Descriibe Mechanical Ventilation and Cardiac Performance

During spontaneous breathing, the intrathoracic and intrapulmonary pressures are negative, while with mechanical ventilation, they are positive. This has significant effects on heart function. The positive intrathoracic pressure during mechanical ventilation reduces venous inflow into the right side of the heart by reducing the pressure gradient for filling. In other words, the preload to the heart is reduced. The positive pressure also impacts on the outer surface of the ventricles, reducing diastolic filling. Positive pressure transmitted from the lung through to the pulmonary vessels also increases the afterload to the right ventricle. When the ventricle is failing, positive pressure on its surface might facilitate emptying during systole. This latter effect may be visualized like a hand squeezing the ventricle during systole. Positive pressure ventilation (by NIV or intubation) might improve the function of a failing ventricle by this mechanism.