Sympathetic nerve activity and central haemodynamics during mechanical ventilation with positive end-expiratory pressure in rats.

The aim of this study was to examine the effects of mechanical ventilation with increasing levels of positive end-expiratory pressure (PEEP) on sympathetic nerve activity (SNA), cardiac output (CO), stroke volume (SV), heart rate (HR), central blood volume (CBV), total peripheral resistance (TPR), mean arterial pressure (MAP), pulse pressure (PP) and right and left atrial transmural pressure in chloralose anaesthetized rats before and after vagotomy. Changing ventilatory pattern from spontaneous breathing (SB) to artificial ventilation with 10 cm H2O PEEP in intact animals caused a significant fall in CO, SV and CBV (42, 48 and 17%, respectively) and an increase in SNA, HR and TPR (90, 13 and 83%, respectively). The MAP increased slightly but significantly from 103 +/- 4 to 107 +/- 4 mmHg while PP decreased from 48 +/- 2 to 37 +/- 3, from spontaneous breathing (SB) to 10 cm H2O PEEP. Transmural left atrial pressure decreased significantly from 4.5 +/- 0.3 to 3.0 +/- 0.4 mmHg. After vagotomy, MAP and CO were significantly lower at 10 cm H2O PEEP and PP and SV were significantly lower at all levels of positive end-expiratory pressure than the corresponding prevagotomy values. In spite of a greater fall in MAP and PP during PEEP after vagotomy, the absolute and relative increase of SNA was significantly lower compared to corresponding prevagotomy values. We conclude that reflex cardiovascular adjustments elicited by ventilation with PEEP are not solely due to unloading of arterial baroreceptors as has been claimed by others. Unloading of cardiac receptors with tonically active inhibitory afferents in the vagi is probably also of great importance for the excitation of the sympathetic nervous system during mechanical ventilation with PEEP.