Learning Objectives for PeriFACTS Case #681: Upon completion, the learner will be able to:
- Describe characteristics of a sinusoidal fetal heart rate pattern.
- Differentiate a sinusoidal fetal heart rate pattern from a pseudosinusoidal fetal heart rate pattern.
- Interpret a saltatory/marked variability pattern from a pre-terminal fetal heart rate pattern.
The challenge of fetal heart rate interpretation is made that much more complex when pattern recognition and terminology seem not to agree. In this PeriFACTS related reading, we will discuss two areas of frequent confusion: 1.) sinusoidal versus pseudosinusoidal fetal heart rate patterns and 2.) saltatory or marked variability fetal heart rate changes. The reader should be alerted that explanation of the first is very straightforward. That cannot be said for the second.
It is critical that obstetric care providers are able to quickly and accurately distinguish between sinusoidal and pseudosinusoidal fetal heart rate (FHR) patterns. Although they may seem similar in appearance, the two patterns are associated with significantly different fetal outcomes. Despite extensive discussion in the obstetric community, there continues to be a lack of consensus regarding the characteristics that constitute a "true" sinusoidal FHR pattern. The most widely accepted criteria used to identify “true" sinusoidal FHR patterns (Figure 1.6) are the following (modified using current NIH nomenclature from Mondanlou and Freeman who published their findings in 1982):
- A stable FHR baseline between 110 to 160 bpm (i.e., normal FHR baseline range) with regular oscillations.
- Oscillations of the sinusoidal wave form above and below an imaginary baseline, although baseline is indeterminable with a sinusoidal pattern.
- Amplitude of oscillations of 5 to 15 bpm (rarely is the amplitude of oscillations >15 bpm). Baseline variability is minimal or absent.
- Frequency of oscillations of two to five cycles/min.
- Absence of FHR accelerations.
A sinusoidal fetal FHR pattern is defined as a pattern of fixed, uniform fluxuations of the FHR that creates a pattern resembling successive geometric sine waves. It frequently is described as undulating and smooth and is characterized by the absence of variability. Here is one area of confusion. These oscillations do not qualify as “variability,” but instead should be referred to as sinusoidal oscillations. Sinusoidal FHR patterns are associated with increased perinatal morbidity and mortality and poor perinatal outcome.
Incidence of a Sinusoidal Fetal Heart Rate Pattern
The incidence of a "true" sinusoidal FHR pattern is rare, reportedly between 0.3 to 1.7%. This figure may not reflect accurately the actual incidence since, historically, standard criteria have not been applied consistently.
Physiology of a Sinusoidal Fetal Heart Rate Pattern
The physiologic basis for the development of a "true" sinusoidal FHR pattern continues to be elusive. Fetal heart rate variability is known to be affected by fetal hypoxemia. Mild fetal hypoxia initiates an adrenergic (sympathetic) response resulting in fetal hypertension. Baroreceptors detect an increase in fetal blood pressure and reflexively activate a vagal (parasympathetic) response. Thus, acute or mild fetal hypoxia triggers an increase in autonomic nervous system activity (both sympathetic and parasympathetic) that results in an increase in both STV and LTV. Prolonged/severe fetal hypoxia with acidemia results in decreased FHR variability, presumably by the effects of hypoxia and acidosis on the fetal central nervous system (CNS).3 Elliot et al. noted that a sinusoidal FHR pattern, demonstrated by a fetus with known erythroblastosis fetalis, continued to be present in the neonatal period and did not resolve with neonatal oxygen administration. In fact, the pattern did not resolve until the infant received a blood transfusion, which suggests that increased hemoglobin concentrations may have alleviated true tissue hypoxia. Research in animal models (fetal lambs) has shown that anemic fetuses exhibiting sinusoidal FHR patterns have elevated levels of arginine vasopressin, a pituitary hormone with vasopressor and antidiuretic effects. However, administration of arginine vasopressin, in the absence of cerebral ischemia, did not result in sinusoidal FHR patterns. Both elevated levels of arginine vasopressin and cerebral ischemia were necessary to elicit sinusoidal FHR patterns.
"True" sinusoidal FHR patterns are associated with the following fetal conditions that result in either severe fetal anemia or severe/prolonged fetal hypoxia with acidosis.
