Advanced practice note
Oxygen delivery is not a number. It is a chain of assumptions.
A physiology-first note on using cardiac output, haemoglobin, arterial oxygen content, and tissue demand as a connected clinical model.
Oxygen delivery is often reduced to a formula, but the formula is only useful when its assumptions are visible. Cardiac output, arterial oxygen content, haemoglobin concentration, and saturation are linked variables, not independent targets. A reasonable-looking saturation can coexist with a delivery problem when flow or haemoglobin is inadequate.
The perioperative task is to decide which link in the chain is limiting and whether the clinical context makes that limitation meaningful. A falling mixed venous saturation, a widening lactate trend, changing urine output, peripheral temperature, capillary refill, and the surgical field may all contribute more than a single arterial blood gas value.
This is why a response to hypotension should not begin and end with a blood pressure. The more useful frame is perfusion: what is the likely mechanism, what has changed in the delivery-demand balance, and which intervention most directly tests that hypothesis?