StarScan Holter calculation of Heart Rate adjusted QT time (QTc, QTnc and QTlc)
General:
The QT interval begins at the onset of the QRS complex and terminates at the end of the T wave. It
represents the time of ventricular depolarization and repolarization. It is useful as a measure of
repolarization and is influenced by electrolyte balance, drugs, and ischemia. The QT interval is
inversely related to heart rate.
QT interval
The QT interval is measured from the beginning of the QRS complex to the end of the T wave. The
QT interval as well as the corrected QT interval are important in the diagnosis of long QT syndrome
and short QT syndrome. The QT interval varies based on the heart rate, and various correction
factors have been developed to correct the QT interval for the heart rate.
The most commonly used method for correcting the QT interval for rate is the one formulated by
Bazett.
Page 1 of 8
StarScan Holter calculation of Heart Rate adjusted QT time (QTc, QTnc and QTlc)
The Q-T interval represents the time for both ventricular depolarization and repolarization to occur,
and therefore roughly estimates the duration of an average ventricular action potential. This
interval can range from 0.2 to 0.4 seconds depending upon heart rate. At high heart rates,
ventricular action potentials shorten in duration, which decreases the Q-T interval. Because
prolonged Q-T intervals can be diagnostic for susceptibility to certain types of tachyarrhythmias, it
is important to determine if a given Q-T interval is excessively long. In practice, the Q-T interval is
expressed as a "corrected Q-T (Q-Tc)" by taking the Q-T interval and dividing it by the square root
of the R-R interval (interval between ventricular depolarizations). This allows an assessment of the
Q-T interval that is independent of heart rate. Normal corrected Q-Tc intervals are less than 0.44
seconds.
Several formulas have been proposed to adjust the QT-interval for the heart rate. The most
commonly used QT correction (QTc) formula is the one postulated by Bazett in 1920 (QTc=QT/RR1/2).
Other common formulas include the nomogram method (QTNc=QT+correcting number), the
Friderica formula (QTFc=QT/RR1/3) and the linear regression equitation (QTLc=QT+0.154x[1-RR]).
Recently, the adequacy of Bazett's formula has been questioned because it seems the QTc
overcorrects the measured QT-interval at fast heart rate and undercorrects it at low heart rates.
In a study, Karjalainen et al. measured QT-intervals in 324 rest ECGs of healthy young men. The
sample was weighted for low and high heart rates. A curve relating QT-intervals and heart rates
from 40 to 120 beats per minute was constructed. The QT-interval at 60 beats per minute was used
as the reference value, and an adjusting nomogram for different heart rates was created. The
reliabilities of the nomogram and three earlier QT correction equitations were tested in the study
group and in 396 middle-aged men.
The nomogram method (see below) adjusted the QT-interval most accurately over the whole range
of heart rates on the basis of smallest mean-squared residual values between measured and
predicted QT-intervals.
The Friderica formula gave the best correction at low, but failed at high heart rates.
The linear regression equitation (Framingham Study) was reliable at normal, but failed at low and
high heart rates.
The Bazett formula performed poorest at all heart rates.
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StarScan Holter calculation of Heart Rate adjusted QT time (QTc, QTnc and QTlc)
The four major causes of a prolonged QT interval:
1.
Electrolyte abnormalities:
Hypokalemia and hypocalcemia
2.
Drugs: (also associated with torsades de pointes)
o
Class Ia antiarrhythmic agents: quinidine, procainamide, disopyramide
o
Class Ic agents: propafenone
o
Class III agents: amiodarone, bretylium, dofetilide, n-acetylprocainamide, sematilide, sotalol
o
Psychotropic agents: tricyclic antidepressants, tetracyclic antidepressants, phenothiazines, haloperidol
o
Antihistamines: astemizole, terfenadine
o
Antibiotics: erythromycin, trimethoprim-sulfamethoxazole
o
Antifungals: ketoconazole, itraconazole
o
Serotonin antagonists: ketanserin, zimeldine
o
Chemotherapeutics: pentamidine, possibly anthracyclines
o
Miscellaneous: bepridil, cisapride, prednisone, prenylamine, probucol, chloral hydrate
o
Toxins and poisons: organophosphate insecticides, anthopleurinn-A, liquid protein diets, some herbs
3.
Congenital long Q-T syndromes:
While congenital long QT syndromes are rare, identification of a patient with this problem may allow for life-saving
therapy to be instituted. It should be searched for in any young patient who presents with syncope or presyncope.
4.
A miscellaneous group, including patients with:
o
Third-degree and sometimes second-degree A-V block
o
At the cessation of ventricular pacing
o
Left ventricular hypertrophy (usually minor degrees of lengthening)
o
Myocardial infarction (in the evolutionary stages where there are marked repolarization abnormalities)
o
Significant active myocardial ischemia
o
Cerebrovascular accident (subarachnoid hemorrhage)
o
Hypothermia
The four causes of a short QT interval:
1.
Hypercalcemia
2.
Digitalis
3.
Thyrotoxicosis
4.
Increased sympathetic tone
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StarScan Holter calculation of Heart Rate adjusted QT time (QTc, QTnc and QTlc)
Alternative 1: Bazett formula QTc = QT / sqrt RRI
Reference: Bazett HC. An analysis of time intervals of electrocardiolgram. Heart
1920;7:353-70
Bazett's formula is:
,
Where QTc is the QT interval corrected for rate, and RR is the interval from the onset of one QRS
complex to the onset of the next QRS complex, measured in seconds. However, this formula tends
to not be accurate, and over-corrects at high heart rates and under-corrects at low heart rates.
Page 4 of 8
StarScan Holter calculation of Heart Rate adjusted QT time (QTc, QTnc and QTlc)
Alternative 2: Nomogram correction method for QT time by
Karjalainen et al.
QTnc = QT + correcting number
Reference: Karjalainen J, Viitasalo M, Manttari M, Manninen V. Relation between QT
intervals and heart rates from 40 to 120 beats/min in rest electrocardiograms of men and a
simple method to adjust QT interval values. J Am Coll Cardiol. 1994 Jun;23(7):1547-53.
References:

