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Original article

Kerala Heart J  2016; 6(2):xx-xx.  



Effect of QRS width on QT interval


Cicy Bastian,RaihanathulMisiriya Kamarudheen Jameela, Sudhayakumar


Corresponding autho
r


Dr.CicyBastian

Associate professor of Cardiology,
Govt.T D medical college, Alappuzha,
Kerala, India.

cicybastian@hotmail.com

Mob. 9447986336



Abstract

Background.

In the surface ECG, ventricular depolarization is represented by the QRS and ventricular repolarization by the ST and T which together forms the JT interval. When the QRS duration is normal, we take QT interval as a measure of repolarization. But when there is QRS widening, what happens to this QT interval? If there is prolongation of the QT , is it due to prolongation of depolarization alone or  prolongation of repolarization?  We tried to find out this by analyzing 50 cases of intermittent narrow and wide QRS.

Methods.

50 patients with intermittent right bundle branch block (RBBB), pacing in Sick sinus syndrome (SSS), pacing in bradyarrhythmias during Myocardial infarction (MI), and intermittent preexcitation were included in the study. Patients with pre-existing broad QRS were excluded from the study. Simultaneous  single channel 12 lead  ECG was taken during narrow QRS and broad QRS . QT interval was calculated as the mean QT of six leads in which QT could be easily measured.  JT interval was  calculated by subtracting QRS duration from QT interval. Corrected QT interval (QTc) was calculated by Bazetts formula.  From this corrected JT ( JTc) was also calculated. All the measurements were taken during narrow QRS rhythm and broad QRS rhythm.

Results

As the QRS width prolonged to 110 ms or more , QT and QTc were also prolonged. There was no change in JT and JTc.

Conclusion

JT and JTc intervals are more reliable than QT and QTc intervals  in case of wide QRS.

Key words

Broad QRS, JT interval ,QRS width , QT interval.

 

Introduction

Normal Electrocardiographic complex

P waves represents depolarization of the atrial myocardium

Q (q) wave the initial negative deflection resulting from ventricular depolarization

R (r) wave the first positive deflection during ventricular depolarization

S (s) wave the first negative deflection of ventricular depolarization that follows the first positive deflection

Normal QRS characteristics:

This is the measurement of total ventricular depolarization time. It is measured from the onset of the Q (or R if there is no Q) to the termination of the S wave.

Normal QRS duration is 0.04 0.11 seconds. QRS width often varies in different leads. The widest QRS measurement on the 12 lead ECG is the correct one. Best leads to measure are usually leads 1 and V1.

Amplitude should not be smaller than 6mm in leads  l , ll, and lll and nor should it be taller than 25 30 mm in the precordial leads.

QRS refers to waves over 5 mm and qrs refers to waves less than 5mm.

The upper limit of normal is 0.1 seconds.

QT interval

Measurement of the time between the start of depolarization of the ventricle of the heart and the end of repolarisation of the ventricles. It measures the duration of the electrical systole.

Normal QT duration

QT ≤  0.38 if Heart rate (HR) is 80/mt or more

Add 0.02 sec for every 10 bpm below 80

Eg. Normal QT is 0.46 if HR 40-50.

HR determined QT

115 84 bpm              QT   0.30 to 0.37 sec.

83 72 bpm                QT   0.32 to 0.40 sec.

71 63 bpm                QT   0.34 to 0.42 sec. 

62 56 bpm                QT   0.36 to 0.43 sec.

55 45 bpm                QT   0.39 to 0.40 sec. 

Measurement of QT interval

Best leads to measure QT are Lead II  , V2 or V3

Often the end of the T wave is not clear. In such cases the end of the T wave can be extrapolated by using the Tangent method. The start of the QT interval is defined as the first deflection of the QRS complex. The end of the QT interval is defined as the  intersection of the descending  part of the  T wave ( positive T wave ) with the isoelectric line. 1

If a U wave appears immediately after the T wave has returned to the baseline, the QT interval is measured as the nadir between T and U waves. If it is not clear whether a second hump occurs before the T wave has returned to baseline, it is included in the QT interval.

Corrected QT interval  ( QTc )

The QT interval is a reflection of the action potential in the cardiac cells. Homogeneous or heterogeneous changes in the action potential duration lead to alteration of QT interval  ( in addition to morphological changes of T and U waves.) such changes can be due to change in heart rate and autonomic tone. They can also be markers of bnormal repolarisation , depolarization or both as a result of electrolyte disturbances , cardiac disease , drugs and congenital long QT syndromes.

