Limitations of arterial pulse pressure variation and left ventricular stroke volume variation in estimating cardiac pre-load during open heart surgery.

ABSTRACT

BACKGROUND: In addition to their well-known ability to predict fluid responsiveness, functional pre-load parameters, such as the left ventricular stroke volume variation (SVV) and pulse pressure variation (PPV), have been proposed to allow real-time monitoring of cardiac pre-load. SVV and PPV result from complex heart-lung interactions during mechanical ventilation. It was hypothesized that, under open-chest conditions, when cyclic changes in pleural pressures during positive-pressure ventilation are less pronounced, functional pre-load indicators may be deceptive in the estimation of ventricular pre-load. METHODS: Forty-five patients undergoing coronary artery bypass grafting participated in this prospective, observational study. PPV and SVV were assessed by pulse contour analysis. The thermodilution technique was used to measure the stroke volume index and global and right ventricular end-diastolic volume index. Trans-oesophageal echocardiography was used to determine the left ventricular end-diastolic area index. All parameters were assessed before and after sternotomy, and, in addition, after weaning from cardiopulmonary bypass before and after chest closure (pericardium left open). Patients were ventilated with constant tidal volumes (8 +/- 2 ml/kg) throughout the study period using pressure control. RESULTS: SVV and PPV decreased after sternotomy and increased after chest closure. However, these changes could not be related to concomitant changes in the ventricular pre-load. The stroke volume index was correlated with SVV and PPV in closed-chest conditions only, whereas volumetric indices reflected cardiac pre-load in both closed- and open-chest conditions. SVV and PPV were correlated with left and right ventricular pre-load in closed-chest-closed-pericardium conditions only (with the best correlation found for the right ventricular end-diastolic volume index). CONCLUSIONS: SVV and PPV may be misleading when estimating cardiac pre-load during open heart surgery.