Ventilation‐Perfusion Distribution During Inhalation Anaesthesia

Abstract

Ventilation-perfusion (VA/Q) ratios were studied by means of an inert gas elimination technique in healthy subjects with an average age of 51 years in the supine posture (a) when awake, (b) during inhalational anaesthesia, spontaneously breathing, (c) during mechanical ventilation, and (d) when a positive end-expiratory pressure (PEEP) was applied. In the awake subject a bimodal distribution of VA/Q was recovered in most patients, one mode centered around the ratio of 1 and another, smaller mode, within low VA/Q-regions. Any shunt was less than 3% of cardiac output. With anaesthesia and spontaneous breathing, the low VA/Q mode was reduced and the shunt increased to an average of 6.2%. With mechanical ventilation, the major VA/Q mode was widened while the shunt was further increased in 4 of 10 subjects (mean 8.6%). With PEEP, the shunt was reduced and a new mode within high VA/Q-regions appeared. The shunt and low VA/Q-regions appeared. The shunt and low VA/Q-regions may be explained in terms of airway closure while the high VA/Q mode with PEEP may be attributed to the development of a zone I.

Matching positive end-expiratory pressure to intra-abdominal pressure improves oxygenation in a porcine sick lung model of intra-abdominal hypertension

“…While increased levels of PEEP have been suggested to improve lung function [9], this approach carries the risk of regional pulmonary overdistension injury [11,16] and hemodynamic compromise [6]. …”

Time-cycled inverse ratio ventilation does not improve gas exchange during anaesthesia

“…Improved gas exchange has been observed; ~8 but in other studies the decrease in shunt was counterbalanced by a decrease in cardiac output, resulting in no change in (A-a)DO2 . 33 The putative beneficial effect of IRV in respiratory failure may be due to the respiratory waveform or airway pressures. In neonates with severe lung disease, ventilated with various I:E ratios and airway pressure waves, optimal oxygenation was achieved with the I:E ratio and pressure wave combination that produced the highest mean AWP.…”

Mathematical Modelling of Pulmonary Gas Exchange

“…In anesthesiological research, the MIGET has been used to describe gas exchange abnormalities following different types of anesthesia: inhalation [28À32], intravenous [33,34], and epidural [35,36]; and after numerous interventions during anesthesia: surgery [37À39], variation in positive end-expiratory pressure (PEEP) [29,40], different modes of mechanical ventilation [41], CO 2 pneumoperitoneum [42], or infusion of inotropic agents [43]. The findings of these studies consistently show an increased shunt and _ V = _ Q mismatch following anesthesia, with increases in PEEP reducing the shunt fraction.…”