Impact of positive pressure ventilation on mean systemic filling pressure in critically ill patients after death.

Mean systemic filling pressure (P) defines the pressure measured in the venous-arterial system when the cardiac output is nil. Its estimation has been proposed in patients with beating hearts by building the venous return curve, using different pairs of right atrial pressure/cardiac output during mechanical ventilation. We raised the hypothesis according to which the P is altered by tidal ventilation and positive end-expiratory pressure (PEEP), which would challenge this extrapolation method based on cardiopulmonary interactions. We conducted a two-center, noninterventional, observational, and prospective study, using an arterial and a venous catheter to measure the pressure in the circulatory system at the time of death in critically ill, mechanically ventilated patients with a PEEP. Arterial (P) and venous pressures (P) were recorded in five conditions: at end expiration and end inspiration with and without PEEP and finally once the ventilator was disconnected. P and P did not differ in any experimental conditions. Tidal ventilation increased P and P by 2.4 and 1.9 mmHg, respectively, whereas PEEP increased both values by 1.2 and 1 mmHg, respectively. After disconnection of the ventilator, P and P were 10.0 ± 4.2 and 9.9 ± 4.2 mmHg, respectively. P increases during mechanical ventilation, with an effect of tidal ventilation and PEEP. This calls into question the validity of its evaluation in heart-beating patients using cardiopulmonary interactions during mechanical ventilation. The physiology of the mean systemic filling pressure (P) is not well understood in human beings. This study is the first report of a tidal ventilation- and positive end-expiratory pressure-related increase in P in critically ill patients. The results challenge the utility and the value estimating P in heart-beating patients by reconstruction of the venous return curve using varying inflation pressures.