Hyperventilation in the treatment of metabolic acidosis does not adversely affect pulmonary gas exchange

Abstract

Background: Hyperventilation has been recommended to increase blood pH during metabolic acidosis. However, hypocapnia may adversely affect arterial blood oxygenation, especially in the presence of lung disease. We therefore studied the effects of metabolic acidosis, with and without normalization of pH by hyperventilation, on pulmonary gas exchange in dogs with permeability pulmonary edema. Methods: Six pentobarbital-anesthetized dogs were administered 0.06 ml/kg of oleic acid at least 150 min before study. Ventilation was set with an inspired O2 fraction of 0.90 and a tidal volume of 18 ml/kg, and the respiratory rate was adjusted to alter the arterial CO2 tension (Pa(CO2)) per the experimental protocol. The protocol in random order was (1) normal pH (7.36 ± 0.01)/normal Pa(CO2) (39 ± 1 mmHg); (2) low pH (7.20 ± 0.01)/normal Pa(CO2) (40 ± 1 mmHg); (3) low pH (7.18 ± 0.01)/hyperventilation with inspired CO2 (Pa(CO2) = 40 ± 1 mmHg); and (4) normal pH (7.35 ± 0.01)/hyperventilation with low Pa(CO2) (24 ± 1 mmHg). In phases 2-4, the pH was slowly reduced by intravenous infusion of 2 N hydrochloric acid. The pH was normalized in phase 1 where necessary by infusion of sodium bicarbonate. The pH in phase 4 was normalized by reducing the Pa(CO2) by increasing the respiratory rate. Gas exchange was assessed by the multiple inert-gas elimination technique. Results: The hemodynamic measurements remained constant throughout the protocol. Arterial O2 tension increased from 244 ± 55 to 293 ± 49 mmHg in the presence of metabolic acidosis (P < 0.05). Hyperventilation to normalize the pH during metabolic acidosis (phase 4), increased arterial O2 tension (313 ± 44 mmHg, P < 0.05), and reduced shunt (from 20 ± 5% to 12 ± 3%, P < 0.05) compared with normal acid-base conditions (phase 1). No change in shunt was observed with hyperventilation compared with metabolic acidosis alone (phase 2). The decrease in pulmonary shunt was not attributable to the direct effects of hyperventilation, because shunt was increased (20 ± 5%) when Pa(CO2) was normalized during hyperventilation by inspiration of CO2 (phase 3). Conclusions: Hyperventilation to normalize blood pH during hydrochloric acid- induced metabolic acidosis did not adversely affect pulmonary gas exchange in dogs with permeability pulmonary edema.