| disease | Uterine Atony |
Labor forces include uterine contractions, abdominal wall and diaphragmatic muscle contractions, and levator ani muscle contractions, with uterine contractions being the primary component. During childbirth, abnormal rhythm, symmetry, polarity, or changes in intensity and frequency of uterine contractions are referred to as abnormal uterine contractility. Clinically, this often results from obstructive dystocia caused by abnormalities in the birth canal or fetal factors, which increase resistance to fetal passage through the birth canal, leading to secondary abnormal labor forces. Abnormal uterine contractility is clinically classified into two categories: uterine inertia and hypertonic uterine contractions, each of which can further be divided into coordinated and uncoordinated uterine contractions.
bubble_chart Etiology
It is usually caused by a combination of several factors, with common reasons including:
1. Cephalopelvic disproportion or abnormal fetal position: The fetal presenting part is obstructed from descending and cannot closely adhere to the lower uterine segment and cervix, thus failing to trigger reflexive uterine contractions, leading to secondary uterine inertia.
2. Uterine factors: Poor uterine development, uterine anomalies (such as bicornuate uterus, etc.), overdistension of the uterine wall (e.g., twins, macrosomia, polyhydramnios, etc.), or multiparity causing degeneration of uterine muscle fibers or uterine fibroids can all lead to uterine inertia.
3. Psychological factors: Primiparas [especially elderly primiparas over 35 years old] may experience excessive mental stress, leading to dysfunction of the cerebral cortex, insufficient sleep, reduced food intake during labor, and excessive physical exhaustion, all of which can result in uterine inertia.
5. Drug effects: Inappropriate use of large doses of sedatives and analgesics during labor, such as morphine, chlorpromazine, pethidine, or barbiturates, can inhibit uterine contractions.
bubble_chart Clinical Manifestations
According to the time of occurrence, it can be divided into primary and secondary types. Primary uterine inertia refers to the condition where uterine contractions are weak at the onset of labor, the cervical os fails to dilate as expected, the fetal presenting part does not descend as anticipated, and the labor process is prolonged. Secondary uterine inertia refers to the condition where uterine contractions are normal at the onset of labor but weaken at a certain stage of labor progression (often during the active phase or the second stage), leading to slow or even halted labor progress. Uterine inertia can be classified into two types, each with distinct clinical manifestations.
1. Coordinated uterine inertia (hypotonic uterine inertia): The uterine contractions maintain normal rhythm, symmetry, and polarity, but the contraction force is weak, with low intrauterine pressure (<2.0 kPa), short duration, long and irregular intervals, and fewer than 2 contractions per 10 minutes. At the peak of uterine contractions, the uterine body does not bulge or harden, and pressing the uterine fundus with fingers may still leave a depression. Labor is prolonged or halted. Due to the low intrauterine tension, the impact on the fetus is minimal.
2. Uncoordinated uterine inertia (hypertonic uterine inertia): The polarity of uterine contractions is reversed, with contractions not originating from the bilateral uterine cornua but from one or multiple points in the uterus, resulting in uncoordinated rhythm. During contractions, the uterine fundus is not strong, while the middle or lower segments are. The uterine wall does not fully relax during the intervals, manifesting as uncoordinated uterine contractions. These contractions cannot dilate the cervical os or facilitate the descent of the fetal presenting part, rendering them ineffective. The mother experiences persistent lower abdominal pain, tenderness aversion, dysphoria, restlessness, dehydration, electrolyte imbalance, intestinal distension, and urinary retention. Fetal-placental circulation may be compromised, leading to fetal distress. Examination reveals tenderness in the lower abdomen, unclear fetal position, irregular fetal heart rate, slow or absent cervical dilation, delayed or halted descent of the fetal presenting part, and prolonged labor.
(1) Prolonged latent phase: The period from the onset of regular labor contractions to cervical dilation of 3 cm is called the latent phase. For primiparas, the normal latent phase lasts about 8 hours, with a maximum limit of 16 hours. Exceeding 16 hours is termed a prolonged latent phase (Figure 1).
Figure 1: Schematic diagram of prolonged latent phase of cervical dilation
(2) Prolonged active phase: The period from cervical dilation of 3 cm to full dilation is called the active phase. For primiparas, the normal active phase lasts about 4 hours, with a maximum limit of 8 hours. Exceeding 8 hours is termed a prolonged active phase (Figure 2).
