Extensive microvascular thrombosis has been demonstrated in histological studies of ARDS [39, 40]. Angiographic studies showed the extent of microvascular obstruction correlated with the severity of respiratory failure and with mortality [38, 39]. Microvascular thrombosis increases lung dead space and the increase in dead space or its bedside surrogate ventilatory ratio was shown to be an independent marker of mortality in ARDS [43, 44]. Microvascular thrombosis also causes increased pulmonary vascular resistance, which may result in right heart failure [45].