Comparison of amplitude-modulated (direct) and frequency-modulated ambulatory techniques for recording ischemic electrocardiographic changes

Continuous ambulatory electrocardiographic monitoring of ST-segment configuration has become a useful technique for evaluation of myocardial ischemia. Concern that direct or amplitude-modulated (AM) recording and playback systems have inherent limitations that cause inaccurate ST-segment recordings has led to preference for frequency-modulated (FM) devices. To determine the accuracy of AM and FM ambulatory electrocardiographic systems, the signal was compared from the same set of 2 bipolar leads simultaneously recorded by standard electrocardiography and AM and FM recorders in 14 patients during treadmill exercise. Also, simultaneous AM and FM recorders were compared in 9 ambulatory patients in 16 monitoring sessions. The AM recording system accurately reproduced ST segments recorded during treadmill exercise (range 4.0 mm of ST-segment depression to 2.0 mm of ST elevation) when measured at the J point (r = 0.91, p less than 0.0001), and 0.08 second after the J point (r = 0.95, p less than 0.0001). FM recording was equally accurate (r = 0.89 and 0.95, respectively, p less than 0.0001). Similarly, during ambulatory recording, the AM technique accurately recorded maximal ST depression in each episode as recorded by the FM device (28 episodes, range 0 to 3 mm of ST depression, r = 0.85, p less than 0.0001). Both AM and FM ambulatory electrocardiographic systems can accurately reproduce ST-segment deviation associated with ischemia and can be used to monitor transient ST-segment changes in patients with coronary artery disease.