Figure 2: Schematic diagram of prolonged active phase
(3) Arrested active phase: After entering the active phase, if cervical dilation ceases for more than 2 hours, it is called an arrested active phase.
(4) Prolonged second stage: For primiparas, if the second stage exceeds 2 hours, or for multiparas, if it exceeds 1 hour without delivery, it is termed a prolonged second stage.
(5) Arrested second stage: If there is no progress in fetal head descent for 1 hour during the second stage, it is called an arrested second stage.
(6) Prolonged fetal head descent: During the active advanced stage, when cervical dilation reaches 9–10 cm, if the fetal head descent rate is less than 1 cm per hour, it is termed prolonged fetal head descent.
(7) Arrested fetal head descent: If the fetal head remains stationary for more than 1 hour, it is called arrested fetal head descent.
These seven types of abnormal labor progress can occur individually or in combination. When the total duration of labor exceeds 24 hours, it is termed prolonged labor, which must be avoided.
bubble_chart Treatment Measures
1. Coordinated uterine contraction lack of strength Whether primary or secondary, once coordinated uterine contraction lack of strength occurs, the first step is to identify the cause, checking for cephalopelvic disproportion and abnormal fetal position, and assessing cervical dilation and fetal descent. If cephalopelvic disproportion is found and vaginal childbirth is deemed impossible, a seasonal epidemic cesarean section should be performed promptly. If no cephalopelvic disproportion or abnormal fetal position is detected and vaginal childbirth is considered feasible, measures to strengthen uterine contractions should be considered.
(1) First stage of labor
1) General management: Relieve mental tension, encourage rest, and promote adequate food intake. For those unable to eat, intravenous nutrition supplementation can be provided, such as 500–1000 ml of 10% glucose solution with 2 g of vitamin C. In cases of acidosis, 5% sodium bicarbonate should be supplemented. For hypokalemia, potassium chloride should be administered via slow intravenous drip. If the mother is excessively fatigued, 10 mg of diazepam can be given via slow intravenous injection or 100 mg of pethidine via intramuscular injection. After some time, uterine contraction strength may improve. For primiparas with cervical dilation less than 3 cm and intact membranes, a warm soapy water enema can be administered to stimulate intestinal peristalsis, expel feces and gas, and promote uterine contractions. If spontaneous urination is difficult, induction methods should be attempted first; if unsuccessful, catheterization should be performed, as emptying the bladder can widen the birth canal and enhance uterine contractions.
2) Strengthening uterine contractions: If uterine contractions remain weak after general management and coordinated uterine contraction lack of strength is confirmed, with no significant progress in labor, the following methods can be used to strengthen contractions:
① Artificial rupture of membranes: For cervical dilation of 3 cm or more, no cephalopelvic disproportion, and engaged fetal head, artificial rupture of membranes can be performed. After rupture, the fetal head directly presses against the lower uterine segment and cervix, triggering reflexive uterine contractions and accelerating labor progress. Some scholars advocate artificial rupture of membranes even if the fetal head is not engaged, believing it can promote fetal descent into the pelvis. Before rupture, check for cord presentation, and perform the procedure during a contraction interval. After rupture, the operator’s fingers should remain in the vagina for 1–2 contractions until the fetal head descends before removal. Bishop proposed a cervical maturity scoring system to predict the effectiveness of measures to strengthen contractions (see Table 1). If the score is 3 or below, artificial rupture of membranes is likely to fail, and other methods should be considered. Scores of 4–6 have a success rate of about 50%, 7–9 about 80%, and above 9 almost always succeed.
Table 1 Bishop Cervical Maturity Scoring System
| Indicator | Score | |||
| 0 | 1 | 2 | 3 | |
| Cervical dilation (cm) | 0 | 1–2 | 3–4 | 5–6 |
| Cervical effacement (%) (uneffaced = 2 cm) | 0–30 | 40–50 | 60–70 | 80–100 |
| Fetal station (ischial spine level = 0) | -3 | -2 | -1–0 | +1～+2 |
| Cervical Firmness | Firm | Medium | Soft | |
| Cervical Os Position | Posterior | Mid | Anterior | |
②Intravenous Diazepam (Valium): Diazepam can relax cervical smooth muscles, soften the cervix, and promote cervical dilation. It is suitable for cases of slow cervical dilation and cervical edema. The usual dose is 10mg administered intravenously, which can be repeated every 2 to 6 hours. The effect is enhanced when used in combination with oxytocin.
