= Coronary oxygen extraction fraction
The \b[coronary oxygen extraction fraction] is the fraction of oxygen carried into a represented myocardial territory that is removed before its blood leaves. Under a matched steady-state inflow–outflow model,
$$
E=\frac{C_a-C_v}{C_a}=1-\frac{C_v}{C_a},\qquad
\dot V_{\mathrm O_2}=Q(C_a-C_v)=D_{\mathrm O_2}E.
$$
Here $C_a$ and $C_v$ are <arterial oxygen content> and <venous oxygen content>, $Q$ is volume flow, $D_{\mathrm O_2}$ is <coronary oxygen delivery>, and $\dot V_{\mathrm O_2}$ is <coronary oxygen uptake>. Extraction is dimensionless; uptake is an amount per time.
With chosen contents $C_a=0.20$ and $C_v=0.08\,\mathrm{mL\ O_2\,mL^{-1}}$, $E=0.60$. At flow $250\,\mathrm{mL\,min^{-1}}$, uptake is $30\,\mathrm{mL\ O_2\,min^{-1}}$. Maintaining that same uptake at flow $200$ and unchanged $C_a$ would require $E=0.75$. Higher extraction in this model can compensate for lower delivery; it is not by itself evidence of a higher total uptake.
The ideal algebraic limit $E\le1$ gives $Q\ge\dot V_{\mathrm O_2}/C_a$. For the example this is $150\,\mathrm{mL\,min^{-1}}$. That is only a conservation bound: it does not establish that complete extraction is physiologically attainable or provide a clinical flow threshold.
Content and saturation should not be confused. When hemoglobin-bound oxygen dominates and hemoglobin concentration is the same in the paired samples, a simplified relation is
$$
E\simeq\frac{S_a-S_v}{S_a}.
$$
It is a relative extraction fraction, not merely the saturation difference $S_a-S_v$. <Hemoglobin> binding and dissolved oxygen are distinct contributions to blood oxygen transport.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 22.5, Oxygen Transport]{ref}
Sampling must represent the same flow path. A <coronary sinus> sample represents its drained venous mixture, not automatically every myocardial region. The sampling site, timing and territory therefore matter. An independent volumetric-flow measurement, such as the primary thermodilution method using saline infusion and temperature sensing, supplies a different component of the uptake calculation.<Direct volumetric blood flow measurement in coronary arteries by thermodilution by Wilbert Aarnoudse and others>[Abstract, Methods and measured volumetric flow]{ref}
Calibration errors can affect ratios differently from products. A common multiplicative content factor cancels from $E$, but a common additive offset does not: adding $0.02$ to both example contents gives $E=0.12/0.22\simeq0.545$. Matching samples, measuring the intended quantity and specifying the common-error model are therefore part of the extraction estimate.
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