Repolarisation disorders are responsible for life threatening arrhythmias like Torsade de pointes. The purpose of heart rate correction is to obtain a standardized value that woud have been measured in the same subject if the heart rate was 60 beats per minute ( QTc). Thus this QTc value will now become independent of the heart rate and measure repolarisation changes. It will thus be a surrogate marker of the risk of Torsade de pointes.

The concept of QTc appeared in 1920, when Bazette introduced his square root formula. This formula which was obtained from data on 39 young men has been questioned because it over corrects QT at fast heart rate and undercorrects at low heart rate. Thus at at slow heart rates, which is one of the predisposing factors of torsade initiation , Bazett correction can easily mask substantial QT prolongation by undercorrecting. This can hide the pro arrhythmic toxicity of drugs which cause slowing of heart rate. An alternative , cube root correction of Fridericia , corrects better than Bazett but again is not reliable at fast heart rates. Compared to these non linear correction formulae, linear regression correction obtained from large population data like the Framingham heart study linear correction are still better.

However the QT does not adapt to changes in heart rate immediately. It takes more than 2 minutes for the QT to adapt ( QT/RR hysteresis ). Hence correction needs to be done at steady heart rates. The concept of heart rate correction ignores the dynamicity of QT/RR relationship. The QT interval is also under autonomic control. Therefore different modes of heart rate changes , eg. Fast heart rate due to parasympathetic withdrawal versus sympathetic overactivity , lead to different direct and reflex effects effects on QT prolongation. Hence the standard QTc correction formula will not be representative of of the actual repolarisation milieu. Ideally, each individual should have his own correction derived from multiple ECG s at different heart rates and conditions to get the ideal correction constant. Since this is not feasible a compromise can be made by using formula which have been developed and validated in a large population based cohort like the Framingham study.

Alternatively, a table of lower and upper limits of QT interval for ifferent RR cycle lengths can be used by clinicians for reference purpose. This method obviates the need of using any QT correction formula. The reliability of this model results from its deviation in a large population based sample. Inspite of all fallacies of Bazetts correction , it is still being used clinically. The reason probably lies in its simplicity and the fact that all clinical data signaling the risk of torsade are derived from this formula only. But in the near future we feel that the Bazetts correction will be replaced by a better formula like Fridericia. We also need to be aware that in addition to prolonged QT , an abnormally short QT can also carry dangerous implications of arrhythmogenicity. The QT also represents depolarization events and hence in the presence of depolarization abnormalities like Left Bundle Branch Blocks (LBBB) and pre excitation syndromes, QTc will not be representative of repolarization abnormalities and should not be commented upon.2 An alternative interval the JT has been proposed in such cases.

QTc is the QT corrected for heart rate  ( QT decrease as the heart rate increases ).

Correction is done by the use of normogram or using

Bazetts formula  ( exponential square root ).3

QT/RR in sec . limitation is that it under corrects at low HR

Fridericia formula  ( exponential cube root )

QT/RR 1/3 in sec.                    Male                       Female

          Normal  (ms)                         <430                                <450

            Borderline (ms)                              431 450             451-470

            Prolonged (ms)                               >450                                 >470

Corrected QT interval is associated with higher risk of mortality in patients with coronary heart diseases  ( CHD ) and in the general population. However the QTc is typically not evaluated when QRS duration  ( QRSd ) is more than 120 ms, because increased QRSd contributes to QT prolongation. In these circumstances the JT interval has been proposed as a more valid way to assess ventricular repolarization. To allow for variation in heart rate, corrected JT interval (JTc) was defined as QTc QRSd .

Q-U interval : This measures the interval from the beginning of the Q wave to the end of the U wave.

JT interval : JT interval reflects repolarization alone , not both depolarization and repolarization.

JT interval measures the interval from the beginning of J point ( ST  T junction ) to the end of T wave. Duration varies from 0.15 to 0.4 sec. JT interval is the sum of ST and T wave duration.

JT interval = QT interval QRSd

JTc interval = JT interval corrected for heart rate.

JTc interval = QTc QRSd .

 

The fallacies of QT correction

There are various issues involved in the measurement of the QT interval especially regarding the ending of the T wave and dfferent morphological pattern of T U complex. The other issue is significant spontaneous variability in in the QT interval resulting in spurious QT prolongation and unnecessary concern.

The QT interval corrected for heart rate (QTc) is the traditional m,ethod of assessing the duration of repolarization . A prolonged QTc interval is associated with a higher risk of arrhythmia typically Torsade de pointes.

