Arterialized Earlobe Samples for Blood Gas Monitoring
ABSTRACT & COMMENTARY
Synopsis: In 115 patients, comparison of blood obtained from a radial artery puncture and arterialized blood from an earlobe indicated close agreement for PCO2 but not for PO2.
Source: Sauty A, et al. Eur Respir J 1996;9:186-189
Sauty et al analyzed arterial and arterialized blood to determine the extent of agreement between PO2 and PCO2. Subjects were 115 consecutive patients seen in their pulmonary function lab for various conditions. Arterialization was accomplished by applying a vasoactive cream (Hymenoptera Venom Cream) to the earlobe for 5-10 minutes. A trained technician then used a scalpel blade to incise the earlobe at its most dependent part and collected the sample in a heparinized glass capillary tube. Arterial samples were obtained via puncture of the radial artery. Both samples were analyzed in the same blood gas analyzer within two minutes of each other.
Values for arterial samples ranged from 39.0 mm Hg to 98.3 mm Hg for PaO2 and from 25.7 to 54.6 mm Hg for PaCO2. When comparisons were made between arterial and arterialized values, the correlation coefficients were r = 0.928 (P < 0.0001) for PO2 and r = 0.957 (P < 0.0001) for PCO2. Despite the highly significant correlation, regression lines were slightly different from lines of identify, particularly for PO2.
Differences between the two methods (arterial vs, arterialized) and 95% confidence intervals for the lower and upper limits of agreement were also reported. Arterialized earlobe PO2 was lower than arterial PO2 in most cases, and the difference increased as arterial PO2 increased. For PCO2, however, the mean difference was close to zero (mean, 0.5 ± 1.5; rang, -5.1 to 5.9) and 95% CI showed little difference (mean plus, 2 SD = 2.9-3.9; mean minus, 2 SD = -1.9 to -2.8)
COMMENT BY LESLIE A. HOFFMAN, RN, PhD
Fluid from the cut earlobe is a mixture of blood from capillaries and venules. When vasodilation is used to increase earlobe blood flow relative to O2 consumption, normal arteriovenous (a-v) oxygen content differences are reduced. If sufficient vasodilation is achieved, arterial and venous PO2 and PCO2 values tend to converge and the arterialized values resemble the arterial values.
In COPD patients, sampling arterialized earlobe blood has been advocated as a simple and reliable method for PO2 and PCO2 determination (Thorax 1994; 49:364). Experience of the authors, who used the arterialized technique as standard practice for several years, did not support this conclusion and the current study was conducted to identify reasons for the discrepancy. Their findings indicated that PO2 was usually lower in the earlobe than in arterial blood, and the limits of agreement between the two methods were fairly wide.
Two possible explanations for these differences were given: a larger sample size in the current study and more subjects with an arterial PaO2 in the normal range. The main cause of underestimation of arterial PO2 in earlobe samples is insufficient arterialization of blood, corresponding to a certain venous admixture. The effect of a given venous admixture in earlobe blood depends on the (a-v)O2 difference (i.e., the larger the (a-v)O2 difference, the wider the discrepancy between earlobe and arterial PO2). Because the (a-v)O2 difference is comparatively large in subjects with normal PaO2, a small venous admixture in earlobe blood will result in a greater discrepancy between the two values. In support of this assertion, PO2, when in the normal range, was often markedly underestimated by the earlobe sample. Conversely, differences (arterial vs arterialized) in PCO2 values were close to zero, suggesting that arterialized blood obtained from an earlobe could serve as a substitute for arterial blood in patients not currently instrumented with an arterial line.
While obviously not a "gold standard," this method might be appropriate for monitoring patients with COPD during an exacerbation or patients weaning from mechanical ventilation who no longer have an arterial line if there are questions about ventilatory status.