Changes in end-tidal carbon dioxide tension following sodium bicarbonate administration: correlation with cardiac output and haemoglobin concentration.

An intravenous administration of sodium bicarbonate (NaHCO3) forms excess CO2, resulting in an immediate increase in end-tidal carbon dioxide tension (PETCO2). We hypothesized that the time until PETCO2 reached a maximum, and the magnitude of the increase in PETCO2 are influenced by cardiac output and haemoglobin concentration, respectively. To test this hypothesis, we examined changes in PETCO2 following an intravenous administration of NaHCO3 at different levels of cardiac output and haemoglobin concentration. We administered 0.2 mmol.kg-1 of 8.4% NaHCO3 into the vena cava in 15 anesthetized dogs under mechanical ventilation of 20 breaths per min. Cardiac output was increased by dopamine infusion, and decreased by blood withdrawal under halothane anaesthesia. Haemoglobin concentrations were changed by haemodilution with hydroxyethyl starch. When control measurements were taken, time-max (the time until the increase in PETCO2 reached a maximum) was 4 +/- 0.2 breaths-time, and delta CO2-max (the magnitude of the increase in PETCO2) was 0.90 +/- 0.04 kPa (6.6 +/- 0.3 mmHg). Cardiac output was inversely correlated with time-max (r = 0.94, P < 0.0001), while it revealed a poor correlation with delta CO2-max. Haemoglobin concentration showed a significant correlation with delta CO2-max (r = 0.736, P < 0.005), but not with time-max. We concluded that the time course and the magnitude of changes in PETCO2 following intravenous administration of NaHCO3 reflect changes in cardiac output and haemoglobin concentration, respectively.