346
Special Correspondence
Ancsth Anala Vol. 57, May-June 1978
Special Correspondence CARDIOPULMONARY EFFECTS OF PEEP AND CPAP To the Editor: We would like to commend Drs. Sturgeon, Douglas, Downs, and Dannemiller (56:633641, 1977) for undertaking the study PEEP and CPAP: Cardiopulmonary Effects During Spontaneous V e n t i l a t i o n , which has helped to clarify the frequently misunderstood effects of PEEP and CPAP during spontaneous ventilation. The authors defined only one disadvantage which they felt was inherent in the application of PEEP-the increase in the work of breathing. However, we feel that no increase in the work of breathing actually existed. The authors made what we consider an erroneous assumption that “respiratory effort was greater during spontaneous ventilation with PEEP than during spontaneous ventilation with ambient airway presure or CPAP.” They based this assumption on finding that “the absolute change in intrapleural pressure between inspiration and exhalation was nearly four times greater during PEEP than during CPAP or ambient airway pressure.” The PEEP system the authors describe appears to be constructed similarly to that of an MA-1 ventilator circuit, which we like to refer to as a zero-flow PEEP device. The CPAP system which appears in the article is the direct opposite of the MA-1 circuit because it requires a continuous flow of gas to generate the end-expiratory pressure. The zero-flow PEEP device maintains positive pressure by compressing a gas volume in the system. As an example, with a standard MA-1 circuit, the amount of compressed volume at a PEEP setting of 10 cm H20 is approximately 3.5 ml/cm H 2 0 or 35 ml. The evacuation of 35 ml from the circuit results in decompression of the gas and “0” airway pressure is achieved. Obviously, a minimal respiratory effort is necessary to remove 35 ml from the circuit, particularly when the gas volume is subject to 10 cm H,O positive pressure.1 Therefore, the intrapleural pressure of 4.9 mm Hg which the authors recorded at endexhalation with the PEEP system was the result of a small volume of gas compressed in the tubing circuit. It then becomes appar-
ent that the fall in intrapleural pressure between the above 4.9-mm Hg value and the appropriate resting value of -1.9 mm Hg is more a function of tubing decompression rather than an actual increase in patient effort. Another point of issue is one which relates to the subatmospheric end of pleural pressure recordings. The increase in subatmospheric pressure from the control endinspiratory value (-6.2 mm Hg) to the value given for the PEEP end-inspiratory position (-9.2 mm Hg) is greatly affected by the type of one-way valve employed and by the location of this valve in the tubing circuit. The amount of flow resistance that the one-way valve will offer is directly dependent upon its geometric structure. In addition, if the one-way valve is not placed very close to the patient’s airway, or if the valve is on the distal port (in relation to the patient) of a humidifying device, a significant increase in patient effort can be observed. When one accounts for the decompression of system gases, (figure, item 1) and the
‘1
CONTROL PEEP CPAP FIGURE. (1) Minimal patient effort-due mainly to decompression of tubing circuitry. (2) Actual patient effort. (3) Actual patient effort, but could be significantly minimized with proper type and placement of one-way valve. (Figure taken from C. L. Sturgeon, Jr, et al, 56:636, 1977)
Anesth Analn Vol. 57, Mey-June 1978
Special Correspondence
type and placement of the one-way valve, (figure, item 3) the remaining absolute intrapleural pressure difference for PEEP with spontaneous ventilation (figure, item 2) is no greater than the pressure difference noted with the control or CPAP groups. Based on the above comments, and on the fact that no other measurable parameter in the article indicated an increase in the work of breathing, it appears that a misinterpretation of the data led the authors to describe a disadvantage of PEEP with spontaneous breathing which probably is of little
347
clinical significance when the vagaries of the circuitry are understood. Larry W. Zwagil, RRT Director, Respiratory Therapy St. Joseph Hospital Thomas M. Jarboe, MD Pulmonary Division Lexington Clinic Lexington, Kentucky 78227
REFERENCE 1. Zwagil LW, Pursley DM: In defense of PEEP with open system IMV. Respiratory Care 22:804, 1977
k
