Another explanation for decreased oxygen consumption in lactic acidosis

Another explanation for depressed oxygen consumption in these patients is also possible, however. If the blood pH is very low, glucose utilization is decreased [2] because the glycolytic enzyme phosphofructokinase is pH dependent - with decreasing pH, its activity is also decreasing [3]. Glucose utilization is an oxygen-consuming process:The consequence of decreased utilization of glucose is thus also decreased oxygen consumption.The patients reported by Protti and colleagues had on admission very low blood pH of 6.93 ± 0.20 and systemic oxygen consumption of 67 ± 28 ml/min/m2 [1]. Systemic oxygen consumption 'normalized within the next 48-72 hours' and 'Systemic O2 consumption was positively associated with arterial pH' (P < 0.001). According to Tables 2 and 3 [1], arterial pH reached normal values on days 2 to 3.Depressed oxygen consumption in patients reported by Protti and colleagues can thus be explained by their very low blood pH.Alessandro Protti and Luciano GattinoniWe thank Dr Rosival for his stimulating comment.Whether acidosis has an impact on oxygen consumption (VO2) remains unclear. In vitro, several studies have demonstrated that tissue VO2 only starts to diminish when the pH falls below 6 to 6.5 [4,5]. In vivo, both animal and clinical studies have reported normal, or even increased, whole-body VO2 during severe acidosis [6,7]. Accordingly, we have observed no correlation between VO2 and arterial pH among 762 critically ill patients, at the time of admission to intensive care (R2 = 0.00, P = 0.88 on linear regression analysis) [8].In order to directly address the issue raised by Dr Rosival, we equipped two healthy, sedated and mechanically ventilated pigs with a metabolic module (to record VO2) and a pulmonary artery catheter (to compute the global oxygen delivery). Following baseline recordings, one animal received a continuous intravenous infusion of metformin whereas the other received lactic acid. Arterial pH, VO2 and oxygen delivery were recorded