Uterine Contractility and Dystocia

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Chapter 11 Uterine Contractility and Dystocia

Richard A. Bashore, Dotun Ogunyemi, Robert H. Hayashi

Although the definition of dystocia is “difficult childbirth,” the term is used interchangeably with dysfunctional labor and characterizes labor that does not progress normally. The problem may be caused by (1) abnormalities of the “powers”: ineffective uterine expulsive forces; (2) abnormalities of the “passenger”: abnormal fetal lie, malpresentation, malposition, or fetal anatomic defects; or (3) abnormalities of the “passage”: maternal bony pelvic contractures, resulting in mechanical interference with the passage of the fetus through the birth canal. The cause or causes of abnormal labor should be determined as accurately as possible so that an effective and safe management plan can be developed.

image Physiologic Changes of Labor

The pregnant uterus is a large smooth muscle organ consisting of billions of smooth muscle cells. Each smooth muscle cell becomes a contractile element when the intracellular ionic calcium concentration increases to trigger an enzymatic process that results in the formation of the actin-myosin element. Stimulation of oxytocin or prostaglandin receptors on the plasma membrane further activates the formation of the actin-myosin element.

Contractions occur in localized areas of the uterus during gestation, but during parturition, the entire uterus contracts in an organized way to empty itself. These coordinated smooth muscle contractions occur as a result of the involvement and action of special gap junction structures. Gap junctions are protein structures that form along the interface of two smooth muscle cell membranes and that act by promoting the movement of action potentials throughout the myometrium.

During labor, two distinct segments of the uterus are formed. The upper segment actively contracts and retracts to expel the fetus while the lower segment, along with the dilating cervix, becomes thinner and passive and is referred to as the lower uterine segment (LUS). A physiologic retraction ring forms at the interface between the two segments. With obstructed labor, the thinning of the LUS becomes very pronounced and is called the pathologic retraction ring of Bandl, and if the obstruction is not relieved, uterine rupture may occur.

The pregnant cervix contains collagen, smooth muscle, and ground substance and must be structurally altered from a firm, intact sphincter to a soft, pliable, dilatable structure through which the products of conception can pass at the appropriate time. These structural changes are the result of collagenolysis and increased hyaluronic acid, with a decrease in dermatan sulfate, which favors increased water content. These changes probably occur in response to an increase in the estrogen-to-progesterone ratio, prostaglandin E2, and enzymatic remodeling of cervical tissue.

image Normal Labor

Labor is diagnosed by regular, painful uterine contractions that increase in frequency and intensity with progressive cervical effacement or dilation.

The early part, or latent phase, of labor is involved with softening and effacement of the cervix with minimal dilation. This is followed by a more rapid rate of cervical dilation, known as the active phase of labor, which is further divided into the acceleration (maximal slope) and deceleration phases. The descent of the fetal presenting part usually begins during the active phase of labor, then progresses at a more rapid rate toward the end of the active phase and continues after the cervix is completely dilated. A useful method for assessing the progress of labor and detecting abnormalities in a timely manner is to plot the rate of cervical dilation and descent of the fetal presenting part (Figure 11-1).

image

FIGURE 11-1 Graphic plot of cervical dilation (green) and descent of the fetal presenting part (red) during labor.

(From Cohen WR, Friedman EA [eds]: Management of Labor. Gaithersburg, MD, Aspen Publishers, 1983, p 13.)

Normal cervical dilation and descent of the fetus take place in a progressive manner and occur within a well-defined time period. Dysfunctional labor occurs when rates of dilation and descent exceed these time limits. The phase of labor during which the abnormality occurs and the configuration of the abnormal labor curve may indicate the potential causes for the abnormal labor.

image Abnormalities of the Latent Phase of Labor

The normal limits of the latent phase of labor extend up to 20 hours for nulliparous patients and up to 14 hours for multiparous patients. A latent phase that exceeds these limits is considered prolonged and may be caused by dysfunctional labor, premature or excessive use of sedatives or analgesics, fetal malpositions, or fetal size. A long, closed, firm cervix requires more time to efface and to undergo early dilation than does a soft, partially effaced cervix, but it is doubtful that a cervical factor alone causes a prolonged latent phase. Many patients who appear to be developing a prolonged latent phase are shown eventually to be in false labor or prelabor, with no progressive dilation of the cervix.

The outcome of a prolonged latent phase is generally favorable for both the mother and the fetus, provided that no other abnormalities of labor subsequently occur.

MANAGEMENT

A prolonged latent phase caused by premature or excessive use of sedation or analgesia usually resolves spontaneously after the effects of the medication have disappeared. Therapeutic rest with morphine sulfate or an equivalent drug has been shown to be effective in ruling out prelabor; women in true labor wake up in active labor, whereas those in prelabor stop contracting.

If a definite diagnosis of prolonged latent phase of labor is made or there are reasons to expedite delivery, augmentation of labor by oxytocin can be performed. This is accomplished by the addition of 10 U of oxytocin to 1000 mL of intravenous solution for a final concentration of 10 mU of oxytocin to each 1 mL of solution. A number of protocols have been suggested for the infusion of oxytocin. Oxytocin can be given as a low dose, in which the infusion is begun at a rate of 1 to 2 mU/minute and is increased in 1- to 2-mU/minute increments every 15 to 40 minutes until the desired frequency and intensity are obtained, or a maximum of 20 to 40 mU/minute is reached. The higher-dose infusion method is begun at a rate of 6 mU/minute, with incremental increases of 6 mU/minute every 15 to 40 minutes until uterine contractions of the desired frequency and intensity are obtained or a maximum of 40 mU/minute is reached.

Amniotomy or artificial rupture of the membranes may be considered as part of the management of the latent phase of labor. An associated risk is an increased incidence of chorioamnionitis.

image Abnormalities of the Active Phase of Labor

When the cervix dilates to about 3 to 4 cm, the rate of dilation progresses more rapidly. Cervical dilation of less than 1.2 cm/hour in nulliparous women and 1.5 cm in multiparous women constitutes a protraction disorder of the active phase of labor. During the latter part of the active phase, the fetal presenting part also descends more rapidly through the pelvis and continues to descend through the second stage of labor. A rate of descent of the presenting part of less than 1 cm/hour in nulliparous women and 2 cm/hour in multiparous women is considered to be a protraction disorder of descent (Figure 11-2). If a period of 2 hours or more elapses during the active phase of labor without progress in cervical dilation, an arrest of dilation has occurred; a period of more than 1 hour without a change in station of the fetal presenting part is defined as an arrest of descent (Figure 11-3).

image

FIGURE 11-2 Normal dilation (green) and descent (red) curves of normal labor and curves depicting protracted dilation and descent abnormalities of labor.

(Modified from Friedman EA: Labor: Clinical Evaluation and Management, 2nd ed. New York, Appleton-Century-Crofts, 1978, p 65.)

image

FIGURE 11-3 Normal dilation (green) and descent (red) curves of normal labor and curves depicting arrest disorders of dilation and descent.

(Modified from Friedman EA: Labor: Clinical Evaluation and Management, 2nd ed. New York, Appleton-Century-Crofts, 1978, p 66.)

Etiology of active phase abnormalities include inadequate uterine activity, cephalopelvic disproportion, fetal malposition, or conduction anesthesia. The maternal pelvis should be evaluated, and the presenting fetal part should also be evaluated under these conditions.

MANAGEMENT

The American College of Obstetricians and Gynecologists (ACOG) recommends the use of oxytocin for all protraction and arrest disorders

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