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Dynamic filling index: a novel parameter to monitor circulatory filling during minimized extracorporeal bypass

^a Department of Cardiothoracic Surgery, Cardiovascular Research Institute Maastricht – CARIM, Maastricht University Medical Center – MUMC, Maastricht, The Netherlands
^b Department of Biomedical Engineering/Biophysics, Cardiovascular Research Institute Maastricht – CARIM, Maastricht University Medical Center – MUMC, Maastricht, The Netherlands
^c Department of Anesthesiology and Pain Treatment/Department of Intensive Care Medicine, Cardiovascular Research Institute Maastricht – CARIM, Maastricht University Medical Center – MUMC, Maastricht, The Netherlands

Received 5 December 2008; received in revised form 23 March 2009; accepted 25 March 2009.

^_* Corresponding author. Address: Maastricht University Medical Center, Department of Cardiothoracic Surgery, P. Debeyelaan 25, PO Box 5800, 6202 AZ Maastricht, The Netherlands. Tel.: +31 43 3877070; fax: +31 43 3875075. (Email: antoinepsimons{at}ctc.unimaas.nl).

Objective: To evaluate the dynamic filling index, a novel parameter to monitor changes in venous return and drainable volume, in circulatory assisted patients. Minimized extracorporeal bypass systems lack volume buffering capacity, demanding tight control of drainable volume to maintain bypass flow. Therefore, with patients on minimized bypass quantitative assessment of venous drainable volume is crucial. Methods: In seven patients undergoing coronary artery bypass grafting using minimized extracorporeal bypass we utilized luxation of the heart to induce a reduction in venous return. The speed of the centrifugal pump was transiently and periodically reduced to monitor resultant changes in bypass flow. The dynamic filling index, a measure of drainable volume, was calculated as flow/speed. Results: With luxation, the dynamic filling index was significantly reduced (from 2.4 ± 0.2 to 2.0 ± 0.2 ml/rotation, p = 0.001; 95% confidence interval of mean difference: 0.23–0.46 ml/rotation), whereas routinely recorded parameters, like bypass flow, pump inlet and arterial line pressure, did not change significantly. The intra-measurement reproducibility for the dynamic filling index was 0.5 ml/rotation (20% of the mean), suggesting good potential for this parameter to monitor on-pump venous return in patients. Conclusion: The dynamic filling index can detect small changes in venous return and drainable volume which remain unrevealed by routinely recorded parameters. This index could be a valuable tool to monitor and control circulatory filling in individual patients supported by minimized extracorporeal bypass.

Key Words: Minimized extracorporeal bypass • Cardiopulmonary bypass • Blood volume • Venous drainage • Venous return • Centrifugal pump