Objective-To quantify the errors associated with manual measurement of QT intervals and to determine the source of the errors. Design-A randomised study of QT measurement by four cardiologists of electrocardiograms plotted on paper in presentations with different noise levels, paper speeds, amplifier gains, and with and without a second QRST complex to indicate the RR interval. Subjects-Four electrocardiograph leads (I, aVR, VI, V5) recorded in eight healthy people relaxing in a semirecumbent position. Main outcome measures-Manual measurement of QT interval in 512 electrocardiograms (eight subjects x four leads x eight presentations x two repeats) by each of four cardiologists. Results-QT intervals measured were significantly longer with greater amplifier gain: by 8 ms for a doubling of gain (p < 0.005), equivalent to a doubling of T wave height. QT intervals measured were significantly longer at slower paper speeds: by 11 ms when paper speed was reduced from 100 to 50 mmis (p < 0.001) and by 16 ms when speed was further reduced from 50 to 25 mmis (p < 0 001).Neither the presence of noise nor the presence of a second QRST complex altered the mean QT measurements. There were consistent differences in the measurements between cardiologists, amounting to a maximum mean difference of 20 ms.

The proximal coronary sinus in patients with AVJRT was significantly different from a control population. The ostium was 44% larger and remained more dilated to at least 10 mm from the ostium. The appearance was like a wind sock in AVJRT patients and tubular in the control patients. These findings may have important implications for arrhythmia pathogenesis in such patients.

These observations enlarge and extend our knowledge of the functional repertoire of complex reentrant tracts that occur in infarct-related ventricular tachycardia. The use of common portions of a reentrant tract by several tachycardias is confirmed. Utilization of alternate pathways can account for changes in configuration and cycle length. Spontaneous and induced block can occur at points of entry and exit in a reentrant tract and may identify optimal targets for ablation attempts. Further advances will require greater emphasis on diastolic activation mapping.