J Am Coll Cardiol. 1994 Jun;23(7):1554-6.

J Am Coll Cardiol. 1995 Feb;25(2):551.

J Am Coll Cardiol. 1995 Feb;25(2):551-2.

J Am Coll Cardiol. 1994 Jun;23(7):1547-53.
Relation between QT intervals and heart rates from 40 to 120 beats/min in rest
electrocardiograms of men and a simple method to adjust QT interval values.
Karjalainen J, Viitasalo M, Manttari M, Manninen V.
Central Military Hospital, Helsinki, Finland.
OBJECTIVES. The aim of this study was to establish the relation between QT intervals and a wide range of rest
heart rates in men. These data provided the basis of a simple method for adjusting the QT interval for heart rate.
BACKGROUND. Earlier correction equations give conflicting results, especially at low and high heart rates.
METHODS. The QT intervals were measured in 324 electrocardiograms of healthy young men. The sample was
weighted for low and high heart rates. A curve relating QT intervals and heart rates from 40 to 120 beats/min was
constructed. The QT interval at 60 beats/min was used as the reference value, and an adjusting nomogram for
different heart rates was created. The reliabilities of the nomogram and three earlier QT correction equations were
tested in the study group and in 396 middle-aged men.
RESULTS. The nomogram method presented (QTNc = QT + correcting number) adjusted the QT interval most
accurately over the whole range of heart rates on the basis of smallest mean-squared residual values between
measured and predicted QT intervals. The Fridericia formula (QTFc = QT/RR1/3) gave the best correction at low,
but failed at high, heart rates. The linear regression equation (QTLc = QT + 0.154[1 - RR], Framingham Study) was
reliable at normal, but failed at low and high, heart rates. The Bazett formula (QTc = QT/RR1/2) performed poorest
at all heart rates. The relation between QT and RR intervals was determined by three linear regressions expressing
the slopes 0.116 for heart rates < 60 beats/min, 0.156 for heart rates from 60 to 100 beats/min and 0.384 for heart
rates > 100 beats/min.
CONCLUSIONS. The QT-RR relation over a wide range of heart rates does not permit the use of one simple
adjustment equation. A nomogram providing, for every heart rate, the number of milliseconds that the QT interval
must be corrected gives excellent adjustment.
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StarScan Holter calculation of Heart Rate adjusted QT time (QTc, QTnc and QTlc)
Table 1. Nomogram for Measured QT-intervals to obtain a heart rate-adjusted QT value.
Heart Rate
(beats/min)
QT-interval
correction
Heart Rate
(beats/min)
QT-interval
correction
Heart Rate
(beats/min)
QT-interval
correction
40
-59
67
16
94
56
41
-55
-50
-47
-44
-40
-36
-32
-29
-26
-23
-21
-18
-16
-13
-11
-9
-6
-4
-2
0
2
5
7
9
11
14
68
18
95
57
69
20
96
59
70
23
97
60
71
25
98
61
72
27
99
63
73
29
100
65
74
31
101
67
75
32
102
69
76
34
103
71
77
35
104
73
78
37
105
75
79
38
106
77
80
40
107
79
81
41
108
81
82
42
109
83
83
43
110
85
84
44
111
87
85
45
112
89
86
46
113
91
87
47
114
93
88
48
