Uterine Atony

Definition

Why It Matters

• It is the most common cause of postpartum hemorrhage (PPH), accounting for the majority of cases (>50-70% of all PPH cases).
• PPH is the leading cause of maternal death worldwide, and uterine atony is its primary driver.
• Despite routine prophylactic oxytocin after delivery, uterine atony still complicates 4% to 6% of all pregnancies.

Risk Factors ("The 4 T's" framework)

• Prolonged labor
• Oxytocin augmentation during labor (receptor desensitization)
• Chorioamnionitis / uterine infection
• Use of volatile anesthetic agents (dose-dependent uterine relaxation - risk increases above 0.5 MAC)
• Magnesium sulfate use (smooth muscle relaxation)
• Tocolytics (e.g., terbutaline)
• Preeclampsia

Clinical Presentation

• Soft, boggy uterus on bimanual or abdominal examination
• Uterine fundus feels large and flabby instead of firm and contracted
• Excessive vaginal bleeding in the immediate postpartum period (within first 24 hours = primary PPH)
• Signs of hypovolemia may be delayed - up to 30% blood volume loss can occur before blood pressure drops, as pregnancy-related plasma volume expansion (40% increase) masks early signs

Management

• Bimanual uterine massage - place a fist in the anterior fornix and compress the fundus against the suprapubic hand; this is the first-line mechanical intervention.
• Uterine compression alone can often stimulate contraction.

• Oxytocin - the standard first drug. WHO recommends 20 IU in 1L crystalloid after cesarean delivery. Bolus doses >10 units should be avoided - they cause significant hemodynamic compromise (hypotension, tachycardia, nausea, headache).

• Methylergonovine (Methergine) 0.2 mg IM - causes intense, prolonged uterine contraction. Contraindicated in hypertension and cardiac disease (causes vascular smooth muscle constriction, can raise BP severely or cause coronary spasm).
• Carboprost (Hemabate, PGF2α) 0.25 mg IM, repeated every 15-90 min up to 2 mg maximum - stimulates uterine contractions. Contraindicated in asthma (causes bronchoconstriction, pulmonary hypertension).
• Misoprostol (PGE1 analogue) 600-800 mcg orally/sublingually/vaginally - useful when oxytocin is unavailable or when oxytocin receptors are desensitized.

• Intrauterine balloon tamponade (e.g., Bakri balloon, Jada System).
• The Jada System (vacuum-induced hemorrhage-control device) achieves definitive control in >90% of patients at a median of 3 minutes.

• B-Lynch compression sutures
• Pelvic vessel ligation or embolization
• Peripartum hysterectomy - most emergency hysterectomies are performed for PPH from uterine atony that cannot be controlled.

Key Anesthetic Consideration

How Each Factor Causes IPH (Immediate/Primary Postpartum Hemorrhage)

IPH here = Primary PPH, i.e., hemorrhage within 24 hours of delivery. The dominant mechanism for ALL of these factors is uterine atony - failure of the myometrium to contract and seal the open placental bed vessels.

Understanding the Core Mechanism First

1. Myometrial fatigue/exhaustion - muscle can't contract despite stimulation
2. Impaired contractility - drugs or inflammation block the contraction machinery
3. Structural/surgical disruption - direct tissue injury causing hemorrhage

1. Prolonged Labour >12 Hours and Prolonged Second Stage

• The uterine muscle contracts repeatedly and forcefully for an extended period (>12 hours first stage, >1-2 hours second stage).
• Like any striated or smooth muscle subjected to prolonged work, the myometrium becomes exhausted and depleted of energy substrates (ATP, glycogen).
• The actin-myosin contractile apparatus can no longer sustain adequate tone postpartum.
• The result is a soft, boggy, non-contracting uterus immediately after delivery.
• Additionally, prolonged oxytocin exposure during a long labour leads to oxytocin receptor downregulation (see point 8), compounding the problem.

2. Caesarean Section

• Direct surgical incision of the lower uterine segment disrupts myometrial continuity - the incised muscle cannot contract as effectively around the cut edges, leading to bleeding from the uterine wound itself.
• General anesthesia for caesarean (especially volatile agents at >0.5 MAC) causes dose-dependent uterine relaxation - halothane, isoflurane, sevoflurane all inhibit myometrial contractility.
• Blood loss during surgery leads to dilutional coagulopathy and further impairs haemostasis.
• Regional anesthesia does not cause this problem, but the surgical trauma itself is unavoidable.
• Caesarean section also requires oxytocin administration intraoperatively, and suboptimal dosing or desensitization can leave the uterus atonic.
• Placenta praevia (common indication for CS) is independently associated with uterine atony and accreta.

