expiratory times were restricted to 1.0/2.0 sec and 1.5/2.5 sec. RESULTS
Injector. The entrainment ratio of the injector venturi at varying driving gas pressures was derived from the following formula: Entrainment ratio= total injector volume—driving gas volume total injector volume The ratio increased from 0.46 to 0.54 with a decrease in driving gas pressure (over the range used). Tidal volume. Tidal volume was reduced by a decrease in compliance and an increase in airway resistance (fig. 4). At a compliance of 0.05 litre/cm H 3 O with a driving pressure of 4.0 kg/cm 1 and airway resistance of 5 cm H,O/litre/sec, the tidal volume was 700 ml. At a compliance of 0.02 litre/cm H , 0 with a driving pressure of 3 kg/ cm2 and airway resistance of 200 cm H a O/litre/ sec, the tidal volume was 175 ml. These volumes were obtained with an inspiratory/expiratory ratio of 1.0/2.0 sec. However, the tidal volume increased by less than 1%, when the inspiratory phase was increased to 1.5 sec. All the measurements were reproducible using this mechanical system. Oxygenation. The concentration of oxygen in the gas mixture delivered to the artificial lung was calculated using the volumes measured with stage
1
^ 200
100
_L 5
0
_L 20 50 Airway resistance cm. H2O/ itre/sec.
200
FIG. 4. Delivered tidal volumes.
I venturi entrainment of both air and 100% oxygen; in both cases oxygen was the driving gas and air was entrained at stage II. With stage I air entrainment (fig. 5), the minimum oxygen concentration in the mixture was 53%. A decrease in compliance and an increase in airway resistance resulted in an increase in the delivered oxygen concentration, when air or oxygen were entrained at stage I (figs. 5 and 6). The oxygen concentrations at different driving pressures were derived using the following expression: %Oi=[DGV+(ViXFO»)+(V 3 x0.21)]xl00 Total delivered volume DGV=driving volume; Vi=stage I entrained volume; FOj=entrained Oj concentration; Vi=stage II entrained volume. Stage I and II
80 Inspiratory time 1.0 sec
70 "Compliance f OJEBre/ cm.HzO
60
50 40
Driving pressure
4.0kg. cy 2 L
2.0 kg. cm
Compliance f 0.05 Kre/ _cm.H20 L
5
50
200
Airway resistance en.
FIG. 5. Oxygen delivered (%)—venturi entraining air, stages I and II.
AUTOMATIC FLOW INTERRUPTION BRONCHOSCOPE
387
100 — Compliance
DISCUSSION
In patients undergoing bronchoscopy, hypoxia and cm.ri20 * 2.0 kg. cm* hypercapnia will occur if ventilation is inadequate 90 Inspiratory time 1.0 sec and if the concentration of oxygen in the inspired gas is low. Therefore, a mechanical ventilating ...» ? system must be designed to maintain pulmonary •* 80 -Compliance A ^ ventilation and a high inspired oxygen concentra0.05 litre/ tion. cm.H20 If oxygen is used as the driving gas in a venturi 70 bronchoscope system, the concentration of oxygen in the ventilating gases depends on dilution by air entrainment. It appears from the results presented 60 in this study that air entrainment is influenced by two factors: patient respiratory resistance and J_ 50 injector/venturi design. The relationship between 200 5 50 the percentage of air entrained (stage U) and comAirway resistance cm. H20/litre/sec. pliance at three different values for airway resistFIG. 6. Oxygen delivered (%). Venturi entraining oxy- ance is shown in figure 8. The percentage of air gen+air. entrained at a fixed compliance decreases with increasing airway resistance and therefore if oxygen Time cycling. The delivered oxygen concentrais used as the driving gas, reduced entrainment of tion increased when the venturi entrained air or air results in an increase in delivered oxygen conoxygen if the respiratory time was prolonged above centration and this compensates to some extent for 1 sec at good (0.05 litre/cm H,O) compliance and the reduced tidal volume. low (5 cm H,O/litre/sec) airway resistance (fig. 7). The effect of patient respiratory resistance on air However, no appreciable increase in delivered oxyentrainment is confirmed (fig. 6), lower resistances gen concentration was observed with poor complihaving a relatively more marked influence on enance (figs. 5 and 7). In both cases the driving gas trainment at lower driving gas pressures and resultcontinued to flow throughout the inspiratory cycle ing in higher delivered oxygen concentration. and passed peripherally down the bronchoscope. The entrainment of air, assuming a value for patient resistance, is also dependent on the design 100r of the injector. Analysis of the results obtained by Komesaroff and McKie (1972), using a modified s 90 Venturi cannula in their "bronchoflator" indicates stage II > entraining air entrainment values of between 22.5 and 45% oxygen c of the tidal volume. Using an injector incorporating o con ntirati
deli
I
80
S
70
Driving Pressure 4.0 kg. cm 2
Venturi entraining air
.Q2ftrefcm.H2q-
60
50
0.05 litre/cm. H2O
50 40
50 _ 200 .
