Myocardial Infarction

• Introduction

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• ST Elevation Infarction
• Thrombolytic Therapy- Reducing Mortality
• Thrombolytic Therapy- Additional Benefits

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• Assessing Extent of Infarction
• ECG and Q Waves

Myocardial Infarction

Myocardial Infarction- Definition

The advent of highly sensitive and specific new markers of myocardial injury such as troponin I and troponin T lead to re-definition of myocardial infarction. Patients that previously were labelled as having unstable angina were now said to have had myocardial infarcts on the basis of significant troponin elevations. Most of these patients, that have not had previous myocardial infarcts, will still have good systolic function if the acute event is associated with only a troponin elevation without significant CK enzyme elevation.

Myocardial Infarction- With ST-Elevation

ST elevation infarction is almost always due to thrombotic occlusion of a coronary artery. It can be the first manifestation of ischaemic heart disease (IHD), or may occur in patients with recent onset angina or in those with chronic ischaemic heart disease.

The initial dramatic gains were obtained with use of thrombolytic therapy and low dose aspirin. Further advances were made with the use of primary angioplasty. More recently, trials have shown the value of transferring patients to interventional centers for primary angioplasty and alternatively ensuring frequent use of percutaneous intervention after thrombolytic therapy.

The occluded artery, in a significant proportion of patients, can re-open spontaneously. Early treatment is aimed at reducing myocardial damage by attempting to re-open the patent artery as soon as possible.

One of three approaches may be adopted:

• Administer thrombolytic therapy
• Proceed to primary angioplasty
• this might occur in hospitals with on-site interventional catheter labs, or
• this might occur after transfer from non-interventional centers incurring some delays
• Administer thrombolytic therapy with frequent use of percutaneous intervention. Percutaneous intervention
• may be done early if there seems to be failure of reperfusion in patients with large infarction, or
• may be done later to give patients the benefit of percutaneous intervention- this seems very likely to benefit patients with significant viable myocardium in the infarct zone, it might provide benefit to patients with little viable myocardium by maintaining an open infarct artery.

Myocardial Infarction- Without ST-Elevation

Also known as Non-ST Elevation Myocardial Infarction or NSTEMI

Those patients who present without ST elevation tend to have a non-occlusive thrombus in the infarct related coronary artery. The exception is many patients with circumflex artery disease that present with ST depression rather than ST elevation and attempting to recognise these patients early and consider immediate percutaneous intervention is important.

It is long been recognised that patients with relatively minor or small myocardial infarcts have the same prognosis at those who present with ST elevation infarction (who generally have moderate or large infarcts).

Anecdotal evidence proved the value of percutaneous intervention in this setting, when patients with refractory angina obtained dramatic benefits and seemed to be at low risk of recurrent problems. Not surprisingly, this period was marked by significant variations in treatment patterns, overseas and here in NZ, undoubtedly influenced by access to resources and also whether cardiologists in individual centers were "early adopters" or "conservatively inclined".

A trial from the Veteran's Hospitals in the United States of America (the VANQWISH trial) published in 1998 raised doubts about the value of an early invasive approach. Critics pointed to the relative high rates of mortality after bypass surgery as a confounding factor. Given the controversy generated, the "early adopters" and "conservatively inclined" seemed to maintain their positions with little apparent change to practise.

The FRISC II trial, published in 1999, proved that an invasive approach with almost routine angiography and very high early use of percutaneous intervention and bypass surgery was superior to the "conventional" approach. The conventional approach in this trial, however, was very conservative by North American or Australian standards, but probably similar to the "routine" practise in many New Zealand centers. Publication of the one year followup results in 2000 seemed to confirm the need to change our approach and provided "hard data" with which to try to make some calculation of "cost-benefits".

For the North Americans, the FRISC II trial seemed less relevant. The low rates of intervention in the "conservatively treated" group was not comparable with their "routine" practise and this led to the TACTICS trial. The TACTICS study was published in 2001 and included the use of a glycoprotein IIb-IIIa blocker- tirofiban- and also compared a "conservative" with an "invasive" approach. The TACTICS study showed a trend towards reduced mortality but a definite reduction in reinfarction with a more "invasive" approach.