Arterial blood gas analysis

Arterial blood gas analysis applies blood gas analysis to a sample taken from an artery, providing a time-specific observation of systemic arterial gas tensions and acid-base state. Arterial oxygen partial pressure concerns oxygen tension, while arterial carbon-dioxide partial pressure and pH provide different information. The arterial qualifier is a sampling condition, not merely a label attached to any blood result.^[ref]

The measurement chain includes obtaining an identified sample, preserving its gas composition and analyzing it with a checked instrument. Air contamination, fluid contamination and analysis delay can change the specimen after it leaves the circulation. A reproducible analyzer reading cannot repair a specimen that no longer represents the intended physiological state.^[ref]

Equal oxygen tension need not mean equal arterial oxygen content. An original illustrative carrier model writeswhere $H$ is hemoglobin concentration, $s$ its oxygen-bound fraction and $h$ the carrier capacity per gram. Choose $h=1.34$ mL/g, $\alpha =0.003$ mL/(dL·mmHg), $s=0.98$ and $P_{\mathrm{a}{O}_2}=100$ mmHg. Samples with $H=15$ and $H=7.5$ g/dL then contain about 20.00 and 10.15 mL O${}_2$/dL, respectively. They have identical stipulated tension and saturation but very different oxygen content. The constants define the illustration; these are not patient results or diagnostic thresholds.

Interpretation also depends on the conditions when the sample was obtained. Inspired oxygen fraction, ventilator settings when applicable and sampling time help distinguish a changed respiratory state from a changed measurement condition. A later pulse oximetry reading is neither a simultaneous sample nor a measurement of carbon-dioxide tension.

Arterial blood gas analysis supplies evidence about gas transport and acid-base balance, but no individual number alone identifies the entire cause of an abnormal result. Measured co-oximetry and calculated oxygen saturation should also be distinguished when unusual hemoglobin species matter.^[ref]