= 20
_L
_L
5 50 200 Airways resistance cm. H2O/litre/sec. FIG. 7. Delivered oxygen concentration with inspiratory time 1.5 sec.
1
10
20 30 40 * Air entrained FIG. 8. Bronchoscope entrainment 1values. Driving pressure 4.0 kg/cm .
BRITISH JOURNAL OF ANAESTHESIA
388 a venturi entraining oxygen (stage I), we have reduced the mavimnm bronchoscope (stage II) entrainment to 28%. At the same time it can be seen (fig. 8) that an increase in airway resistance, or compliance, will reduce this value considerably. It is probable that these lower values are caused by reduction in the bronchoscope to injector pressure differential, decreasing the venturi effect at the injector tip. Careful attention to venturi design is also required if high delivered oxygen concentrations are to be achieved. To overcome the problem of low delivered oxygen concentration associated with many venturi bronchoscopes at present in use, a method of supplying oxygen as the driving gas, and to the venturi, is suggested (fig. 9). These findings confirm those of Bennetts (1973), who indicated that if Entonox is used as the driving gas for injector systems the delivered concentration of nitrous oxide may be reduced by 50%. The bronchoscope and ventilator tested were found to be capable of maintaining good tidal flow (fig. 4). The maximum tidal volume delivered was 0.7 litre with a good compliance and low airway resistance. As expected, tidal volume decreased with increasing respiratory resistance; however the mavimnm pressure in the lung simulator did not exceed 17 cm of water. This autoregulation of the tidal volume by the injector would appear to be a safety factor when the bronchoscope is passed peripherally. This system is suitable for use in patients with obstructive airway disease and conversely with restrictive pulmonary lesions because tidal flow can be achieved and modified by changes in driving gas pressure in the presence of increased airway resistance and reduced compliance. Ventilation of the opposite lung occurs via the side vents even when the bronchoscope has been introduced into a main bronchus (fig. 10). Foot pedal
;—Spring
—
- To Injector
To venturi
FIG. 10. Ventilation pattern, bronchoscope in right main bronchus.
Although changes in the tidal volume were found to be minimal, when the inspiratory time was greater than 1 sec, injector gas continued to flow. This resulted in the delivered oxygen concentration increasing above that found with a shorter inspiratory time. It would seem therefore that to some extent previously distributed tidal gases were "washed out" by oxygen. The level of this wash out phase will of course be determined by the point at which rapid flow ceases in the bronchoscope. However, better distribution of gases within the lungs may be achieved by prolonging the inspiratory time and the advantage of this would probably outweigh the reduction in minute volume in patients with high respiratory resistance. Analysis of function is somewhat complex as it would appear to be a mixture of three patterns. Firstly, the injector system behaves as a time cycled constant pressure generator. Second, the pattern of air entrainment at the bronchoscope may be considered analagous to the inspiratory phase of a pressure cycled constant pressure generator. Finally, an insufflation phase may be superimposed. It appears from the results obtained in this laboratory investigation that the automatic flow interruption venturi bronchoscope will enable bronchoscopy to be performed with more satisfactory puhnonary ventilation than has hitherto been obtained using other techniques.
Flow valve +ve
w
Gas supply
I Const, presi . red. valve
FIG. 9. Dual supply system for venturi bronchoscope.
ACKNOWLEDGEMENTS
We thank Mr J. Lythe, medical artist for Nottingham University, for his help with the drawing, and also Richard Wolf for supplying the bronchoscope and time cycled unit used in this study. REFERENCES
Bennetts, F. E. (1973). General anaesthesia for bronchoscopy. Br. J. Hosp. Med., 10, 71.
