A physio-chemical mathematical model of the effects of O2 and CO2 diffusion between the blood and gas phases of venous blood sampled in vacuum tubes

Measurement of blood acid-base status is usually performed using standard blood gas syringes. This contrasts with venous blood for other purposes which are usually sampled in vacuum tubes which include a quantity of air. This study reports changes in blood sampled from normal participants over 90 min in standard blood gas syringes, and in vacuum tubes designed for blood gas sampling. It proposes a mathematical model based on physio-chemical processes to simulate changes occurring in the vacuum tubes due to contamination with air. Venous blood samples were taken from 20 normal participants, into standard blood gas syringes and analysed at baseline and 90 min; and 2 ml and 4 ml vacuum tubes analysed after 20 and 90 min. A mathematical model was proposed describing diffusion of O₂ and CO₂ between blood and gas phases of the vacuum tubes, and used to simulate corresponding measured values of plasma pH, pCO₂, pO₂, glucose, and lactate. Simulated and measured values were compared with Bland-Altman and correlation analysis. Differences between measured and model simulated values were (bias ± SD): pH -0.01 ± 0.01, pCO₂ 0.03 ± 0.17 kPa, pO₂ 0.88 ± 1.73 kPa, glucose -0.01 ± 0.11 mmol/l and lactate 0.06 ± 0.18 mmol/l. All linear correlations between measured and simulated values had R² > 0.90. This paper illustrates that the effects of sampling in vacuum tubes can be accurately simulated with a mathematical model of physio-chemistry. While further evaluation is necessary, this may indicate a role for this model in the use of vacuum tubes for routine acid-base and oxygenation analysis.