Influence of pleural pressure variations on cardiovascular system dynamics: a model study

A mathematical model of the human cardiovascular system (CVS) is used to study the effect of different respiratory manoeuvres on the circulation. The model simulates the normal CVS and the interaction between the heart and the intrathoracic pressure. The vascular system is represented by resistive, capacitive and inertial elements whereas the ventricles are assumed to function according to the time-varying elastance concept based on their transmural pressures. The model predicts that normal inspiratory effeort effects an increase in the venous return, an increase in the pulmonary flow and a slight decrease in the left ventricular stroke volume (LVSV), which represents a decrease in ejection due to the increased LV transmural pressure. A step decrease in pleural pressure to -40 mm Hg, representing the Müller manoeuvre (MM), accentuates these findings, showing a decrease in LVSV in spite of an increase in the LV end-diastolic volume (LVEDV), LV end-systolic volume (LVESV) and the LV filling pressure, expressed as the mean left atrial transmural pressure. Simulating intermittent positive pressure ventilation (IPPV) with added positive end expiratory pressure (PEEP) shows an 18·6 per cent decrease in the cardiac output compared with quiet respiration. The calculated results of the model are in good agreement with available experimental data, suggesting that most of these findings may be explained by basic haemodynamic principles in the uncontrolled CVS.