Comparison and Correlation Between Arterial and Capillary Blood Gas Values in Mechanically Ventilated Adults

Abstract

BACKGROUND The essence of critical care medicine is the application of physiological principles to the care of seriously ill patients. Arterial puncture apart from being painful and technically difficult carries the risk of complications like local haematoma, infection, thrombosis, embolisation or ischaemia of the digits. The purpose of this study was to investigate the correlation between simultaneous arterial blood gas and arterialised earlobe blood samples. MATERIALS AND METHODS This was a descriptive study. Blood sample was collected simultaneously from left radial artery and ear lobe. The ear lobe was arterialised using hot mops for 3 minutes, a bold puncture was made with a lancet and blood was collected in pre-heparinised capillary tubes anaerobically without squeezing the ear lobe. Arterial sample was collected from the left radial artery in preheparinised 2 mL syringes by 22-G needle after modified Allen's test. All samples were collected from the patients after 12 hours of initiation of mechanical ventilation and the samples were analysed using a point-of-care blood gas analyser. RESULTS A total of 82 paired samples were analysed. The baseline demographics were comparable. On Bland-Altman's analysis of pH the mean difference was 0.01, while Limits of Agreement (LOA) were between +0.07 to-0.05 units and all dates were between two standard deviations. On Bland-Altman analysis for PaO2 the mean difference was 18.6 mmHg, while the limits of agreement (LOA) were between 14.2 mmHg to 23.0 mmHg and all the data were with two standard deviations. On Bland-Altman analysis for pCO2 and bicarbonate, we found scattered beyond two standard deviations. Regression analysis was done to find the convection formula for capillary to arterial. PaO2, it was 0.9 (pCO2) + 32.6 mmHg. CONCLUSION Capillary blood gas analysis can be used to estimate the pH and PaO2, but may not be of value for calculating pCO2 or bicarbonate.