Fetal Oxygen Supply

Fetal oxygen supply must be maintained during labor to prevent fetal compromise and to promote newborn health after birth.

There are numerous ways in which fetal oxygen supply can decrease, including:

1. Reduction of blood flow through the maternal vessels as a result of maternal hypertension (chronic hypertension, preeclampsia, or gestational hypertension), hypotension (caused by supine maternal position, hemorrhage, or epidural anesthesia), or hypovolemia (caused by hemorrhage)

2. Reduction of the oxygen content in the maternal blood as a result of hemorrhage or severe anemia

3. Alterations in fetal circulation occurring with

   • Compression of the umbilical cord: transient (e.g. during uterine contractions) or prolonged (e.g. resulting from cord prolapse)

   • Partial placental separation or complete abruption

   • Head compression, which causes increased intracranial pressure and vagal nerve stimulation with an accompanying decrease in fetal heart rate

4. Reduction in blood flow to the intervillous space in the placenta secondary to 

   • 1. • Uterine hypertonia (generally caused by excessive exogenous oxytocin) 

        • Deterioration of the placental vasculature associated with maternal disorders such as hypertension or diabetes mellitus

Likewise, uterine activity (UA) can be identified as normal (e.g. frequency every 2-5 minutes, moderate to strong in strength) or abnormal (e.g. frequency greater than every 5 minutes, mild strength)

Fetal Compromise

Fetal compromise is the term used when the fetal heart rate is not stable when contractions occur.  The goals of intrapartum FHR monitoring are to identify and differentiate:

• Normal (reassuring) patterns

• Abnormal (nonreassuring) patterns, which can indicate fetal compromise

Abnormal fetal heart rate patterns are those associated with fetal hypoxemia, which is a deficiency of oxygen in the blood. If uncorrected, hypoxemia can deteriorate to severe fetal hypoxia, an inadequate supply of oxygen at the cellular level that can cause metabolic acidosis. Metabolic acidosis can lead to acidemia, or increased hydrogen ion content in the blood, thereby decreasing its pH. Metabolic acidemia may be a marker of clinically significant interruption of fetal oxygenation.

The purpose of EFM is to assess the adequacy of fetal oxygenation during labor. If the monitor demonstrates evidence of interruption, further evaluation can be initiated, or interventions implemented to improve fetal oxygenation. If these actions are not successful, the monitor can provide information to assist in making decisions regarding the optimal timing and method of birth to avoid the potential consequences of fetal hypoxia.  Two modes of EFM are: External Mode and Internal Mode.

It must be noted that studies show that there are no differences in outcomes for babies who have EFM.  In fact, they lead to more interventions being done during labor.  

The ideal method of fetal assessment during labor, intermittent auscultation (IA) or electronic fetal monitoring (EFM), continues to be debated. Some clinicians prefer the use of IA in low-risk women because it promotes mobility during labor, may be used with hydrotherapy, and provides a more natural birthing experience. The continued reliance on EFM in the United States is most likely because of staffing patterns, staffing mix, and the increased use of defensive practices in a litigious environment.

Nurses must evaluate many factors to determine whether a FHR pattern is normal or abnormal. Evaluation of these factors are based on the presence of other obstetric complications, progress in labor, use of analgesia or anesthesia, and time interval until birth. As such, interventions are based on clinical judgment of a complex, integrated process.

Nurses who care for patients during labor and birth are legally responsible for:

• Correctly interpreting FHR patterns

• Initiating appropriate nursing interventions based on those patterns

• Documenting the outcomes of those interventions

• Timely notification of the physician or nurse–midwife in the event of abnormal FHR patterns

• Initiating the institutional chain of command should differences in opinion arise among health care providers concerning the interpretation of the FHR pattern and the intervention required

External Mode

External fetal monitoring uses transducers placed onto the maternal abdomen to assess FHR and UA.  The data collected from the monitors is shown on a computer monitor and can also be printed out onto paper that will trace the fetal heart rate and contraction pattern using sensors that are attached to the mother using elastic belts.  

