Effects of lung recruitment and PEEP after CPB on pressure-absolute volume curves

Introduction

Impaired lung function is common after cardiopulmonary bypass (CPB). We hypothesized that due to collapse tendency 1) the effect of a lung recruitment maneuver (LR) on a pressure-absolute lung volume (P-V) curve would be minimal, 2) but if LR is followed immediately by PEEP the curve would shift upwards and the slope would be steeper.

Methods

Sixteen patients (48–75 years) after CABG in CPB were studied postoperatively every 0.5 h during 3 h while mechani cally ventilated (FiO₂ 1.0) with measurements of end-expiratory lung volume (EELV) and blood gases. Eight patients were randomized to LR (45 cmH₂O airway pressure 2 × 20 s) after which PEEP was set 1 cmH₂O > LIP obtained from a static P–V curve (PEEP group), while the 8 other were randomized to LR only (ZEEP group). Three inspiratory P–V (including EELV) curves were obtained in both groups. In the PEEP-group, the first curve (A ◎) was obtained before LR and PEEP, the second (B ○) during PEEP (14 ± 3 cmH₂O, mean ± SD) 2.5 h after LR, and the third (C △) 0.5 h after removal of PEEP, i.e. 3 h after LR. In the ZEEP group, P–V curves were obtained at similar times.

Statistics

ANOVA and Wilcoxon signed rank test.

Results

In the ZEEP group, no change in PaO₂, lung volume or PV-relations occurred during the study. In the PEEP group, PaO₂ increased by 16 ± 15 kPa (P < 0.002) after LR and PEEP and was unchanged during the study. EELV increased by 1120 ± 235 ml (P < 0.0001) and remained stable until removal of PEEP. The 3 P–V curves are shown in the figure (A ≠ B or C, P < 0.03).

Figure

Discussion

In patients after CPB, LR without subsequent PEEP had no effect. However, when LR was followed by PEEP, EELV increased and the P–V curve became steeper and shifted upwards. Furthermore, the curve remained the same 0.5 h after PEEP-removal. This and the unchanged PaO₂ indicate that no new lung collapse occurred after removal of PEEP and suggest that PEEP might have a sustained stabilizing effect on lung structures in these patients.