Patterns in oxygenation, dead space ventilation, lung mechanics and CO₂ kinetics after prolonged cardiopulmonary bypass: an analysis by the CO₂ SMO-PLUS

Cardiopulmonary bypass (CPB) alters pulmonary function and its duration is related to postoperative problems. The lung is the first organ to initiate a clinical response to this inflammatory reaction, translated as an acute lung injury, with ventilation, oxygenation and pulmonary mechanics alterations. The aim of this study was to investigate the immediate changes in lung function after a prolonged (more than 2 h) CPB in cardiac surgeries.

Twelve patients (36-85 years; 56-92 kg) were studied before (B) and after (A) CPB. Respiratory monitoring was achieved by intermittent measurements of arterial blood gases, dead space ventilation (VD/VT), and continuous evaluation of expired carbon dioxide (PETCO₂), CO₂ flow (VCO₂), inspired fraction of oxygen (FiO₂) and expired tidal volumes (VT), minute ventilation/kg weight (VE/Kg), inspiratory and expiratory flows, pressures and mechanics (static compliance-CSt), utilizing a solid state/single beam nondispersal infrared unit-main stream capnography with a fixed orifice differential pressure pneumotach (CO₂SMO-PLUS-Novametrix). PaO₂/FiO₂ was calculated. Data were gathered before and after CPB, with the chest closed and stable hemodynamics. Normality was evaluated by Kolmogorov-Smirnov test, modified by Lilliefors. Equal variances were assessed by Levene median test. Comparisons between the groups were performed by Student's t-test and Mann-Whitney Rank Sum test. Significance level was 5%.

The mean CPB time was 151.08 ± 38.85 (120-245 min). There were no statistical differences in VE/kg (87.08 ± 14.54 versus 88.24 ± 12.31), FiO₂ (0.47 ± 0.4 versus 0.50 ± 0.4), WmOB (1.34 ± 0.5 versus 1.34 ± 0.4), RawI, RawE, Cdyn and VD/VT. There were significant differences in PaO₂ (153.72 ± 61.11 versus 106.90 ± 38.43), pH (7.46 ± 0.06 versus 7.39 ± 0.09), PaCO₂ (33.86 ± 6.47 versus 40.47 ± 7.85) and CSt (47.60 ± 14.90 versus 42.08 ± 12.80).

Thus, in patients after prolonged CPB, the only change in pulmonary mechanics was the static compliance values. On the other hand, oxygenation was greatly impaired. An increase in CO₂ values was seen despite the same minute ventilation and VD/VT ratio, probably due to a high VCO₂.