1190 CARDIOPULMONARY CONSEQUENCES OF POSITIVE END-EXPIRATORY PRESSURE (PEEP)

Abstract

Cardiopulmonary adaptation to PEEP was studied in 7 normal spontaneously breathing dogs during stepwise increase of PEEP to 3,6,9,12 and 15 cm H2O. Each pressure was sustained for 20 mins during which time a steady state was achieved. [To obviate protocol bias pressure changes were also made at random. The response to reduction of PEEP was also monitored, as was a control group kept at 1.0 cm H2O PEEP]. Increasing levels of PEEP produced [1] no change of tidal volume and inspiratory time; [2] a progressive fall of respiratory rate and minute volume; [3] progressive decrease of passive expiratory flow; and [4] generation of active expiration which became pre-inspiratory at the highest pressures. These responses occurred immediately and thus may be reflex in nature. As PEEP was increased, functional residual capacity (FRC) increased, specific pulmonary compliance fell, and progressively less of the applied airway pressure was transmitted to an esophageal balloon used for estimating pleural pressure. Whereas right atrial and pulmonary arterial pressures increased with increasing PEEP, cardiac output was not altered when compared with the control group. Thus PEEP may induce reflex changes in respiratory rhythm, as well as mechanical changes related to FRC and compliance. The former tend to compromize alveolar ventilation and work of breathing. The latter affect both ventilatory work and cardiovascular performance. (Supported by the NHLBI, NIH grant numbers HL 16137 and HL 07060).