Myocardium at risk: Reasons and methods for measuring the extent
Alessia Gimelli
Daniele Rovai
FESC
If a coronary artery is acutely occluded, the downstream myocardium becomes severely under-perfused and at risk of necrosis. The area of myocardium at jeopardy of infarction is usually referred to as area at risk. It is intuitive that the extent of myocardium at risk downstream from an occluded coronary artery does not have the dimension of an area, but rather that of a volume or a mass. Indeed, the extent of myocardium at risk is usually expressed in grams or in percentage of ventricular myocardium. However, because the initial experiments were carried out by injecting dyes or radionuclide-labeled microspheres upstream from the occluded coronary artery, and because the myocardium at risk was defined postmortem as the non-stained or non-labeled area in cross-sectional slices of the heart, the term area at risk has become common and hence entered the medical dictionary.1,2 If a flow tracer is injected into a coronary artery proximal to its occlusion, and a second flow tracer is injected after the occlusion upstream the occluded artery, it becomes evident that the extent of myocardium perfused by a given open artery is larger than the myocardium at risk when the same vessel is occluded (Figure 1A, B). This difference is on average 20% in the canine heart, and is due to the collateral circulation.3 It has been traditionally assumed that coronary arteries are functional end-arteries under physiological conditions. However, even in the absence of coronary stenoses or in entirely normal hearts, brief periods of coronary
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occlusion produced by balloon inflation are not followed
by electrocardiographic signs of myocardial ischemia in
20%-25% of patients. This is attributed to collateral
circulation.4 This phenomenon is more prominent in the
case of preexisting collaterals, as observed in patients
with chronic coronary artery disease. Thus, the extent of
the area at risk after coronary occlusion is influenced
both by an anatomical factor (the size and hierarchy of
the occluded vessel) and by a series of functional factors
(largely related to the collateral circulation). For this
reason, an acute coronary occlusion causes a smaller
infarction and a less severe hemodynamic impairment in
an older patient with extensive coronary atherosclerosis
and well-developed collaterals than in a younger patient
without preexisting collaterals. Another functional
factor is how rapid is the occlusion. If the occlusion is very
slow (as it can be obtained in experimental animals and
as it occurs in patients with coronary lesions slowly
evolving toward total occlusion), collaterals have the
time to develop and to minimize the extent of
myocardium at risk. This goes in parallel with the common
observation of patients with even complete occlusion of
a proximal coronary artery and absence of myocardial
infarction. Finally, if the perfusion territory is small, as
when a secondary branch is occluded, collateral vessels
can totally obscure the area of myocardium at risk.
WHY MEASURE THE EXTENT OF MYOCARDIUM
AT RISK
There are several reasons for measuring the extent
of the area at risk in the clinical setting. First of all it is
important to have an idea of how large the infarction can
become. It is well known that the size of a myocardial
infarction depends upon several factors, including
duration of coronary occlusion, presence of a residual
stenosis, myocardial preconditioning, and myocardial
oxygen consumption during occlusion (as seen with the
effects of beta blockers). However, the infarction can
never become larger than myocardium at risk. In other
words, knowing the area at risk of necrosis during
coronary occlusion provides us information on the
worst-case scenario, which could be useful in clinical
decision-making. If this information was available,
prompt revascularization could be indicated in case of a
large risk area not accompanied by clear
electrocardiographic changes; alternatively, revascularization could
be postponed in the case of a very small risk area.
Other information that can be derived from the
assessment of the area at risk is the extent of salvaged
myocardium, which requires an estimate of myocardial
infarct size as compared to the area at risk (Figure 1C,
D). This information has been very useful in evaluating
the different reperfusion strategies over the past few
decades.
Finally, the location of the myocardium at risk
(rather than its extent) can be useful in identifying the
infarct-related artery. This is mostly true in patients with
extensive coronary artery disease in whom information
on the culprit vessel to be revascularized is not obvious.
HOW TO MEASURE THE EXTENT OF
MYOCARDIUM AT RISK
Although a variety of methods is currently available
to measure the risk area, each of them shows several
limitations in the clinical setting. An elevation of the ST
segment in the electrocardiographic leads reflecting the
area of myocardium at risk is the typical sign of acute (...truncated)