③Intravenous Oxytocin Infusion: This is suitable for cases with coordinated uterine contractions lacking strength, good fetal heart rate, normal fetal position, and cephalopelvic disproportion. Add 2.5U of oxytocin to 500ml of 5% glucose solution, making each drop contain 0.33mU of oxytocin. Start at 8 drops/min (2.5mU/min) and adjust based on the strength of contractions, usually not exceeding 10mU/min (30 drops/min). Maintain intrauterine pressure during contractions at 6.7–8.0kPa (50–60mmHg), with contractions occurring every 2–3 minutes and lasting 40–60 seconds. For less sensitive cases, the oxytocin dose can be increased.
During oxytocin infusion, a dedicated person should monitor contractions, listen to the fetal heart rate, and measure blood pressure. If contractions last more than 1 minute or changes in fetal heart rate are detected, the infusion should be stopped immediately. The half-life of oxytocin in maternal blood is 2–3 minutes, and its effects subside quickly after discontinuation. Sedatives may be added if necessary to suppress its effects. If blood pressure rises, the infusion rate should be slowed. Due to oxytocin's antidiuretic effect, water reabsorption increases, potentially leading to oliguria, so vigilance for water intoxication is required.
④Prostaglandin (PG) Application: Both prostaglandin E2 and F2α promote uterine contractions. Routes of administration include oral, intravenous infusion, and local application (placed in the posterior vaginal fornix). Intravenous infusion of PGE2 at 0.5μg/min and PGF2α at 5μg/min usually maintains effective uterine contractions. If contractions remain weak after 30 minutes, the dose may be increased as appropriate, with a maximum dose of 20μg/min. Side effects of prostaglandins include excessive uterine contractions, nausea, vomiting, headache, tachycardia, blurred vision, and superficial phlebitis, so caution is advised.
⑤Acupuncture: This can enhance uterine contractions. Commonly used acupuncture points include Hegu (LI4), Sanyinjiao (SP6), Taichong (LR3), Zhongji (CV3), and Guanyuan (CV4), using strong stimulation and retaining the needles for 20–30 minutes. Ear acupuncture may target the uterus, sympathetic, and endocrine points.
If labor fails to progress or signs of fetal distress appear after the above measures, a cesarean section should be performed promptly.
（2）Second Stage of Labor: If there is no cephalopelvic disproportion but uterine contractions are weak, oxytocin infusion should be used to strengthen contractions and promote labor progress. If the biparietal diameter of the fetal head has passed the ischial spine level, await spontaneous delivery or perform an episiotomy, vacuum extraction, or forceps delivery. If the fetal head is not engaged or signs of fetal distress are present, a cesarean section should be performed.
(3) The third stage of labor: To prevent postpartum metrorrhagia, when the fetal anterior shoulder appears at the vaginal orifice, 0.2mg of ergot alkaloid can be administered intravenously, along with 10-20U of oxytocin via intravenous drip to enhance uterine contractions, promote placental detachment and delivery, and facilitate the closure of uterine sinusoids. If the labor is prolonged or the membrane rupture duration is extended, antibiotics should be administered to prevent infection.
2. Uterine incoordination and lack of strength in uterine contractions. The principle of management is to regulate uterine contractions and restore their polarity. Administer strong sedatives such as 100mg of pethidine or 10-15mg of morphine intramuscularly to ensure the mother gets adequate rest. Upon waking, coordinated uterine contractions usually resume. The use of oxytocin is strictly prohibited until uterine contractions return to a coordinated state. If the above measures fail to correct the incoordinate uterine contractions, or if there are signs of fetal distress or cephalopelvic disproportion, a cesarean section should be performed. If the incoordinate uterine contractions are controlled but remain weak, methods to strengthen uterine contractions, as used in cases of coordinated uterine contractions with lack of strength, may be applied.
Pregnant women should receive prenatal education to alleviate their anxieties and fears, helping them understand that pregnancy and childbirth are natural physiological processes. Currently, maternity wards both domestically and internationally provide comfortable labor rooms (where partners and family members can accompany the mother) and family-style wards, which help reduce the mother's tension, boost her confidence, and prevent uterine inertia caused by mental stress. During childbirth, mothers are encouraged to eat more, and intravenous nutrition can be supplemented if necessary. Excessive use of sedatives should be avoided, and checks for cephalopelvic disproportion should be conducted, as these are effective measures to prevent uterine inertia. It is also important to ensure timely emptying of the rectum and bladder, with warm soapy water enemas or catheterization performed if necessary.
