POSITIVE END-EXPIRATORY PRESSURE DECREASES MESENTERIC BLOOD-FLOW DESPITE NORMALIZATION OF CARDIAC-OUTPUT

Positive end-expiratory pressure (PEEP) is a commonly used adjunct to mechanical ventilation and is known to have deleterious effects on car diac output (CO). Its effects on regional blood flow are not well know n. We evaluated the effect of PEEP on the mesenteric microcirculation and CO. Sprague-Dawley rats were treated with mechanical ventilation a nd either no PEEP (Control) or increasing levels of PEEP (PEEP). Using in vivo video-microscopy, mesenteric Al arteriolar optical Doppler ve locities and Al and A3 (the first- and third-order arterioles branchin g off the feeding mesenteric arcade) intraluminal diameters were measu red (n = 6/group). In a separate set of experimental animals, CO was d etermined by thermodilution technique (n = 5/group). Additionally, aft er the PEEP group attained a PEEP level of 20-cm H2O PEEP, two boluses of 2 mt 0.9 normal saline (NS) were given intravenously. The Control groups had the same determinations performed over the same time course as the PEEP group but were not exposed to any PEEP. Mesenteric blood flow (MBF) was calculated from vessel diameter and red blood cell velo city. The MBF and CO fell progressively as PEEP was increased from 10- to 15- to 20-cm H2O pressure. MBF was reduced 75% (p < 0.05) and the CO was reduced 31% (p < 0.05) from baseline at 20-cm H2O pressure PEEP . After 4 mt normal saline, the MBF was still 45% below baseline (p < 0.05) while the CO had returned to baseline. In conclusion, both MBF a nd CO are decreased significantly with increasing PEEP. Fluid resuscit ation will reverse the adverse effects of PEEP on CO, but there is onl y partial correction of the negative effects on MBF.