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Cardiopulmonary Consequences of Positive End- Expiratory Pressure

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Cardiopulmonary Consequences of Positive End- Expiratory Pressure Pediat. Res. 13: 1112-1120 (1979) Cardiac output pulmonary compliance cardiovascular function pulmonary function positive end-expiratory pressure Cardiopulmonary Consequences of Positive End- Expiratory Pressure BERNARD H. HOLZMAN AND EMlLE M. SCARPELLI Pediatric Pulmonary Division, Albert Einstein College of Medicine. Bronx, New York, USA Summary of the lungs (before PEEP) has predictive value with regard to the effects of PEEP on cardiorespiratory function. Positive end-expiratory pressure (PEEP), while of major benefit These experiments also revealed that slowing of respiratory in the therapy of pulmonary diseases characterized by low func- frequency and displacement of expiration from inspiration is a tional residual capacity (FRC), is frequently associated with de- regular response to the application of PEEP and that these effects pression of cardiac output (CO) and specific dynamic compliance increase as PEEP is increased. Thus, PEEP produces a postinspir- (Csp). The cardiopulmonary consequences of sequential increases atory pause followed by an active expiration that immediately of PEEP (3, 6, 9, 12, 15 cm HzO) in normal dogs were studied precedes the next inspiration at the highest PEEP. The immediacy utilizing an apparatus that permits measurement of FRC by helium with which these changes occur suggests that they are reflex in . dilution while the animal remains on PEEP. It was found that origin. increasing levels of PEEP had no effect on tidal volume (VT) or Although the term continuous positive airway pressure (CPAP) inspiratory time, but appeared to lengthen expiratory time by is used often to describe the application of positive pressure is delaying expiratory flow until the preinspiratory period, and led to spontaneously breathing subjects such as ours, we have elected to facilitated expiration. This response occurred immediately upon use the term PEEP. PEEP conveys the fact that the applied changes in the level of PEEP and is felt to be reflex in nature. pressures are imposed quantitatively at end-expiration, whereas FRC increased and Csp fell as PEEP was increased. This was airway pressures vary (perhaps becoming subatmospheric), during reflected in a fall in transmission of applied airway pressure to an inspiration. esophageal balloon. Pulmonary hypertension did not develop in our animals, but net right atrial (RA) pressure was elevated by PEEP. There was no difference in cardiac index (CI) between the MATERIALS AND METHODS control and study groups. Both the magnitude of applied airway pressure and the extent to which it is transmitted across the lungs Anesthesia was induced in 12 healthy spontaneously breathing to the pleural space appear to be determinants of possible effects mongrel dogs (13-25 kg) with sodium pentobarbital, 30 mg/kg iv, of PEEP on CO and may explain the diversity of results among and maintained with additional iv doses (1-2 mg/kg) when seem- reported studies of PEEP. Transmission of pressure is related ingly purposeful motor activity occurred. One to three additional directly to Csp. doses were required to maintain a steady level of anesthesia as judged by clinical signs. A cuffed endotracheal tube was inserted and catheters were placed in the RA via the external jugular vein Speculation and aorta (Ao) via the femoral artery. A Swan-Ganz catheter was Transmission of PEEP to the pleural space is dependent on the inserted through a femoral vein and advanced to the main pul- level of PEEP and on the preexisting state of the lungs. Trans- monary artery (PA), where the position was confirmed by pressure mission of applied airway pressure will be greatest when the Csp tracings. Pleural pressure (Ppl) was recorded (52) from an intra- of the lungs is highest; it may be reduced significantly if either esophageal balloon using the technique of Milic-Emili el al. (32). insufficient or excessive levels are used for a given clinical situa- Inspiratory and expiratory flow rate (QI and QF:)were recorded tion. The pleural pressure vs. applied PEEP relationship thus from a heated Fleisch pneumotachograph (53); VT was determined provides a sensitive index of changing status of the lungs, as well by electronic integration of inspiratory air flow (54). All measure- as an assessment of therapeutic efficacy. ments, including RA, PA, and Ao pressures were displayed on a six-channel pen recorder (55). Gas pressures and pH of arterial and mixed venous blood, and inspired and expired oxygen ten- PEEP can be most effective in the treatment of neonatal respi- sions were measured in a specially modified microanalytic system ratory distress and other diseases characterized by low pulmonary (56). Hemoglobin-oxygen saturation was determined from nom- dynamic compliance (C), low FRC, and intrapulmonary right-to- ograms for dogs (39). RA, Ao, and