During labor and delivery, fetal hypoxemia can increase the risk of significant morbidity and mortality. Causes of fetal hypoxemia include any number of insults to uterine blood flow, such as normal contractions, umbilical cord compression, poorly functioning placentas, maternal hypotension, and chronic maternal vascular disease. Possible outcomes include cerebral palsy, neonatal seizures, neonatal hypoxic ischemic encephalopathy, and intrapartum fetal death, or stillbirth. Since there are no direct tests to diagnose fetal hypoxemia, electronic fetal monitoring (EFM) serves as an indirect assessment of the status of the fetus. Monitoring of the fetal heart rate (FHR) is the primary modality used to assess the well-being of the fetus. Normal FHR measurements will accurately predict fetal well-being, while abnormal tracings suggest hypoxemia and acidemia. Timely identification and intervention can prevent fetal death and neurologic injury.
FHR patterns reflect the cardiovascular and neurologic responses to hypoxemia and acidemia. When the oxygen delivery to the fetus decreases, the healthy fetus initially responds with tachycardia in order to increases cardiac output and oxygen delivery. A significant increase in systemic vascular resistance then helps to redistribute blood flow from nonessential vascular beds (peripheral, splanchnic) to vital organs such as the brain and heart. In response to this vasoconstriction, reflex bradycardia occurs, which is reflected as a “deceleration” through EFM. In addition, the fetus also compensates by decreasing oxygen consumption (through slower breathing and movements) and relying upon anaerobic glycolysis. A healthy fetus should be able to tolerate brief hypoxic episodes. If the hypoxia is severe, prolonged, or uncorrected, these compensatory processes become overwhelmed, leading to fetal acidosis and injury.
METHODS OF FETAL HEART RATE ASSESSMENT
Manual auscultation involves using either a fetal stethoscope or hand-held Doppler ultrasound device to measure the FHR intermittently, usually during and immediately after a contraction. The only data obtained is the FHR. Manual auscultation does not provide information about heart rate variability or deceleration patterns. This form of monitoring is not used as commonly today, perhaps because it requires dedicated 1:1 patient:nursing care ratios. However, if performed at the recommended intervals (every 15 minutes during first stage of labor, every 5 minutes during second stage), manual auscultation has an equivalent efficacy as continuous EFM when it comes to reducing long-term neurologic complications and low Apgar scores.
Continuous Electronic FHR Monitoring (EFM)
Continuous EFM can be measured through both invasive and noninvasive approaches. The preferred method uses a Doppler ultrasound device that is placed on the patient’s abdomen and secured with a belt. The device measures the blood flow in major fetal vessels and calculates FHR by determining the intervals between myocardial contractions. Invasive monitoring of FHR involves placing an internal electrode within the fetal scalp. This electrode measures the fetal electrocardiogram and calculates heart rate based on the R–R interval. It provides a clear, more accurate measurement. With both approaches, an ...