389
AUTOMATIC FLOW INTERRUPTION BRONCHOSCOPE Giesecke, A. H., Gerbshagen, H. U., Dortman, C , and Lee, D. (1973). Comparison of the ventilating and injection bronchoscopes. Anesthesiology, 38, 298. Grant, P. A. (1971). Ventilation during bronchoscopy. Can. Anaesth. Soc. J., 18, 178. Komesaroff, D., and McKie, B. (1972). The "bronchoflator": a new technique for bronchoscopy under general anaesthesia. Br. j . Anaesth., 44, 1057. Pender, J. W., Winchester, L. W., Jamplis, R. W., Lillington, G. A., and McClenahan, J. B. (1968). Effects of anaesthesia on ventilation during bronchoscopy. Anesth. Analg. (Cleve.), 47, 415. Sanders, R. D. (1967). Two ventilating attachments for bronchoscopes. Del. Med. J., 39, 170. Spoerel, W. E. (1969). Ventilation through an open bronchoscope. Can. Anaesth. Soc. J., 16, 61. LA BRONCHOSCOPIE AVEC INTERRUPTION AUTOMATIQUE DE PASSAGE: ETUDE EN LABORATOIRE RESUME
Une Evaluation en laboratoire d'un bronchoscope de ventilation incorporant un dispositif d'interruption auto-
matique de passage suggere que ce dernier peut assurer la ventilation pulmonaire de maniere sure. BRONCHOSKOPIE MIT AUTOMATISCHER DURCHZUGSUNTERBRECHUNG—EINE LABORATORIUMSUNTERSUCHUNG ZUSAMMENFASSUNG
Eine laboratoriumsmassige Untersuchung eines Beluftungs-Bronchoskops mit autornatischer Durchzugsunterbrechung ergab, dass es eine sichere pulmonare Beluftung bietet. BRONCOSCOPIA CON INTERRUPCION AUTOMATICA DE FLUJO: ESTUDIO DE LABORATORIO SUMARIO
La valoraci6n de laboratorio de un broncoscopio ventilado con mechanismo para interrupci6n automitica de flujo demostro quc proporciona ventilaci6n pulmonar segura.
AUTOMATIC FLOW INTERRUPTION BRONCHOSCOPE: A LABORATORY STUDY A. D. JARDINE, M. J. HARRISON AND T. E. J. HEALY SUMMARY
A laboratory assessment of a ventilating bronchoscope incorporating automatic flow interruption suggests that it will safely provide adequate ventilation. There are techniques designed to avoid inadequate pulmonary ventilation during bronchoscopy, but none is free from the risk of hypoxia and hypercarbia. The application of the venturi system by Sanders in 1967 provided a new approach, allowing simultaneous ventilation and bronchoscopy. Several authors (Pender et al., 1968; Spoerel, 1969; Grant, 1971; Giesecke et al., 1973) have shown that adequate ventilation and oxygenation can be achieved in clinical practice, using instruments incorporating this principle. However, despite Spoerel's suggestion (1969) in favour of automatic interruption of the ventilating gas flow, manual operation is still used invariably. This study reports the assessment of a bronchoscope incorporating the venturi principle and time cycled automatic flow interruption of the ventilating gases. Tests have been performed to determine the effects of the time cycled interruption on the tidal volume, the oxygen concentration and the volume of air entrained. EQUIPMENT
The bronchoscope incorporates distal side vents and has been modified proximally to accept a detachable venturi pattern injector (fig. 1) with a needle diameter 0.8/1.0 mm. Unlike most venturi systems (fig. 2), the injector does not pass to a point distal from its point of entry into the bronchoscope. Furthermore, encroachment by the injector into the bronchoscope lumen is minimal. The injector is connected by a plastic pressure hose to the outlet of a ventilator. The ventilator allows gas pressure, flow and cycling time to be varied. The gas supply pressure used (4.2 kg/cm1, 60 Lb./sq. in.) corresponds with current United Kingdom piped gas practice although the ventilator will function with pressures up to 6.0 kg/cm1, 85 Lb./sq. in.). A. D . JARDINE, M.B., CH.B.; M . J. HARRISON, M.B., B.S., F.F.AJLC.S.; T. E. J. HEALY, M.B., B.S., B.SC., D.A.,
F.F.A.RX.S., Department of Anaesthetic Studies, General Hospital, Nottingham.