To the Editor: We thank Mr. Zwagil and Dr. Jarboe for providing us with the opportunity to clarify further an important misconception regarding the clinical application of PEEP and CPAP. Each point raised by Zwagil and Jarboe will be discussed, but not necessarily in the order presented in their letter. We agree that an understanding of circuit function and respiratory physiology is necessary to ensure optimal patient care and response to therapy. We also agree that injudicious choice of circuitry may impose significant inspiratory flow resistance. Therefore, we used an IMV valve with minimal flow resistance, a flowover humidifier, and large-bore smooth tubing. We also ensured that no acute angles existed in the patient’s breathing circuit. The flow resistance of our circuitry was calculated to be 3.6 cm H,O/ L/sec, which is clearly insufficient to cause intrapleural pressure changes of the magnitude suggested by Zwagil and Jarboe. Had we employed the circuit they described, pressure changes might have been much greater. Zwagil and Jarboe mistakenly assume that a continuous flow of gas is required to generate end-expiratory pressure during CPAP. On the contrary, the continuous flow of gas serves to maintain positive airway pressure during inhalation. Differences between CPAP and PEEP are reflected in the degree of change in inspiratory airway pressure, which is dependent upon the balance between the gas-flow rate into the system and the patient’s peak inspira.tory flow rate. During CPAP, we provided a continuous gas flow at a rate that exceeded the patient’s peak inspiratory flow rate, so that the drop in airway pressure was minimal. During
PEEP, continuous gas flow was not provided. Therefore, not until airway pressure was drawn subambient did the IMV valve open, thereby allowing a significant change in lung volume to occur. With the level of PEEP employed in our study, this system required a large initial drop in inspiratory airway pressure, and any increase in lung volume must equal the gas volume compressed within the ventilator circuit and the patient’s lungs. Our system had a gas compressiontubing expansion factor of 6 ml/cm H,O, so that 90 ml of gas was compressed when PEEP was 15 cm H20.Assuming barometric pressure was 760 torr, H 2 0 vapor pressure was 47 torr, and functional residual capacity (FRC) was 2 L, we calculated, according to Boyle’s law, that 30 ml of gas would be compressed within the patient’s lungs when PEEP is 15 cm H20. Therefore, with our PEEP system, lung volume would increase by 120 ml before the IMV valve opened. Zwagil and Jarboe incorrectly assumed that this change in volume occurred without significant respiratory work. Analogy may be made with the patient who is required to drop airway pressure 15 cm H20 to cycle an assist-mode ventilator. The respiratory effort (work by the respiratory muscles) imposed by the ventilator is best appreciated by simple observation. We can better understand these phenomena, however, by analyzing intrapleural pressure changes and pressure-volume relationships during PEEP and CPAP. Positive end-expiratory pressure increased intrapleural pressure to + 4.9 torr. ThereFore, transpulmonary pressure (airway-intrapleural pressure) increased,‘ as did FRC. Thus, -1.9 torr was no longer the “appropriate resting value” for intrapleural pressure. During initial inhalation there could
Special Correspondence
Ancsth Anala Vol. 57, May-June 1978
Special Correspondence CARDIOPULMONARY EFFECTS OF PEEP AND CPAP To the Editor: We would like to commend Drs. Sturgeon, Douglas, Downs, and Dannemiller (56:633641, 1977) for undertaking the study PEEP and CPAP: Cardiopulmonary Effects During Spontaneous V e n t i l a t i o n , which has helped to clarify the frequently misunderstood effects of PEEP and CPAP during spontaneous ventilation. The authors defined only one disadvantage which they felt was inherent in the application of PEEP-the increase in the work of breathing. However, we feel that no increase in the work of breathing actually existed. The authors made what we consider an erroneous assumption that “respiratory effort was greater during spontaneous ventilation with PEEP than during spontaneous ventilation with ambient airway presure or CPAP.” They based this assumption on finding that “the absolute change in intrapleural pressure between inspiration and exhalation was nearly four times greater during PEEP than during CPAP or ambient airway pressure.” The PEEP system the authors describe appears to be constructed similarly to that of an MA-1 ventilator circuit, which we like to refer to as a zero-flow PEEP device. The CPAP system which appears in the article is the direct opposite of the MA-1 circuit because it requires a continuous flow of gas to generate the end-expiratory pressure. The zero-flow PEEP device maintains positive pressure by compressing a gas volume in the system. As an example, with a standard MA-1 circuit, the amount of compressed volume at a PEEP setting of 10 cm H20 is approximately 3.5 ml/cm H 2 0 or 35 ml. The evacuation of 35 ml from the circuit results in decompression of the gas and “0” airway pressure is achieved. Obviously, a minimal respiratory effort is necessary to remove 35 ml from the circuit, particularly when the gas volume is subject to 10 cm H,O positive pressure.1 Therefore, the intrapleural pressure of 4.9 mm Hg which the authors recorded at endexhalation with the PEEP system was the result of a small volume of gas compressed in the tubing circuit. It then becomes appar-
