The reliability of rate-pressure product as an index of myocardial oxygen consumption in atrial synchronized versus fixed rate ventricular pacing.

The product of heart rate and blood pressure was tested as an index of myocardial oxygen consumption (MVO2) and compared with directly determined MVO2 during ventricular demand (VVI) fixed rate pacing and atrial synchronized (VAT) pacing at rest and during exercise. Systolic brachial artery pressure, pulmonary wedge pressure and MVO2 were similar in the two pacing modes and showed similar response to exercise. The correlation between rate-pressure product and MVO2 was closer with VAT than with VVI pacing (r = 0.74 and r = 0.64, respectively), and the latter value was not improved by using the product of atrial rate and systolic pressure (r = 0.61). The rate-pressure product was significantly higher during VAT pacing compared to VVI during exercise, although MVO2 was similar. The similarity of MVO2 during exercise indicated some other contributory factor than heart rate in VVI pacing, probably increase of contractility and/or volume. Because such factors are not included in currently used indices of MVO2 assessments must be interpreted cautiously, particularly in cases of complete heart block with VVI pacing.