• Stage I Entrapment Gas flow
Venturi Stage I
Driving gas FIG. 1. Venturi bronchoscope.
25-30% Oxygen
Air entrainment 20-40% (Stage!)
Air
Air entrainment 45-50% (Stage I )
Gas supply Oxygen 60 (Lb./sq.in.) FIG. 2. Venturi and injector bronchoscope. METHOD
An artificial lung (Manley lung ventilator performance analyser with improved calibration features) was connected to the distal (patient) end of the bronchoscope and the side vents were occluded to prevent gas loss. In clinical use, gas leakage between bronchoscope and trachea would be expected and therefore the variable leak incorporated in the artificial lung was set at its itiavimiim value (fig. 3). The total delivered tidal volume and its component volumes were measured under different conditions. All measurements were repeated five times and the mean of the readings used to construct the graphs. The component volumes were driving gas, air or oxygen entrained by the venturi and air entrained at the proximal (operator) end of the bronchoscope. During measurements of
386
BRITISH JOURNAL OF ANAESTHESIA
Bronchoscope
Variable compliance Variable air leak,'
700
Inspiratory time 1.0 sec Compliance 0.05 itrd/cm.
1
600
500
Rubber seals Variable airway resistance
~~
Pressure
9au9e
§ 1
m
o>
FIG. 3. The experimental model.
injector gas volume an eye piece was used to occlude the operator end and prevent air entrainment at stage II (fig. 1) and in addition a rubber seal was used to occlude the venturi at stage I (fig. 1), during measurement of driving gas volume. All volumes were assessed at a driving gas pressure of 4, 3 and 2 kg/cm 1 for both good (0.05 litre/cm HaO) and poor (0.02 litre/cm H,O) simulated compliances, and with varying (5, 20, 50 and 200 cm H,O/litre/sec) airway resistance. Inspiratory/ expiratory times were restricted to 1.0/2.0 sec and 1.5/2.5 sec. RESULTS
Injector. The entrainment ratio of the injector venturi at varying driving gas pressures was derived from the following formula: Entrainment ratio= total injector volume—driving gas volume total injector volume The ratio increased from 0.46 to 0.54 with a decrease in driving gas pressure (over the range used). Tidal volume. Tidal volume was reduced by a decrease in compliance and an increase in airway resistance (fig. 4). At a compliance of 0.05 litre/cm H 3 O with a driving pressure of 4.0 kg/cm 1 and airway resistance of 5 cm H,O/litre/sec, the tidal volume was 700 ml. At a compliance of 0.02 litre/cm H , 0 with a driving pressure of 3 kg/ cm2 and airway resistance of 200 cm H a O/litre/ sec, the tidal volume was 175 ml. These volumes were obtained with an inspiratory/expiratory ratio of 1.0/2.0 sec. However, the tidal volume increased by less than 1%, when the inspiratory phase was increased to 1.5 sec. All the measurements were reproducible using this mechanical system. Oxygenation. The concentration of oxygen in the gas mixture delivered to the artificial lung was calculated using the volumes measured with stage
1
^ 200
100
_L 5
0
_L 20 50 Airway resistance cm. H2O/ itre/sec.
200
FIG. 4. Delivered tidal volumes.
I venturi entrainment of both air and 100% oxygen; in both cases oxygen was the driving gas and air was entrained at stage II. With stage I air entrainment (fig. 5), the minimum oxygen concentration in the mixture was 53%. A decrease in compliance and an increase in airway resistance resulted in an increase in the delivered oxygen concentration, when air or oxygen were entrained at stage I (figs. 5 and 6). The oxygen concentrations at different driving pressures were derived using the following expression: %Oi=[DGV+(ViXFO»)+(V 3 x0.21)]xl00 Total delivered volume DGV=driving volume; Vi=stage I entrained volume; FOj=entrained Oj concentration; Vi=stage II entrained volume. Stage I and II
80 Inspiratory time 1.0 sec
70 "Compliance f OJEBre/ cm.HzO
60
50 40
Driving pressure
4.0kg. cy 2 L
2.0 kg. cm
Compliance f 0.05 Kre/ _cm.H20 L
5
50
200
Airway resistance en.
