Clinical Research Article
 

A Study of heart rate variability and QT dispersion in patients of Acute
ST Elevation Myocardial Infarction


A. Wahab, M.S. Zaheer, M.U. Rabbani, S. Wahab
Centre of cardiology & Cardiovascular Research Department of Medicine,
Department of Radiodiagnosis, J.N. Medical College, Aligarh Muslim University, Aligarh.

Abstract

Aims and Objectives: To study the effect of Heart Rate Variability (HRV) and QT dispersion (QTd) in patients presenting with Acute ST elevation myocardial infarction (STEMI).

Methods: 100 patients of AMI were evaluated by history and examination. Electrocardiogram (ECG) and Holter monitoring was done in all patients. HRV, QTd, Standard Deviation of the mean RR interval (SDNN) and Standard Deviation of the average of RR interval measured every five minutes during 24 hr (SDANN) were calculated. Results were statistically analyzed using Student ‘t’ test, Paired ‘t’ test and Pearson’s formula.

Results: The most common presenting symptoms were chest pain (88%) and dyspnoea (50%). Tachycardia was seen in 565 while congestive heart failure was present in 29% patients. Mean SDNN was 61.4±24.4 ms and SDANN was 50±18.2 ms. HRV values were significantly decreased while QTd was higher in patients who died in comparison to patients who survived. A strong negative correlation was seen between HRV and QTd.

Conclusions: Markers of autonomic regulation of heart like HRV and QTd  provide valuable information about the future course of events in a patient following acute STEMI which can be utilized to plan the future course of management in patients especially predisposed to adverse and catastrophic outcomes.

Key words: Heart Rate Variability (HRV), Acute ST Elevation Myocardial Infarction (STEMI) , Q T Dispersion (QTd) , Standard Deviation of the mean RR interval (SDNN), Standard Deviation of the Average of RR interval measured every five minutes during 24 hours (SDANN)
INTRODUCTION

Coronary heart disease (CHD) has plagued man since time immemorial. With the passage of time contribution of CHD to the global disease burden has been increasing by leaps and bounds 1. Acute Myocardial infarction (AMI) represents one end of the spectrum of CHD. Despite remarkable advances in the treatment of acute myocardial infarction substantial early and late mortality remains. About 50% of death due to AMI occurs within 1 hr of the event and are attributable to arrhythmias mainly, late causes of death include electromechanical dissociation, cardiac rupture, cardiogenic shock etc. Full understanding and recognition of these changes is still lacking but several investigators suggest that alterations in the level and kind of autonomic control to the heart affect early and long term prognosis of the patient after AMI 2, 3. Experimental evidence of the association between propensity for lethal arrhythmias and either enhanced sympathetic or reduced Vagal activity has led to development of quantitative and qualitative markers of autonomic activity. Heart Rate Variability (HRV) has been suggested as one such marker of automatic tone of the heart. Analysis of HRV provides valuable

valuable information on the underlying neural control of the heart and therefore serves as a powerful and independent predictor of mortality after AMI 4,5 . Another important parameter that reflects changes in local myocardial milieu is QT dispersion (QTd). QT dispersion reflects differences in the local myocardial repolarization and hence the electrophysiological environment. Clinical interest in QTd on the surface ECG is based on the observation that regional heterogeneity of action potential in adjacent cardiac muscle tissue can initiate and sustain ventricular arrhythmias especially in vulnerable myocardium like that in ischemic heart disease (IHD)6,7 .
MATERIAL AND METHODS:

The present study was conducted on patients admitted with the diagnosis of acute ST elevation myocardial infarction. All the subjects were informed and explained the nature of the investigation undertaken. In all 100 patients of acute myocardial infarction were subjected to a complete evaluation in terms of history and examination as per the protocol and were then taken up for further specialized investigations.
Correspondence: Dr. M. Shoaib Zaheer, 4/3 - C, Anoopshahar road, Near SSP Residence, AMU, ALIGARH - 202 002 (UP), India.
E-mail: mszaheer@gmail.com
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Patients were included in the study if they fulfilled all of the following criteria:

  • History suggestive of acute coronary syndrome within the preceding 48 hrs of admission.
  • ST segment and T wave changes typical of myocardial infarction.
  • Positive cardiac markers: cardiac troponin T or CPK – MB

Patients were excluded in the study if they had any of the following:

  • Presentation after 48 hrs.
  • Prior ECG showing QRS duration > 120 msec (LBBB or RBBB)
  • Previously on drugs affecting QRS interval or heart rate variability
  • Diagnosed case of conditions affecting QT dispersion or heart rate variability: Diabetes, CHF, valvular heart disease, 
  • Non-ST elevation Myocardial Infarction.

