2.3 Fetal Tachycardia and Bradycardia

Fetal Tachycardia

Fetal tachycardia is a baseline FHR above 160 bpm that persists for 10 minutes or longer. Because it is a sustained shift in the baseline, brief upward swings (accelerations) do not count toward it. Tachycardia is frequently an early, compensatory finding rather than a sign of established compromise, and its significance hinges heavily on the accompanying variability and the presence or absence of decelerations. Tachycardia with retained moderate variability is far more reassuring than tachycardia with minimal or absent variability.

The single most common cause of fetal tachycardia is maternal fever and intrauterine infection. Chorioamnionitis classically announces itself first as a rising fetal baseline, often before maternal temperature or other clinical signs become obvious, so a creeping baseline should prompt a search for infection.

Causes of Fetal Tachycardia

A practical exam distinction is sinus tachycardia versus a true fetal tachyarrhythmia. A baseline of 165-180 bpm that varies and responds to maternal cooling or hydration is usually sinus tachycardia. A fixed rate near or above 200-240 bpm with little variability suggests a primary fetal arrhythmia such as supraventricular tachycardia, which is a cardiac rather than an oxygenation problem and may require fetal cardiology evaluation.

Fetal Bradycardia

Fetal bradycardia is a baseline FHR below 110 bpm that persists for 10 minutes or longer. Like tachycardia, it is a baseline finding, not a transient dip, and it carries different implications depending on the cause and the surrounding tracing.

The reassuring clue with congenital heart block is that variability is often preserved despite the low rate, because the slow rate is a conduction problem rather than a CNS oxygenation problem. By contrast, bradycardia from hypoxia/acidemia is typically accompanied by loss of variability and a deteriorating pattern — a far more dangerous picture demanding immediate action.

Distinguishing a Low Baseline from a Prolonged Deceleration

The most heavily tested timing rule in this section separates a baseline change from a prolonged deceleration. The dividing line is 10 minutes:

In practice, a sudden fall to 90 bpm that recovers within 6 minutes is a prolonged deceleration, but if that 90 bpm rate persists past 10 minutes, it is reclassified as a bradycardic baseline. The same logic governs the high end: a sustained rate above 160 bpm for 10 minutes is tachycardia, not a prolonged acceleration. When you see a low or high rate on the strip, your first question is always how long has it lasted? — the duration, not the depth, decides whether you are describing an event or a new baseline.

Worked Example: During the second stage, the FHR abruptly falls from a baseline of 140 bpm to 85 bpm and stays there. At minute 4, this is a prolonged deceleration (a drop of 15+ bpm lasting 2 to under 10 minutes), and the team begins intrauterine resuscitation — reposition the patient, give an IV fluid bolus, stop oxytocin, and assess for cord prolapse. If, despite these measures, the rate remains at 85 bpm and crosses the 10-minute mark, it is no longer a prolonged deceleration: it is reclassified as a bradycardic baseline of 85 bpm, a finding that — particularly if variability is lost — points toward worsening hypoxia and the need to expedite delivery.