Ventilation and acid-base changes in deep coma due to barbiturate or tricyclic antidepressant poisoning

Forty-two adults in deep coma due to uncomplicated overdosage with barbiturates or tricyclic antidepressants were studied to determine the effects of the drugs and the depth of coma on ventilation and acid-base balance. All the patients were breathing spontaneously. The usual acid-base disturbance was a mixed respiratory and metabolic acidosis. No differences in effects of barbiturates and tricyclic antidepressants could be demonstrated in either grade of coma studied. Patients who were completely unresponsive to painful stimuli were significantly more acidemic (arterial [H+] 51.4 nmole/liter c.f. 44.4 mole/liter) and hypoxic (PaO2 6.6 kPa c.f. 10.5 kPa) than those who showed a minimal response to pain. These findings could not be explained by differences in alveolar ventilation, and it is suggested that they are due to ventilation/perfusion imbalance.