The patients were evaluated on the basis of history and clinical examination Besides routine investigations standard 12 lead standard ECG was done in all cases on admission, after 4 hrs, 24 hrs, 48 hrs and on 7th day of admission on ECG strips running at speed of 25 mm/sec and at a setting of 1mv=10mm.QT interval was measured manually from the onset of QRS complex to the end of T wave. The end of T wave was considered the point of return to the isoelectric line. ECGs in which the QT interval was not measurable in more than 8 leads were excluded from the study. If U waves were present then QT interval was taken from the beginning of QRS complex to the lowest point between T and U wave. QT correction was deduced using Bazett’s formula i.e.
QTC =   QT dispersion  was calculated as QTd = QTmax – QTm­in
In addition holter monitoring was also done on every patient within 48 hours of developing symptoms. All patients in this study were put on 3-channel 24 hr holter monitoring for determining heart rate variability. The Standard Deviation of the mean RR interval (SDNN), Standard Deviation of the average of RR interval measured every five minutes during 24 hr (SDANN) was deduced using specialized software (Zymed 1810 Digitrak Plus version 2.5a ). The results were then visually analysed and manual corrections made wherever required.
Statistical Analysis was done using Student ΄t΄ test for unpaired samples to compare the differences between two groups. Paired ΄t΄ test was used to check the significance of difference between observed values within the same group while Pearson’s formula was used to calculate the coefficient of correlation between two variables and the significance of ‘r’ was tested.

RESULTS:

Out of 100 patients taken in the study 71% were males and 29% were females.The maximum number of cases were >50 years old with the largest group belonging to 51-60 year strata. None of the patient was >75 years of age. Female: Male ratio progressively increased as the age of the patients gradually increased to reach a value of 1:1 after 70 years.Most commonly patient presented with chest pain , being present in 88% patients followed by dyspnea in 50% patients. Other prominent symptoms were vomiting, palpitations and sweating that was seen in about 1/3rd   of patients.
The most common physical finding was tachycardia that was present in 56% of   patients, congestive heart failure (CHF) developed in 44 patients with 29 % of patients demonstrating evidence of right sided heart failure. Hypotension was present in 20% of patients.
The mean SDNN of all patients was 61.4 ±  24.4 ms and SDANN was 50 ± 18.2 ms. There was marked difference in HRV parameters between anterior and non-anterior wall infarction .Those with anterior wall myocardial infarction had statistically significant lower SDNN and SDANN as compared to inferior /lateral wall MI (Table 1) . SDNN values ranged from 144.3 to 42.1 ms and SDANN from 96.8 to 31.3 ms. In patients with anterior wall MI the maximum SDNN value was 82.5 and minimum value was 42.1 whereas SDANN values for the same were 64.8 and 31.3 milliseconds  respectively.   

Table 1: Heart Rate Variability Distribution in Myocardial Infarction


Heart rate Variability
(in ms)

Acute Myocardial Infarction

Anterior Wall   MI

(1)

Non Anterior Wall MI
(2)

p value
1-2

SDNN

61.4 ± 24.4

52.5 ± 10.7

74.7 ± 32

<0.001

SDANN

50.9 ± 18.2

44.5 ± 10

60.7 ± 22.8

<0.001

HRV values were significantly decreased  in patients who expired when compared with those who survived with SDANN values being more decreased (Table 2). The mean SDNN and SDANN values were 43 ± 1.1 and 34.6 ± 2.9 milliseconds in the patients who died during the hospital course whereas in those who survived it was 64.0 ± 25.1 and 53.2 ± 18.9 ms. All patients who died had SDNN and SDANN < 50ms. We found that SDNN value < 40 ms and SDANN < 45ms was significantly associated with mortality (p<0.001).

Table 2: Heart Rate Variability

Expired Versus Survivors

HRV(in ms)

Expired

Survived

p value

SDNN

43  ± 1.1

64.0 ± 25.1

< 0.05

SDANN

34.6 ± 2.9

53.2 ± 18.4

< 0.001

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Heart rate Variability and QTD in STEMI
 

Table 3: QT Dispersion in Acute Myocardial Infarction

Time

QT Dispersion
(in milliseconds)

On Admission

124.5 ± 22.9

After 4 Hours

114 ± 24.7

After 24 Hours

110 ± 22.6

After 48 Hours

108 ± 29.6

On Seventh Day

94.3 ± 16.7


During QTd  analysis the maximum dispersion occurred on admission being
124.5 ± 22.9 ms, it declined progressively with time to 110 ± 22.6 ms after 24 hours and 94.3 ±  16.3 ms on the 7th day of admission (Table 3). The maximum QT dispersion was 172 ms and minimum was 94 ms in the study group on admission.  Opposite to results of   HRV we found that, patients who expired had higher   QT dispersion on presentation (164 ± 10.4 ms) in comparison to survivors (119.1 ± 18.6 ms) (Table 4) .The difference between the two

