pH effects on lactate and excess lactate in relation to O2 deficit in hypoxic dogs.

Both hypoxemia and alkalemia increase arterial lactate levels, but excess lactate (XL) may be better related to the true O2 deficit and less subject to the pH effect upon glycolysis. To test this idea, 50 anesthetized (30 ml/kg pentobarbital sodium) and paralyzed dogs in five groups were ventilated with low O2 mixtures. Arterial pH was altered by CO2, hyperventilation, and HCO3- so that the average pH of the five groups were 6.99, 7.21, 7.34, 7.53, and 7.54 (+ propranolol). Pyruvate and lactate levels rose at rates roughly corresponding to pH. In all animals, rise of lactate (deltaL), XL and net O2 deficit were linear with time and the slope accurately described the rate of rise. When the ratio of XL rate deltaL rate was plotted against pH, it increased toward unity as pH decreased to 6.9. The ratio of deltaL rate to true O2 deficit rate also showed a significant pH effect by increasing with pH. Ratio of XL rate to true O2 deficit rate, on the other hand, was independent of pH. XL may correct for red cell lactate production which is not affected by O2 supply but is influenced by pH.