HYPOTENSION: A PHYSIOLOGIC DIFFERENTIAL DIAGNOSIS

A patient presents with hypotension after a syncope event. Clearly the issue was a temporary lack of perfusion to the brain related to the low pressure. But what caused the drop? The physiologic formula for blood pressure can help.

MAP = CO x SVR

(CO = SV x HR)

The heart rate (HR) is normal. You see no evidence of sepsis and doubt the systemic vascular resistance (SVR) has dropped. Vasovagal reactions temporarily drop the heart rate or SVR, but this does not fit that, with ongoing hypotension. Something caused the stroke volume (SV) to drop. The laboratory results show anemia, and the rectal examination is heme positive. This is an occult gastrointestinal hemorrhage.

Take home points:

MAP = SV x HR x SVR

A drop in blood pressure suggests a drop in stroke volume, heart rate, or systemic vascular resistance

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