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UNSW Embryology

Cardiovascular System Development - Embryonic Heart Rate

© Dr Mark Hill (2008)

Acknowledgements

Introduction

Heart rate (beats / minute) is a measurement that can be made from early in development (when the heart first starts) through embryonic and fetal stages into labor and birth. It is a common clinical diagnostic tool, but data can be variable between countries and institutions.

Early ultrasonographic measurement of embryonic heart rate (EHR) shows a steady increase from Stage 9-10 (75 beats/minute) to Stage 18 (130 beats/minute) and on to Stage 20, following which a gradual decrease in EHR occurs (More? Embryonic Heart Rate). This increase correlates with heart development and a low EHR is used as an indicator of developmental failure and likely abortion. Late stethoscope measurements of fetal heart rate can monitor fetal stress and identifies the characteristic "lub-dub" heart valve sounds.

Human Fetus (17 Week) heart beat

An article by Wisser and Dirschedl (1994) studied human ERH by serial ultrasound correlated heart development with the change in ERH, which increased to Stage 20 (22 mm crown rump length) then decreased. Wisser J, Dirschedl P Embryonic heart rate in dated human embryos. Early Hum Dev. 1994 May;37(2):107-15. (More? Embryonic Heart Rate)

Page Links: Introduction | Some Recent Findings | Embryonic Heart Rate | Low EHR | Fetal Heart Rate | Heart Sounds | Heart Rate and Embryo Sex | Heart Rate During Birth |References | Glossary

Some Recent Findings

Smith JF Jr. Fetal health assessment using prenatal diagnostic techniques. Curr Opin Obstet Gynecol. 2008 Apr;20(2):152-6. Review.

"Fetal heart-rate monitoring remains the main form of fetal assessment for high-risk pregnancies. The additional assessments afforded by the analysis of ST and T-wave changes of the fetal electrocardiogram hold promise for improving the predictive value of fetal heart-rate assessments."

Mongiovì M, Pipitone S. Supraventricular tachycardia in fetus: how can we treat? Curr Pharm Des. 2008;14(8):736-42. Review.

"The normal fetal cardiac rhythm is characterized by a regular heart rate ranging between 100 and 160 -180 beats/min with a normal 1:1 atrioventricular electromechanical relationship during each cardiac cycle. Fetal tachycardia occurring in approximately 0.5% of all pregnancies and it is an important cause of fetal morbidity and mortality. ...Reliable diagnosis in utero of fetal arrhythmia is possible by ultrasound examination of the fetal heart. In fact pulsed wave Doppler guided by two-dimensional echocardiography provided important information on cardiac rhythm as it study the blood flow from different chambers."

Doubilet PM, Benson CB. Outcome of first-trimester pregnancies with slow embryonic heart rate at 6-7 weeks gestation and normal heart rate by 8 weeks at US. Radiology. 2005 Aug;236(2):643-6.

"Singleton pregnancies with an embryonic heart rate measured on a 6.0-7.0-week US scan were identified. The heart rate was classified as slow if it was fewer than 90 beats per minute prior to 6.3 weeks or fewer than 110 beats/min at 6.3-7.0 weeks, normal if it was 100 or more beats/min at less than 6.3 weeks or 120 or more beats/min at 6.3-7.0 weeks, or borderline if it was 90-99 beats/min prior to 6.3 weeks or 110-119 beats/min at 6.3-7.0 weeks."

Embryonic Heart Rate

In a later study by Coulam etal. (1996), normal successful human gestations were defined by EHR criteria at different early embryonic (34-56 days from last menstrual period) developmental stages (at the earliest stages when embryo length is difficult to measure gestational sac diameters are included). Coulam CB, Britten S, Soenksen DM. Early (34-56 days from last menstrual period) ultrasonographic measurements in normal pregnancies. Hum Reprod. 1996 Aug;11(8):1771-4.

