Physiology of Normal Labor

PHYSIOLOGY OF NORMAL LABOR

On average, labor commences 40 ± 2 weeks follow-ing the last menstrual period. The factors involved in the initiation of labor likely involve distention of the uterus, enhanced myometrial sensitivity to oxy-tocin, and altered prostaglandin synthesis by fetal membranes and decidual tissues. Although circu-lating oxytocin levels often do not increase at the beginning of labor, the number of myometrial oxy-tocin receptors rapidly increases. Several prodromal events usually precede true labor approximately 2–4 weeks prior to delivery: the fetal presenting part settles into the pelvis (lightening); patients develop uterine (Braxton Hicks) contractions that are char-acteristically irregular in frequency, duration, and intensity; and the cervix softens and thins out (cervi-cal effacement). Approximately 1 week to 1 h before true labor, the cervical mucous plug (which is often bloody) breaks free (bloody show).

True labor begins when the sporadic Braxton Hicks contractions increase in strength (25–60 mm Hg), coordination, and frequency (15–20 min apart). Amniotic membranes may rupture spontaneously prior or subsequent to the onset of true labor. Following progressive cervical dilation, the con-tractions propel first the fetus and then the placenta through the pelvis and perineum. By convention, labor is divided into three stages. The first stage is defined by the onset of true labor and ends with

complete cervical dilation. The second stage begins with full cervical dilation, is characterized by fetal descent, and ends with complete delivery of the fetus. Finally, the third stage extends from the birth of the baby to the delivery of the placenta.

Based on the rate of cervical dilation, the first stage is further divided into a slow latent phase fol-lowed by a faster active phase (Figure 40–3). The latent phase is characterized by progressive cervical effacement and minor dilation (2–4 cm). The subse-quent active phase is characterized by more frequent contractions (3–5 min apart) and progressive cervi-cal dilation up to 10 cm. The first stage usually lasts 8–12 h in nulliparous patients and about 5–8 h in multiparous patients.

Contractions during the second stage occur 1.5–2 min apart and last 1–1.5 min. Although con-traction intensity does not appreciably change, the parturient, by bearing down, can greatly augment intrauterine pressure and facilitate expulsion of the fetus. The second stage usually lasts 15–120 min and the third stage typically 15–30 min.

The course of labor is monitored by uterine activity, cervical dilation, and fetal descent. Uterine activity refers to the frequency and magnitude of uterine contractions. The latter may be measured directly, with a catheter inserted through the cer-vix, or indirectly, with a tocodynamometer applied externally around the abdomen. Cervical dilation and fetal descent are assessed by pelvic examination. Fetal station refers to the level of descent (in centi-meters) of the presenting part relative to the ischial spines (eg, –1 or +1).

Effect of Labor on Maternal Physiology

During intense painful contractions, maternal min-ute ventilation may increase up to 300%. Oxygen consumption also increases by an additional 60% above third-trimester values. With excessive hyper-ventilation, Paco₂ may decrease below 20 mm Hg. Marked hypocapnia can cause periods of hypoven-tilation and transient maternal and fetal hypoxemia between contractions. Excessive maternal hyper-ventilation also reduces uterine blood flow and pro-motes fetal acidosis.

Each contraction places an additional burden on the heart by displacing 300–500 mL of blood from the uterus into the central circulation (analogous to an autotransfusion). Cardiac output rises 45% over third-trimester values. The greatest strain on the heart, however, occursimmediately after delivery, when intense uterine contraction and involution suddenly relieve inferior vena caval obstruction and increase cardiac output as much as 80% above late third trimester values.

Effect of Anesthetic Agents on Uterine Activity & Labor

A. Inhalational Agents

Sevoflurane, desflurane, isoflurane, and halothane depress uterine activity equally at equipotent doses; all cause dose-dependent uterine relaxation. Low doses (<0.75 MAC) of these agents, however, do not interfere with the effect of oxytocin on the uterus. Higher doses can result in uterine atony and increase blood loss at delivery. Nitrous oxide has minimal, if any, effects.

B. Parenteral Agents

Opioids minimally decrease the progression of labor; ketamine, in doses of less than 2 mg/kg, appears to have little effect.

C. Regional Anesthesia

The administration of epidural analgesia is usually based upon the patient’s choice, and it is often uti-lized for patients with

maternal or fetal factors that increase the likelihood of prolonged labor or cesarean delivery (Table 40–2). Current evidence indicates that dilute combinations of a localanesthetic (eg, bupivacaine, 0.125% or less) and an opioid (eg, fentanyl, 5 mcg/mL or less) for epidural

or combined spinal–epidural (CSE) analgesia do not prolong labor or increase the likelihood of operative delivery.

