What is coronary steal syndrome from kdt 3- 5 mark pha
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I now have all the content needed from KDT (Katzung's Basic and Clinical Pharmacology) and supporting texts. Here is a fully structured 3-5 mark answer:
Coronary Steal Syndrome
(From KDT - Katzung's Basic and Clinical Pharmacology, 16th Ed.)
Definition
Coronary steal is the term given to the action of nonselective coronary arteriolar dilators in patients with partial obstruction (stenosis) of a portion of the coronary vasculature, whereby blood flow is "stolen" or diverted away from the ischemic myocardium toward the well-perfused regions.
Mechanism
Under normal (drug-free) conditions in a patient with coronary artery disease:
- Arterioles supplying the ischemic myocardium (downstream from a stenosis) are already maximally dilated due to local metabolic autoregulatory factors (adenosine, hypoxia, CO2 accumulation, etc.)
- Resistance vessels in the well-perfused (normal) regions retain their capacity to dilate further in response to exercise or drugs
When a potent nonselective arteriolar dilator is administered:
- It can only cause further vasodilation in the normal (non-ischemic) regions (because ischemic vessels are already maximally dilated)
- This reduces vascular resistance preferentially in normal regions
- Blood flow is therefore diverted ("stolen") away from the ischemic zone into the normal zone
- The net result is a worsening of ischemia in the already compromised myocardium
(Medical Physiology, Boron & Boulpaep) also explains that vasodilators decrease the pressure at the branch point upstream from the stenosis, further reducing perfusion pressure to the ischemic area.
Drugs that Cause Coronary Steal
| Drug | Mechanism |
|---|---|
| Dipyridamole | Inhibits adenosine reuptake → increases extracellular adenosine → nonselective coronary arteriolar dilation |
| Adenosine | Acts on A2A receptors → potent coronary resistance vessel dilation |
| Regadenoson | Selective A2A agonist, used in stress testing - labeled with warning for this effect |
| Isoflurane (volatile anesthetic) | Potent coronary vasodilator (historically controversial) |
| Sodium nitroprusside | Significant intra-myocardial vasodilation - not recommended in active myocardial ischemia |
Clinical Significance
- Dipyridamole is an extremely effective coronary dilator but is not effective in angina precisely because of coronary steal
- In patients with unstable angina, transient coronary steal can precipitate myocardial infarction
- Adenosine and regadenoson carry black-box/labeled warnings about this effect
- Dipyridamole is therefore contraindicated in unstable angina
- This is also why dipyridamole stress testing can unmask ischemia - it deliberately provokes coronary steal for diagnostic purposes (e.g., pharmacological stress imaging)
Key Point for Exam
"In ischemic myocardium, arterioles are already maximally dilated by local metabolic factors. A nonselective dilator cannot dilate these vessels further but dilates vessels in normal zones, thereby lowering upstream pressure and diverting blood away from the ischemic region - this is coronary steal."
- Katzung's Basic and Clinical Pharmacology, 16th Ed., p. 318 (Special Coronary Vasodilators / Coronary Steal section)
- Medical Physiology (Boron & Boulpaep), p. 826 (Vasodilator drugs and coronary steal)
- Lippincott Illustrated Reviews: Pharmacology, p. 446 (Dipyridamole - coronary steal phenomenon)
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