If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
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This month’s stumper
The diagram shows four phases of coronary blood flow in the left coronary artery during one complete cardiac cycle.
During which of the following phase(s) shown in the diagram is the left ventricle contracting?
A. Phase 1.
B. Phase 2.
C. Phase 3.
D. Phase 4.
E. Phases 1 and 2.
F. Phases 2 and 3.
G. Phases 3 and 4.
H. Phases 4 and 1.
The correct answer is F.
Kaplan Medical explains why
The graph plots left coronary flow (in mL/min) during five sequential phases of the cardiac cycle as follows:
- 1 = Diastole.
- 2 = Isovolumic ventricular contraction.
- 3 = Ejection.
- 4 = Isovolumic ventricular relaxation.
- 1 = Diastole.
Left ventricular contraction occurs during phases 2 and 3.
Flow through the systemic circulation is driven by the pressure gradient between aorta and right atrium. In practice, this means that flow through systemic capillary beds largely follows aortic pressure during the cardiac cycle. Flow rises during systole when the aortic valve is open and falls off slowly during diastole when the aortic valve is closed ("diastolic runoff"). The aortic pressure profile appears at top below:
Left coronary flow is unique because it drops sharply with the onset of isovolumic contraction and then rises to a maximum during diastole, as shown in the figure above. This is because the left coronaries run between the muscle fibers that make up the left ventricle. When these muscle fibers contract, they compress and, in some regions of the myocardium, collapse the coronary supply vessels through "extravascular compression."
These vessels rely on pressure within the aorta to maintain their patency, but the ventricle must generate pressures in excess of aortic pressure to eject blood, and hence the vessels collapse. Flow is halted and, because arteries do not have valves, may actually reverse direction as blood is squeezed out of vessels and backward toward the aorta (note the dip below zero on the y-axis during phase 2).
Once aortic valve opens and aortic pressure climbs to a maximum (ejection; phase 3), flow through coronaries in the subepicardial region may be restored. Flow thus rises and falls during phase 3, mirroring the aortic pressure curve.
During diastole, the myocardium relaxes, and the extravascular compressive forces are removed. The left coronaries now regain full patency and flow surges to a peak (phase 4). Note that while left coronary flow is interrupted temporarily during systole, flow during diastole is more than sufficient to meet the needs of the myocardium in a healthy individual.
The right ventricle generates much lower pressures during systole than does the left (mean pulmonary arterial pressure ~ 10-20 mm Hg), so extravascular compression is not so much of a concern here. Flow through the right coronaries resembles that of most other systemic capillary beds.
Why the other answers are wrong
Choice A, D, E, G and H: The ventricle is relaxing during phases 1 and 4, which excludes these choices as correct answers.
Choices B and C: Both include only one of two phases during which the ventricle is contracting, which means that they are incorrect.
Tips to remember
- Left coronary blood flow falls sharply and may reverse direction during ventricular contraction.
- This flow pattern is caused by extravascular compression and collapse of coronary supply vessels during systole. The compression is due to contraction of myocytes surrounding the supply vessels.
- Left coronary flow peaks during diastole when compressive forces are removed.
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.