Effects of carboxyhemoglobin on pulse oximetry in humans

Carboxyhemoglobin (HbCO)-induced reading errors of the Biox 3700 (version J, Ohmeda) pulse oximeter were determined in 6 healthy volunteers rendered hypoxic (Sa(O)(2) from 65-100%) by breathing mixtures of air in nitrogen. The oximeter reading (Sp(O)(2)) before and after cigarette smoking was compared with oxyhemoglobin percentage (%HbO(2)). Mean HbCO levels were; 3.0 +/- 1.0 (SD) % before cigarette smoking and 5.2 +/- 1.7% after smoking, whereas mean methemoglobin was unchanged as 0.5 +/- 0.1%. The correlations of the Sp(O)(2) (y) with %HbO(2) (x) were; y = 1.01x - 0.30 (r = 0.990, n = 21, P < 0.001) when %HbCO was less than 2.5, and y = 1.01x + + 3.21 (r = 0.964, n = 33, P < 0.001) when %HbCO was above 5.0%. The reading error, (Sp(O)(2) - %HbO(2)), could be expressed as a function of %HbCO; 1.06 x %HbCO(2) - 2.49 (r = 0.669, n = 83, P < 0.05). Thus, the Sp(O)(2) is approximately the sum of %HbCO(2) and (%HbCO - 2.5), and overestimates %HbO(2) in the high levels of HbCO. The pulse oximeter should be used with caution in patients with the elevated level of %HbCO.