From the Guidelines
The normal cardiothoracic (CT) ratio in a fetus is 0.25 to 0.35 when using area and less than 0.5 when using circumference 1.
Key Points
- The cardiothoracic ratio is calculated by the area or circumference occupied by the heart in diastole to thoracic area or circumference.
- This ratio is used to assess relative fetal heart size and identify potential hemodynamic compromise.
- A normal cardiothoracic ratio is essential for fetal well-being, and abnormalities may indicate conditions such as congenital heart disease or fetal anemia.
Assessment of Fetal Heart Size
- The cardiothoracic ratio is a crucial marker of fetal heart size, with values outside the normal range indicating potential cardiac dysfunction.
- Other markers of hemodynamic compromise include increased fetal heart size, valvular regurgitation, abnormal Doppler waveforms, and decreased ventricular wall shortening 1.
- Fetal echocardiography is a valuable tool for assessing fetal cardiac function and identifying potential complications early on.
From the Research
Normal Cardiothoracic Ratio in a Fetus
The normal cardiothoracic (CT) ratio in a fetus can be determined through various studies, which have established reference ranges for this measurement.
- The CT ratio remains constant throughout gestation, with the 5th, 50th, and 95th percentiles of the CT ratio in 21-38 weeks gestational age being 0.32,0.39, and 0.48, respectively 2.
- Another study found that the mean CT ratio values increased slightly with gestational age, rising from 0.38 at 11 weeks to 0.45 at 20 weeks 3.
- A study established z score reference ranges of fetal cardiothoracic diameter ratio, which showed that the CT ratio was slightly increased with advanced gestational age, biparietal diameter, and head circumference 4.
- The CT ratio can be used to evaluate heart disease in fetuses, with an increased ratio indicating potential cardiac abnormalities, such as Ebstein's anomaly, tricuspid dysplasia, atrioventricular septal defect, and complete heart block 5.
- The measurement of the CT ratio is essential in the evaluation of fetal hydrops, as an increased value may point to the diagnosis of an intermittent fetal tachycardia if the fetus is assessed during a period of sinus rhythm 5.
- The CT ratio can be measured using various methods, including ultrasound and MRI, with the latter providing more accurate measurements 2, 6.
- The reference ranges for CT ratio vary depending on the method used and the gestational age of the fetus, with most studies agreeing that the CT ratio remains relatively constant throughout pregnancy 2, 6, 3, 4.