An indication of fetal distress and an urgent need to deliver the baby, to avoid fetal loss is known as fetal bradycardia. The recognition, especially intrapartum recognition, is considered highly important for pregnancy management, and the wellbeing of the fetus.
Transient episodes of bradycardia conversely occur during obstetric scanning but are considered benign in nature.
They have shown no clinical implications and so far these are indicated as an occurrence of response towards vagal stimulation due to partial cord occlusion.
- These are primarily caused by pressing the transducer on the maternal abdomen. The episodes of bradycardia resolve invariably when external pressure is reduced and both the situation can be considered as secondary bradycardia.
- Fetal Bradycardia does not constitute any harmful effects on the mother or the child.
- Persistent bradycardia, irrespective of being intermittent or not, is documented during an ultrasound examination at the time of routine antenatal visits with the doctor.
- Once bradycardia is diagnosed persistent in nature, regular monitoring on the fetal status is made to ensure clarification of the mechanism.
- Slow fetal heart rate resulting from fetal anomalies which are closely related to the heart, occurring due to cardiac conduction system or myocardium itself, is referred to as primary bradycardia or primary fetal bradycardia.
Bradycardia once diagnosed, thereafter the primary concern of the doctor is to provide normal cardiac rhythm and in understanding the electrophysiological principles that underpin rhythm assessment by USG. This is made into a natural life indicator towards highlighting the features that allow proper and correct diagnosis of different mechanisms of bradycardia.
General Considerations Of Bradycardia
Generally, a doppler device is used to listen to fetal heart rates. Proper documentation of the recording is a common practice in pregnancy management to reduce the risk of pregnancy symptoms that affect the mother and the child.
The normal heart rate of a fetus range between 120 breaths per minute to 160 breaths per minute, which is considered the normal rate. fetal heart rate below 100 breath for a minute is considered bradycardia. For baseline fetal heart rate there are specific gestational age-related nomograms.
Understanding The Normal Sinus Rhythm
The normal cardiac rhythm begins at the sinus node located in the right atrium of the human body. Just like a natural pacemaker, it is the function of the sinus node to dictate the frequency of overall cardiac contraction. The impulse generated from cardiac contraction triggers an actual contraction and travels to the atrioventricular node.
Here it reaches the ventricles via conduction cells. These conduction cells are named as the “bundle of his”, they lie at the right and left bundle branch along with the Purkinje fibers. Finally, these triggers lead to myocardial depolarization and ventricular contraction.
Therefore one cardiac cycle equals an atrial contraction sequentially followed by a ventricular contraction, with only a slight delay. An understanding of this is of essential importance as these events constitute fundamental knowledge and the arrhythmic pattern can be analyzed well.
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Differential Diagnosis Of Bradycardia
Fetal bradycardia is a result of electrophysiological mechanism and management strategies depend on achieving a correct diagnosis. The normal sinus rhythm is followed by an atrial contraction and ventricular contraction, which takes place after a short time interval. It is of utmost importance that the ventricle and the atrium activities are simultaneously registered so that the analysis of rhythmic disturbance is well noted.
The most frequently used ultrasound techniques to determine bradycardia include the echocardiography of M-mode and a pulse wave Doppler. The analysis of the atrium and ventricle emotion of the Doppler flow waveform is very well-noted because of the electrical stimulation.
Other diagnostic modalities include the ones less commonly used:
- fetal electro magnetocardiography
- fetal electrocardiography
Two different forms of bradycardia can take place namely, irregular and regular bradycardia. The regularity is ascertained by constant monitoring of the fetal heart rate or by visual inspection, with the help of recording. This analysis allows a proper diagnosis of the underlying electrophysiological mechanism of any form of arrhythmia.
Management Of fetal Bradycardia
The antepartum part of management is targeted towards a particular type of fetal bradycardia. Here are some of the management guidelines followed.
- Blocked actual premature contractions - This type of bradycardia requires no fetal therapy, as these are premature atrial contractions and not pathologic in nature
- Sinus bradycardia – If fetal distress or hypoxia causes sinus bradycardia, fetal therapies are used to treat the condition. For children who suffer from persistent fetal sinus bradycardia are comprehensively evaluated by a pediatric cardiologist. Infants may require defibrillators, beta-blockers or pacemakers for long term treatment.
- Fluorinated steroids - For example, dexamethasone is given to women who suffer from prolonged bradycardia intervals to prevent the progression of complete heart block. Steroids are looked upon and advised in light of the fact that they help diminish autoimmune response and cardiac inflammatory injuries.
- Congenital heart block - So far as heart diseases are concerned, heart blocks cannot be treated in utero. The continental heart block may improve with certain therapies that are immune-mediated. These include steroids, intravenous immunoglobulin, and application of therapeutic strategies including the recent use of hydroxychloroquine.
Complications of fetal bradycardia include the two most common ones:
- Low ventricular rate which is approximately 55 breath per minute. This may happen along with a non-reactive fetal heart rate associated with the risk of hydrops fetalis
- Hydrops fetalis is the case, where the prognosis is extremely poor because of the presence of a ventricular block. Universally the hydrops fetalis is fatal. A course of steroids are given to improve the condition, called myocarditis if hydrops fetalis is diagnosed in the early stages of pregnancy, termination of pregnancy is usually advised.
Prognosis And Outcome
The underlying etiology of fetal bradycardia determines the prognosis required. It also includes the gestational age, structural heart defects, and the clinical situation of the mother. Bradycardia is related to an increased risk of intrauterine fetal demise, and neonatal demise.
Immune-mediated heart block without hydrops is better but children are at the risk of requiring pacemakers after birth or later in life. Patients diagnosed with fetal bradycardia are at a high risk of long term complications of autoimmune disorders that may include neurological problems, renal insufficiency, and also cardiovascular diseases.
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