Fetal Tachycardia

Fetal tachycardia is elevated heart rate in which the heart rate exceeds 160 beats per minute (bpm) for at least 10 minutes.

The baseline fetal heart rate, or the normal fetal heart rate, is 110-160 bpm.

Tachycardia is elevated above this baseline.

You can see from this waveform that the heart rate starts off at about 170 bpm and then rises to about 200 bpm. These heart rates are sustained for well over 10 minutes.

Each box represents 10 seconds of time. So each 6 boxes represents 60 seconds, or 1 minute. You can visualize 6 boxes easily by looking for the bold vertical line after each 6 boxes. All these represent 1 minute, so it's easier to visualize the complete time interval. If you count all the boxes, you will see that this graph represents 11 minutes. This is above the 10 minutes needed to diagnose fetal tachycardia.

You can see the heart rate is elevated above the 160 bpm for longer than 10 minutes. Thus, it's a definitive diagnosis of fetal tachycardia.

You can see that the tachycardia is constant regardless of whether is uterus is contracting or resting. The tachycardia persists during contraction and resting periods. So it's not correlated to the contraction cycles.

Causes of Fetal Tachycardia

Fetal tachycardia can exist for a number of reasons. Possible causes are maternal fever, maternal dehydration, maternal or fetal hypoxia, fetal acidosis, maternal or fetal hypovolemia, fetal cardiac dysrhythmias, maternal severe anemia, maternal hyperthyroidism, and different types of drugs.

Having an infection and a fever increases metabolic demand on the body. Therefore, the body needs more oxygen delivered throughout it than if a person was afebrile or without a fever. In order to meet this increased oxygen demand, the heart rate increases, in order to pump blood quicker throughout the body. And this is why infection increases heart rate and can put a baby in a tachycardic state.

Maternal dehydration can cause tachycardia, because the mother is in a state of hypovolemia, or low blood volume. Since the blood volume is so low and deficient in the body, the body has to quickly move around what blood is in the body in order to circulate it throughout the body.

Hypoxia, either maternal or fetal, can cause tachycardia or bradycardia. If it is tachycardia that results, the reason is as follows. If the respirations of the mother is depressed or there is insufficient oxygen exchange to the fetus such as because of cord compression, hypoxia can occur. Since there is so little oxygen in the blood, to compensate for this, the body pumps the blood faster throughout the body via the heart pumping quicker. If the oxygen in the blood was rich, the heart could pump at a normal rate. But since the oxygen is so low, the heart has to pump faster to deliver what little oxygen is in the blood faster systemically through the body.

Fetal acidosis, like fetal sepsis, increased metabolic demand. Acidosis can arise from things such as a hypoxic state. Due to this, the body looks to deliver more oxygen throughout the body to overcome this acidotic state. Therefore, it the heart pumps faster to deliver oxygenated blood throughout the body. This is also why when a person is in a state of acidosis, he or she hyperventilates in order to compenstate for the acidosis. By hyperventilating, the person blows off more carbon dioxide than oxygen. Therefore, more CO₂ is blown off than oxygen. This increases the oxygen of the body in relation to CO₂ and this helps to alkalize the acidotic state. The tachycardic state then ensures that this newly held oxygen ratio is distributed quickly throughout the body. Fetal acidosis can also cause bradycardia, which is a more dire situation, because the fetus is compensating for the acidotic state.

Fetal cardiac dysrhythmias causes tachycardia because many dysrhythmias such as supraventricular tachycardia cause a rapid heartbeat that fails to pump blood properly systematically. The heart is basically quivering or fluttering but not producing sufficient thrust to pump blood throughout the body. Therefore, there is decreased cardiac output, which is the amount of blood pumped out by the heart. Since the body isn't getting sufficient blood and, therefore, oxygen, during a cardiac dysrhythmia such as supraventricular tachycardia, the heart has to pump blood faster in order to compensate for the lack of blood volume being pumped out by the heart. This is so that the body can be properly oxygenated. Tachycardia is compensation for the cardiac dysrhythmia.

Severe maternal anemia will cause tachycardia. Anemia is a shortage of red blood cells. Since red blood cells transport oxygen in blood, a lack of red blood cells cause a deficiency of oxygen. Therefore, the heart needs to pump blood quicker in order to properly oxygenate the body.

Maternal hyperthyroidism causes tachycardia because hyperthyroidism causes an overactive thyroid. Therefore, with an overactive thyroid, the person has a much faster metabolism than normal. Therefore, the person's body requires greater oxygen demand. To get this increased demand, the heart rate speeds up to get blood flowing through the body faster.

Also differing drugs can cause tachycardia in fetus. These include drugs such as bronchodilators, decongestants, stimulant drugs, and terbutaline.

Related Resources

Fetal Bradycardia- Explained

Early Decelerations- Explained

Late Decelerations- Explained

Variable Decelerations- Explained

Fetal Heart Rate Variability- Explained