- Chronic fetal anemia associated with erythroblastosis fetalis, usually from Rh sensitization
- Acute, intrapartum asphyxia
- Fetal-maternal hemorrhage
- In-utero, fetal hemorrhage
Interventions for a Sinusoidal Fetal Heart Rate Pattern
When a "true" sinusoidal FHR pattern is noted, immediate intervention is required. Continuous, careful, fetal assessment, including direct ultrasound observation for signs of fetal hydrops, anomalies, or placental abnormalities, is necessary. Assessment for active fetal movement is important in quickly assessing fetal status. Ultrasound provides an opportunity to perform a biophysical profile and quantitative fetal health parameters. Percutaneous umbilical blood sampling (PUBS) under ultrasound guidance is a technique that permits fetal hemoglobin levels to be assessed in utero and blood transfusions to be given directly to the fetus if clinically indicated. A maternal serum Kleihauer-Betke test, which quantitates the presence of fetal blood in maternal circulation, may be helpful if a fetal-maternal bleed is suspected. Preparations for a possible emergent delivery should be initiated if intrauterine resuscitation is not appropriate.
Pseudosinusoidal Fetal Heart Rate Pattern
A sinusoidal-like FHR pattern, often referred to as "pseudosinusoidal" ("false" sinusoidal), is a FHR pattern that closely resembles a sinusoidal pattern (Figure 1.7). Pseudosinusoidal FHR patterns, however, usually are transient, resolve spontaneously without intervention, and are associated with good fetal outcomes. Usually, a pseudosinusoidal FHR pattern retains periods of moderate variability and/or an occasional FHR acceleration.
Incidence of a Pseudosinusoidal Fetal Heart Rate Pattern
Pseudosinusoidal FHR patterns frequently are encountered during the antepartum, intrapartum, and neonatal period. In a study by Murphy et al. that included a sample of 1,520 patients, 15% demonstrated pseudosinusoidal FHR patterns. Pseudosinusoidal FHR patterns frequently are observed after maternal administration of narcotic analgesics during labor.
Physiology of a Pseudosinusoidal Fetal Heart Rate Pattern
The cause of pseudosinusoidal FHR patterns is unknown. Research suggests that pseudosinusoidal FHR patterns, when observed after maternal narcotic analgesic administration, result from rhythmic fetal breathing, sucking, or hiccoughing movements in the absence of other fetal movements. An undulating pattern often appears within five minutes of intravenous narcotic administration and spontaneously resolves without intervention within 30 minutes. The pattern of undulation usually is interspersed with an occasional acceleration or period of moderate variability. Pseudosinusoidal FHR patterns have been observed after administration of Demerol®, Nubain®, morphine, Stadol®, and Nisentil®. These medications commonly are used to alleviate pain during labor and delivery. Pseudosinusoidal FHR patterns, therefore, are encountered frequently by obstetric care providers.
Interventions for a Pseudosinusoidal Fetal Heart Rate Pattern
Often the most important intervention for a pseudosinusoidal FHR pattern is for the care provider to rule out the presence of fetal acidosis and anemia and to distinguish the pattern from a "true" sinusoidal one. The development of a pseudosinusoidal FHR pattern following medication administration, in the absence of fetal acidosis and anemia, has not been associated with increased perinatal morbidity and mortality. Therefore, it does not necessitate intervention other than continued observation.
In numerous articles and National Society reports, the terms “saltatory” and “marked variability” have been described interchangeably but with some variations in definition and clinical significance (Figure 1.8). Broadly defined, this pattern consists of FHR fluctuations >25 bpm occurring either 3 to 6, or >6 times per minute. Some investigators have associated this pattern with a lower Apgar score, suggesting acute hypoxia. This relationship has been noted in the animal model, and clinically in which hypoxemia was documented.
In this latter report, a woman, at term, was inadvertently given an inappropriate nitrous oxide-oxygen mixture that produced a 5-minute period of maternal hypoxemia. The documented FHR pattern showed an initial bradycardia and loss of variability. Once the woman was appropriately ventilated with the correct level of oxygen, the FHR returned to a normal baseline but exhibited marked variability. Fortunately, the newborn, delivered several hours later, had an Apgar score of 9 and 10 at 1 and 5 minutes, respectively. Others have stressed that in the absence of other FHR abnormalities, it is not an ominous pattern. For example, Obrien-Abel (1992) identified ten such tracings in 433 consecutive FHR monitor tracings (2.3%). All ten were at term, six occurred one to four hours after maternal ephedrine administration, three were following uterine hyperstimulation, and one followed a cervical examination. All ten newborns delivered with Apgar scores of 8/9 or 9/9. Others have found that a saltatory pattern is benign if the FHR tracing is otherwise reassuring.
The real confusion comes from witnessing a pre-terminal fetal heart rate pattern (Figure 1.9) that also is referred occasionally to as a saltatory pattern. The pre-terminal pattern is indeed linked to a marked increase in perinatal mortality. The etiology of this pattern is unclear but most likely reflects heart failure with profound hypoxemia and metabolic acidosis. While debate regarding the clinical significance of a saltatory pattern continues, there should be no debate regarding the ominous significance of a pre-terminal fetal heart rate pattern. Prompt delivery is the only approach to this devastating heart rate pattern.
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