Table 4: QT Dispersion : Survivor  Versus  Non Survivors

QT Dispersion

Survived

Expired

p Value

On Admission

119.1 ± 18.6

164 ± 10.4

<0.001

After 4 Hours

107.9 ± 18.5

158.7 ± 19.8

<0.001

After 24 Hours

105.6 ± 17.2

160 ± 16.6

<0.001

After 48 Hours

99.4 ± 16.9

171.3 ± 27.6

<0.001

Table 5: Correlation Between HRV and QTd

QT Dispersion

Correlation with SDNN

p value

Correlation with SDANN

p value

On Admission

-0.53

<0.01

-0.54

<0.01

After 4 Hours

-0.5

<0.01

-0.5

<0.01

After 24 Hours

-0.48

<0.01

-0.47

<0.01

After 48 Hours

-0.46

<0.01

-0.46

<0.01

On 7th Day

0.005

>0.01

-0.02

>0.01

was significant (p<0.001) .No mortality occurred after 48 hours and this relationship was maintained during 48 hours.
When we analyzed the relationship  between HRV and QTd it was found that there is a strong negative correlation between the two. The highest correlation was present on admission (r = -0.53 and r = -0.54 p <0.01) for both SDNN and SDANN. With time this significant correlation progressively decreased and on the 7th day it became insignificant (Table 5).                                                                                                                                       

DISCUSSION:

The primary finding of this study is that measurement of HRV and QTd early in patients with STEMI is a useful prognosticator,

 

their analysis can predict patients at higher risk of adverse outcomes, furthermore both these variables are strongly correlated not only with mortality but also with each other.
The mean age of study subjects was 53.5± 10.0 years. The maximum number of cases were ≥ 50 years (72%), within this group the incidence was highest in 51-60 years (36%).
A interesting finding was that none of the patient was more than 75 years of age . Females constituted around 30% of patients with the maximum number lying in 51-60 year age group however their ratio with males increased progressively with age to reach a value of 1:1 after 70 years.
The most common complaint in patients diagnosed to be suffering from STEMI has usually been chest pain 8,9 . It’s incidence was 96%  in the study of Richman8 .Other common presentation during various studies had been diaphoresis  nausea , dyspnea , light-headedness8,9 . In our study the incidence of chest pain was slightly less at 88% followed by dyspnea in 50%. Vomiting was seen in 40% of patients and profuse sweating episode in 38%.
In the present study HRV analysis was done within 48 hrs of symptom onset . The mean SDNN was 61.4±24.4 & mean SDANN 50±18.2 ms. Patients with anterior wall MI had significantly lower HRV parameters (SDNN 52.5±10.7 Vs 74.7±32 and SDANN 44.5±10 Vs 60.7±22.8) as compared to non-anterior wall MI (p value < 0.001).Though some previous studies had shown similar results10,11,12, these results have shown significant variability13,14. The significant difference in HRV between anterior and non anterior wall myocardial infarction can be attributed to the large size of anterior wall MI thereby causing greater perturbation of autonomic control and renin-angiotensin system.
When HRV parameters were compared between patients who survived and those who did not we found SDNN values were significantly lower in patients who did not survive (43±1.1 Vs 64.0±25.1, p < 0.05). Similar results were obtained when SDANN was compared (34.6±2.9 versus 53.2±18.4, p < 0.001). Moreover significant association with mortality was observed when SDNN values did not exceed 40 ms and also when SDANN value did not exceed 45 ms, this finding of our study has important implication in the respect that these cut-off values can be used to mark patients likely to have adverse outcome during the course of their hospital stay, similar results had been obtained by others though their cut-off values for adverse outcome had been slightly higher15.

In the present study mean QT dispersion was highest on the day of admission (124.5±22.9 ms) and it declined progressively to 110±22.6 after 24 hrs and to 94.3±16.7 on seventh day of admission. Patients of anterior wall MI had significantly greater QTd than non-anterior wall MI (on admission 137.3±16.6 versus 101.8±13.1 p<0.001). This difference was maintained throughout the course of hospital stay of the patients. Similar results had been obtained by other authors in their studies16,17
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though these studies evaluated QT dispersion after 12 hours of admission, unlike ours where we evaluated QTd on admission and subsequently at 4 hours,24 hrs, 48 hrs and on the 7th day of admission.
Finally we wanted to know whether there is any correlation between HRV and QTd. Though Qiang18 had found a significant correlation between QTd and HRV in his study on patients with Acute MI, his study was limited to the analysis of association between QTd and SDNN .When we compared HRV with QTd a strong negative correlation was seen to exist between QTd and both SDNN and SDANN. The strongest correlation was seen at admission   (r =-0.53 and -0.54, p<0.01 respectively) after which the strength of association progressively declined to become insignificant by the 7th day of admission.
Conclusion:
To conclude it can be said that markers of autonomic regulation of heart like HRV and QTd  provide valuable information about the future course of events in a patient following acute ST elevation MI and these variables can be utilized to tailor the pace and course of management in patients especially predisposed to adverse and catastrophic outcomes. Moreover more tight regulation of autonomic tone in such patients may better the prognosis in such patients.



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