In the surface ECG, ventricular depolarization is represented by QRS and ventricular repolarization by ST and T which together forms the JT interval. When the QRS duration is normal, usually we take QT interval as a measure of repolarization. But when there is QRS widening, what happens to this QT interval? If there is prolongation of QT , is it due to prolongation of depolarization alone or is there any prolongation of repolaization?

We tried to find out this by analyzing 50 cases of intermittent narrow and wide QRS.

MATERIALS AND METHODS

50 patients in different clinical settings such as intermittent RBBB, pacing in sick sinus syndrome, pacing in brady arrhythmias during myocardial infarction and , intermittent preexcitation were analyzed. Patients already having broad QRS were excluded.

After a detailed clinical examination, simultaneous 12 lead ECG was taken for all patients during narrow QRS rhythm and broad QRS . in case of patients on temporary pacemaker , ECG was taken during patients own rhythm and pacemaker rhythm at the same rate. 

QRS duration was measured from the first deflection of the Q wave ( or R wave when the Q wave was absent ) and the end of the S wave  ( defined as the point of minimum voltage in the terminal phase of the QRS complex ). The QT interval was measured from the beginning of the QRS complex to the end of the T wave , where the end of the T wave was defined as the point where the T wave merges with the isoelectric line.1 QT interval was calculated as the mean QT of 6 leads in which QT could be easily measured. The JT interval was calculated by subtracting the QRS duration from the QT interval. The RR interval was taken as the mean RR interval over the 30 sec. recording period and the corrected QT interval ( QTc) was calculated by using the Bazetts formula: QTc = QT/√RR in seconds. The JTc interval was calculated by subtracting the QRS duration from the QTc interval . ST segment and T wave duration were also measured in the same leads where QT was measured.

All the measurements were taken during narrow QRS rhythm and broad QRS rhythm.

 

Statistical analysis

The data were tabulated and categorical variables were compared by chi-square test. A probability value of less than 0.05 was considered significant.

RESULTS.

50 patients who satisfied the criteria of the present study were enrolled. Most of the patients were elderly males who presented with myocardial infarction and bradyarrhythmias who underwent pacing. Male to female ratio was 3:1. 22 patients  (44%) underwent pacing following myocardial infarction and bradyarrhythmias, 8 patients  (16%) underwent pacing for sick sinus syndrome. 4 patients (8%) had intermittent pre excitation. 16 patients  (32%) had intermittent RBBB and acut myocardial infarction.

 

Table 1

Electrocardiographic parameters

 

Narrow QRS

Wide QRS

P value

Mean QRS

0.072

0.12

<0.0001

Mean QT

0.39

0.43

<0.0001

Mean QTc

0.44

0.49

<0.0001

Mean JT

0.32

0.31

NS

Mean JTc

0.37

0.37

NS

 

 

 

When the mean QRS width increased from 72ms to 120ms, QT interval and QTc also increased from 390ms to 430ms and from 440ms to 490ms respectively (P < 0.0001). mean JT and JTc did not show any significant change. ( table 1 )

 

 


  

 Table 2

Pacing for Bradyarrhythmias .

 

Myocardial infarcton

Sick sinus syndrome

ECG parametres

Narrow

QRS

Broad

QRS

Narrow

QRS

Broad

QRS

QRS width in seconds

0.077

0.125

0.06

0.13

QT in seconds

0.374

0.42

0.436

0.51

QTc

0.45

0.5

0.47

0.54

JT

0.297

0.297

0.38

0.38

JTc

0.37

0.37

0.41

0.41

 

 

Pacing and QRS width

When QRS with increased in patients with acute myocardial infarction and brady arrhythmias , QT interval and QTc interval also increased . JT , JTc remained the same.

When QRS width increased in patients with pre excitation , QT interval and QTc interval also increased. JT and JTc had no change.

 

Intermittent pre excitation.

When QRS width increased in patients with pre excitation , QT interval and QTc interval showed no change. JT  and JTc shortened.

 

Table 3  intermittent pre excitation

ECG parametres

Narrow QRS

Broad QRS

QRS width in seconds

0.08

0.1

QT

0.4

0.4

QTc

0.4

0.4

JT

0.32

0.3

JTc

0.32

0.3

 

 

Intermittent RBBB

When QRS width increased in patients with transient RBBB, QT interval and QTc also increased. JT and JTc showed no change.

 

Table 4  intermittent RBBB

ECG parametres

Narrow QRS

Broad QRS

QRS width in seconds

0.07

0.12

QT

0.38

0.43

QTc

0.44

0.49

JT

0.31

0.31

JTc

0.37

0.37

 

                           

On further analysis, when the QRS width increase was to >110ms both  QT and QTC increased without any change in JT and JTc.