When greater concentrations of local anesthetic (eg, bupivacaine, 0.25%) are used for continuous epidural analgesia, the second stage of labor may be prolonged by approximately 15–30 min. Intense regional analgesia/anesthesia can remove the urge to bear down during the second stage (Ferguson reflex), and motor weakness can impair expulsive efforts, often prolonging the second stage of delivery. Use of dilute local anesthetic–opioid mixtures can preserve motor function and allow effective push-ing. Intravenous fluid loading (crystalloid boluses) is often used to prevent or reduce the severity of hypo-tension following an epidural injection. So-called fluid loading does not reduce the incidence of hypotension and has been shown to reduce endog-enous oxytocin secretion from the pituitary and transiently decrease uterine activity. Epinephrine-containing local anesthetic solutions could theoreti-cally prolong the first stage of labor if absorption of epinephrine from the epidural space results in sig-nificant systemic β-adrenergic effects. Prolongation of labor is generally not clinically observed with very dilute (eg, 1:400,000) epinephrine-containing local anesthetics.

D. Vasopressors

Uterine muscle has both α and β receptors. α₁-Receptor stimulation causes uterine contraction, whereas β₂-receptor stimulation produces relax-ation. Large doses of α-adrenergic agents, such as phenylephrine, in addition to causing uterine arte-rial constriction, can produce tetanic uterine con-tractions. Small doses of phenylephrine (40 mcg) may increase uterine blood flow in normal parturi-ents by raising arterial blood pressure. In contrast, ephedrine has little effect on uterine contractions.

E. Oxytocin

Oxytocin (Pitocin) is usually administered intrave-nously to induce or augment uterine contractions or to maintain uterine tone postpartum. It has a half-life of 3–5 min. Induction doses for labor are 0.5–8 mU/min. Complications include fetal dis-tress due to hyperstimulation, uterine tetany, and, less commonly, maternal water retention (antidiuretic effect). Rapid intravenous infusion can cause transient systemic hypotension due to relaxation of vascular smooth muscle; reflex tachycardia may also be noted.

Uterine atony is the most common cause of severe postpartum hemorrhage. Immediate admin-istration of oxytocin after delivery is a standard measure to prevent this complication. Despite this practice, uterine atony complicates 4–6% of pregnan-cies. The concentration of volatile anesthetics should be reduced to 0.5 MAC in obstetric patients undergo-ing general anesthesia for cesarean delivery to avoid the uterine-relaxing effects of these drugs. Second-line oxytocics are methylergonovine (Methergine) and carboprost tromethamine (Hemabate).

F. Ergot Alkaloids

Methylergonovine (Methergine) causes intense and prolonged uterine contractions. It is therefore given only after delivery (postpartum) to treat uterine atony. Moreover, because it also constricts vascular smooth muscle and can cause severe hypertension if given as an intravenous bolus, it is usually admin-istered only as a single 0.2 mg dose intramuscularly or in dilute form as an intravenous infusion over 10 minutes.

G. Prostaglandins

Carboprost tromethamine (Hemabate, prostaglan-din F_2α) is a synthetic analogue of prostaglandin F₂ that stimulates uterine contractions. It is often used to treat refractory postpartum hemorrhage. An ini-tial dose of 0.25 mg intramuscularly may be repeated every 15–90 min to a maximum of 2 mg. Common side effects include nausea, vomiting, broncho-constriction, and diarrhea. It is contraindicated in patients with bronchial asthma. Prostaglandin E₁ (Cytotec, rectal suppository) or E ₂ (Dinoprostone, vaginal suppository) is sometimes administered and has no bronchoconstricting effect.

H. Magnesium

Magnesium is used in obstetrics both to stop pre-mature labor (tocolysis) and to prevent eclamptic seizures. It is usually administered as a 4 g intra-venous loading dose (over 20 min) followed by a 2 g/h infusion. Therapeutic serum levels are consid-ered to be 6–8 mg/dL. Serious side effects include hypotension, heart block, muscle weakness, and sedation. Magnesium in these doses and concentra-tions intensifies neuromuscular blockade from non-depolarizing agents.

H. β₂ Agonists

The β₂-adrenergic agonists ritodrine and terbutaline inhibit uterine contractions and are used to treat premature labor.