In one line: oxygen the body uses per minute, divided by the oxygen each liter of blood gives up on one pass, equals liters of blood per minute.
In one line: oxygen the body uses per minute, divided by (how much O₂ a liter of blood can carry × the fraction of it unloaded on one pass), equals liters of blood per minute.
CO
Cardiac output. Liters of blood pumped per minute, which is what the equation solves for. Without a shunt this is both pulmonary and systemic flow.
VO₂
Oxygen consumption. The body's metabolic demand for O₂ per minute. At steady state it equals what the lungs take up. It is usually assumed (about 125 mL of O₂/min per m² of body surface area) rather than measured, which makes it the largest source of error. If true demand is higher than assumed (fever, sepsis, agitation, work of breathing), the calculated CO is too low.
CaO₂
Arterial O₂ content. Oxygen carried in each 100 mL of blood leaving the lungs: 1.34 × Hb × SaO₂. Normal is about 20 mL of O₂/dL of blood.
CvO₂
Mixed venous O₂ content. Oxygen left in each 100 mL of blood returning to the lungs after the tissues have taken their share: 1.34 × Hb × SvO₂. Normal is about 15 mL of O₂/dL of blood.
CaO₂ − CvO₂
A–V O₂ difference. Oxygen each 100 mL of blood gives up in the tissues, and picks back up in the lungs. Normal is about 4–5 mL of O₂/dL of blood. When flow falls, tissues take more from each unit of blood to meet the same demand, so the gap widens. That inverse relationship is the equation.
× 10
Unit conversion. O₂ per volume of blood is calculated per dL of blood. There are 10 dL in a liter, so × 10 dL/L gives mL of O₂ per L of blood. Then (mL of O₂/min) ÷ (mL of O₂/L of blood): mL of O₂ cancels and L of blood/min is left.
1.34
O₂ per gram of hemoglobin. mL of O₂ bound per g of Hb when fully saturated (Hüfner's constant; values of 1.34–1.39 are quoted).
Hb
Hemoglobin, the carrier (g of Hb/dL of blood). Less Hb means less O₂ per liter. To meet the same VO₂, more liters must flow and the saturation gap narrows. A low SvO₂ in anemia can reflect low carrying capacity rather than low flow.
SaO₂
Arterial saturation. Fraction of Hb binding sites loaded as blood leaves the lungs. Use it as a fraction (95% = 0.95).
SvO₂
Mixed venous saturation. Fraction still loaded in PA blood after the whole body has extracted what it needs. Normal is 65–75%.
SaO₂ − SvO₂
Saturation gap. How much of the Hb's loading is given up in one pass. Normal is about 25 points (98% → 73%). This is what the cath lab measures directly. It sits in the denominator and is small, so errors count: a 3-point error in SvO₂ moves CO by about 10%.
[ denominator ]
O₂ unloaded per liter. 1.34 × Hb × (SaO₂ − SvO₂) × 10, in mL of O₂/L of blood. The units cancel as (mL of O₂/g of Hb) × (g of Hb/dL of blood) × (dL/L). Divide VO₂ (mL of O₂/min) by this: mL of O₂ cancels and L of blood/min is left.
0.0031 × PO₂
Dissolved O₂ (optional). O₂ dissolved in plasma rather than bound to Hb: 0.0031 mL of O₂/dL of blood per mmHg. At a PaO₂ of 100 it is only about 0.3 mL of O₂/dL of blood, so it is usually ignored. It matters on high FiO₂, when PaO₂ is several hundred mmHg. In the saturation form it is added as 0.0031 × (PaO₂ − PvO₂) inside the bracket, before the × 10.