Changes in the distribution of ventilation and perfusion associated with separation from mechanical ventilation in patients with obstructive pulmonary disease

Abstract

A trial of separation from mechanical ventilation may induce an abnormal respiratory pattern and a maldistribution of ventilation- to-perfusion ratios (V(A)/Q), especially in patients with chronic obstructive pulmonary disease. This study was designed to assess the effects of three different modes of ventilation on the distribution of global and also regional V(A)/Q in eight patients with chronic obstructive pulmonary disease recovering from acute respiratory failure who remained dependent on mechanical ventilation after more than 5 days of attempted separation from the ventilator. V(A)/Q distribution was assessed using the multiple inert gas and isotopic scanning methods after 30 min each of controlled mechanical ventilation (CMV), 10 cmH2O inspiratory pressure support, and spontaneous breathing (SB). Controlled ventilation was provided at a respiratory rate ranging from 12 to 18 breaths per min and a tidal volume of 8 ml·kg-1. In comparison to CMV, SB resulted in a decrease in tidal volume (from 512 ± 144 to 301 ± 102 ml, P < 0.01), and an increase in respiratory rate (from 15.5 ± 3.2 to 27.3 ± 15.0 breaths per min, P < 0.05), which increased dead space (+7.1% of minute ventilation), cardiac output (+36%), and the perfusion to areas of low V(A)/Q (+8.9% of cardiac output) (P < 0.05, P < 0.001, and P < 0.05, respectively). Isotopic scans revealed a horizontal craniocaudal difference of V(A)/Q in all modes, with the lowest V(A)/Q zones at the basal part of the lungs (mean basal V(A)/Q 0.58 in SB and 1.05 in CMV). During SB, this craniocaudal difference of V(A)/Q was highly correlated to the dispersion of perfusion (dispersion around the mean [log SD(Q)], r = 0.87; P < 0.01). Moreover, the patients with the smallest tidal volume during SB showed the lowest caudal V(A)/Q ratios (r = 0.88, P < 0.01), the largest craniocaudal gradient in V(A)/Q (r = 0.77, P < 0.05), and the largest amount of perfusion in the areas of low V(A)/Q ratios (r = -0.71, P < 0.05). We conclude that in our patients, the change from CMV to SB, induced an abnormal breathing pattern with small tidal volumes. This resulted in a maldistribution of V(A)/Q ratios that was not improved by pressure support at a level of 10 cmH2O.