History Of The Transvenous Pacing Biology Essay

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A number of studies have analyzed the incidence of CAVB and its impact on the prognosis of patients with inferior wall acute MI, both in the prethrombolytic era and later in the thrombolytic era. However little is known about the characteristics and the implications of CAVB in patients of our population .

INCIDENCE

Earlier western studies of thrombolytic era {Thrombolysis in Myocardial Infarction (TIMI) II Trial} show an incidence of AVB of about 12% 2 However reported incidence appears to be far more in our population than in western population. Recent data from our population shows an incidence of AVB of about 30%, and that of third degree AVB of about 21%10 Other studies have reported an incidence of High degree AVB of 23.6% 8 and 29.4% 7 In fifty percent of cases heart block evolves through gradual progression of conduction delay while in others heart block appears abruptly. 80 In the present study AVB developed in 29%, with high degree AVB in 24.8% which collaborates with the contemporary recent local literature.

AV node has dual blood supply from LAD through the first perforator artery as well as from the AV nodal artery originating from either RCA or left circumflex

artery. This causes absence of severe necrosis in the AV node even after interruption of flow to the dominant artery 85-86

The two most common explanations to explain the aetiology of the heart block are 1) interruption of blood supply to AV node and 2) and increased vagal tone as a result of Bezold-Jarisch reaction .81 The accumulation of metabolites, such as adenosine and potassium is also suggested as a likely reason for the AVB.

The presence of complete AVB in acute inferior wall MI is an indicator of a larger infarct size. 88,42, 81 Autopsy studies in patients with AVB showed larger areas of infarction than infarction of the AV node as a possible cause behind AV block 85-86

IMPLICATIONS

The prognostic implication of heart blocks in inferior MI is not well understood. Although inferior MI is considered to have a benign course significant mortality is reported when accompanied with AVB. The mortality is documented to be about 7.1% 2 18.6% 8 up to 23% with high degree AV block and 29% with third degree heart block 81 As with earlier literature 7,,87 , patients in present study with complete AVB were older and exhibited a higher in-hospital mortality ( 7.1% of AVB group ).The incidence of syncope and post MI angina was also more in

patients having AVB as compared to those without AVB in present study as is reported in other studies. 10

About 8% of patients have high degree heart block on hospital arrival; two thirds of the patients develop high degree heart block within 24 hours of admission. Nearly all the rest of patients develop heart block within 3 days of admission. 81 The heart block is usually responsive to atropine or isoproterenol, and does not require placement of a temporary pacemaker, and rarely ever requires permanent pacing 81

In our study heart block reverted to sinus rhythm after atropine in 11 patients (21%), after thrombolysis in 5 patients (10%).

In inferior wall myocardial infarction, advanced age, female gender,, complete atrioventricular block are independent predictors of poor in hospital outcome. Thrombolysis is generally useful, especially in these high-risk subgroups 55

FIBRINOLYSIS

Thrombolytic therapy in acute inferior MI has been shown to reduce the incidence of complete AVB in recent literature 16, 21 from 10% to15% in the pre-thrombolytic period to 6% to10%7 in the thrombolytic era. This small decrease is due to short therapeutic window (no more than 6 hours) in inferior MI which limits the patients eligible for thrombolysis. 2

However other studies documented a reduced duration but not a reduced incidence of AV block with fibrinolytic therapy. Patients receiving thrombolysis in inferior MI had an incidence of AVB of 13.8% as compared to 8% in those not receiving thrombolysis. With median duration of block reduced to 75 minutes from 24 hours in thrombolytic group when compared to those not receiving thrombolysis. TPM was implanted in 43 %of fibrinolytic group as compared to 84.6% of non fibrinolytic group.60

In the present study 84.5% of total with 70.9% of AVB and 89.9% of non AVB group received thrombolysis.23.9% of patients receiving thrombolysis developed AVB and 62.5% of those not receiving thrombolysis had AVB contrary to the recent literature showing a much more incidence of AVB block in non fibrinolytic group. TPM implanted in 15.5% patients (52.7% of AVB group. Our study revealed that reperfusion therapy was associated with a significant shortening of the duration of AVB compared with conservative treatment .This finding is in accordance with a previous studies.90

TRANSVENOUS PACING

Although placement of transvenous catheter is life saving especially if hemodynamic compromise is evident and the arrhythmias are not responding to atropine ,their implementation is not without hazards and is associated with potential risk for arrhythmias, perforation of ventricular wall, cardiac tamponade and infectious complications. Temporary pacing has been

associated with multiple complications; the frequency of complications ranges from 13.7% to 33% of patients in literature series 92-93

An alternative to Transvenous pacing is use of transcutaneous pacemaker but the technique is generally not well tolerated and requires sedation and analgesia which may further compromise the patient hemodynamically, and therefore is not commonly used.

PERMANANT PACING

Long-term permanent pacing after complete AVB is uncommon, being reported in less than 1.9% in literature.94

This relatively low incidence may be related to the ACC/ AHA Guidelines that recommend permanent pacing only when complete AVB is persistent. Among our hospital survivors, only 02 (1.03%) patients required permanent pacing with persistent AVB 94

Conclusions

Third degree AVB occurred in acute inferior MI in about 22% of patients in the study under discussion. Complete AVB usually had an early onset and was commonly transient (especially in patients receiving thrombolysis). Complete AVB developed more frequently in the elderly patients, and was accompanied with a much more extensive infarct. Thrombolytic treatment not only greatly decreased the percentage of patients developing AVB, but it also reduced the duration for which AVB was present, lead to an overall and reduction in in-hospital mortality rate. Only 02 patients required a permanent pacemaker in their hospital stay.. The presence of high degree AVB in acute inferior myocardial infarction is an represents an extensive infarct size. 3,5,6 In our study patients developing high degree AVB in the setting of acute inferior MI were much older than those with no AV blocks , and were more frequently accompanied by RV infarction which is consistent with earlier literature. Cardiogenic shock, as well as larger infarct size was also more commonly observed in these cases.

Reperfusion

Over the last 20 years, the in-hospital mortality resulting from high degree AVB in association with inferior wall myocardial infarction has greatly declined. , which is probably due to the increased implementation of fibrinolysis and administration of other related drugs. Reperfusion therapy has been reported to reduce the incidence of complete AVB in acute inferior MI in recent studies, 16 ,23

This reduction is reported to be as much as from 10 to15 percent in pre-thrombolytic era to about 6 to10 percent observed recently 7 in the thrombolytic era. This relatively only small decrease in incidence can be explained by the short therapeutic window that is less than 6 hours in inferior wall myocardial infarction limiting the patient eligible for the thrombolytic treatment. In early studies about one-third of high degree AVB were of late onset and developed after 24 hours of symptom onset. 2 This ratio was considerably reduced in current study to one-fifth. Only 5% of patients who were given thrombolytic treatment were found to develop late onset complete heart block as compared to 19 percent developing late onset complete heart block that were conservatively managed. This difference however was not significant statistically, perhaps as a result of small patient number. Study under discussion also exhibited that thrombolytic treatment was accompanied with a considerable reduction in the duration of high degree AVB when compared to conservative therapy alone.

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