What are the causes of late decelerations (late decels)?

Causes of Late Decelerations in Fetal Heart Rate Monitoring

Late decelerations are primarily caused by uteroplacental insufficiency, which leads to decreased oxygen supply to the fetus during uterine contractions. 1

Primary Mechanism and Definition

Late decelerations are visually apparent, usually symmetric decreases in fetal heart rate with the following characteristics:

• Begin after the onset of uterine contraction
• Nadir occurs after the peak of contraction
• Recovery occurs after the end of contraction 1

Major Causes of Late Decelerations

1. Uteroplacental Insufficiency

• Primary underlying mechanism: Reduced oxygen transfer across the placenta 1
• Occurs when uterine contractions temporarily decrease blood flow and oxygen delivery to the intervillous space 2

2. Transient Causes

• Maternal hypotension (common reversible cause) 1, 3
• Uterine hyperstimulation (excessive contractions or increased uterine tone) 1, 3
• Medications that affect uterine blood flow

3. Pathological Conditions

• Placental insufficiency (reduced placental function or reserve) 2
• Reduced uterine blood supply (leads to lower baseline FHR and smaller decelerations) 2
• Reduced placental diffusion capacity 2
• Maternal conditions affecting oxygen delivery:
  • Maternal sickle cell crisis 4
  • Severe anemia
  • Cardiovascular disease

Severity Assessment

The significance of late decelerations depends on:

1. Frequency of occurrence:

   • Occasional (<50% of contractions): Lower risk of acidemia (1% PPV) 5
   • Recurrent (≥50% of contractions): Higher risk of acidemia 5

2. Associated FHR patterns:

   • Absence of accelerations and reduced variability with recurrent late decelerations: >50% positive predictive value for fetal acidemia (pH <7.1) 5
   • Normal variability within late decelerations: Better prognosis 3

3. Duration and depth:

   • Deeper decelerations may indicate more severe compromise 2
   • Prolonged decelerations (>2 minutes but <10 minutes) indicate more significant hypoxic stress 3

Clinical Implications

• Late decelerations represent a chemoreflex response to hypoxemia 3
• When persistent, they indicate potential fetal compromise requiring intervention 1
• If a late deceleration pattern continues for >10 minutes, it may progress to terminal bradycardia with risk of hypoxic-ischemic brain injury 3

Management Considerations

• Identify and treat reversible causes (maternal position change, IV fluids, oxygen, discontinue oxytocin) 1
• Late decelerations with absent variability and recurrent pattern are classified as Category III (abnormal) tracings requiring immediate intervention 1
• Subtle late decelerations may be difficult to visualize but can be detected using a straight edge along the baseline 1

Important Pitfalls to Avoid

1. Missing subtle late decelerations: They may be shallow and difficult to visualize
2. Confusing with other deceleration patterns: Unlike early decelerations (head compression) or variable decelerations (cord compression)
3. Ignoring gestational age: Premature, term, and post-term fetuses respond differently to hypoxic stress 6
4. Overlooking associated FHR patterns: The combination of late decelerations with absent variability and accelerations significantly increases risk of acidemia 5