The three negative effects of mechanical ventilation on the cardiac output; Integrated model of the circulatory and respiratory systems

Introduction: Mechanical ventilation (MV) can decrease the cardiac output (CO) and oxygen delivery to the periphery, which increases the mortality rate and requires transition to extra-corporal membranous oxygenation. It is well-known that MV decreases CO by (1) decreasing the RV venous return and (2) increasing the pulmonary vascular resistance (PVR) and the RV afterload. However, the effects on LV filling is vague, and some suggest that high inspiratory pressure may augment LV filling. The cardiopulmonary interactions are complex since the pulmonary circulation passes through two compartments, intrapulmonary and pleural compartments, with two different pressures. Understanding the complex cardiopulmonary interactions is essential for optimization of MV.

Methods: The integrated model of the cardiovascular and respiratory systems simulates the entire circulation and respiratory mechanics. The pulmonary vessel diameters are determined by their transmural pressure gradient; the focal intravascular pressure minus the surrounding alveolar or pleural pressures. The vessel diameters determine the PVR. The vessels can collapse when the alveolar pressure increases and the transmural pressure approaches zero.

Results: The integrated model deciphers how MV parameters (e.g. plateau pressure and the inspiration:expiration ratio) adversely affect the CO. It also presents the effects of lung compliance (lung disease severity) on the transpulmonary pressure, i.e. alveolar (P_alv) minus pleural (P_pl) pressures, and CO. High P_alv (intrapulmonary circulation) compared to lower P_pl (around the RV) increases the RV afterload. MV increases the PVR, especially during inspiration. MV can cause collapse of the postcapillary vessels, during inspiration. The collapse is at the venous side, with the lower intravascular pressure. This collapse shifts lung regions from zone 3 to zone 2. This phenomenon increases the PVR and the RV afterload and decreases the LV filling.

Conclusions: MV can reduce CO by decreasing the RV preload, increasing the PVR, and causing a collapse of postcapillary vessels, especially during inspiration.