(More about stages? Carnegie Stage table)

Low Embryonic Heart Rate

Low ERH is correlated with embryonic loss and is often used as a predictor for risk of spontaneous miscarriage, the majority of ERH research articles focus on this aspect as an indicator. "When a slow embryonic heart rate is detected at 6.0-7.0 weeks, the likelihood of subsequent first-trimester demise remains elevated (approximately 25%) even if the heart rate is normal at follow-up. In such pregnancies, at least one follow-up scan in late first trimester is warranted." Doubilet PM, Benson CB. Outcome of first-trimester pregnancies with slow embryonic heart rate at 6-7 weeks gestation and normal heart rate by 8 weeks at US. Radiology. 2005 Aug;236(2):643-6.

Fetal Heart Rate

12 week fetal heart rate

Later in development, Fetal Heart Rate (FHR) can also be measured using ultrasound doppler techniques. Baseline fetal heart rate (FHR) ranges (normal limits) in many studies ranges from 120 to 160 beats/minute.

FHR decreases from 30-33 weeks to 34-35 weeks, and steadily falls from 36-37 weeks within the range from 130 to 140 beats / minute. An article by Park etal. (2001) found fetal heart rate (FHR) as 143 beats / minute at term (37-40 weeks), another study by Druzin et al. (1986) reported 132 beats / minute. FHR data is variable due to retrieval method, analysis of FHR parameters, different institutions, different countries, and collection under different conditions.


Human Fetal Heart Rate Changes

(Graph from: Park MI, Hwang JH, Cha KJ, Park YS, Koh SK. Computerized analysis of fetal heart rate parameters by gestational age. Int J Gynaecol Obstet. 2001 Aug;74(2):157-64.)

Heart Sounds

Embryonic heart sounds can be detected by Doppler during ultrasound examination. Fetal heart sounds can be detected by stethoscope.

The characteristic "lub-dup" sounds are associated with closing of heart valves.

The first sound (lub) occurs as atrioventricular valves close and signifies beginning of systole (contraction)

The second sound (dup) occurs when semilunar valves close at the beginning of ventricular diastole (relaxation)

Heart Rate and Embryo Sex

Embryonic heart rate has been suggested as an indicator of sex (male/female), scientific research to date does not support any correlation between EHR and the sex of the embryo/fetus.

"Baseline fetal heart rate was not significantly different between male and female fetuses." Druzin ML, Hutson JM, Edersheim TG. Relationship of baseline fetal heart rate to gestational age and fetal sex. Am J Obstet Gynecol. 1986 May;154(5):1102-3. | Glahn RP, et al. Evaluation of sex differences in embryonic heart rates. Poult Sci. 1987 Aug;66(8):1398-401. | DuBose TJ, et al. Fetal heart rate (FHR) is not an indicator of the baby's sex. J Ultrasound Med. 1988 Apr;7(4):237-8.

Heart Rate During Birth

Fetal heart rate is often monitored electronically during labor as a diagnostic and clinical management tool. At term, the normal, unmedicated fetus before labor has an average heart rate of 140 beats/minute. Preterm fetus (at 20 weeks gestational age) has a mean heart rate of 155 beats/minute.

Basal FHR

Normal 110-160 (beats/minute)

Tachycardia (Moderate) 161-180 (Marked) 181 or more

Bradycardia (Moderate) 100-119 (Marked 90) or less

Transitory FHR Changes

Variable 5-15 beats per minute amplitude

Accelerations Increased by 15 or more

Decelerations - Early Decreased by 10-40, Late Decreased by 5-60, Variable Decreased by 10-60, occasionally more

Tachycardia Causes: Fetal hypoxia, Maternal fever, amnionitis (inflammation of the amnion), Drugs, Maternal hyperthyroidism, Fetal anemia, Fetal heart failure and Fetal cardiac tachydysrhythmias

Bradycardia Causes: Fetal hypoxia, Fetal acidosis, Fetal cardiac bradydysrhythmias, Drugs, Hypothermia and Reflex

(data from NCBI Bookshelf hstat6.section.700#711)

References

Search Pubmed Now: heart[TITL]+rate[WORD]+development[WORD]

Reviews on Embryonic Heart Rate

Articles on Embryonic Heart Rate

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