Methods of Increasing FRC in Acute Respiratory Failure

Abstract

The goal of respiratory therapy to increase functional residual capacity (FRC) is not only to improve oxygenation, so important this effect may be, but to reestablish pertinent and normal ventilation of all regions of the lung. In acute respiratory failure (ARF), ventilation is endangered by destabilization of alveoli [1, 2]. Apart from interstitial edema the clinical picture will be determined by the pathophysiological consequences of increased pulmonary retraction and decreased lung volume leading to reduced compliance, hypoventilation, shunting, and hypoxemia. In addition fluid balance is disturbed and extravasation will increase further [3] (Fig. 1). Respiratory therapy at increased FRC does not influence the initial or the causative mechanisms of ARF, but it will confine their influence on pulmonary volume and therefore mechanics and gas exchange because it counteracts volume loss and prevents it becoming a causative factor per se. From that point of view ventilation with increased FRC is a struggle against progressive pulmonary retraction. The improvement of gas exchange that usually results is important and a useful monitor of successful therapy, however it is not indispensable for justification of ventilatory patterns with increased FRC.