Lower oxygen delivery on bypass associated with higher 30-day mortality

Design

Retrospective single-centre cohort

Population

4,358 adults having cardiac surgery on cardiopulmonary bypass, 2018 to 2024

Primary outcome

30-day mortality

Effect

Adjusted OR 1.16 per 10 mL/min/m² fall in DO2i (95% CI 1.09 to 1.23)

This retrospective cohort at one tertiary centre included 4,358 adults having cardiac surgery on cardiopulmonary bypass between 2018 and 2024. Indexed oxygen delivery was calculated as a time-weighted average from pump flow, arterial saturation, haemoglobin and body surface area.

Thirty-day mortality was 1.7%. Each 10 mL/min/m² fall in oxygen delivery was associated with 16% higher odds of death (adjusted OR 1.16, 95% CI 1.09 to 1.23). Adjusted mortality was 1.69% at the 25th centile of oxygen delivery against 0.74% at the 75th. Lower oxygen delivery was also associated with acute kidney injury and longer stays, but not with stroke.

The design cannot separate oxygen delivery from the reasons it was low, such as anaemia, sicker patients or longer, more complex operations. The authors call for interventional trials.

In practice, oxygen delivery is a single number that combines pump flow and haemoglobin, and it is worth tracking during bypass alongside the usual targets.

• Track indexed oxygen delivery during bypass, combining pump flow and haemoglobin.
• Discuss with the perfusionist how pump flow can compensate when haemoglobin is low.
• Recognise low oxygen delivery as a marker of higher risk of kidney injury and death.
• Do not transfuse to a delivery target on this evidence alone; trials are needed.

Why it matters

It gives the bypass team one integrated number to watch, rather than flow and haemoglobin separately.

Don't overread it

This is an observational association; it does not show that raising oxygen delivery lowers mortality.

The statistics, in plain English

An adjusted odds ratio of 1.16 per 10-unit fall means risk rises steadily as delivery falls. With only 75 deaths, the estimates are reasonably but not very precise, and residual confounding is likely.

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