Critical capillary oxygen partial pressure and lactate threshold in patients with cardiovascular disease

AbstractObjectives. The aim of this study was to determine the relation between femoral vein oxygen partial pressure (Po2) and lactate increase during exercise in patients with cardiovascular disease.Background. Considerable controversy surround; the relation between the increase in lactate during exercise and the oxygen supply to the exercising muscles. We assumed that femoral vein Po2would be a measure of end-capillary Po2during leg-cycling exercise and that it would decrease to a “floor” level when the critical capillary Po2(the Po2below which the capillary-mitochondrial difference would be too low to allow oxygen consumption) was reached. At the critical capiliary Po2, anaerobic metabolism should take place, and lactate shoulds increase in the effluent blood.Methods. Tea patients with cardiovascular disease performed two 6-min constant work rate tests (moderate and heavy intensity) and an incremental exercise test to the symptom-limited maximum on a cycle ergometer. Femoral vein blood was repeatedly sampled through a percutaneous catheter before and during each exercise test.Results. The Po2rapidly decreased toward a minimal value with increasing oxygen uptake for all three tests in all patients. After reaching its nadir (18.2 ± 2.0 mm Hg), the Po2remained unchanged in five patients but increased in the other five patients despite the further increase in work rate and oxyzen uptake. The relation between Po2and oxygen uptake was characteristic for each patient and independent of the protocol used for the study. Femoral vein lacate did not change appreciably until Po2reached the minimal (critical) value. Thereafter, it dramatically increased without a further decrease in Po2. The minimal Po2was posisively correlated with the peak oxygen uptake (r = 0.70, p = 0.01).Conclusions. During leg-cycling exercise, muscle capillary Po2reaches a minimal value in the midrange of the subjects' work capacity before lactate concentration increases in patients with cardiovascular disease. The lack of further decrease in Po2at the oxygen uptake at which lacate starts to increase suggests that the minimal capillary Po2is the “critical” capillary Po2