ST segment alternans during percutaneous transluminal coronary angioplasty--a case report. The occurrence of electrical alternans of the ST segment has been reported in patients with variant angina. The authors encountered a patient with typical electrical alternans of the ST segment in leads V4 through V6, which developed during percutaneous transluminal coronary angioplasty (PTCA) of the proximal left anterior descending artery. Hemodynamic pulsus alternans of the aortic pressure tracing was not observed during electrical alternans, and a Ca2+ blocker could not prevent this phenomenon during PTCA. 