Errors in predicting functional capacity for postmyocardial infarction patients using a modified Bruce protocol

Clinically, the modified Bruce protocol is widely used to predict functional capacity in postmyocardial infarction (post-MI) patients. However, it has been suggested that post-MI patients have lower oxygen uptakes for standard workloads. In order to study this, we measured oxygen uptake (VO2) and venous blood lactic acid concentration in 12 post-MI patients and 12 normal male subjects during a modified Bruce treadmill protocol. During the first four stages of the protocol, mean oxygen uptake was significantly lower (1.0 to 6.2 cc X kg-1 X min-1; p less than 0.001) for the post-MI patients than for the normal male subjects. Venous blood lactic acid concentrations were different only at stage 4. However, a higher respiratory exchange ratio was observed for post-MI patients (p less than 0.001 at stage 3), suggesting an increased anaerobic metabolism and adequate buffering of lactic acid. The post-MI patient's measured VO2 for three stages of the protocol ranged from 1.8 to 7.3 cc X kg-1 X min-1 lower than the Bruce prediction for cardiac patients. In addition, maximal measured VO2 for the post-MI patients ranged from 3.7 to 11.2 cc X kg-1 X min-1 lower than predicted VO2 derived from the normal subjects. These data suggest myocardial damage may slow oxygen uptake kinetics, thus increasing the oxygen deficit at standard workloads during progressive exercise. An increased respiratory exchange ratio secondary to the buffering of lactic acid suggests that anaerobic metabolism may compensate for this oxygen deficit.