PA pressures, Ppl, QI, QK,and left shunts (5, 17, 18, 21, 25, 28, 44, 46, 47). The use of PEEP, VT were recorded continuously. Gas pressures, pH, CO, and FRC however, may be associated with undesirable effects including were determined at the end of each control and test period. lowering of CO (14, 15, 17, 24, 26, 28, 44,46) and overdistention All dogs were observed for 30 min after the placement of of alveoli leading to a fall in C (5, 6, 12, 27, 44). In the present catheters-and endotracheal tube-pneumotachograph assembly studies, the time-course of changes in relevant cardiopulmonary ("control period I," zero end-expiratory pressure (ZEEP)). The parameters induced by PEEP has been evaluated in normal, PEEP apparatus for the application of PEEP and maintenance of spontaneously breathing, anesthetized dogs. The objective was to PEEP during FRC determination (modified after Berman et al. study, comprehensively in each subject, the broad spectrum of (a)), was then connected to the endotracheal tube-pneumotacho- changes that may be induced by PEEP and their possible inter- graph assembly with humidified air flow through the system relations, and to compare these with control animals that were maintained at 2.0-2.5 liter/min to prevent rebreathing. This flow handled identically except for PEEP. These studies indicate that rate produced a baseline PEEP of 0.6 cm H20, which has been the extent to which pulmonary C is altered is the important termed "minimal PEEP" (MEEP). After 20 min ("control period determinant of PEEP transmission to the thorax and of the 2," MEEP), the dogs were divided randomly into two groups: consequent effects of PEEP on intrathoracic pressures and cardio- Seven dogs (mean weight, 19.9 kg) were designated the "study vascular function. It is also concluded that the antecedent status group." Experimental protocol for this group included placement CARDIOPULMONARY CONSEQUENCES OF PEEP 11 13 of the expiratory limb of the PEEP-apparatus under water se- frequency in the control group for the duration of the procedure. quentially at 3. 6, 9, 12, and I5 cm HzO. Each PEEP setting was In the study group, however, respiratory frequency fell progres- maintained for 20 min. Five dogs (mean weight, 21.5 kg) were sively with increasing PEEP up to 9 cm HzO.after which there designated the "control group." These dogs were monitored for was no further change. This relative bradypnea was associated five successive 20-min periods ("control periods 3-7") with the with no change of inspiratory time, with an increase of expiratory PEEP apparatus in place and baseline gas flow of 2.0-2.5 liter/ time. and with a displacement of peak expiratory flow from the min, but without the expiratory tube under water. i.e.. with end-inspiratory period (Fig. 2). Thus. there developed a pause continuous MEEP. between inspiration and peak expiration. and. at higher levels of CO and C1 were calculated by a modification of the conven- PEEP. peak expiration could be characterized as "preinspiratory." tional method using the Fick principle (see Appendix). Surface Coincidentally, expiration became active as judged from the area was determined from the formula pleural pressure tracings (Fig. 3). When PEEP was reduced. the inspiratory-expiratory patterns assumed their previous configura- m2 = 0.1 I2 x body wt (kg)":' (46). tion (Fig. 4). These effects were apparent as soon as the PEEP FRC was determined by the closed circuit helium dilution method level was changed and they were sustained as long as a particular using the apparatus described by Berman er a/. (8) ("PEEP- PEEP was maintained. apparatus"), which permits measurements while the animals re- main on PEEP. Helium concentration was determined with a helium analyzer (57) calibrated with air and known concentrations of helium in air. Helium equilibration was achieved within 60 sec in each study. Dynamic C was calculated as VT divided by the difference in esophageal pressures at points of zero airflow. Csp was calculated as C divided by FRC. Unpaired data were analyzed by Student's r test. RESULTS Measurements were begun for both control and study groups after endotracheal intubation (ZEEP) and continued after place- ment of the PEEP-apparatus with gas flow of 2.0-2.5 liter/min (MEEP). Thereafter, the control group was maintained at MEEP. whereas PEEP was increased sequentially in the study group, as described. TIDAL VOLUME u I I I I I I I [~]PEEP(c~H~o)ZEEP MEEP 3 6 9 I2 I5 At ZEEP, VT was 148 f 52 ml in the control group and 1 13 f 79 ml in the study group. VT increased in both groups with MEEP, [*]CONTROL PERIOD 1 2 3 4 5 6 7 but the change was not significant (P < 0.10). With time (control PROBABILITIES: group) and PEEP (study group), VT increased, but there was no significant difference between the groups. At I5 cm H20 PEEP. [a] CONTROL PERIODS vr ZEEP f V.r was 189 103 ml and at the equivalent time period in the [o] STUDY PERIODS vr ZEEP <005 '0025 <0.01 ‘0 025 COO25 control group V.r was 184 f 45 ml.
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