FIG. 5. Oxygen delivered (%)—venturi entraining air, stages I and II.
AUTOMATIC FLOW INTERRUPTION BRONCHOSCOPE
387
100 — Compliance
DISCUSSION
In patients undergoing bronchoscopy, hypoxia and cm.ri20 * 2.0 kg. cm* hypercapnia will occur if ventilation is inadequate 90 Inspiratory time 1.0 sec and if the concentration of oxygen in the inspired gas is low. Therefore, a mechanical ventilating ...» ? system must be designed to maintain pulmonary •* 80 -Compliance A ^ ventilation and a high inspired oxygen concentra0.05 litre/ tion. cm.H20 If oxygen is used as the driving gas in a venturi 70 bronchoscope system, the concentration of oxygen in the ventilating gases depends on dilution by air entrainment. It appears from the results presented 60 in this study that air entrainment is influenced by two factors: patient respiratory resistance and J_ 50 injector/venturi design. The relationship between 200 5 50 the percentage of air entrained (stage U) and comAirway resistance cm. H20/litre/sec. pliance at three different values for airway resistFIG. 6. Oxygen delivered (%). Venturi entraining oxy- ance is shown in figure 8. The percentage of air gen+air. entrained at a fixed compliance decreases with increasing airway resistance and therefore if oxygen Time cycling. The delivered oxygen concentrais used as the driving gas, reduced entrainment of tion increased when the venturi entrained air or air results in an increase in delivered oxygen conoxygen if the respiratory time was prolonged above centration and this compensates to some extent for 1 sec at good (0.05 litre/cm H,O) compliance and the reduced tidal volume. low (5 cm H,O/litre/sec) airway resistance (fig. 7). The effect of patient respiratory resistance on air However, no appreciable increase in delivered oxyentrainment is confirmed (fig. 6), lower resistances gen concentration was observed with poor complihaving a relatively more marked influence on enance (figs. 5 and 7). In both cases the driving gas trainment at lower driving gas pressures and resultcontinued to flow throughout the inspiratory cycle ing in higher delivered oxygen concentration. and passed peripherally down the bronchoscope. The entrainment of air, assuming a value for patient resistance, is also dependent on the design 100r of the injector. Analysis of the results obtained by Komesaroff and McKie (1972), using a modified s 90 Venturi cannula in their "bronchoflator" indicates stage II > entraining air entrainment values of between 22.5 and 45% oxygen c of the tidal volume. Using an injector incorporating o con ntirati
deli
I
80
S
70
Driving Pressure 4.0 kg. cm 2
Venturi entraining air
.Q2ftrefcm.H2q-
60
50
0.05 litre/cm. H2O
50 40
50 _ 200 .
= 20
_L
_L
5 50 200 Airways resistance cm. H2O/litre/sec. FIG. 7. Delivered oxygen concentration with inspiratory time 1.5 sec.
1
10
20 30 40 * Air entrained FIG. 8. Bronchoscope entrainment 1values. Driving pressure 4.0 kg/cm .
BRITISH JOURNAL OF ANAESTHESIA
388 a venturi entraining oxygen (stage I), we have reduced the mavimnm bronchoscope (stage II) entrainment to 28%. At the same time it can be seen (fig. 8) that an increase in airway resistance, or compliance, will reduce this value considerably. It is probable that these lower values are caused by reduction in the bronchoscope to injector pressure differential, decreasing the venturi effect at the injector tip. Careful attention to venturi design is also required if high delivered oxygen concentrations are to be achieved. To overcome the problem of low delivered oxygen concentration associated with many venturi bronchoscopes at present in use, a method of supplying oxygen as the driving gas, and to the venturi, is suggested (fig. 9). These findings confirm those of Bennetts (1973), who indicated that if Entonox is used as the driving gas for injector systems the delivered concentration of nitrous oxide may be reduced by 50%. The bronchoscope and ventilator tested were found to be capable of maintaining good tidal flow (fig. 4). The maximum tidal volume delivered was 0.7 litre with a good compliance and low airway resistance. As expected, tidal volume decreased with increasing respiratory resistance; however the mavimnm pressure in the lung simulator did not exceed 17 cm of water. This autoregulation of the tidal volume by the injector would appear to be a safety factor when the bronchoscope is passed peripherally. This system is suitable for use in patients with obstructive airway disease and conversely with restrictive pulmonary lesions because tidal flow can be achieved and modified by changes in driving gas pressure in the presence of increased airway resistance and reduced compliance. Ventilation of the opposite lung occurs via the side vents even when the bronchoscope has been introduced into a main bronchus (fig. 10). Foot pedal
;—Spring
—
- To Injector
To venturi
FIG. 10. Ventilation pattern, bronchoscope in right main bronchus.
