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Changes in physiological dead space/tidal volume ratio after pleural fluid drainage

We aimed at evaluating physiological dead space/tidal volume ratio (VDph/VT) in patients with unilateral free pleural effusion without significant parenvchymal lesion, before (T1), immediately after (T2) and 48 hours (T3) after drainage of pleural fluid. We studied 57 patients, with unilateral effusion (32 male and 25 female) aged 61.9±10.3 years (mean±SD) of whom 36 had amaller-sized effusion(group A) while 21 a larger-sized one (group B) .Each patient, after resting in comfortable sitting position for five minutes, began breathing room air via a Rudolph valve and collecting expired air in a connected 200 lt Douglas bag. We simultaneously obtained a 2 ml sample of arterial blood for gas analysis, while mixedexpired air was analyzed for P E CO2.The same procedure was repeated 30 minutes (T2) and 48 hours after the procedure (T3). We calculated VDph/VT using Bohr9s equation (VDph/VT= PaCO2-P E CO2/PaCO2).In group A, from T1 to T2 and T3, VDph/VT was respectively [mean (SD)] 0.43 (0.1), 0.45 (0.1), 0.41(0.1) [p<0.001]. In group B, from T1 to T2 and T3, VDph/VT was respectively 0.39 (0.1), 0.46 (0.1) and 0.41 (0.1) [p<0.001].
 We can conclude that draining an effusion is associated with an immediate increase in VDph/VT which returns to near pre-procedure values after 48 hours. This is especially more prominent after removing fluid from larger effusions, propably amenable to a more significant change in the pattern of breathing.