THAM administration reduces pulmonary carbon dioxide elimination in hypercapnia - an experimental porcine study.

BACKGROUND

In a previous study, we found a rebound of arterial carbon dioxide tension (PaCO ) after stopping THAM buffer administration. We hypothesized that this was due to reduced pulmonary CO elimination during THAM administration. The aim of this study was to investigate this hypothesis in an experimental porcine hypercapnic model.

METHODS

In seven, initially normoventilated, anesthetized pigs (22-27 kg) minute ventilation was reduced by 66% for 7 h. Two hours after commencing hypoventilation, THAM was infused IV for 3 h in a dose targeting a pH of 7.35 followed by a 2 h observation period. Acid-base status, blood-gas content and exhaled CO were measured.

RESULTS

THAM raised pH (7.07 ± 0.04 to 7.41 ± 0.04, P < 0.05) and lowered PaCO (15.2 ± 1.4 to 12.2 ± 1.1 kPa, P < 0.05). After the infusion, pH decreased and PaCO increased again. At the end of the observation period, pH and PaCO were 7.24 ± 0.03 and 16.6 ± 1.2 kPa, respectively (P < 0.05). Pulmonary CO excretion decreased from 109 ± 12 to 74 ± 12 ml/min (P < 0.05) during the THAM infusion but returned at the end of the observation period to 111 ± 15 ml/min (P < 0.05). The estimated reduction of pulmonary CO2 elimination during the infusion was 5800 ml.

CONCLUSIONS

In this respiratory acidosis model, THAM reduced PaCO , but seemed not to increase the total CO elimination due to decreased pulmonary CO excretion suggesting only cautious use of THAM in hypercapnic acidosis.