A patient with significant coronary artery disease develops rapid atrial fibrillation with a ventricular rate of 140 bpm, compared to their resting rate of 70 bpm. Why is this particularly dangerous for coronary perfusion?
AHigher heart rates reduce aortic systolic pressure, decreasing the driving force for coronary flow
BTachycardia both increases myocardial oxygen demand and shortens diastole — the phase when coronary perfusion primarily occurs — simultaneously reducing supply
CRapid rates cause the coronary arteries to spasm, mechanically obstructing flow independent of the cardiac cycle
DHigher heart rates increase LVEDP, which dilates the ventricle and compresses the coronary arteries from inside
Tachycardia creates a double jeopardy for the ischemic heart. First, the increased heart rate elevates myocardial oxygen demand (demand side). Second, when heart rate doubles, the cardiac cycle halves — but systole shortens only slightly, so diastole is disproportionately compressed. Since left coronary flow primarily occurs during diastole (systolic compression restricts subendocardial vessels), less diastolic time means less perfusion time. A diseased vessel that can barely meet resting demand faces both increased need and reduced delivery time simultaneously.
Question 2 Multiple Choice
Which of the following changes would most directly REDUCE coronary perfusion pressure?
AAn increase in heart rate from 60 to 90 bpm during mild exercise
BA rise in aortic diastolic pressure from 80 to 90 mmHg
CA fall in aortic diastolic pressure combined with a rise in left ventricular end-diastolic pressure (LVEDP)
DAn increase in coronary vasodilation driven by adenosine release
Coronary perfusion pressure ≈ aortic diastolic pressure − LVEDP. Anything that lowers aortic diastolic pressure (hypotension, aortic regurgitation) or raises LVEDP (heart failure, volume overload) narrows this gradient and reduces coronary flow. Option C describes both changes occurring simultaneously — the worst scenario, which occurs in decompensated heart failure or severe aortic regurgitation. Options A and B do not directly reduce the perfusion pressure gradient. Option D (vasodilation) increases flow by reducing resistance, not by changing pressure.
Question 3 True / False
Coronary flow reserve refers to the heart's ability to increase coronary blood flow above resting levels in response to increased metabolic demand.
TTrue
FFalse
Answer: True
Coronary flow reserve is the ratio of maximal coronary flow (during peak vasodilation, e.g., during exercise or pharmacologic stress) to resting flow — normally four to five times resting levels. It represents the vasodilatory capacity held in reserve. A significant stenosis may not reduce resting flow (because autoregulatory vasodilation compensates) but will erode coronary flow reserve, producing ischemia only under stress. This is why exercise stress testing can detect coronary disease that is invisible at rest.
Question 4 True / False
Because both ventricles contract during systole, the right and left coronary arteries are equally compressed during systole and deliver similar flow patterns throughout the cardiac cycle.
TTrue
FFalse
Answer: False
The right and left coronary arteries behave very differently. The left ventricle generates 120 mmHg or more during systole — enough to compress the subendocardial vessels nearly shut, so left coronary flow primarily occurs during diastole. The right ventricle develops much lower pressures (25–30 mmHg), so right coronary vessels are less compressed and receive flow throughout the cardiac cycle. This asymmetry explains why the left ventricular subendocardium is most vulnerable to ischemia, and why conditions that shorten diastole or reduce aortic diastolic pressure affect the left heart disproportionately.
Question 5 Short Answer
Why is the left ventricular subendocardium the region most vulnerable to ischemic injury during episodes of reduced coronary perfusion?
Think about your answer, then reveal below.
Model answer: The subendocardium (innermost layer of the left ventricular wall) is the last region to receive blood and the first to be deprived. During systole, high left ventricular pressure compresses the intramyocardial coronary vessels against the ventricular wall from inside, with the greatest compression in the subendocardium closest to the high-pressure cavity. This means subendocardial vessels are essentially occluded during systole and rely entirely on diastolic flow — a shorter window than the epicardial vessels experience. Additionally, the subendocardium has the highest metabolic demand (it works hardest, stretches most during contraction), so its oxygen extraction is already near maximum. When coronary perfusion pressure drops or diastolic time shortens, the subendocardium loses its supply first and has the least reserve.
This explains the characteristic pattern of ischemia seen on electrocardiograms (subendocardial ST depression) and why conditions like tachycardia, hypotension, and elevated LVEDP produce subendocardial ischemia before transmural infarction. It also explains why the subendocardial zone is the first region affected in demand-ischemia scenarios (e.g., hypertensive hypertrophy, aortic stenosis) — the region with the highest demand and most restricted supply is always the first to fail.