Transcutaneous Pco2 and Po2: A multicenter study of accuracy

A multicentcr study used 756 samples from 251 patients in 12 institutions to compare arterial (PaO₂, PaCO₂) with transcutaneous (PsO₂, PsCO₂) oxygen and carbon dioxide tensions, measured usually at 44°C. Of these samples, 336 were obtained from 116 neonates, 27 from 25 children with cystic fibrosis, and 140 from 40 patients under general anesthesia. Ninety-one patients were between 4 weeks and 18 years of age, 32 were between 18 and 60 years, and 12 were over 60. The ratio of transcutaneous to arterial P(s/a)CO₂ was 1.01 ±0.11 with PaCO₂ less than 30 mm Hg, increasing to 1.04 ±0.08 at PaCO₂ greater than 40 mm Hg. Mean bias and its standard deviation (PsCO₂ — PaCO₂) were + 1.3 ± 3.9 mm Hg in the entire group, + 1.8 ± 4.2 mm Hg in neonates (NS). Bias was +0.2 ± 2.7 mm Hg when PaCO₂ was less than 30 mm Hg (N = 175, NS), 1.0 ± 3.4 with 30 < PaCO₂ < 40 (n = 329,p < 0.001), and +2.04 ± 4.00 mm Hg with 40 < PaCO₂ < 70 (n = 229,p < 0.001). These data suggest that, using transcutaneous PCO₂ monitors with inbuilt temperature correction of 4.5%/‡C, the skin metabolic offset should be set to 6 mm Hg. The linear regression was PsCO₂ =1.052(PaCO₂)-0.56, Sy·x = 3.92, R = 0.929 (n = 756); and PsCO₂ = 1.09(PaCO₂)-1.57, Sy·x = 4.17, R = 0.928 in neonates (n = 336). The use of vasopressors and vasodilators had no significant effect on bias or its standard deviation or on regression slope and intercept (n = 78). In cystic fibrosis patients, bias and standard deviation were 0.0 ± 1.7 mm Hg (n = 27). Under anesthesia, PsCO₂ = 1.07PaCO₂-1.58, with bias and standard deviation = 0.6 ± 3.5 (n = 140). For oxygen, at PaO₂ ≤ 80 the ratio P(s/a)O₂ = 1.05 ± 0.16 in nconates and 0.93 ± 0.21 in older patients, but when PaO₂ > 80, P(s/a)O₂ fell to 0.88 ± 0.18 in neonates and 0.74 ± 0.21 in older patients. The errors were significantly greater (p < 0.001) in older patients than in neonates above but not below 80 mm Hg, and within both groups errors were significantly greater above than below 80 mm Hg.