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The Pco2, Po2 and pH of arterial blood and arterialized capillary blood from the ear lobe were compared in 13 patients at rest and during submaximal exercise. The mean capillary Pco2 was .6 mmHg lower than arterial Pco2 at rest and 1 mmHg lower in exercise. The variation in comparisons (S.D. 1.4 mmHg) at rest and exercise, was greater than between duplicate capillary samples (S.D. .7 mmHg). The mean capillary Po2 was 3.3 mmHg higher than arterial at rest, and 1.7 mmHg higher in exercise: the variation was also greater than for Pco2 (S.D. 5.2 mmHg at rest, 4.2 mmHg in exercise) and greater than in duplicate samples (S.D. 2.6 mmHg). A comparison was made of calculated cardiac output (indirect Fick method) and dead space: tidal volume ratio during exercise, using arterial Pco2 and capillary Pco2; cardiac output was systematically overestimated by 6% (S.D. 9.3%) and Vd/Vt ratio underestimated by 10% (S.D. 9.3%) when capillary PCo2 was used in the calculations. Capillary blood samples are an acceptable alternative to arterial blood, for measurement of blood gases and pH, as long as blood flow is brisk enough to fill a large (125 μl) glass capillary tube within 30 seconds. The accuracy of the estimates is greatest for pH, and least for Po2. The PCO2 and Po2 measurements are sufficiently precise for their use in the calculation of cardiac output, Vd/Vt ratio, alveolar-arterial Po2 difference and venous admixture effect during exercise.
McEvoy et al. (Wed,) studied this question.