Introduction

Myocardial infarction is a common presentation of ischemic heart disease. Ischemic heart disease is the leading cause of death in developed countries, but third to AIDS and lower respiratory infections in developing countries. In India, Ischemic heart disease is the leading cause of death. Although a relatively new epidemic in India, it has quickly become a major health issue with deaths due to Ischemic heart disease expected to double during 1985-2015.Mortality estimates due to Ischemic heart disease vary widely by state, ranging from 10% in Meghalaya to 49% in Punjab (percentage of all deaths). Punjab (49%), Goa (42%), Tamil Nadu (36%) and Andhra Pradesh (31%) have the highest Ischemic heart disease related mortality estimates. State-wise differences are correlated with prevalence of specific dietary risk factors in the states. Moderate physical exercise is associated with reduced incidence of Ischemic heart disease in India (those who exercise have less than half the risk of those who don't). Ischemic heart disease also affects Indians at a younger age (in their 30s and 40s) when compared to developed countries.

QTc dispersion is an important marker that reflect variations of ventricular repolarisation and arrythmogenic potential and sudden death. This has been proved by extensive studies^3,5,6,8,9,10 done earlier. This study is based on various studies suggesting significant reduction in QTc dispersion after thrombolytic therapy in acute myocardial infarction^1,5,8,9

Aims Of The Study

To calculate the QT, QTc, QTd, QTcd in all patients admitted with acute myocardial infarction and to analyze the difference of QT parameters in patients treated with thrombolytic agents (streptokinase) against those not treated with thrombolytic agents (streptokinase).

Materials & Methods

A total of 102 patients admitted in Chennai Medical College Hospital Research centre, Tiruchirapalli,Tamilnadu, India with Acute Myocardial infarction were taken up for the study. A proper Ethics approval was obtained from the Institutional Ethics Committee of Chennai Medical College Hospital Research centre, Tiruchirapalli. All patients were followed for a period of 8±2 days during their stay in the hospital. The patients who had Acute Myocardial infarction based on, (a) Chest pain >30 minutes, Chest pain not relieved by rest or nitrates, ST elevation >1mm or 0.1mv in ≥2 limb leads, ST elevation >2mm or 0.2mv in 2 ≥ precordial leads, b) NSTEMI were included in the study. Treatment with early thrombolytic therapy(within 12 hours of onset of chest pain) and without thrombolytic therapy was noted. Patients who had a contraindication for thrombolytic therapy (for those patients who were treated with thrombolytic therapy),patients who were on drugs affecting QT interval like quinidine, procainamide, tricyclics& tetracyclics depressants,astemizole,digitalis were excluded from the study. Patients with acute carditis, atrial fibrillation, bundle branch blocks, hypertrophic cardiomyopathy history of prior coronary bypass surgery,with serum potassium <3.5 mmol/l or > 5.0mmol/l and patients with congenital long QT Syndromes were also excluded from the study.

In Patients admitted for Acute Myocardial infarction, a standard 12 lead ECG was taken at paper speed of 25 mm/s at admission and before discharge(day 8±2).From these ECG’s taken in all patients the following parameter were calculated.1.QT INTERVAL: It was measured from the first deflection of the QRS complex to the point of T wave to the isoelectric TP baseline. It represents the total duration of ventricular activity i.e the sum of ventricular depolarisation and repolarisation.2. QTc INTERVAL: QT interval shortens with tachycardia and lengthens with bradycardia. So it is corrected using Bazett’s formula.RR interval is measured between two consecutive R waves.Normal range of QTc is 0.35 to 0.43 sec.3.QT & QTc Dispersions: They are defined as the difference between the maximum and minimum QT,QTc in each of the 12 leads studied.The QT parameters were correlated between two groups taking into the following variables- site of infarction, age of the patient, diabetes, hypertension, smoking, alcoholism. Since the peak creatine kinase level have no correlation with QT parameters, it was not considered.

