When the MAP drops and you reach for phenylephrine, the display tells you whether it worked. It does not tell you what you did to flow.
So make it a two-number check. If the MAP is low and the pulse pressure is wide, you are looking at vasodilatation and a vasoconstrictor is the right answer. If the MAP is low and the pulse pressure is already narrow, the stroke volume is small, and squeezing a small stroke volume harder raises the number on the screen without moving any more blood.
The second pattern is the one that hides. The MAP comes back up, the anaesthetic chart looks clean, and the kidney spends the case at a stroke volume index around half of normal. Ask instead whether the patient is underfilled or the heart is failing to eject — and if you have a cardiac output monitor available for that patient, this is the case to use it on.
- Low MAP plus wide pulse pressure suggests vasodilatation — vasoconstrictor is reasonable
- Low MAP plus narrow pulse pressure suggests low stroke volume — think volume or inotrope
- A MAP restored by vasopressor alone with a narrowing pulse pressure is not a success
- Say the pulse pressure out loud during handover, not just the MAP range
Why it matters
The number an anaesthetist is trained to restore can be restored in a way that makes perfusion worse.
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