ent that the fall in intrapleural pressure between the above 4.9-mm Hg value and the appropriate resting value of -1.9 mm Hg is more a function of tubing decompression rather than an actual increase in patient effort. Another point of issue is one which relates to the subatmospheric end of pleural pressure recordings. The increase in subatmospheric pressure from the control endinspiratory value (-6.2 mm Hg) to the value given for the PEEP end-inspiratory position (-9.2 mm Hg) is greatly affected by the type of one-way valve employed and by the location of this valve in the tubing circuit. The amount of flow resistance that the one-way valve will offer is directly dependent upon its geometric structure. In addition, if the one-way valve is not placed very close to the patient’s airway, or if the valve is on the distal port (in relation to the patient) of a humidifying device, a significant increase in patient effort can be observed. When one accounts for the decompression of system gases, (figure, item 1) and the
‘1
CONTROL PEEP CPAP FIGURE. (1) Minimal patient effort-due mainly to decompression of tubing circuitry. (2) Actual patient effort. (3) Actual patient effort, but could be significantly minimized with proper type and placement of one-way valve. (Figure taken from C. L. Sturgeon, Jr, et al, 56:636, 1977)
Anesth Analn Vol. 57, Mey-June 1978
Special Correspondence
type and placement of the one-way valve, (figure, item 3) the remaining absolute intrapleural pressure difference for PEEP with spontaneous ventilation (figure, item 2) is no greater than the pressure difference noted with the control or CPAP groups. Based on the above comments, and on the fact that no other measurable parameter in the article indicated an increase in the work of breathing, it appears that a misinterpretation of the data led the authors to describe a disadvantage of PEEP with spontaneous breathing which probably is of little
347
clinical significance when the vagaries of the circuitry are understood. Larry W. Zwagil, RRT Director, Respiratory Therapy St. Joseph Hospital Thomas M. Jarboe, MD Pulmonary Division Lexington Clinic Lexington, Kentucky 78227
REFERENCE 1. Zwagil LW, Pursley DM: In defense of PEEP with open system IMV. Respiratory Care 22:804, 1977
k
To the Editor: We thank Mr. Zwagil and Dr. Jarboe for providing us with the opportunity to clarify further an important misconception regarding the clinical application of PEEP and CPAP. Each point raised by Zwagil and Jarboe will be discussed, but not necessarily in the order presented in their letter. We agree that an understanding of circuit function and respiratory physiology is necessary to ensure optimal patient care and response to therapy. We also agree that injudicious choice of circuitry may impose significant inspiratory flow resistance. Therefore, we used an IMV valve with minimal flow resistance, a flowover humidifier, and large-bore smooth tubing. We also ensured that no acute angles existed in the patient’s breathing circuit. The flow resistance of our circuitry was calculated to be 3.6 cm H,O/ L/sec, which is clearly insufficient to cause intrapleural pressure changes of the magnitude suggested by Zwagil and Jarboe. Had we employed the circuit they described, pressure changes might have been much greater. Zwagil and Jarboe mistakenly assume that a continuous flow of gas is required to generate end-expiratory pressure during CPAP. On the contrary, the continuous flow of gas serves to maintain positive airway pressure during inhalation. Differences between CPAP and PEEP are reflected in the degree of change in inspiratory airway pressure, which is dependent upon the balance between the gas-flow rate into the system and the patient’s peak inspira.tory flow rate. During CPAP, we provided a continuous gas flow at a rate that exceeded the patient’s peak inspiratory flow rate, so that the drop in airway pressure was minimal. During
PEEP, continuous gas flow was not provided. Therefore, not until airway pressure was drawn subambient did the IMV valve open, thereby allowing a significant change in lung volume to occur. With the level of PEEP employed in our study, this system required a large initial drop in inspiratory airway pressure, and any increase in lung volume must equal the gas volume compressed within the ventilator circuit and the patient’s lungs. Our system had a gas compressiontubing expansion factor of 6 ml/cm H,O, so that 90 ml of gas was compressed when PEEP was 15 cm H20.Assuming barometric pressure was 760 torr, H 2 0 vapor pressure was 47 torr, and functional residual capacity (FRC) was 2 L, we calculated, according to Boyle’s law, that 30 ml of gas would be compressed within the patient’s lungs when PEEP is 15 cm H20. Therefore, with our PEEP system, lung volume would increase by 120 ml before the IMV valve opened. Zwagil and Jarboe incorrectly assumed that this change in volume occurred without significant respiratory work. Analogy may be made with the patient who is required to drop airway pressure 15 cm H20 to cycle an assist-mode ventilator. The respiratory effort (work by the respiratory muscles) imposed by the ventilator is best appreciated by simple observation. We can better understand these phenomena, however, by analyzing intrapleural pressure changes and pressure-volume relationships during PEEP and CPAP. Positive end-expiratory pressure increased intrapleural pressure to + 4.9 torr. ThereFore, transpulmonary pressure (airway-intrapleural pressure) increased,‘ as did FRC. Thus, -1.9 torr was no longer the “appropriate resting value” for intrapleural pressure. During initial inhalation there could
