Intensive buffering can keep pH above 7.2 for over 4 h during apnea: an experimental porcine study

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 (PaCO₂ ) after stopping THAM buffer administration. We hypothesized that this was due to reduced pulmonary CO₂ 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 CO₂ were measured.

RESULTS

THAM raised pH (7.07 ± 0.04 to 7.41 ± 0.04, P < 0.05) and lowered PaCO₂ (15.2 ± 1.4 to 12.2 ± 1.1 kPa, P < 0.05). After the infusion, pH decreased and PaCO₂ increased again. At the end of the observation period, pH and PaCO₂ were 7.24 ± 0.03 and 16.6 ± 1.2 kPa, respectively (P < 0.05). Pulmonary CO₂ 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 PaCO₂ , but seemed not to increase the total CO₂ elimination due to decreased pulmonary CO₂ excretion_, suggesting only cautious use of THAM in hypercapnic acidosis.

THAM reduces CO2-associated increase in pulmonary vascular resistance - an experimental study in lung-injured piglets

INTRODUCTION

Low tidal volume (VT) ventilation is recommended in patients with acute respiratory distress syndrome (ARDS). This may increase arterial carbon dioxide tension (PaCO2), decrease pH, and augment pulmonary vascular resistance (PVR). We hypothesized that Tris(hydroxymethyl)aminomethane (THAM), a pure proton acceptor, would dampen these effects, preventing the increase in PVR.

METHODS

A one-hit injury ARDS model was established by repeated lung lavages in 18 piglets. After ventilation with VT of 6 ml/kg to maintain normocapnia, VT was reduced to 3 ml/kg to induce hypercapnia. Six animals received THAM for 1 h, six for 3 h, and six serving as controls received no THAM. In all, the experiment continued for 6 h. The THAM dosage was calculated to normalize pH and exhibit a lasting effect. Gas exchange, pulmonary, and systemic hemodynamics were tracked. Inflammatory markers were obtained at the end of the experiment.

RESULTS

In the controls, the decrease in VT from 6 to 3 ml/kg increased PaCO2 from 6.0±0.5 to 13.8±1.5 kPa and lowered pH from 7.40±0.01 to 7.12±0.06, whereas base excess (BE) remained stable at 2.7±2.3 mEq/L to 3.4±3.2 mEq/L. In the THAM groups, PaCO2 decreased and pH increased above 7.4 during the infusions. After discontinuing the infusions, PaCO2 increased above the corresponding level of the controls (15.2±1.7 kPa and 22.6±3.3 kPa for 1-h and 3-h THAM infusions, respectively). Despite a marked increase in BE (13.8±3.5 and 31.2±2.2 for 1-h and 3-h THAM infusions, respectively), pH became similar to the corresponding levels of the controls. PVR was lower in the THAM groups (at 6 h, 329±77 dyn∙s/m(5) and 255±43 dyn∙s/m(5) in the 1-h and 3-h groups, respectively, compared with 450±141 dyn∙s/m(5) in the controls), as were pulmonary arterial pressures.

CONCLUSIONS

The pH in the THAM groups was similar to pH in the controls at 6 h, despite a marked increase in BE. This was due to an increase in PaCO2 after stopping the THAM infusion, possibly by intracellular release of CO2. Pulmonary arterial pressure and PVR were lower in the THAM-treated animals, indicating that THAM may be an option to reduce PVR in acute hypercapnia.