Stenosis

Figure 2. The effect of stenosis on mycocardial flow. At rest (curve A), flow is maintained until a critical stenosis of approximately 90% is reached. With stress (curve B), flow increases by a factor of 4. As the degree of stenosis increases, flow is maintained until approximately a 50% stenosis is reached. In this example, at rest there is no difference in flow between myocardium supplied by a normal artery and that supplied by an artery with a 70% stenosis. However, at stress flow will be reduced to myocardium supplied by the stenotic artery.

is encouraged to continue exercising an additional 1-2 minutes to allow tracer uptake during the time of maximal coronary flow.

The diagnostic accuracy of the examination is dependent on the ability of the patient to exercise sufficiently to induce maximal vasodilatation. A frequently used index to determine if the patient has exercised adequately is whether the patient has attained his target heart rate (THR). The THR is 85% of the predicted maximum heart rate (PMHR) where the PMHR = (220 - age) beats per minute.

Pharmacologic Agents

Pharmacologic vasodilatation with adenosine or dipyridamole is indicated for patients who are unable to exercise or are unable to increase their heart rate (rate-limiting medications) and patients with a left bundle branch block (LBBB) or paced ventricular rhythm.

Due to delayed septal activation in patients with LBBB, there is a corresponding delay in relaxation of the septum resulting in less time for diastolic flow. At rest, this

effect is negligible but as the heart rate increases (and diastole shortens) flow to the septum may be compromised in the absence of coronary stenosis. Vasodilator stress is preferred in this circumstance because it is not associated with a significant increase in heart rate.

Vasodilator stress with adenosine or dipyridamole is contraindicated in patients with significant reactive airway disease. Methylxanthines such as caffeine and theophylline competitively block adenosine receptors and must be avoided for 24 hours prior to the examination.

Exercise and dobutamine increase myocardial oxygen demand and in the presence of hemodynamically significant coronary stenosis result in myocardial ischemia. Vasodilator stress with dipyridamole or adenosine usually creates flow heterogeneity by causing a greater increase in coronary blood flow in normal coronary arteries compared with coronary arteries with significant stenosis. Although myocardial ischemia occurs much less commonly with these agents, there is the potential for the development of ischemia due to a decrease in distal perfusion pressure and/or the development of a coronary steal. Side effects occurring with vasodilator stress can be rapidly reversed by the intravenous administration of aminophylline (100-200 mg over 2-5 min).

Adenosine

Adenosine activates specific receptors in vascular smooth muscle resulting in smooth muscle relaxation and vasodilatation. This results in approximately a fourfold increase in coronary flow. Adenosine is administered intravenously at 140 ^g/ kg per minute for three minutes followed by radiotracer injection into a different vein and continuation of the adenosine infusion for an additional three minutes.

Dipyridamole

Dipyridamole blocks the cellular re-uptake of endogenously produced adenosine. It is administered as an intravenous infusion of 0.56 mg/kg over 4 minutes. The radiotracer is injected after an additional 4 minutes (i.e., 8 minutes after the start of the dipyridamole infusion) when there is maximal increase in coronary flow. The use of low-level supplemental exercise (e.g., Bruce stage I) following the infusion is used in some centers. It serves to lessen symptoms as well as to improve image quality by reducing infradiaphragmatic splanchnic activity.

Dobutamine

Dobutamine is a beta adrenergic agonist with both positive chronotropic and inotropic effects resulting in coronary arteriolar dilatation secondary to an increase in myocardial oxygen demand. Dobutamine is given as an intravenous infusion in incremental doses starting at 5 ^g/kg/min and gradually increasing, at three-minute intervals, to 40 ^g/kg/min. If there is a submaximal increase in heart rate, atropine is often administered during maximal dobutamine infusion. Dobutamine is less potent than either adenosine or dipyridamole for maximizing coronary blood flow (two to threefold increase in coronary flow) and may be associated with a lower sensitivity in the detection of CAD. It is primarily used for patients with reactive airway disease in whom adenosine and dipyridamole are contraindicated.

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