In the kidney, reduction in cardiac index and venous return causes, via a sympatho-adrenergic reaction, vasoconstriction of the afferent renal arterioles with reduction and redistribution of the renal blood flow from the cortical to the juxtaglomerular nephrons. This reduces the glomerular filtration rate and sodium excretion. Increase in venous return and cardiac output, due to the periodic fall in intrathoracic pressure during spontaneous inspiration, should significantly improve kidney perfusion and function during partial ventilatory support. Inpatients with ARDS, spontaneous breathing with IMV leads to an increase in glomerular filtration rate and sodium excretion [45, 46]. Compatible with these results, in patients with ARDS, kidney perfusion and glomerular filtration rate increase during spontaneous breathing with APRV/BiPAP as compared to pressure-limited ventilation with equal  airway pressure limits or Ve. Although cardiac index has been highest during mechanical ventilation with low Vt resulting in hypercapnia, kidney perfusion and glomerular filtration rate were lower than during spontaneous breathing with APRV/BiPAP (Fig. 7). This indicates that maintained spontaneous breathing is favorable for the perfusion and function of the kidney in patients requiring ventilatory support due to severe pulmonary dysfunction.
Reduction of cardiac output and venous return causes, via a sympatho-adren-ergic reaction, vasoconstriction and lower blood flow in the portal vein and thereby in the liver and the splanchnic area. Preliminary data in patients requiring ventilatory support for ALI suggest that maintained spontaneous breathing may be beneficial for the liver function . In addition, using colored microspheres in pigs with oleic acid-induced lung injury spontaneous breathing during APRV/BiPAP may be advantageous for the perfusion of the splanchnic area .
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