Errors in manual measurement of QT intervals.

A. Murray, N. B. McLaughlin, J. P. Bourke, J. C. Doig, S. S. Furniss, R. W. Campbell

Regional Medical Physics Department, Freeman Hospital, Newcastle upon Tyne.

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, V1, 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 mm/s (p < 0.001) and by 16 ms when speed was further reduced from 50 to 25 mm/s (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. CONCLUSIONS--Manual measurement of QT interval is significantly affected by the paper speed used to plot the electrocardiogram and by electrocardiogram gain, and hence also T wave amplitude. Manual QT measurement also differed consistently with different cardiologists.

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