Although changes in the tidal volume were found to be minimal, when the inspiratory time was greater than 1 sec, injector gas continued to flow. This resulted in the delivered oxygen concentration increasing above that found with a shorter inspiratory time. It would seem therefore that to some extent previously distributed tidal gases were "washed out" by oxygen. The level of this wash out phase will of course be determined by the point at which rapid flow ceases in the bronchoscope. However, better distribution of gases within the lungs may be achieved by prolonging the inspiratory time and the advantage of this would probably outweigh the reduction in minute volume in patients with high respiratory resistance. Analysis of function is somewhat complex as it would appear to be a mixture of three patterns. Firstly, the injector system behaves as a time cycled constant pressure generator. Second, the pattern of air entrainment at the bronchoscope may be considered analagous to the inspiratory phase of a pressure cycled constant pressure generator. Finally, an insufflation phase may be superimposed. It appears from the results obtained in this laboratory investigation that the automatic flow interruption venturi bronchoscope will enable bronchoscopy to be performed with more satisfactory puhnonary ventilation than has hitherto been obtained using other techniques.
Flow valve +ve
w
Gas supply
I Const, presi . red. valve
FIG. 9. Dual supply system for venturi bronchoscope.
ACKNOWLEDGEMENTS
We thank Mr J. Lythe, medical artist for Nottingham University, for his help with the drawing, and also Richard Wolf for supplying the bronchoscope and time cycled unit used in this study. REFERENCES
Bennetts, F. E. (1973). General anaesthesia for bronchoscopy. Br. J. Hosp. Med., 10, 71.
389
AUTOMATIC FLOW INTERRUPTION BRONCHOSCOPE Giesecke, A. H., Gerbshagen, H. U., Dortman, C , and Lee, D. (1973). Comparison of the ventilating and injection bronchoscopes. Anesthesiology, 38, 298. Grant, P. A. (1971). Ventilation during bronchoscopy. Can. Anaesth. Soc. J., 18, 178. Komesaroff, D., and McKie, B. (1972). The "bronchoflator": a new technique for bronchoscopy under general anaesthesia. Br. j . Anaesth., 44, 1057. Pender, J. W., Winchester, L. W., Jamplis, R. W., Lillington, G. A., and McClenahan, J. B. (1968). Effects of anaesthesia on ventilation during bronchoscopy. Anesth. Analg. (Cleve.), 47, 415. Sanders, R. D. (1967). Two ventilating attachments for bronchoscopes. Del. Med. J., 39, 170. Spoerel, W. E. (1969). Ventilation through an open bronchoscope. Can. Anaesth. Soc. J., 16, 61. LA BRONCHOSCOPIE AVEC INTERRUPTION AUTOMATIQUE DE PASSAGE: ETUDE EN LABORATOIRE RESUME
Une Evaluation en laboratoire d'un bronchoscope de ventilation incorporant un dispositif d'interruption auto-
matique de passage suggere que ce dernier peut assurer la ventilation pulmonaire de maniere sure. BRONCHOSKOPIE MIT AUTOMATISCHER DURCHZUGSUNTERBRECHUNG—EINE LABORATORIUMSUNTERSUCHUNG ZUSAMMENFASSUNG
Eine laboratoriumsmassige Untersuchung eines Beluftungs-Bronchoskops mit autornatischer Durchzugsunterbrechung ergab, dass es eine sichere pulmonare Beluftung bietet. BRONCOSCOPIA CON INTERRUPCION AUTOMATICA DE FLUJO: ESTUDIO DE LABORATORIO SUMARIO
La valoraci6n de laboratorio de un broncoscopio ventilado con mechanismo para interrupci6n automitica de flujo demostro quc proporciona ventilaci6n pulmonar segura.