Blood gas estimations from arterialized capillary blood versus arterial puncture: are they different?

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Abstract
Blood gas estimations from arterialized capillary blood versus arterial puncture: are they different? The answer to this question is that the oxygen tension (PO2) of arte- rial blood must be higher than the PO2 of so-called arte- rialized blood flowing freely from the ear lobe after it has been pierced by a scalpel. This is because there is a gradient of PO2 from around 13 kPa (98 mmHg) at the arterial end of the capillary bed to 5 kPa (38 mmHg) at the venous end. Fluid collected from the cut ear lobe is a mixture of blood from capillaries and venules. Never- theless, it is well-known that under certain circumstances, the differences are so small that the arterial and arteri- alized estimations are, for practical purposes, identical. How is this possible? As already mentioned, the normal arteriovenous dif- ference for PO2 at rest is 8 kPa (60 mmHg), increasing to 10 kPa (75 mmHg) on light exercise and to 70.7 kPa (530 mmHg) at least when breathing 100% oxygen. This difference can be reduced by increasing ear lobe blood flow relative to oxygen consumption by vasodilatation, either by heat or by application of a vasoactive cream. No one knows the magnitude of the changes induced in the human ear lobe by such manoeuvres, but it would be interesting to find out! For example, increasing the ratio of blood flow to oxygen consumption fivefold would reduce the arteriovenous oxygen content differ- ence from 5 mL per 100 mL to 1 mL per 100 mL, and the arteriovenous PO2 difference to 4.0 kPa (30 mmHg), assuming a normal PO2 of 13 kPa (98 mmHg). Provided sufficient vasodilatation could be achieved, arterial and venous PO2 in the ear lobe would tend to converge, and the arterialized PO2 would come to resemble the arte- rial PO2.

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