- Email: [email protected]
Applicability of correcting the QT interval for heart rate
Letters
to the Editor
Applicability of correcting for heart rate
the
QT interval
To the Editor: Correcting the QT interval for heart rate (QTc) is a common and well established practice. It is assumed the QT interval consistently shortens as heart rate increases, based on studies by Bazett,’ by Fridericiat by Simonson and associates,” and by others. Many of these studies contain experimental bias that limit evaluation of the QT interval changes; e.g., analysis of heart r,ate/QT variation was between individuals rather than within individuals. The present questions arose from a critical electrocardiographic review that was part of a study that evaluated the cardiovascular, pulmonary, and neuromuscular effects of an experimental bronchodilator (clenbuterol) in 11 patients with reversible obstructive airways disease. Each subject had 48 measurements of QT interval following both adrenergic drugs or placebo. Three dosage strengths of the new bronchodilator were compared to two of a standard (terbutaline) and a placebo. QT intervals were measured and rounded to 10 msec. intervals, while heart rates were obtained from R-R intervals and were rounded to even beats per minute. Data were analyzed by analysis of variance and linear regression. Dose-related increases in heart rate were observed following administration of each bronchodilator and after a meal. Analysis of variance revealed residual changes in QTc following correction by the formula of Bazett.’ No residual changes were observed using either of the correction formulas of Simonson and colleagues.” This suggests the latter formula was a better corrector of QT for heart rate than the more commonly used square root of the R-R interval formula of Bazett.’ The present data were tested further by QT interval versus heart rate regression analysis, which revealed an inverse relationship (Fig. 1) similar to the log linear formula of Simonson and associates3 While the population data agree, line C, there was a disparate relationship in some individuals-lines A and B. Individual B showed marked changes in the QT interval at nearly the same heart rate, and individual A demonstrated an invariant QT with marked heart rate changes. The coefficient of determination (P) representing the entire population demonstrates the same thing; i.e., 75% of the variation in QT is not explained by heart rate. Therefore, population statistics such as those of Simonson and coworkem as well as ours, may be inappropriate when applied to a specific ECG. While this has been recognized by Simonson and associates and by others, it is not generally appreciated. The misapplication of statistical inference to a cardiac pharmacologic decision based on a prolonged QTc would be misleading in instances where the patient’s QT varies widely at the same heart rate, or is invariant as heart rate changes. Thus, rate correcting an individual’s QT interval is appropriate when known rate-related QT changes are present. Carl V. Manion, M.D. Thomas L. Whitsett, M.D. Michael F. Wilson, M.D.
\ \
400
Aem*
0 lndlvldurl h Individual
\
\ A.
.
390
.
l
all
A B
other
\ \
360
,..*A*
:
l
\
QT Intorvrl (mrao)
370
\
360
,
.
Y:42.29-.0913 r 3 0.672
l
.
AD\
X
3SCi I\ ,,*...:.
.:
.
340
.
330
.
\ .b.
. .
. . iii
320
a.0
. :**
. .
\ :*
.:.“.:
\
.
..
. w* C
\ 310
i-1
l **
\
‘I
B
“do-;0 r;o do Heart
160
1;o
Rate
Fig. 1. A plot of all the QT intervals and heart rate values. Regression coefficients for individual A (squares) and individual B (triangles) represent the extremes in variance from that of the entire group’s (C). The linear regression constants for the formula Y = A - Bx are shown.
Department of Medicine University of Oklahoma Health Sciences Center, College of Medicine and Oklahoma City VA Medical Center Oklahoma City, Okla. 73190 REFERENCES
1. Bazett, H. C.: An analysis of the time-relations of electrocardiograms, Heart 7:353, 1920. 2. Fridericia, L. A.: Die Systolendauer im Elektrokardiogram bei normalen Menschen und bei Herzkranken, Acta Med. Stand. 53:489, 1920. 3. Simonson, E., Cady, L. D., Jr., and Woodbury, M.: The normal Q-T interval, AM. HEART J. 63:747, 1962.
Improved
circulation
by coronary
bypass?
