Abstract
Positive end-expiratory pressure (PEEP) is commonly used in the treatment of critically ill patients whose sympathetic nervous system is stressed; however, PEEP's actions on sympathetic nervous system activity are unknown. We therefore measured the plasma noradrenaline response (an index of sympathetic nervous system activity) to graded doses of PEEP in nine mongrel dogs. After 30 minutes at each level of PEEP, plasma noradrenaline concentrations increased from baseline mean values of 300 (SD 108) pg/ml to 388 (SD 225) pg/ml (P< 0.05) at 5 cm, 433 (SD 255) pg/ml (P <0.01) at 10 cm and 1194 (SD 882) pg/ml (P<0.01) at 20 cm water pressure of PEEP. The increases in plasma noradrenaline concentrations correlated inversely (r= - 0.43, P< 0.01) with PEEP-induced changes in cardiac output. Plasma adrenaline levels did not change significantly in response to 5 or 10 cm of PEEP; however, plasma adrenaline increased, while heart rate and mean arterial blood pressure fell, at 20 cm water pressure of PEEP (P < 0.05). Within 15 minutes after discontinuation of PEEP, the plasma catecholamine concentrations returned to baseline levels. Conclusions: 1. PEEP significantly increases sympathetic nervous system activity in a rapid, dose- dependent, reversible manner; 2. the PEEP-induced increases in sympathetic activity may explain the reductions in organ blood flow which others have observed following the initiation of PEEP; 3. PEEP-related changes in sympathetic nervous system activity are a consequence of PEEP- induced reductions in cardiac output.