Cardiovascular system: atrial contribution to ventricular filling: practice question

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Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Loss of the late-diastolic top-up of ventricular filling that a stiff ventricle depends on

Atrial systole normally adds roughly a fifth of ventricular filling, and more when the ventricle is stiff and fills poorly in early diastole. Atrial fibrillation abolishes this atrial contribution, so end-diastolic volume, stroke volume and blood pressure fall while left atrial pressure rises, causing breathlessness.

Option by option

• A. A fall in afterload because the irregular rhythm lowers aortic impedance

  Afterload does not fall with an irregular rhythm, and a lower afterload would not cause hypotension and congestion.

• B. Loss of the late-diastolic top-up of ventricular filling that a stiff ventricle depends on (correct)

  Correct. Without effective atrial contraction a non-compliant ventricle loses an important part of its filling.

• C. Loss of the Bainbridge reflex, which removes the heart rate response to venous return

  The Bainbridge reflex concerns heart rate response to atrial stretch and is not the reason stroke volume falls when atrial systole is lost.

• D. Failure of isovolumetric contraction because the SA node no longer paces the ventricle

  The ventricle is still activated through the AV node, so isovolumetric contraction occurs normally.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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