Similarly to intermittent auscultations, an ultrasound transducer measures the fetal heart and works by reflecting high-frequency sound waves off a moving interface: in this case, the monitor is attached to the mother using elastic straps.  The FHR and UA data is collected on a specially formatted paper.  It is sometimes difficult to reproduce a continuous and precise record of the FHR because the fetal and maternal movements can introduce artifacts.  Weak FHR signals may be caused by maternal obesity, occiput posterior position of the fetus, anterior attachment of the placenta.

The toco transducer measures uterine activity (UA) transabdominally. The device is placed over the fundus above the umbilicus and also held securely in place using an elastic belt. Uterine contractions or fetal movements depress a pressure-sensitive surface on the side next to the abdomen. The toco transducer can measure and record the frequency and approximate duration of UCs but not their intensity. The intensity of the contractions is measured by palpation. The method is especially valuable for measuring UA during the first stage of labor in patients with intact membranes or for antepartum testing. If the patient is obese, the toco transducer may be unable to detect the exact frequency and duration of UA.  Because most electronic fetal monitors are designed for assessing UA in a term pregnancy, it may not be sensitive enough to detect preterm UA. Additionally, the placement of external transducers often must be readjusted as the patient or fetus changes position.

Internal mode, which uses a fetal scalp electrode (FSE) applied to the fetal presenting part to assess the FHR and an intrauterine pressure catheter (IUPC) to assess UA and uterine resting tone

The technique of continuous internal FHR or UA monitoring provides a more accurate assessment of fetal well-being during labor than external monitoring because it is not interrupted by fetal or maternal movement or affected by maternal size. Membranes must be ruptured, the cervix sufficiently dilated (at least 2 to 3 cm), and the presenting part, which is usually the fetal head, low enough to allow placement of the spiral electrode or intrauterine pressure catheter, or both.  Internal and external modes of monitoring may be combined (i.e., internal FHR with external UA or external FHR with internal UA) without difficulty.

Internal monitoring of the FHR is accomplished by attaching a small spiral electrode to the fetus' presenting part.

For UA to be monitored internally, an intrauterine pressure catheter (IUPC) is introduced into the uterine cavity. The catheter has a pressure-sensitive tip that measures changes in intrauterine pressure. The catheter is compressed during a contraction, and pressure is placed on the pressure transducer. Pressure is then converted into a pressure reading in millimeters of mercury.

There must be at least two minutes of interpretable baseline data in a 10-minute segment of tracing. After 10 minutes of tracing is observed, the approximate mean rate is rounded to the closest 5 bpm interval.

Normal FHR range is 110 to 160 bpm.

Clinical Significance of Bradycardia and Tachycardia

• Persistent tachycardia in the absence of periodic changes does not appear to be serious in terms of neonatal outcome, especially if tachycardia is associated with maternal fever.

• Tachycardia is abnormal when associated with late decelerations, severe variable decelerations, or absent variability.

• Baseline bradycardia alone is not specifically related to fetal oxygenation.

• The clinical significance of bradycardia depends on the underlying cause and the accompanying FHR patterns, including variability, accelerations, or decelerations.

It can be either periodic or episodic. It may occur in association with fetal movement or spontaneously. If accelerations do not occur spontaneously, they can be elicited by fetal scalp or vibroacoustic stimulation.

Another possible cause of accelerations is transient compression of the umbilical vein, resulting in decreased fetal venous return and a reflex rise in heart rate. Similar to moderate variability, accelerations are considered an indication of fetal well-being. Their presence is highly predictive of a normal fetal acid–base balance (absence of fetal metabolic acidemia).

Clinical Significance

Acceleration with fetal movement signifies fetal well-being and represents fetal alertness or arousal states.

Mnemonic to Remember Fetal Heart Rate Patterns

Nursing Management of Abnormal Patterns

Five essential components of the FHR tracing that must be evaluated regularly are:

• Baseline rate

• Baseline variability

• Accelerations

• Decelerations

• Changes or trends over time

Whenever one of these five essential components is assessed as abnormal, corrective measures must be taken immediately. The purpose of these actions is to improve fetal oxygenation.

