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Comprehensive and Detailed Explanation From Exact Extract–Based NCC C-EFM References:

Maternal hyperthermia—most commonly from infection—causes a rise in fetal temperature, which increases fetal metabolic rate. The fetus responds by increasing heart rate to meet the increased oxygen demand.

Effects include:

Increased fetal oxygen consumption

Enhanced fetal cardiac output

Resultant tachycardia, often 160–180 bpm

This mechanism is repeatedly outlined in NCC’s physiology domain, AWHONN, Menihan, Simpson, and Creasy & Resnik.

Option A is incorrect because maternal fever does not reduce perfusion.

Option C is incorrect because catecholamines are often elevated, not inhibited.

Thus, the mechanism is increased fetal metabolism.

Comprehensive and Detailed Explanation From Exact Extract–Based NCC C-EFM References:

Sustained fetal supraventricular tachycardia (SVT) often produces heart rates > 200–240 bpm, causing:

Poor ventricular filling

Decreased stroke volume

Reduced cardiac output

Congestive heart failure

Progressive fluid accumulation

NCC and AWHONN emphasize that untreated SVT leads to hydrops fetalis, characterized by:

Ascites

Pleural effusion

Pericardial effusion

Skin edema

Why the other answers are incorrect:

A. Fetal anemia — Causes tachycardia but is not caused by SVT.

C. Neonatal pacemaker — Pacemakers treat heart block, not SVT.

Correct answer: B. Hydrops fetalis

Comprehensive and Detailed Explanation From Exact Extract NCC-Recommended Sources

Oxygen saturation refers to the percentage of hemoglobin binding sites occupied by oxygen. NCC physiology resources, including Simpson & Creehan and Creasy & Resnik, define oxygen saturation as the “ratio of oxyhemoglobin to total hemoglobin”—the same definition used in fetal oxygenation discussions.

Oxygen affinity refers to hemoglobin’s tendency to bind oxygen (related to the oxyhemoglobin dissociation curve).

Oxygen carrying capacity refers to the total amount of oxygen hemoglobin can transport, independent of current saturation.

AWHONN and Menihan emphasize that fetal oxygenation assessment is dependent on understanding oxygen saturation, not affinity or carrying capacity, when discussing fetal hypoxemia and gas exchange.

Comprehensive and Detailed Explanation From Exact Extract–Based NCC C-EFM References:

NICHD/NCC definitions:

Prolonged deceleration: decrease in FHR ≥15 bpm lasting 2 to 10 minutes

Bradycardia: baseline FHR <110 bpm lasting ≥10 minutes

The differentiating factor is duration, not rate and not contraction relationship.

Before 10 minutes → prolonged deceleration

At or beyond 10 minutes → new baseline → bradycardia

Thus, the factor that differentiates the two is length of time it lasts.