= Hypoxemia
= Hypoxaemia
{synonym}
\b[Hypoxemia], also spelled \b[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.<Arterial Blood Gas (ABG) Test by MedlinePlus>[What is an arterial blood gas test?; Is there anything else I need to know?: measured partial pressure versus estimated saturation]{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 $C_a=0.20\,\mathrm{L\,L^{-1}}$ and $Q=5\,\mathrm{L\,min^{-1}}$ give $1\,\mathrm{L\,min^{-1}}$; 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.
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