3. Operative Vaginal Delivery (Forceps / Ventouse)

• Operative vaginal delivery is used when the second stage is prolonged or when maternal pushing is inadequate - both of which already predispose to myometrial exhaustion (see point 1).
• The procedure itself increases the risk of genital tract lacerations (cervical, vaginal, perineal) which directly cause hemorrhage independent of uterine tone.
• Forceps application can cause uterine lower segment trauma.
• The increased intrauterine manipulation raises the risk of retained placental fragments, which physically prevent uterine contraction (a placental fragment keeps the uterine cavity open and the myometrium cannot close down around it).
• Mediolateral episiotomy is often performed alongside (see point 4), adding to blood loss.

4. Mediolateral Episiotomy

• The perineum and vaginal walls have a rich blood supply (branches of the internal pudendal artery).
• A mediolateral episiotomy (cut at ~45° from midline into the bulbocavernosus and transverse perineal muscles) transects these vessels.
• Unlike midline episiotomy, the mediolateral cut involves more muscle and larger vessels, making it inherently more vascular.
• Blood loss from the episiotomy wound itself directly contributes to IPH volume.
• If the repair is delayed or inadequate, a haematoma can form in the ischiorectal or paravaginal space, concealing significant ongoing blood loss.
• This mechanism is direct vascular injury / laceration rather than atony - it falls under the "Trauma" category of the 4 T's.

5. Pyrexia in Labour

• Intrapartum fever (pyrexia) is most often a sign of underlying infection/chorioamnionitis (see point 6) or epidural-related fever.
• Inflammatory mediators (prostaglandins, cytokines - IL-1β, TNF-α, IL-6) released during febrile states and infection paradoxically impair myometrial contractility at the cellular level despite initially stimulating contractions.
• Endotoxins from bacterial infection interfere with the calcium-dependent mechanisms needed for smooth muscle contraction.
• Pyrexia also causes maternal dehydration and metabolic derangement, depleting energy stores needed for sustained myometrial contraction.
• High fever itself can cause direct myometrial cell dysfunction.

6. Chorioamnionitis

• Chorioamnionitis = infection/inflammation of the chorion, amnion, and often the myometrium itself.
• Bacterial toxins and inflammatory cytokines directly infiltrate the myometrium, causing:
  • Impaired actin-myosin cross-bridge cycling
  • Disruption of gap junctions needed for coordinated uterine contraction
  • Nitric oxide release (causes smooth muscle relaxation)
• The infected, inflamed myometrium simply cannot generate the contractile force needed to achieve hemostasis postpartum.
• Chorioamnionitis is one of the most potent risk factors for atony - it is listed explicitly in Miller's Anesthesia as a primary risk factor.
• Additionally, sepsis from chorioamnionitis can trigger DIC (disseminated intravascular coagulation), adding a coagulopathic component to the hemorrhage.

7. Induction of Labour

• Induction of labour almost always involves prolonged exogenous oxytocin infusion, which leads to downregulation and desensitization of myometrial oxytocin receptors over time.
• When the baby is delivered and you need the uterus to contract postpartum, the receptors are already partially exhausted and respond poorly to both endogenous and exogenous oxytocin.
• Induction also frequently leads to longer labours (especially when the cervix is unfavourable), compounding myometrial fatigue (point 1).
• The uterus that has been "worked" by artificial oxytocin stimulation for many hours is effectively pre-exhausted before delivery even occurs.

8. Prolonged Oxytocin Use and Magnesium Sulphate Use

• Prolonged continuous oxytocin infusion causes homologous desensitization - the myometrial oxytocin receptors are internalized and downregulated.
• Receptor numbers decrease by up to 90% after prolonged exposure.
• When postpartum oxytocin is given to treat/prevent atony, there are very few receptors left to respond - the drug is ineffective.
• This is why the textbooks note that in oxytocin-desensitized patients, misoprostol (PGE1) is preferred as it works through a different receptor system (prostaglandin receptors, not oxytocin receptors).

• Magnesium is a calcium antagonist - it competes with calcium at the level of the cell membrane and intracellular contractile machinery.
• Uterine smooth muscle contraction is entirely calcium-dependent (calcium binds calmodulin → activates myosin light-chain kinase → phosphorylates myosin → cross-bridges form with actin → contraction).
• By blocking calcium entry and intracellular calcium mobilization, magnesium directly relaxes uterine smooth muscle (this is why it is used as a tocolytic).
• After delivery, magnesium serum levels remain elevated for some time, and the uterus remains hypotonic.

Summary Table