EVALUATION OF THE FETUS

Fetal well-being is of major importance to both the obstetrician and anesthesiologist and should be addressed in the preanesthetic evaluation of the parturient. An understanding of current methods available to evaluate fetal well-being allows better communication with other staff members of the labor delivery suite, especially when faced with emergency situations.

Electronic fetal heart rate (FHR) monitoring was first developed in the 1960s; it allows continuous monitoring during labor onto a paper chart.^[48] Continuous FHR monitoring is now routinely used during labor for most women in developed countries. FHR can be monitored externally by surface ultrasound or internally with a fetal scalp electrode. The normal FHR varies between 120 and 160 beats/min. Variability of 5 to 25 beats/min occurs as a result of the opposing effects of the sympathetic and parasympathetic innervation of the atrial pacemaker. Reduced variability may be observed during quiescent phases of the fetal activity cycle (corresponding to "sleep") or be pharmacologically induced by drugs administered to the mother (e.g., meperidine). It also occurs during fetal hypoxia and acidosis.

FHR decelerations are classified according to their timing related to uterine contractions. If they occur in the absence of contractions, they are almost always pathologic. Early decelerations mirror uterine contractions measured with a tocograph (internal or external) and are generally related to vagal discharge in response to fetal head compression. Variable decelerations vary in both configuration and timing of uterine contractions. These decelerations usually show an abrupt rise and fall and are caused by compression of the umbilical cord. Late decelerations occur after the peak of the uterine contraction and are an indication of fetal compromise. Fetal scalp blood pH sampling may be used in combination with evaluation of FHR patterns to assess the fetal status in labor. It should be noted, however, that there is not an exact correlation between the severity of fetal hypoxia and the depth of FHR deceleration.^{[49] [50]} In fact, although fetal monitoring is almost always used, it has been suggested that the only clinically significant benefit from continuous FHR monitoring is a reduction in neonatal seizures against an increased rate of instrumental deliveries and cesarean sections.^[51] An increase in FHR may also indicate fetal asphyxia, but it may be due to maternal fever or drug effect.

Fetal pulse oximetry is a recent addition to the tools available to monitor the fetus and is being investigated. A probe is inserted through the cervix and placed between the fetal cheek and uterine wall. To date, investigators have reported a wide range of fetal pulse oximetry values varying between 28% and 71%.^[52] Values below 30% are considered abnormal. Persistently low recordings have been demonstrated to lead to fetal acidosis.^[53] This technology may allow early detection of acidosis when used in combination with continuous FHR monitoring. A recent ACOG committee opinion, however, has stated that the Committee on Obstetric Practice "cannot endorse the adoption of this device in clinical practice at this time

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Role of Intrauterine Resuscitation

Intrauterine resuscitation (IUR) refers to the application of certain measures in an attempt to improve both oxygen delivery to the placenta and umbilical blood flow for reversal of fetal hypoxia and acidosis. With a better understanding of the mechanisms at play during acute fetal "distress" in labor, specific IUR maneuvers may be applied to individual patients. Short-lived reductions in fetal oxygenation may be a result of aortocaval compression, uterine hyperstimulation, umbilical cord compression, or maternal hypoxemia. Maneuvers used during such situations include left lateral or knee-chest positioning, discontinuation of oxytocin infusion, supplemental maternal oxygen administration, rapid infusion of crystalloid solution, vasopressor administration to treat hypotension, and terbutaline- or nitroglycerin-induced tocolysis. ^[55] In addition amnioinfusion may be initiated by the obstetrician.

Although few studies have evaluated the use of IUR in a randomized fashion, its application is increasing dramatically. As anesthesiologists, we may become actively involved in IUR, especially during the establishment of regional analgesia and when urgent delivery of the fetus is required by cesarean section or instrumental delivery.^[56] The ability to temporarily improve a problematic FHR can often allow the anesthesiologist to use a neuraxial technique.