Amniotic fluid embolism

Contents

Definition and Epidemiology

Amniotic fluid embolism (AFE) is a rare but catastrophic obstetric emergency caused by entry of amniotic fluid and fetal debris into the maternal circulation, triggering a complex multisystem response. It is a diagnosis of exclusion.

  • Incidence: ~1–2 per 100,000 deliveries
  • Mortality: 20–40% (UK MBRRACE data); one of the leading causes of direct maternal death in developed countries
  • Timing: usually during labour, at delivery, or within 30 minutes postpartum; rare cases during caesarean or second-trimester procedures

Risk Factors

  • Advanced maternal age
  • Multiparity
  • Placenta praevia / placenta accreta spectrum
  • Amnioinfusion, artificial rupture of membranes (ARM)
  • Caesarean section, instrumental delivery
  • Cervical lacerations
  • Uterine hyperstimulation

Pathophysiology

AFE does not behave like a simple mechanical embolism. The pathophysiology is an anaphylactoid reaction or immunological/inflammatory cascade triggered by amniotic fluid components entering maternal circulation:

Phase 1: Cardiovascular and Respiratory Collapse (minutes)

  • Amniotic fluid (containing fetal squames, vernix, lanugo, prostaglandins, cytokines, thromboxane) enters maternal circulation through opened uterine veins or cervical lacerations
  • Triggers massive release of mast cell mediators, complement activation, and cytokine storm
  • Pulmonary vasospasm → acute cor pulmonale (RV failure) → ↑ PVR → ↓ LV preload → ↓ CO → cardiovascular collapse
  • Coronary vasospasm may cause acute myocardial ischaemia
  • Severe hypoxaemia from V/Q mismatch and pulmonary oedema

Phase 2: DIC (minutes to hours)

  • Tissue factor released from amniotic fluid / fetal material → initiation of extrinsic coagulation cascade → massive thrombin generation → fibrinogen consumption → DIC
  • Massive haemorrhage from atonic uterus + DIC = catastrophic coagulopathic bleeding
  • The coagulopathy may unmask or worsen uterine atony
  • Fibrinolysis: tPA released → systemic fibrinolysis → further haemorrhage

Phase 3: Neurological Injury

  • Hypoxic-ischaemic brain injury from cardiac arrest and/or severe hypoxaemia
  • Seizures (from cerebral hypoperfusion or direct neurological effects)

Clinical Presentation

Classic Triad

  1. Cardiovascular collapse (hypotension, cardiac arrest)
  2. Acute respiratory failure (hypoxaemia, dyspnoea)
  3. DIC / massive haemorrhage (excessive bleeding from IV sites, haemorrhage)

Variable Presentation

  • Not all three components always present simultaneously
  • May present primarily as uterine atony with haemorrhage (DIC phase) without obvious cardiac collapse
  • May present as sudden-onset seizure during or immediately after delivery
  • Fetal bradycardia may precede maternal collapse (reflection of uteroplacental insufficiency)

Red Flags During/After Delivery

  • Sudden unexplained agitation, restlessness, dyspnoea
  • Sudden cyanosis
  • Seizure
  • Cardiovascular collapse disproportionate to estimated blood loss
  • Excessive, diffuse bleeding from all sites (DIC)

Diagnosis

AFE is a clinical diagnosis of exclusion — there is no pathognomonic test.

UK Diagnostic Criteria (UKOSS)

All of the following:

  1. Acute hypotension or cardiac arrest
  2. Acute hypoxia (dyspnoea, cyanosis, respiratory arrest, or SpO₂ <90%)
  3. Coagulopathy (DIC or severe haemorrhage with no other explanation)
  4. Onset during labour, caesarean section, D&E, or within 30 minutes of delivery
  5. No other condition to explain signs and symptoms

Differential Diagnoses to Exclude

Condition Distinguishing feature
Pulmonary embolism Usually not immediate DIC; CT-PA positive
Eclampsia Hypertension (usually); seizure; HELLP
High spinal anaesthetic Known spinal procedure; no DIC
Anaphylaxis to drug Drug exposure history; urticaria; no DIC initially
Haemorrhagic shock Clear obstetric source of bleeding; no acute cardiac collapse initially
Septic shock Fever; prodrome; organism identified
Peripartum cardiomyopathy More subacute; no DIC; echocardiography characteristic

Biomarkers (not specific)

  • Complement C3 and C4: may be low (complement activation)
  • Mast cell tryptase: may be elevated (anaphylactoid response)
  • No test confirms AFE; post-mortem histology can show fetal squames in pulmonary vasculature

Cardiovascular Management

Immediate Resuscitation

  • Call for help: obstetric emergency call; activate local major haemorrhage protocol; anaesthetics, haematology, neonatology
  • If arrest: immediate CPR (lateral uterine displacement to reduce aortocaval compression if pregnant); perimortem caesarean section if in arrest at >20 weeks gestation within 4–5 minutes of arrest (improves maternal resuscitation — decompresses aortocaval compression)

Vasopressors and Inotropes

  • Noradrenaline: systemic vasodilation component → first-line vasopressor
  • Adrenaline: for cardiac arrest or profound shock with poor contractility
  • Dobutamine: if RV failure and low cardiac output (echo-guided)
  • Inhaled nitric oxide: if significant RV failure from pulmonary vasospasm

Vasopressin

Vasopressin 40 units IV can be considered in refractory cardiac arrest (ACLS protocol) — may also help with uterine arterial vasoconstriction

VA-ECMO

  • Case reports of VA-ECMO as rescue therapy in refractory cardiac arrest or severe cardiogenic shock post-AFE
  • Should be considered in centres with availability if cardiovascular collapse is refractory

DIC Management

AFE-associated DIC is rapid-onset and severe — massive thrombin generation rapidly consumes fibrinogen, platelets, and clotting factors.

