Pulmonary Microvascular Perfusion during Positive End-Expiratory Pressure (PEEP)

PEEP has been shown to decrease both cardiac output (C.O.) and alveolar capillary perfusion (Q.). Method: We attached an in-vivo microscope apparatus to the lungs of 5 anesthetized dogs through a left thoracotomy to study the effects of PEEP on Q. Q was assessed at baseline with 5 cm H₂O PEEP and during the application of 15 cm H₂O PEEP. To differentiate between the effect of the fall in C.O. on Q. and the effects of other hemodynamic changes with PEEP, C.O. was restored to the control level with dextran-70 infusion. Results: The institution of 15 cm H₂O PEEP effected a dramatic fall in Q. with a concomitant decline in C.O. Infusion of dextran 70 returned C.O. to baseline, but Q. remained 70% below baseline. Photomicrographs showed flat, constricted capillaries with PEEP. Conclusion: These data suggest that PEEP reduces Q. by direct compression of pulmonary capillaries rather than by decreasing cardiac output.