115
95
89
50
116
97
90
51
117
99
91
52
118
101
92
53
119
103
93
55
120
105
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
From: Karjalainen, Viitasalo, Mänttäri, Manninnen. Relation Between QT Intervals and Heart rates From 40 to 120 beats/min in Rest
Electrocardiograms of Men and a Simple Method to Adjust QT Interval Values. JACC Vol. 23, No. 7. June 1994:1547-53
Sagie, Larson, Goldberg, Bengtson, Levy: An Improved Method for Adjusting the QT Interval for Heart Rate (the Framingham Heart Study).
Am J Cardiol 1992;70:797-80
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StarScan Holter calculation of Heart Rate adjusted QT time (QTc, QTnc and QTlc)
Alternative 3: Linear correction method for QT time (QTlc) from
the Framingham Study by Sagie et al.
QTlc = QT + 0.154(1 – RR)
References:

Am J Cardiol. 1993 Feb 15;71(5):504.
An improved method for adjusting the QT interval for heart rate (the
Framingham Heart Study)
Sagie A, Larson MG, Goldberg RJ, Bengtson JR, Levy D.
Framingham Heart Study, Massachusetts 01701.
Several formulas have been proposed to adjust the QT interval for heart rate, the most commonly used being the QT
correction formula (QTc = QT/square root of RR) proposed in 1920 by Bazett. The QTc formula was derived from
observations in only 39 young subjects. Recently, the adequacy of Bazett's formula has been questioned. To evaluate
the heart rate QT association, the QT interval was measured on the initial baseline electrocardiogram of 5,018
subjects (2,239 men and 2,779 women) from the Framingham Heart Study with a mean age of 44 years (range 28 to
62). Persons with coronary artery disease were excluded. A linear regression model was developed for correcting
QT according to RR cycle length. The large sample allowed for subdivision of the population into sex-specific
deciles of RR intervals and for comparison of QT, Bazett's QTc and linear corrected QT (QTLC). The mean RR
interval was 0.81 second (range 0.5 to 1.47), heart rate 74 beats/min (range 41 to 120), and mean QT was 0.35
second (range 0.24 to 0.49) in men and 0.36 second (range 0.26 to 0.48) in women. The linear regression model
yielded a correction formula (for a reference RR interval of 1 second): QTLC = QT + 0.154 (1-RR) that applies for
men and women. This equation corrects QT more reliably than the Bazett's formula, which overcorrects the QT
interval at fast heart rates and under corrects it at low heart rates. Lower and upper limits of normal QT values in
relation to RR were generated.
Page 7 of 8
StarScan Holter calculation of Heart Rate adjusted QT time (QTc, QTnc and QTlc)
QT Analysis notes:
The QT is NOT calculated on Heart Rates below 40 bpm and above 120 bpm.
This calculation is based upon:
a. The rolling heart rate selected by the user in the analysis settings. If the rolling heart
rate is above or below the limits the corrected QT is NOT calculated.
b. If the previous RR interval is above or below the limits the corrected QT is NOT
calculated.
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