The influence of intrathoracic pressure on fluid and electrolyte balance.

s modem respiratory intensive care becomes A increasingly well defined as a subspecialty, it is important to critically evaluate the effect of various types of ventilatory support on the patient as a whole. One aspect of the general care of the respiratory patient with which the physician must deal is fluid and electrolyte balance. The original report of Sladen et al,' suggesting that sodium and fluid retention are associated with positive pressure ventilation, has been followed by other studies with similar findings.*s3 The magnitude of the clinical problems presented in these studies points out the potential importance of this association, but the existing clinical literature does little more than document the phenomenon in a retrospective fashion and does little to elucidate the mechanism in humans. On the other hand, the physiologic literature on the influence of intrathoracic pressure on fluid and electrolyte balance is replete with studies on the mechanism of this influence. Unfortunately, it is at the same time more confusing and often contradictory. The purpose of this article is to briefly review the literature in this field and point out how the thinking has shifted in recent years. It is the author's hope that such a review may crystallize the physiologic aspects of this topic and give some direction to future investigations into the clinical aspects. Until recently the role of the left atrial stretch reflex, as first described by Henry, Gauer and Reeves in 1956: was the cornerstone of physiologic thinking regarding the influence of the chest on urine output. Recently, as will be discussed in detail below, the physiologic importance of left atrial