Long cardiopulmonary bypass times are associated with adverse postoperative outcomes and increased healthcare resource use. It is likely that this effect is pronounced in smaller patients. Previous studies have been criticized for not taking into consideration that prolonged bypass times are often due to higher complexity. The purpose of this study was to investigate the relationship between bypass index (bypass time/body surface area) and adverse postoperative events.

Single-centre, retrospective cohort study including 2413 patients undergoing cardiac surgery on cardiopulmonary bypass from June 2018 to April 2020. Length of hospital stay, as surrogate marker of postoperative morbidity, was selected as primary outcome. The strength of association between bypass index and the primary outcome was assessed with linear regression analysis. Secondary outcomes included new onset renal, pulmonary or cardiac rhythm dysfunction. The predictive value of bypass index was assessed with linear regression analysis; univariate and multiple regression were used to assess the strength of association between Bi and the secondary outcomes.

Bypass index was predictive for length of stay at univariate (Relative Risk (RR): 1.004, P < 0.001) and at multivariable (RR: 1.003, P < 0.001) analysis. The association between bypass index and new renal (mean difference: 14.1 min/m², P < 0.001) and cardiac rhythm dysfunction (mean difference: 12.6 min/m²) was significant. This was not true of postoperative lung dysfunction (mean difference: −1.5 min/m², P = 0.293).

Bypass index, calculated as total bypass time/patient body surface area, is predictive of postoperative morbidity and resource utilization after cardiac surgery on pump.