What is a normal and abnormal cerebroplacental (CPR) ratio?

Normal and Abnormal Cerebroplacental Doppler Ratio

A normal cerebroplacental ratio (CPR) is generally considered to be greater than 1.0, while a CPR less than 1.0 is abnormal and associated with significantly increased risk of adverse perinatal outcomes.

Definition and Calculation

The cerebroplacental ratio (CPR) is calculated by dividing the middle cerebral artery (MCA) pulsatility index by the umbilical artery (UA) pulsatility index:

CPR = MCA PI / UA PI

Normal Values

According to current guidelines and research:

• Normal CPR: Values above 1.0-1.08
• Borderline CPR: Values between 1.0-1.5 (may indicate early compromise)
• Optimal CPR: Values above 1.5 (associated with good outcomes)

Abnormal Values and Clinical Significance

CPR values can be interpreted in several ways:

1. Categorical threshold approach:

   • CPR < 1.0: Clearly abnormal, indicating brain-sparing effect
   • CPR < 1.08: Associated with adverse outcomes 1

2. Gestational age-specific percentiles approach:

   • CPR < 5th percentile for gestational age: Most predictive of adverse outcomes 2
   • CPR < 10th percentile for gestational age: Concerning but less specific

Physiological Basis

The CPR reflects the relationship between cerebral and placental blood flow:

• Normal situation: Higher resistance in cerebral circulation than in placental circulation (CPR > 1)
• Compromised situation: "Brain-sparing effect" where cerebral vasodilation occurs in response to hypoxemia, resulting in decreased MCA PI relative to UA PI (CPR < 1)

Clinical Applications

In Fetal Growth Restriction (FGR)

• CPR has emerged as a predictor of adverse outcomes among fetuses with FGR 3
• It may be an earlier predictor of adverse outcome than biophysical profile (BPP) or individual vessel Doppler measurements 3
• In early-onset FGR (<34 weeks), abnormal CPR is associated with:
  • Lower gestational age at birth
  • Lower birthweight
  • Higher rates of cesarean delivery for fetal distress
  • Higher rates of neonatal acidosis and NICU admission 4

In Appropriate for Gestational Age (AGA) Fetuses

• Even in AGA fetuses, abnormal CPR (< 1) is associated with 91.3% adverse outcome rate compared to 20.2% in those with normal CPR 5
• A cut-off value of 1.49 for CPR in AGA fetuses has been identified with sensitivity of 67.5% and specificity of 68% for predicting adverse outcomes 6

Limitations and Considerations

• The Society for Maternal-Fetal Medicine (SMFM) notes that while CPR has promising predictive utility, there is insufficient evidence to demonstrate its value as a stand-alone test 3
• Clinical trials are needed to evaluate the effectiveness of CPR in guiding clinical management, especially in late-onset FGR 3
• CPR should be considered as part of a comprehensive assessment rather than in isolation

Practical Application

When interpreting CPR results:

1. Consider gestational age-specific reference values when available
2. Integrate with other Doppler parameters (UA, MCA, ductus venosus)
3. Consider clinical context (FGR vs. AGA, early vs. late onset)
4. Use CPR as part of clinical decision-making regarding timing of delivery, especially in high-risk pregnancies

Common Pitfalls

• Relying solely on CPR without considering other clinical and ultrasound parameters
• Using a single categorical threshold rather than gestational age-specific values
• Failing to recognize that even AGA fetuses with abnormal CPR may be at risk
• Not repeating measurements when values are borderline or abnormal

The CPR represents an important tool in fetal assessment that reflects both placental insufficiency and fetal adaptation, providing valuable information about fetal well-being beyond what can be obtained from individual vessel measurements alone.