Results And Observation

Composition of the Study Population ( Table 1). A total of 102 patients were taken up for the study.Of these 56 patients were treated with thrombolytic therapy and 46 patients were not treated with thrombolytic therapy.There were 92 males (90%) and 10 females(10%)- Figure1.Anterior wall infarction constituted 49%,extensive anterior 10% and inferior wall 41%-Figure 2There were only 6 patients with NSTEMI. Diabetes, Hypertension, Smoking were found as main risk factors (Table 2) (Figure4)When considering the age, most of patients were in the age group of 40-49 and 50-59(Table 4)( Figure 5). Age and QT parameters (Table 5)The QT parameters were correlated among different age groups. The QT parameters showed significant variation between the patients treated with thrombolytic therapy and not treated with thrombolytic therapy, in age groups 40-49,50-59,60-69.The other age groups did not show significant statistical variation,as the number of patients was small. QT parameters and Thrombolysis and Site of Infarction (Table 3).The QT parameters were correlated among study groups and it was found that there was significantly greater reductions in QT parameters at day 8±2 in patients(Figure 3) treated with thrombolytic therapy when compared with not treated with thrombolytic therapy.It was noted that anterior wall infarction^5,9 show significantly greater QT,QTc dispersions when compared with inferior wall infarction.These differences in the QT parameters were all statistically significant.

Figure 2

Table: 2 risk factors and myocardial infarction

Figure 3

Table: 3 qt parameters and thrombolysis

Figure 4

QT parameters in thrombolysed patents and site of infarction

Figure 5

Qt parameters and site of infarction in not thrombolysed patents

Figure 6

Table :4 age and site of infarction

Figure 7

Table 5:age & qt parameters in thrombolysed patients

Figure 8

Age & qt parameters in not thrombolysed patients

Figure 9

Figure 1: sex distribution in study population

Figure 10

Figure 11

Figure 2

Figure 12

Figure 3

Figure 14

Figure 6

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Discussion

Acute Myocardial Infarction is a common cause of sudden death in our rural area, where the risk factors of myocardial infarction like diabetes, hypertension, smoking are so prevalent . It has been suggested that QT dispersion (maximal minus minimal ^{QT interval calculated on a standard 12-lead ECG) could reflect regional variations of ventricular repolarization and could provide a substrate for reentry ventricular arrhythmias2,3,4.Analyzing the QT dispersion will definitely be helpful in rural settings as a noninvasive, simple predictor of arrythmogenicity of the heart and hence will aid in the treatment of this life threatening ailment.Previous studies have proven that successful thrombolysis1,7 significantly decreases the QT parameters and thereby the arrythmogenic potential and hence it decreases the risk of sudden cardiac death6,10,12 in patients with acute myocardial infarction.}

The present study evaluates QT dispersion in patients with acute myocardial infarction treated with early thrombolytic therapy when compared with those who were not treated with thrombolytic therapy. As given in most of the studies cited in the review of literature male¹ patients out numbered the females,constituting 90 % of the study population . A total of 60 0f 102(59%) patients were in age group of 40 to 59 years, which is characteristic of incidence of acute myocardial infarction in a developing country. Sex and age of the patients did not influence QT dispersion. With regard to age and occurrence of In the first 3 days, QT dispersion was not different ^{among those patients treated with thrombolysis and those not treated with thrombolysis, whereas on day 8±2, QT dispersion was greater in those not treated with thrombolysis .When correlated totally with all walls of infarction taken together or individually, there were significantly greater reduction in QT,QTc dispersions1 in patients treated with thrombolytic therapy when compared with those who were not treated with thrombolytic therapy.The anterior wall acute myocardial infarction showed significantly greater QT parameters when compared with inferior acute myocardial infarction patients. And there was significantly greater reductions in QT parameters in patients treated with thrombolytic therapy when compared with those who were not treated with thrombolytic therapy at day 8±2 These reductions were also statistically significant.}

Conclusion

Early successful thrombolysis significantly reduces QT dispersion in acute myocardial infarction and hence the ventricular excitability and the subsequent ventricular arrhythmias and the risk of sudden death is substantially decreased by successful thrombolysis. Patients with anterior acute myocardial infarction showed significantly greater QT parameters when compared with inferior acute myocardial infarction patients^5,9. There were significantly greater reduction in QT,QTc dispersions after treatment with thrombolysis than without it^1,7. QT,QTc dispersions are greatest in the early hours of acute myocardial infarction and fall with time and successful thrombolysis^5,8.^.These results can be taken into account in the risk stratification for malignant ventricular tachyarrythymias^3,5,6 and its management and they are another evidence for the benefits of thrombolytic therapy in patients with acute myocardial infarction. The QT dispersion will be definitely helpful in primary health care as a noninvasive, reproducible and simple test to predict the arrythmogenicity of the heart and hence will aid in the more intensive management of this life threatening condition.