Errors in microsphere determination of cardiac output: a computer simulation in fetal sheep

Area of Science:

• Cardiovascular Physiology
• Fetal Medicine
• Medical Imaging

Background:

• Accurate measurement of fetal cardiac output and organ blood flow is crucial for understanding fetal development and well-being.
• The radioactively labeled microsphere technique is a common method for assessing these parameters, but its accuracy in fetal lambs requires thorough evaluation.

Purpose of the Study:

• To simulate and evaluate potential sources of error in measuring cardiac output and distribution using the microsphere technique in fetal lambs.
• To assess the impact of fetal weight, hypovolemia, vascular resistance, and microsphere recirculation on measurement accuracy.

Main Methods:

• A compartmental model was used to simulate microsphere measurements in fetal lambs (0.5–3 kg).
• Systematic errors due to artifactual blood volume changes and microsphere recirculation were analyzed.
• Sensitivity analysis was performed for changes in vascular resistance.

Main Results:

• Systematic error from blood volume changes was <5% for lambs ≥1 kg, increasing in smaller fetuses and with simulated hypovolemia.
• No significant error increase was found for 20% recirculation in a single organ, but 7–14% errors occurred with simultaneous multi-compartment recirculation.
• Recirculation also caused significant errors in measured cardiac output distribution in specific scenarios.

Conclusions:

• The microsphere technique is a reliable method for quantifying fetal cardiac output and organ flow.
• Inherent nonrandom errors are around 10%, likely less than natural variations in fetal cardiac output.
• Careful consideration of experimental design is needed to minimize errors associated with blood volume changes and recirculation.