But when QRS width increase was to < 110ms , there was no significant change in QT and QTc.

Discussion

QT interval is the measurement of the time between the start of depolarization of the ventricles of the heart and the end of repolarization of the ventricles. It measures the duration of the electrical systole. Homogeneous or heterogeneous changes in the action potential duration lead to alteration of the QT interval.

Zhou S H et al examined the influence of QRS duration on the JT and QT intervals in 20687 normal adult subjects and 2865 subjects with various categories of intraventricular conduction delay ( IVCD).4 QRS duration accounted for about 16% of total QT variation , but had a practically negligible effect on JT interval in complete bundle branch blocks. They concluded that it is preferable to use JT  rather than QT  as a more appropriate index of duration of repolarization in IVCD.

Das G evaluated the relative contribution of the depolarization and the repolarization time prolongation to the prolonged QT interval in patients with IVCD.5 The QRS, QT, and JT intervals were measured in 72 subjects with various types of IVCD. The observed intervals in IVCD subjects were compared to similar intervals in 33 healthy individuals in whom there was no evidence of intraventricular conduction abnormalities. The QTc interval was significantly prolonged compared to control group. The prolongation of QTc interval in each category of IVCD subjects was entirely secondary to a prolonged depolarization time , as the JT intervals were not significantly different from those observed in the control group. These observations may provide an explanation for the differential prognosis for subjects with prolonged QT interval with prolonged repolarization time as compared to those with prolonged QT interval and prolonged depolarization time.

Prolonged QTc interval is associated with higher risk of mortality in patients with coronary heart disease ( CHD) and in the general population.6 Crow RS et al assessed JT interval in these patients with CAD  and QRSd ≥ 120ms and concluded that the JTc is a simple measurement that is a significant independent predictor of CAD events in men with wide QRS complex.7

The present study population included 50 patients in 4 different clinical settings. Pacing for brady arrhythmias in acute MI, pacing in patients with sick sinus syndrome and bradyarrhythmias, transient RBBB and intermittent pre excitation were the study groups. As the QRS width prolonged to 110ms or more , QT and QTc prolonged. But there was no change in JT and JTc.

Four patients had intermittent pre excitation. In them , the mean QRS width prolonged to 100ms from 80ms. There was no change in QT and QTc. But JTc shortened. The prolongation of QTc interval in each category of subjects was entirely secondary to a prolonged depolarization time as the JT intervals were not significantly different from those observed in the narrow QRS group.

 

 

Summary and conclusion

Fifty patients in four different clinical settings in which prolonged QRS duration was taken. ECG parameters including QT , QTc , JT ,and  JTc  were measured in narrow QRS settings and broad QRS settings. In all the previous studies, patients with IVCD were compared with the normal population. In the present sudy, wide and narrow QRS were compared in the same patien.

As the mean QRS width increased to >110ms , both QT and QTc increased.

When the QRS width increased to <110ms from basal value, no change was noticed in the QT and QTc intervals

There was no change in the JT and JTc in subjects with prolonged QRS duration.

JT and JTc interval were found to be more reliable index than QT and QTc in the case of wide QRS.

 

 

References

 

1.kautzener J. QT interval measurements : Card. Electrophysiol. Rev. 2002 Sep; 6 (3): 273 7

2.Salim M A , Case C L , Gillette P C . The JT interval as a depolarization independent measure of repolarization : lessons from catheter ablation of the Wolff Parkinson White syndrome . Pacing Clin. Electrophysiol. 1995 ; 18 : 2158 2162 .

3. Bazette H C . An analysis of the time relations of electrocardiogram. Heart . 1920; 7:53 70 .

4.Zhou S. H, Wong S,Rautaharaju P M, Karnik N, Calhoun H P. Should the JT rather than the QT interval be used to detect prolongation of ventricular repolarization ? An assessment in normal conduction and in ventricular conduction defects. J Electrocardiol 1992; 25: Suppl: 131 6

5.Das G. QT interval and  Repolarization time in patients with intraventricular conduction. Indian pacing and electrophysiology journal ( ISSN 0972 6292), 4(4): 156 161 (2004).

6. Kenny R A , Sulton R. The prolonged QT interval : a frequently unrecognized abnormality. Post graduate Med J.  1985; 61: 279 286 .

7.Richard S Crow H D , Peter  J Hannan, Mstat, Aaron R Folsom , M D : increased QRS duration  ( QRSd) contributes to QT interval prolongation. Circulation 2003; 108: 1985 1989

 



 

 

 

 

 

 

 

 

 

 

 

 

 

 

 














































































































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