Effect of different pulses of nitric oxide on venous admixture in the anaesthetized horse

Abstract

Background. Dependent atelectatic lung areas open towards the end of inspiration when the lung opening pressure increases, and recollapse during expiration. We hypothesized that inhaled nitric oxide (NO) counteracts hypoxic vasoconstriction in these collapsing lung areas, resulting in increased pulmonary shunt perfusion. Methods. We administered NO as a pulse and varied the pulse timing during inspiration in equine anaesthesia, where atelectasis develops regularly. Six spontaneously breathing standard breed trotters were studied under isoflurane anaesthesia in lateral recumbency. NO pulsed into the first 30% of inspiration (group NOpl) was assumed to affect open lung areas. To cover more open lung areas NO was then pulsed into the first 60% of inspiration (group NOp2). In a third group, administration between 50 and 80% of inspiration was aimed at the intermittently opening lung areas (group NOp3). Results. With NOp1, venous admixture decreased by 8 (2)% (mean (SEM), P=0.045) and with NOp2 by 10 (1)% (P=0.01). With NOp3, venous admixture reduction was insignificant. Conclusions. Pulsed administration of NO in early inspiration is optimal in reducing right to left vascular shunt in atelectatic equine lung. This reduction is positively correlated with the magnitude of the initial shunt. With administration in early inspiration, NO is mostly taken up by the lung. This prevents NO accumulation and NO2 formation in rebreathing circuits. These findings may be important in humans when atelectasis occurs increasingly with overweight and age during anaesthesia, but also in postoperative intensive care and in ARDS.