The quantitative relation between ST-segment changes and the severity and extent of myocardial ischemia during coronary occlusion remains unclear. This study assesses whether ST-segment changes during percutaneous transluminal coronary angioplasty (PTCA) correlate with the amount of myocardium at risk, measured with technetium-99m hexakis 2-methoxyisobutyl isonitrile (MIBI; also called sestamibi) single-photon emission computed tomography (SPECT). Quantitative continuous dynamic vectorcardiography was performed during PTCA of the left anterior descending coronary artery in 11 patients (mean age 64.3 years) without previous myocardial infarction. Change in the magnitude of the ST vector (STc-VM) was continuously recorded. A standardized protocol of balloon inflations was used and technetium-99m MIBI was injected intravenously at the onset of the third inflation. SPECT imaging was performed 60 minutes later and compared to a rest acquisition. SPECT was quantified by bull's-eye analysis using: (1) the change in the pathologic/normal area count ratio (delta P/N) as an index of the severity of ischemia; and (2) planimetered defect size during PTCA as an indicator of the size of the area at risk. The delta P/N from baseline to balloon occlusion (22 +/- 11%) was correlated, albeit loosely, to the maximum value of STc-VM (245 +/- 186 microV, r = 0.62, p < 0.05), but there was no correlation between the size of the scintigraphic defect and STc-VM. Likewise, the sum of ST-segment elevation was correlated to delta P/N (r = 0.72, p < 0.02), but not to the size of the scintigraphic defect.(ABSTRACT TRUNCATED AT 250 WORDS)