Why the earlobe?

The ‘gold standard’ technique for blood gas analysis is measuring gases directly from an artery, most often the radial artery in the wrist. Arterial blood gas sampling has potential risks, and is considered a painful procedure. Specialist training in arterial blood gas sampling is essential for patient safety and comfort.

Capillary blood sampling can be obtained by a less painful, less invasive procedure, which uses an automated lancet skin puncture at the earlobe. The clinical value of capillary blood gas results depends on how capillary blood accurately reflects pH, PaCO2 and PaO2 of arterial blood.

Capillaries are the smallest blood vessel, arterioles and venules, ultimately connecting the smallest artery to the smallest vein. Subsequently, there is a pH, PaCO2 and PaO2 difference, or gradient, across the capillary network.

Interestingly, during an ELCBG procedure, the puncture of the capillaries also punctures the arterioles and venules. Owing to a higher blood pressure on the arterial side of the circulation, a ‘capillary’ blood sample in fact approximates closer to arterial blood than venous blood.

Furthermore, warming the sample site or introducing vasodilating creams prior to capillary blood sampling increases the blood flow to the earlobe. This method of ‘arterialising’ the sample site has the effect of reducing the difference between the arterial and venous samples, ensuring the properties of the capillary blood sample are as close matched to arterial blood as possible.

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