Cardiology: electrical alternans in pericardial effusion: practice question

A General Medicine question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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General Medicine · Cardiology: electrical alternans in pericardial effusion

Sample item

A patient with a large malignant pericardial effusion has low-voltage QRS complexes whose amplitude and axis change on alternate beats. What explains this beat-to-beat change?

AEvery second beat arises from an ectopic ventricular focusBRespiratory swings in venous return alter ventricular fillingCThe heart swings to and fro within the fluid-filled pericardial sacDAlternate beats are conducted through different AV nodal pathways

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

The answer and the reasoning

C. The heart swings to and fro within the fluid-filled pericardial sac

Electrical alternans is a sign of a large pericardial effusion, usually with tamponade physiology. The heart moves within the fluid, so its electrical axis in relation to the chest leads changes with each beat. Low voltage arises because the fluid dampens the signal.

Option by option

A. Every second beat arises from an ectopic ventricular focus
Ventricular bigeminy gives premature wide beats, and the QRS shape of the sinus beats stays the same.
B. Respiratory swings in venous return alter ventricular filling
Respiratory variation underlies pulsus paradoxus, which is a pulse finding and not a beat-to-beat ECG change.
C. The heart swings to and fro within the fluid-filled pericardial sac (correct)
Correct. The swinging heart changes its position relative to the leads on successive beats.
D. Alternate beats are conducted through different AV nodal pathways
Dual AV nodal pathways cause re-entrant tachycardia and not a regular alternation of QRS amplitude.

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.

Questions about this General Medicine question

What is the answer to this General Medicine question?

The best answer is C, The heart swings to and fro within the fluid-filled pericardial sac. Electrical alternans is a sign of a large pericardial effusion, usually with tamponade physiology.

Why is "Every second beat arises from an ectopic ventricular focus" not the answer?

Ventricular bigeminy gives premature wide beats, and the QRS shape of the sinus beats stays the same.

Why is "Respiratory swings in venous return alter ventricular filling" not the answer?

Respiratory variation underlies pulsus paradoxus, which is a pulse finding and not a beat-to-beat ECG change.

Why is "Alternate beats are conducted through different AV nodal pathways" not the answer?

Dual AV nodal pathways cause re-entrant tachycardia and not a regular alternation of QRS amplitude.

What concept does this question test?

It tests Cardiology: electrical alternans in pericardial effusion in General Medicine. The heart moves within the fluid, so its electrical axis in relation to the chest leads changes with each beat. Low voltage arises because the fluid dampens the signal.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Cardiology: electrical alternans in pericardial effusion?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.