HEMODYNAMIC, SYMPATHETIC AND ANGIOTENSIN-II RESPONSES TO PEEP VENTILATION BEFORE AND DURING ADMINISTRATION OF ISOFLURANE

Background: Positive end-expiratory pressure (PEEP) ventilation and is oflurane anesthesia may opposingly affect the sympathetic nervous and renin-angiotensin systems. This study was performed to elucidate the m odulatory effects of isoflurane anesthesia on the neurohumoral and car diovascular responses to PEEP. Methods: Renin-angiotensin and sympathe tic nervous activity were investigated in mechanically ventilated, nor movolemic, chloralose anesthetized pigs before and during administrati on of 1.4% isoflurane. Arterial angiotensin II (AII) concentrations we re measured and systemic, mesenteric, hepatic and renal spillover of n orepinephrine (NE-SO) were calculated using isotope dilution. Regional hemodynamic variables were investigated in parallel. Results: PEEP10 alone moderately elevated AII levels (+12.5 +/-4.9 pg/ml, P<0.05) and increased systemic (+22+/-2.9 pmol . min . 100g(-1), P<0.05) and notab ly mesenteric (+32+/-9.6 pmol . min . 100g(-1), P<0.05) NE-SO. Blood f low decreased in all vascular beds studied. Except for in the liver, i soflurane generally reduced NE-SO compared to baseline but did not cha nge AII concentrations, Strikingly, the sympathoexcitatory response to PEEP10 was inhibited, whereas AII increased markedly (+284+/-64 pg/ml , P<0.05) during PEEP10 and isoflurane. renal blood flow was significa ntly more reduced during PEEP10 and isoflurane compared to PEEP10 alon e, whereas the magnitude of reductions were similar in the other vascu lar beds. Conclusion: The data suggest that renin-angiotensin activati on is important to attenuate the impact of PEEP ventilation on cardiov ascular performance during administration of the sympathodepressant is oflurane. Interference with the renin-angiotensin system may cause car diovascular decompensation in isoflurane anesthetized patients subject ed to PEEP-ventilation.