To the Editor: I would like to say just a few words concerning the Annotation by Dr. Burch in a recent issue of the JOURNAL (September, 1979, Vol. 98: 404) entitled “Of ‘The quality of life’. ”
May, 1980, Vol. 99, No. 5
to the Editor
Applicability of correcting for heart rate
the
QT interval
To the Editor: Correcting the QT interval for heart rate (QTc) is a common and well established practice. It is assumed the QT interval consistently shortens as heart rate increases, based on studies by Bazett,’ by Fridericiat by Simonson and associates,” and by others. Many of these studies contain experimental bias that limit evaluation of the QT interval changes; e.g., analysis of heart r,ate/QT variation was between individuals rather than within individuals. The present questions arose from a critical electrocardiographic review that was part of a study that evaluated the cardiovascular, pulmonary, and neuromuscular effects of an experimental bronchodilator (clenbuterol) in 11 patients with reversible obstructive airways disease. Each subject had 48 measurements of QT interval following both adrenergic drugs or placebo. Three dosage strengths of the new bronchodilator were compared to two of a standard (terbutaline) and a placebo. QT intervals were measured and rounded to 10 msec. intervals, while heart rates were obtained from R-R intervals and were rounded to even beats per minute. Data were analyzed by analysis of variance and linear regression. Dose-related increases in heart rate were observed following administration of each bronchodilator and after a meal. Analysis of variance revealed residual changes in QTc following correction by the formula of Bazett.’ No residual changes were observed using either of the correction formulas of Simonson and colleagues.” This suggests the latter formula was a better corrector of QT for heart rate than the more commonly used square root of the R-R interval formula of Bazett.’ The present data were tested further by QT interval versus heart rate regression analysis, which revealed an inverse relationship (Fig. 1) similar to the log linear formula of Simonson and associates3 While the population data agree, line C, there was a disparate relationship in some individuals-lines A and B. Individual B showed marked changes in the QT interval at nearly the same heart rate, and individual A demonstrated an invariant QT with marked heart rate changes. The coefficient of determination (P) representing the entire population demonstrates the same thing; i.e., 75% of the variation in QT is not explained by heart rate. Therefore, population statistics such as those of Simonson and coworkem as well as ours, may be inappropriate when applied to a specific ECG. While this has been recognized by Simonson and associates and by others, it is not generally appreciated. The misapplication of statistical inference to a cardiac pharmacologic decision based on a prolonged QTc would be misleading in instances where the patient’s QT varies widely at the same heart rate, or is invariant as heart rate changes. Thus, rate correcting an individual’s QT interval is appropriate when known rate-related QT changes are present. Carl V. Manion, M.D. Thomas L. Whitsett, M.D. Michael F. Wilson, M.D.
\ \
400
Aem*
0 lndlvldurl h Individual
\
\ A.
.
390
.
l
all
A B
other
\ \
360
,..*A*
:
l
\
QT Intorvrl (mrao)
370
\
360
,
.
Y:42.29-.0913 r 3 0.672
l
.
AD\
X
3SCi I\ ,,*...:.
.:
.
340
.
330
.
\ .b.
. .
. . iii
320
a.0
. :**
. .
\ :*
.:.“.:
\
.
..
. w* C
\ 310
i-1
l **
\
‘I
B
“do-;0 r;o do Heart
160
1;o
Rate
Fig. 1. A plot of all the QT intervals and heart rate values. Regression coefficients for individual A (squares) and individual B (triangles) represent the extremes in variance from that of the entire group’s (C). The linear regression constants for the formula Y = A - Bx are shown.
Department of Medicine University of Oklahoma Health Sciences Center, College of Medicine and Oklahoma City VA Medical Center Oklahoma City, Okla. 73190 REFERENCES
1. Bazett, H. C.: An analysis of the time-relations of electrocardiograms, Heart 7:353, 1920. 2. Fridericia, L. A.: Die Systolendauer im Elektrokardiogram bei normalen Menschen und bei Herzkranken, Acta Med. Stand. 53:489, 1920. 3. Simonson, E., Cady, L. D., Jr., and Woodbury, M.: The normal Q-T interval, AM. HEART J. 63:747, 1962.
Improved
circulation
by coronary
bypass?
To the Editor: I would like to say just a few words concerning the Annotation by Dr. Burch in a recent issue of the JOURNAL (September, 1979, Vol. 98: 404) entitled “Of ‘The quality of life’. ”
May, 1980, Vol. 99, No. 5