Altering ventilation-perfusion relationships in ventilated patients with acute lung injury

Abstract

IntroductionSince its description by Ashbaugh and Petty in 1967, the acute respiratory distress syndrome (ARDS) has been a major research focus in pulmonary and critical care medicine. However, despite rigorous investigation, it remains a common and vexing problem with a high (albeit possibly decreasing) mortality and no specific therapy. The cornerstone of managing patients with ARDS continues to be meticulous supportive care. Severe hypoxemia is a defining characteristic and is usually treated by high fractional inspired oxygen concentrations and the application of positive end expiratory pressure (PEEP). Recently, in response to a better understanding of the mechanisms of hypoxemia, the regional distribution of pulmonary blood flow and ventilation, several additional approaches to improving gas exchange in ARDS have been described. Although studies have not yet been reported to allow comment on whether these new approaches alter measures of clinical outcome, they have the potential to markedly improve our ability to manage these patients.In this review, the regional ventilation-perfusion (V A / Q ) relationships seen in ARDS are described as well as a number of new interventions designed to alter these in a favorable fashion.Ventilation-perfusion relationships in ARDS: mechanisms of hypoxemia ARDS results from a variety of predisposing factors which lead to injury of the pulmonary endothelium and alveolar epithelial membrane [1,2]. This is manifest clinically in diffuse pulmonary infiltrates and marked hypoxemia with increased venous admixture as calculated from arterial blood gases [1, 3]. The hypoxemia is usually relatively refractory to increasing F I O 2 but frequently responds in part, to the application of PEEP [4,5].Early investigators [3], using the multiple inert gas elimination technique, found that hypoxemia in ARDS is due primarily to intrapulmonary shunt, with an additional contribution from regions of very low V A /Q in some patients. These findings are consistent with those seen in experimental models of ARDS [6][7][8]. A subsequent study [9] confirmed these findings and demonstrated that the application of PEEP reduced shunt and redistributed blood flow to regions of low or normal V A /Q . Pulmonary hypertension is described in patients with ARDS and in animal models of the syndrome. The duration in pulmonary arterial pressure is usually modest but is associated with an increased mortality [10,11]. In animal models the degree of pulmonary hypertension is considerably greater [12] and in the early stages of injury has been attributed to neurohormonal mediators constricting the pulmonary circulation [13]. In humans, it has been difficult to separate vasoconstriction from the effects of PEEP although both seemingly contribute as the degree of pulmonary hypertension changes with titration of PEEP, and intravenous vasodilators reduce pulmonary vascular pressures and improve cardiac output [14,15, 27, 28]. Unfortunately, this beneficial hemodynamic effect is associated with worsening axygen delivery ...

Lung and 'end organ' injury due to mechanical ventilation in animals: comparison between the prone and supine positions

“…As early as 1976 it was reported that placing patients with ALI/ARDS in the prone position improves their oxygenation [ 9 - 17 ]. Prone positioning improves secretion drainage from the airways, relieving lung compression by the heart and abdomen.…”

An adult with ARDS managed with high-frequency oscillatory ventilation and prone position

“…The effect of the prone position is generally thought to be brought about by better recruitment and/or better ventilation/ perfusion matching [9][10][11]. Because improved oxygenation continued after the patient was returned to the supine position, it is conceivable that better recruitment was obtained with the prone position and was maintained with HFOV after the return to the supine position.…”

Efeito a curto prazo da posição prona na oxigenação de crianças em ventilação mecânica

“…Ao melhorar a relação V/Q, houve também redução da fração de shunt (Qs/Qt). As áreas pouco aeradas na região dorsal são recrutadas com pouca ou nenhuma mudança na ventilação na região ventral do pulmão, provocando, assim, maior homogeneidade na relação V/Q (pela diminuição da proporção de áreas com reduzida V/Q) 11,[26][27][28][29][30] . A melhora da distribuição da relação V/Q na posição prona também pode ser comprovada em seres humanos, observando-se o recrutamento de áreas previamente atelectasiadas, mas sem doença 31 .…”

“…Os valores da paO 2 /FiO 2 obtidos nesta pesquisa (27,7% de melhora no grupo total), após 1 hora de colocação em posição prona, não ficam distantes dos encontrados na literatura (44 a 75%) 28,31,36,38,39 . Provavelmente, nossos resultados tenham relação com a população mista estudada (doença pulmonar obstrutiva e restritiva) e também com o tipo de comprometimento pulmonar envolvido.…”