From the Guidelines
Pseudosinusoidal fetal heart rate patterns are primarily caused by severe fetal anemia, fetal hypoxia, umbilical cord compression, and administration of narcotic medications to the mother, which can lead to alterations in autonomic nervous system regulation of the fetal heart rate. The main causes of pseudosinusoidal fetal heart rate patterns include:
- Severe fetal anemia (from Rh isoimmunization, fetomaternal hemorrhage, or twin-to-twin transfusion syndrome) 1
- Fetal hypoxia
- Umbilical cord compression
- Administration of narcotic medications to the mother
- Fetal sucking or hiccupping
These patterns appear as smooth, undulating waves that resemble true sinusoidal patterns but lack the perfect regularity. When identified on a fetal monitoring strip, immediate evaluation is necessary to determine the underlying cause. Severe fetal anemia typically produces a persistent pattern, while intermittent patterns are more commonly associated with cord compression or narcotic effect. Management depends on the suspected cause and may include fetal blood sampling to assess hemoglobin levels, discontinuation of narcotics if applicable, position changes to relieve cord compression, or emergency delivery if fetal compromise is evident 1. The mechanism behind pseudosinusoidal patterns involves alterations in autonomic nervous system regulation of the fetal heart rate, with decreased variability and rhythmic oscillations resulting from impaired central control mechanisms.
It is essential to note that sleep cycles of 20 to 40 minutes or longer may cause a normal decrease in FHR variability, as can certain medications, including analgesics, anesthetics, barbiturates, and magnesium sulfate 1. Loss of variability, accompanied by late or variable decelerations, increases the possibility of fetal acidosis if uncorrected. Therefore, it is crucial to consider these factors when interpreting fetal heart rate patterns and to prioritize the well-being and safety of the fetus.
In terms of specific conditions that can cause pseudosinusoidal patterns, nonimmune hydrops fetalis (NIHF) is a significant concern, with various etiologies, including cardiovascular, chromosomal, hematologic, infectious, and lymphatic dysplasia 1. The most common causes of NIHF include cardiovascular malformations, chromosomal abnormalities, and hematologic disorders, such as alpha thalassemia.
Overall, pseudosinusoidal fetal heart rate patterns require prompt attention and evaluation to determine the underlying cause and to ensure the best possible outcome for the fetus.
From the Research
Causes of Pseudosinusoidal Fetal Heart Rate Pattern
The causes of a pseudosinusoidal fetal heart rate (FHR) pattern are varied and can be associated with several fetal conditions. Some of the possible causes include:
- Fetal cardiac anomalies, as suggested by a case study where pseudo-sinusoidal heart rate episodes were associated with multiple major cardiac anomalies 2
- Fetal anemia, which can be caused by various factors such as Rh-sensitization, fetal-maternal hemorrhage, or umbilical cord knots 3, 4, 5
- Fetal asphyxia/hypoxia, which can lead to autonomic nervous system dysfunction and an increase in arginine vasopressin levels 3
- Fetal infection, which can cause changes in fetal heart rate patterns 3
- Effects of drugs, particularly narcotics, which can alter fetal heart rate variability and reactivity 3
Fetal Conditions Associated with Sinusoidal Heart Rate Pattern
The sinusoidal heart rate pattern, which is similar to the pseudosinusoidal pattern, has been reported in association with several fetal conditions, including:
- Severe fetal anemia of several etiologies 3, 4, 5
- Fetal cardiac anomalies 2, 3
- Fetal asphyxia/hypoxia 3
- Fetal infection 3
- Fetal sleep cycles and sucking and rhythmic movements of the fetal mouth 3
Definition and Clinical Significance of True Sinusoidal Heart Rate Pattern
A true sinusoidal heart rate pattern is defined by specific criteria, including a stable baseline FHR, amplitude of 5-15 bpm, frequency of 2-5 cycles per minute, and fixed or flat short-term variability 3. This pattern is considered an ominous sign of fetal jeopardy and requires immediate intervention 3. The correct diagnosis of a true sinusoidal heart rate pattern should also include fetal biophysical profile and the absence of drugs such as narcotics 3.