[Maximal effort, maximal heart rate and serum lactic acid in exercise electrocardiography. A comparative study]

In exercise stress testing the quantification of effort on which the value and significance of the test depend is based on two simple and practical criteria: theoretical maximal heart rate for age (MHR) and serum lactic acid levels 3 minutes after exercise. Usually only MHR is used, 85% of MHR being the common cut-off point between "interpretable" maximal or "inframaximal" effort and useless effort. Our study was based on 1 000 exercise stress tests performed over a two year period with the same protocol and equipment. Initially, we determined our values of MHR and serum lactic acid in undoubted maximal effort stress tests, as shown by a levelling out of the heart rate during the last load increments, the last stage of exercise being sustained for at least 1.5 minute. The application of both criteria to all the investigations showed significant differences in the evaluation of the exercise obtained when the maximal measured heart rate was between 85 and 95% of MHR. This was also true, to a lesser extent, in patients over 60 years of age or under 30 years of age with maximal measured heart rate of 85% or less than MHR. A discussion of the theoretical basis of the two criteria helps explain this divergence, attributing it to the inaccuracy and unreliability of the MHR. The only obstacle to the use of serum lactate as the preferential indicator of maximal effort is the need for laboratory facilities and the delay in obtaining the result.(ABSTRACT TRUNCATED AT 250 WORDS)