Prediction of arterial blood gas values from earlobe blood gas values in patients receiving mechanical ventilation

Background

Arterial blood gas (ABG) sampling represents the gold standard method for acquiring patients' acid-base status. The most common complications associated with arterial puncture are pain, arterial injury, and thrombosis with distal ischemia, hemorrhage, and aneurism formation. Earlobe blood gas samplings (EBG) may be useful alternatives to ABG sampling. This study evaluates whether the pH, partial pressure of oxygen (PO₂), partial pressure of carbon dioxide (PCO₂), base excess (BE), and bicarbonate (HCO₃^-) values of EBG accurately predict their ABG analogs for patients treated by mechanical ventilation in an ICU.

Methods

Sixty-seven patients who were admitted to the ICU and treated by mechanical ventilation were included in this descriptive study. Blood for ABG analysis was sampled from the radial or brachial arteries. Blood for EBG analysis was sampled simultaneously from ear lobe by contact with a capillary tube tip. Regression equations and mean percentage-difference equations were derived to predict arterial pH, PCO₂, PO₂, BE, and HCO₃^- values from their EBG analogs.

Results

A total of 67 simultaneous arterial and earlobe blood samples were obtained from 67 patients. The pH, PCO₂, BE, and HCO₃^- ere all significantly correlated in ABG and EBG. In spite of a highly significant correlation, the limits of agreement between the two methods were wide for PO₂. Earlobe values of PO₂ were usually lower than arterial values, with larger differences in the range of normal arterial PO₂. On the other hand, the error and the limits of agreement were smaller for PCO₂. Regression equations for prediction of pH, PCO₂, BE, and HCO₃^- values were: arterial pH (pHa) = 1.81 + 0/76 × earlobe pH (pHe) [r = 0.791, P < 0.001]; PaCO₂ = 11/44 + 0/7 × earlobe PCO₂ (PeCO₂) [r = 0.774, P < 0.001]; arterial BE (BEa) = 1/14 + 0/95 × earlobe BE (BEe) [r = 0.894, P< 0.001], and arterial HCO₃ (HCO₃^- (HCO₃^-a) = 1/41 + earlobe HCO₃^- (HCO₃^-e) [r = 0.874, P < 0.001]. The predicted ABG values from the mean percentage-difference equations were derived as follows: pHa = pHe × 1.001; PaCO₂ = PeCO₂ × 1.04; BEa = BEe × 0.57; and HCO₃^-a = HCO₃^-e ×1.06.

Conclusion

Earlobe blood gas can accurately predict the ABG values of pH, PCO_2' BE, and HCO₃ ^- for patients receiving mechanical ventilation.