Prediction of arterial blood gas values from venous blood gas values in patients with acute respiratory failure receiving mechanical ventilation

Background and Purpose: Arterial blood gas (ABG) analysis is useful in evaluation of the clinical condition of critically ill patients; however, arterial puncture or insertion of an arterial catheter may cause many complications. This study evaluated whether pH, partial pressure of carbon dioxide (PCO₂) and bicarbonate (HCO₃^-) values of venous blood gas (VBG) could accurately predict their ABG analogs for patients with acute respiratory failure treated by mechanical ventilation in an intensive care unit (ICU). Methods: Forty six patients who were admitted to the ICU due to acute respiratory failure and treated by mechanical ventilation were included in this study. Blood for VBG analysis was sampled from the cubital or dorsal palmar veins, while ABG was sampled simultaneously from the radial or brachial arteries via an arterial catheter at the other upper extremity. Regression equations and mean percentage-difference equations were derived to predict arterial pH, PCO₂, and HCO₃^- values from their VBG analogs. The equations were validated by evaluationg VBG and ABG samples from a separative group of 11 patients. Results: A total of 46 paired samples from 46 patients were evaluated. The mean percentage differences between the venous and arterial values divided by venous values for pH, PCO₂, and HCO₃^- were (mean ± SD : ΔpH (1%), 0.50 ± 0.45; ΔPCO₂ (%), 17.09 ± 9.60; and ΔHCO₃^- (%), 9.72 ± 7.73; respectively. Regression equations for prediction of pH, PCO₂ and HCO₃^- values were: arterial pH (pH_a) = 0.45 + 0.94 × venous pH (pH_v) [r = 0.83, p < 0.0001]; partial pressure of arterial CO₂ (P_aCO₂) = 3.06 + 0.76 × partial pressure of venous CO₂ (P_vCO₂) [r = 0.86, p < 0.0001]; and arterial HCO₃^- (HCO_3a^- 2.34 + 0.82 × venous HCO₃^- (HCO_3v^-) [r = 0.91, p < 0.0001]. The predicted ABG values from the mean percentage-difference equations were derived as follows: pH_a = pH_v × 1.005; P_aCO₂ = P_vCO₂ × 0.83; and HCO_3a^- = HCO_3a^- × 0.90. Validation of the regression equations and mean percentage-difference equations revealed only a small (clinically insignificant) variation between the actual and predicted ABG values. Conclusions: Venous blood gas can accurately predict the ABG values of pH, PCO₂ and HCO₃^- for patients with acute respiratory failure being treated with mechanical ventilation.