What is the effect of positive pressure ventilation on cardiac output and kidney perfusion?

Positive pressure ventilation has specific physiological effects on the cardiovascular and renal systems. When positive pressure ventilation is applied, particularly in the setting of mechanical ventilation, it can result in decreased venous return to the heart due to increased intrathoracic pressure. This increase in intrathoracic pressure can compress the great veins (such as the superior and inferior vena cava), leading to reduced preload. Consequently, when preload is diminished, cardiac output is likely to decrease as there is less blood returning to the heart to pump.

As cardiac output decreases, renal perfusion can also be adversely affected. The kidneys are highly sensitive to changes in perfusion pressure, and during hypoperfusion, the glomerular filtration rate (GFR) may fall due to both decreased blood flow and compensatory responses like the activation of the renin-angiotensin-aldosterone system, further constricting vasculature.

Moreover, the increased intrathoracic pressure can lead to decreased perfusion of various organs, including the kidneys, as effective blood flow is compromised. Therefore, positive pressure ventilation generally has a detrimental effect on both cardiac output and kidney perfusion, resulting in decreased function in both systems during its application.