Intrauterine Resuscitation

The nurse must check the FHR. If the baseline rate begins to slow, if absent or minimal variability occurs, or if abnormal (e.g., late, variable, or prolonged) deceleration patterns develop, interventions must be initiated promptly; these three main actions are known as intrauterine resuscitation.  The first action is to turn the patient onto their side to reduce the pressure of the uterus against the ascending vena cava and descending aorta to increase blood flow to the placenta; increasing the intravenous fluids will also assist in this process.  Lastly, oxygen can be administered by a nonrebreather mask at 10 L/min to theoretically provide more oxygen to the fetus. This last intervention has not been shown to improve outcomes, but it is still noted to be an intervention. A vaginal exam can also be done to check for a precipitous descent of the head, a prolapsed cord, and give fetal scalp stimulation (which sometimes can cause a rise in the FHR).  If the FHR and pattern do not become normal immediately, the next step is to notify the nurse–midwife or physician because the patient may need medical intervention to give birth.

The term intrauterine resuscitation is sometimes used to refer to specific interventions initiated when an abnormal FHR pattern is noted.

Basic corrective measures include:

• Providing supplemental oxygen

• Instituting maternal position changes

• Increasing intravenous (IV) fluid administration

These interventions are implemented to improve uterine and intervillous space blood flow. They also serve to increase maternal oxygenation and cardiac output.

Depending on the underlying cause of the abnormal FHR, however, other interventions such as correcting maternal hypotension, reducing uterine activity, and altering second-stage pushing techniques also may be instituted. Based on the FHR response to these interventions, the obstetric health care provider decides whether additional interventions should be instituted or whether immediate vaginal or cesarean birth should be performed.

Fetal Scalp Stimulation and Vibroacoustic Stimulation

FHR acceleration in response to digital or vibroacoustic stimulation is highly predictive of a normal scalp blood pH. Two methods of fetal stimulation used most often in clinical practice are scalp stimulation (using digital pressure during a vaginal examination) and vibroacoustic stimulation (using an artificial larynx or fetal acoustic stimulation device on the maternal abdomen over the fetal head continuously for one to five seconds).

The desired result of these stimulation methods is an acceleration in the FHR of at least 15 bpm for at least 15 seconds.

FHR acceleration indicates the absence of metabolic acidemia. If the fetus does not respond to stimulation with an acceleration, fetal compromise is not necessarily indicated; however, further evaluation of fetal well-being is needed.

Fetal stimulation should be performed at times when the FHR is at baseline. Neither fetal scalp nor vibroacoustic stimulation should be instituted if FHR decelerations or bradycardia are present.

Other Methods of Assessment and Intervention

A major shortcoming of EFM is its high rate of false-positive results. Even the most abnormal patterns are poorly predictive of neonatal morbidity. Therefore, other methods of assessment and intervention have been developed to evaluate fetal status.

Umbilical Cord Blood Acid–Base Determination

A sample of cord blood is a useful adjunct to the Apgar score, especially if there has been an abnormal or confusing FHR tracing during labor or neonatal depression at birth. The procedure is performed by withdrawing blood from both the umbilical artery and the umbilical vein. Both samples are then tested for pH, carbon dioxide pressure (PCO2), oxygen pressure (PO2), and base deficit or base excess.

Umbilical arterial values reflect fetal condition, whereas umbilical vein values indicate placental function.

ACOG and AAP suggest obtaining umbilical artery cord blood values when a newborn has an Apgar score of 5 or above at five minutes of age. Normal findings preclude the presence of acidemia at or immediately before birth. If acidemia is present (e.g., pH <7.20), however, the type of acidemia should be determined (respiratory, metabolic, or mixed) by analyzing the blood gas values.

Fetal Scalp Blood Sampling

This is performed by obtaining a blood sample from the fetal scalp through the dilated cervix after the membranes have ruptured.

Its use is limited by many factors:

• Requirement for cervical dilation and membrane rupture

• Technical difficulty of the procedure

• Need for repetitive pH determinations

• Uncertainty regarding interpretation and application of results