Whether multiarterial grafting (MAG) improves long-term survival compared with single arterial grafting (SAG) during coronary artery bypass grafting (CABG) has remained uncertain because randomized trials and observational studies have reported conflicting findings.
A retrospective Medicare cohort study published in Journal of the American College of Cardiology evaluated whether the apparent survival advantage associated with MAG persisted after accounting for unmeasured confounding using an instrumental variable approach based on surgeon grafting preference.
The analysis included 1,291,314 Medicare beneficiaries who underwent CABG between 2001 and 2019, including 145,554 patients treated with MAG and 1,145,760 treated with SAG.

Findings

• In the conventional non-instrumental variable model, MAG was associated with longer risk-adjusted median survival compared with SAG: 10.74 years (95% CI 10.70-10.79) versus 10.33 years (95% CI 10.31-10.35), a difference of 0.41 years.
• Across 4,164 surgeons, the prior 12-month MAG rate was 7.7% ± 9.5% among SAG recipients and 32.9% ± 25.8% among MAG recipients.
• In the instrumental variable model, median survival was similar between groups: 10.38 years (95% CI 10.29-10.48) for MAG and 10.38 years (95% CI 10.35-10.40) for SAG, a difference of 0.01 years.

The attenuation of survival differences after instrumental variable adjustment suggested that unmeasured confounding may influence observational estimates of MAG benefit.
The findings suggest that the apparent survival advantage associated with MAG in conventional observational analyses may not persist after adjustment for treatment-selection bias.