Intrapartum assessment - Fetal heart rate patterns: Nursing

Intrapartum assessment of the fetal heart rate, or FHR for short, refers to the assessment of the FHR and rhythm in response to uterine activity during the intrapartum period, which refers to the time of pregnancy from the onset of labor to delivery of the newborn and the placenta. Now, fetal monitoring can be external or internal. External fetal monitoring is a noninvasive method to monitor the FHR and uterine activity. An ultrasound transducer is placed on the maternal abdomen over the fetal back to record the FHR. A tocotransducer, sometimes called a tocodynamometer, or toco for short, is a pressure-sensitive button that is placed over the uterine fundus to track uterine activity. Both devices are held in place by belts or bands and then attached to the fetal monitor that can print out or display the FHR and uterine activity.

Now, during the procedure, the client should avoid the supine position because the weight of the uterus and fetus can compress large blood vessels, such as inferior vena cava and aorta. This can decrease cardiac output and cause supine hypotension, which results in less blood pumping into the systemic circulation and the placenta, so less oxygen is delivered to the fetus. To alleviate the pressure on these vessels, the client should be turned to a lateral recumbent position, or a wedge should be placed under the right hip, displacing the uterus to the left and away from the vessels. On the flip side, internal fetal monitoring is an invasive procedure that requires cervical dilation of at least 2 to 3 centimeters and rupture of the fetal membranes. This way, a spiral electrode can be placed through the cervix onto the presenting part of the fetus, most commonly the scalp, to track the FHR.

If uterine contractions also need to be monitored more closely, an intrauterine pressure catheter can be placed inside the uterus.Now, let’s take a look at the fetal heart rate tracing, which is divided into two main parts. The upper part records the FHR, while the lower part records the uterine activity. At the bottom of each part, there’s an X axis which represents the time, while the Y axis on the left, measures the fetal heart beats per minute, as well as the relative strength of a uterine contraction, measured in millimeters of mercury. Now, there are several important parameters that we assess when looking at the FHR. These include the baseline FHR, variability, and periodic changes. First, let’s take a look at the baseline FHR, which refers to the average FHR, meaning it does not include other variables, such as marked variability, periodic changes, or segments with a difference greater than 25 beats per minute.

In other words, we can say that the baseline FHR is where the fetal heart “settles” over a period of 10 minutes. Normally, this is somewhere between 110 and 160 beats per minute and should be rounded to the nearest 5 beats per minute increments. If the baseline FHR drops below 110, it’s considered bradycardia; but if it rises over 160 beats per minute, it’s tachycardia. The next thing to consider is the baseline FHR variability which refers to the fluctuations of the baseline FHR due to the normal irregularities of the fetal cardiac rhythm. Since variability reflects a healthy modulation of the FHR by the sympathetic and parasympathetic nervous system, it is considered one of the most important characteristics of the FHR. Variability is graded based on the amplitude of fluctuations from the baseline as absent where the amplitude of fluctuations is undetectable; minimal, where the amplitude ranges from more than undetectable and 5 or less; moderate, where the amplitude ranges between 6 and 25; and finally marked with amplitude over 25 beats per minute.

Now, let’s focus on accelerations. Accelerations represent a rise in the FHR for at least 15 beats per minute with a duration of at least 15 seconds. Accelerations represent a normal, reassuring sign. Usually, they are associated with fetal movement, transient compression of umbilical vein, vaginal examination, or scalp stimulation. If accelerations last for more than 2 minutes, they are called prolonged accelerations, and if they last for more than 10 minutes, it’s considered a change in the baseline FHR. On the flip side, decelerations describe a decrease in the FHR. Now, based on the time when they occur in relation to uterine contractions, decelerations can be classified into early, late, variable, and prolonged. Early decelerations represent a normal decrease in the FHR heart rate below the baseline but no less than 100 beats per minute. These changes mirror uterine contractions because the lowest point of the FHR matches the peak of the uterine contraction. This gives early decelerations a uniform shape.

Early decelerations occur when a uterus compresses the fetal head, causing changes in intracranial pressure of cerebral blood flow. The fetal autonomic response to is stimulation of the vagus nerve which decreases the FHR. Early decelerations are considered benign so no interventions or corrective measures are needed. On the other hand, late decelerations begin after the contraction begins, the lowest point of the deceleration occurs after the peak of the contraction, and the recovery of the deceleration occurs after the end of contraction. Late decelerations are associated with uteroplacental insufficiency, or decreased oxygen transfer to the fetus. Lack of oxygen along with lactic acidosis triggers the vagus nerve to decrease the FHR. Late decelerations are considered abnormal, and corrective measures are needed, including position changes, fluid and oxygen administration. Next up are variable decelerations, which refer to an abrupt decrease in the FHR for at least 15 beats per minute with a duration of at least 15 seconds, but no longer than 2 minutes.

These decelerations often resemble the letter “U,” “V” or “W” and are called “variable” because their timing, duration, and amplitude fall and rise abruptly, most commonly due to a compression of the umbilical cord. This is because the cessation of blood flow through the umbilical cord triggers a vagal response, resulting in a slowing of the FHR. Variable decelerations can be periodic, meaning related to uterine contractions, or nonperiodic, meaning independent of uterine contractions. If variable decelerations are intermittent and associated with a normal baseline FHR and variability, the fetus is usually not at risk. But, if they are recurrent and prolonged, they can lead to fetal hypoxia and acidosis. Finally, prolonged decelerations are those that last for more than 2 minutes. But, once again, if it lasts longer than 10 minutes it’s considered a change to baseline FHR. Prolonged decelerations can also be periodic, or nonperiodic, and are associated with sustained interruption of fetal oxygen supply.