 Hypoxemia

Hypoxemia, also spelled hypoxaemia, is reduced oxygenation of arterial blood. Its characterization must identify what was measured: arterial oxygen partial pressure, oxygen saturation and arterial oxygen content are related quantities with different meanings. Hypoxia concerns inadequate oxygen availability to tissue, rather than simply naming an arterial measurement.

An arterial blood gas analysis measures the pressure of dissolved oxygen. Pulse oximetry instead estimates oxygen saturation optically; its output is not a direct partial-pressure measurement.^[ref] This distinction explains why the observation and instrument should accompany a report of reduced oxygenation.

A counting example separates saturation from oxygen amount. Suppose 100 available hemoglobin binding sites have 90 oxygen molecules bound: the occupied fraction is $90/100=0.90$. Doubling the number of sites to 200 while keeping that fraction gives 180 bound molecules. The saturation is unchanged although the amount bound has doubled. This is a constructed binding-site count, not a conversion between oxygen saturation and partial pressure.

Likewise, oxygen carried per unit blood and blood delivered per unit time enter different factors. If $C_a$ is arterial oxygen content and $Q$ is blood flow, then the transported amount per unit time is $Q{C}_a$. Stipulated values $C_a=0.20\mathrm{L}{\mathrm{L}}^{-1}$ and $Q=5\mathrm{L}\mathrm{mi}{\mathrm{n}}^{-1}$ give $1\mathrm{L}\mathrm{mi}{\mathrm{n}}^{-1}$; halving $Q$ halves that transport without changing $C_a$. The example shows why one arterial oxygen measurement cannot by itself describe oxygen delivery to every tissue. It is not a diagnostic reference range or a treatment rule.