Effects of mid-line thoracotomy on the interaction between mechanical ventilation and cardiac filling during cardiac surgery

Background. Mid-line thoracotomy is a standard approach for cardiac surgery. However, little is known how this surgical approach affects the interaction between the circulation and mechanical ventilation. We studied how mid-line thoracotomy affects cardiac ®lling volumes and cardiovascular haemodynamics, particularly variations in stroke volume and pulse pressure caused by mechanical ventilation. Methods. We studied 19 patients during elective coronary artery bypass surgery. Before and after mid-line thoracotomy, we measured arterial pressure, cardiac index (CI) and global end-diastolic volume index (GEDVI) by thermodilution, left ventricular end-diastolic area index (LVEDAI) by transoesophageal echocardiography and the variations in left ventricular stroke volume and pulse pressure during ventilation by arterial pulse contour analysis. Results. After thoracotomy, CI increased from 2.3 (0.4) to 2.9 (0.6) litre min±1 m±2, GEDVI increased from 605 (110) to 640 (94) litre min±1 m±2, and LVEDAI increased from 9.2 (3.7) to 11.2 (4.1) cm2 m±2. All these changes were signi®cant. In contrast, stroke volume variation (SVV) decreased from 10 (3) to 6 (2) % and pulse pressure variation (PPV) decreased from 11 (3) to 5 (3)%. Before thoracotomy, SVV and PPV signi®cantly correlated with GEDVI (both P<0.01). When the chest was open, similar signi®cant correlations of SVV (P<0.001) and PPV (P<0.01) were found with GEDVI. Conclusion. Thoracotomy increases cardiac ®lling and preload. Further, thoracotomy reduces the effect of mechanical ventilation on left ventricular stroke volume. However, also under open chest conditions, SVV and PPV are preload-dependent