Pharmacologic Stress Testing

Fig. 6.1

Auto-regulation of flow in normal and hyperemic states (Adapted from Gould et al. [3]). At rest, coronary flow rate is approximately 1 ml/g/min. As coronary stenosis worsens to ~90 %, symptoms develop. During vasodilator pharmacological stress testing, coronary flow rates increase two to fourfold and coronary stenosis can be detected at 60–70 % stenosis

Fig. 6.2

Coronary flow in normal and stenotic arteries during rest and vasodilator stress testing. In normal state (a), flow distal to stenosis is compensated by vasodilation distal to the obstruction. During vasodilator stress testing (b), non-obstructed coronary arteries induce hyperemic response with increased flow. However, in stenotic arteries, the area distal to stenosis is already maximally dilated at rest and cannot further dilate during stress. This creates a relative hypoperfusion on MPI compared to the normally perfused myocardium

The very short half-life of adenosine mandates that the infusion continue during the radiopharmaceutical delivery and uptake (1–2 min). However, the offset of hyperemia is also very rapid. In contrast, dipyridamole and regadenoson have a longer duration of action and may require reversal with aminophylline should side effect develop. Regadenoson is a selective A2A agonist, which limits the stimulation of non-vasodilatory receptors and reduces adverse effects. Additionally, this agent is not depend on weight based dosing.

Dobutamine is most commonly used with echocardiography but may also be used in conjunction with nuclear cardiology methods and CMR. Similar to exercise testing, dobutamine induces a positive inotropic and chronotropic response with increase in heart rate and systolic pressure, thereby, increasing myocardial oxygen consumption and induces ischemia and wall motion abnormalities or perfusion defects in the myocardium supplied by a stenotic coronary artery.

Indications

The indications for pharmacological stress testing is similar to exercise stress testing (see Chap. 4) and according to the 2002 ACC/AHA guidelines [1] (Table 6.1).

Table 6.1

Indications for stress testing

1.	Symptoms suggestive of CAD
2.	Acute chest pain after ACS is ruled out
3.	Recent ACS not treated with coronary angioplasty
4.	Known CAD and change in clinical status
5.	Prior incomplete revascularization
6.	Newly diagnosed cardiomyopathy
7.	Certain cardiac arrhythmias: atrial fibrillation, PVCs, VT
8.	Pre-op cardiac assessment prior to non-cardiac surgery

Contraindications

All pharmacologic testing modalities should be avoided in patients with unstable clinical conditions such as hypotension, decompensated heart failure, unstable coronary syndromes or recent ACS, uncontrolled arrhythmias, and severe aortic stenosis. Absolute contraindication is hypersensitivity to any of the stress agents.

With regards to the use of vasodilator stress, the contraindications are related to the effects on the other adenosine receptors (Table 6.2).

Table 6.2

Contraindications for vasodilator stress testing [2]

1.	Asthma or COPD with active wheezing
2.	Second or third degree AV block
3.	Profound bradycardia (<40)
4.	SBP < 90
5.	Use of methylxanthines (aminophylline, caffeine) in last 12 h
6.	Recent use of dipyridamole
7.	Known hypersensitivity for the stress agent
8.	Critical AS
9.	Use of regadenoson or adenosine in patients taking dipyridamol

In cases of asthma or severe COPD with ongoing wheezing, and high degree AV block, dobutamine is an acceptable alternative. Dobutamine is a positive inotropic and chronotropic agent and should be avoided in situations where the hemodynamic effects may exacerbate existing conditions (Table 6.3).

Table 6.3

Contraindications to dobutamine stress testing