Cardiac Output (Fick) Calculator

Named for the German physiologist Adolph Fick, who first described it in 1870, the Fick principle calculates cardiac output from whole-body oxygen consumption divided by how much oxygen each deciliter of blood picks up crossing the lungs -- the arteriovenous oxygen content difference. Arterial and venous oxygen content are each computed from the standard formula CxO2 = (Hb x 1.34 x SxO2/100) + (0.003 x PxO2), where 1.34 mL O2/g is the amount of oxygen a gram of fully saturated hemoglobin carries and 0.003 mL/dL/mmHg is the small amount dissolved directly in plasma. Cardiac output then equals VO2 divided by the arteriovenous difference (scaled by 10 to convert mL/dL to mL/L), so for a fixed oxygen consumption, a WIDER arteriovenous difference means a LOWER calculated cardiac output -- the tissues are extracting more oxygen from each unit of blood the heart delivers, so the heart needs to pump less blood to deliver the same VO2 -- near this calculator's own default inputs, raising arterial saturation therefore lowers the calculated output rather than raising it. Near those same defaults, arterial saturation is the most sensitive input: it enters both the arterial content term directly and shifts how large the A-V difference is relative to venous content.

That local sensitivity does not hold across this calculator's full declared ranges, though -- hemoglobin alone can swing cardiac output by an order of magnitude more than saturation once it is taken down toward its low end, since a near-zero hemoglobin collapses both oxygen content terms toward the same small dissolved-oxygen residual and drives the A-V difference, and therefore the denominator, toward zero. Because dividing by an arteriovenous difference that shrinks toward zero makes cardiac output blow up toward infinity, this calculator has no upper sanity cap -- a narrow-but-positive A-V difference (from, say, an SaO2 only slightly above SvO2) can produce a wildly unrealistic double-digit or triple-digit "cardiac output" that no human heart could actually generate, so any result well outside the roughly 4-8 L/min normal resting range deserves a hard look at whether the underlying saturations and hemoglobin are clinically plausible together before trusting the number. A real patient's SaO2 should always exceed their SvO2, since blood loses oxygen crossing the tissues rather than gaining it; entering values that invert that relationship drives the A-V difference to zero or negative, and this calculator returns 0 L/min rather than a physiologically meaningless negative cardiac output -- a 0 result is a signal of inconsistent inputs, not a real hemodynamic finding. Body surface area has no effect on cardiac output itself -- it only rescales output into cardiac index (CO/BSA), the size-adjusted figure most useful for comparing patients of different body sizes.