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The use of assumed values of oxygen consumption has become an accepted practice in the calculation of direct Fick cardiac output. A survey showed that the assumed values in common use were derived from basal metabolic rate studies on normal subjects, a use which may not be valid. We have compared previous assumed values based on basal metabolic rate or cardiac catheterization studies with those obtained by direct measurement in 80 patients (age range 38-78 years) with various cardiac disorders. Comparison of the assumed and directly measured values of indexed oxygen consumption and the cardiac index showed large discrepancies, with over half the values differing by more than +/- 10% and many by more than +/- 25% from the measured value. Assumed values of oxygen consumption should be used with caution when calculating cardiac output during cardiac catheterization procedures, because large errors can result. The equations of LaFarge and Miettinen gave the closest approximation to the measured data and their use is recommended in preference to values predicted from basal metabolic rate studies.

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A comparison of the standard 12-lead electrocardiograph with the Mason-Likar lead system widely used for exercise stress testing shows that the two are not 'essentially identical' as was originally claimed. Placement of the limb electrodes onto the torso distorts the electrocardiograph causing a rightward shift of the mean QRS axis, a significant reduction in R-wave amplitude in leads I and aVL, and a significant increase in R-wave amplitude in leads II, III and aVF; the R-wave amplitude of the chest leads is also altered. The so-called 'inferior' leads on the exercise electrocardiography are probably modified anterior/inferior leads, since their R-wave amplitudes correlate closely with those of antero-lateral chest leads. The inferior surface of the heart is not represented in isolation on the exercise electrocardiograph, thus explaining the reported inability of the exercise test to predict the location of coronary artery disease and high incidence of false negative tests in patients with ischaemia limited to the inferior cardiac surface.