Guiding Principle

Fibrinogen is the first factor to fall and the most important to replace in obstetric haemorrhage (see also obstetric haemorrhage page):

  • Fibrinogen target: >2 g/L (obstetric threshold is higher than non-obstetric DIC)
  • Give cryoprecipitate or fibrinogen concentrate (Haemocomplettan, RiaSTAP) immediately

Blood Component Therapy

  • Activate major haemorrhage protocol immediately
  • Packed red cells: transfuse to Hb ≥80 g/L (higher threshold in pregnancy for fetal/neonatal welfare)
  • FFP: 4 units; contains fibrinogen + clotting factors; support coagulation
  • Cryoprecipitate: 2 pools; high-concentration fibrinogen; first-line for fibrinogen replacement
  • Platelets: if <50 ×10⁹/L; risk of platelet consumption in DIC
  • Tranexamic acid: 1 g IV (WOMAN trial demonstrated mortality reduction from postpartum haemorrhage); give early; repeat at 30 minutes if ongoing haemorrhage; inhibits fibrinolysis (tPA-mediated fibrinolysis contributes to AFE haemorrhage)

Recombinant Factor VIIa (rFVIIa)

  • Off-label use for refractory obstetric haemorrhage with DIC
  • Promotes thrombin generation at the site of vessel injury; requires adequate fibrinogen and platelets to be effective
  • Used as salvage when blood components and surgical haemostasis have failed

Obstetric Management

Delivery

  • If AFE occurs before delivery: immediate delivery (caesarean section) — both to resuscitate the fetus and to allow better maternal resuscitation (removes aortocaval compression)
  • Perimortem caesarean: if cardiac arrest, should be performed within 4–5 minutes if no return of spontaneous circulation (ROSC)

Uterine Atony

  • Oxytocin, ergometrine, carboprost (15-methyl-PGF2α) for uterine contraction
  • Uterine balloon tamponade (Bakri balloon)
  • B-Lynch compression suture
  • Uterine artery ligation / internal iliac artery ligation
  • Hysterectomy: definitive — must not be delayed if haemorrhage uncontrolled

Interventional Radiology

  • Uterine artery embolisation if haemodynamically stable and not in theatre

Viva Questions

1. Explain the pathophysiology of amniotic fluid embolism and why it causes cardiovascular collapse followed by DIC.

AFE is not a simple mechanical occlusion of the pulmonary vasculature but an immunological and inflammatory cascade triggered by amniotic fluid components entering maternal circulation. Phase 1 (immediate): amniotic fluid containing fetal squames, vernix, prostaglandins, and other bioactive substances activates mast cells, complement, and cytokine release. Pulmonary vasospasm causes acute cor pulmonale — the RV fails acutely under the increased afterload, the interventricular septum shifts left, and LV preload falls — causing rapid cardiovascular collapse. Coronary vasospasm may also contribute. Phase 2 (minutes to hours): tissue factor and other procoagulant substances in amniotic fluid activate the extrinsic coagulation pathway → massive thrombin generation → fibrinogen consumption → DIC. Simultaneous release of tPA causes systemic fibrinolysis. The combination of consumptive coagulopathy + fibrinolysis + uterine atony produces catastrophic haemorrhage. This two-phase pattern explains why some patients present predominantly with cardiac collapse and others with haemorrhage — the clinical picture depends on which phase predominates and how rapidly it evolves.

2. What are the immediate priorities in a patient who collapses during delivery with suspected AFE?

Immediate: (1) Call for help — obstetric emergency team, anaesthetics, haematology, neonatology; senior colleagues to bedside; activate major haemorrhage protocol; (2) Airway — intubate if GCS falling or in arrest; 100% oxygen; (3) If cardiac arrest: CPR with left lateral tilt or manual uterine displacement to relieve aortocaval compression; perimortem caesarean section if no ROSC within 4–5 minutes (30 weeks or greater); (4) IV access × 2; send urgent bloods: FBC, group and crossmatch, fibrinogen, clotting (ROTEM if available), ABG; (5) Vasopressors: noradrenaline for hypotension; adrenaline for arrest; (6) Blood products: activate major haemorrhage protocol; give FFP 4 units + cryoprecipitate 2 pools immediately (fibrinogen is first to fall); tranexamic acid 1 g IV stat; (7) Echo if available: assess RV function, LV function, guide fluid/vasopressor strategy; (8) DIC management guided by ROTEM/TEG or conventional coagulation — replace fibrinogen aggressively (target >2 g/L); (9) Obstetric surgical haemostasis: oxytocin, uterine balloon, B-Lynch suture, hysterectomy as escalating steps if haemorrhage not controlled.

3. How does AFE differ from massive pulmonary embolism clinically and in its management?

Both cause acute cardiovascular collapse with sudden hypotension, hypoxia, and RV failure. Key differences: AFE occurs in the peripartum period and is almost always associated with DIC and haemorrhage — massive PE rarely causes DIC in the acute setting; massive PE typically presents with pleuritic pain, ECG changes (S1Q3T3, right heart strain pattern), and CT-PA showing vascular filling defects; AFE has no CT-PA finding (it is an immunological response, not a mechanical occlusion). Echocardiography may show similar RV dilation in both. The critical management difference: thrombolysis is the cornerstone of massive PE treatment (alteplase 100 mg over 2 hours); in AFE, thrombolysis would catastrophically worsen DIC-related haemorrhage and is contraindicated. AFE management is resuscitative — vasopressors, DIC management, blood products, obstetric haemostasis. The presence of DIC and peripartum context strongly suggests AFE; CT-PA and clinical exclusion of alternatives are important.