Shock in Obstetrics: A Comprehensive Overview

1. Definition of Shock

Critically: shock can occur with a normal blood pressure, and not all hypotensive patients are in shock. The presence of a base deficit more negative than -4 mEq/L or serum lactate >4.0 mmol/L warrants a presumptive diagnosis of shock even with normal vital signs.

• Rosen's Emergency Medicine, p. 58

2. Definition of Obstetric Shock

3. Types of Shock Encountered in Obstetrics

A. Haemorrhagic / Hypovolaemic Shock (Most Common)

• Antepartum: Placenta praevia, placental abruption, uterine rupture, ectopic pregnancy (early pregnancy)
• Intrapartum: Uterine rupture, cord prolapse with vascular injury
• Postpartum: Uterine atony (most common cause of PPH, ~70%), retained placenta, genital tract lacerations, placenta accreta spectrum (PAS), uterine inversion, coagulopathy (DIC)

• Creasy & Resnik's Maternal-Fetal Medicine, p. 1114

B. Distributive Shock

• Septic shock: Septic abortion, chorioamnionitis, endometritis, pyelonephritis, necrotising fasciitis. Characterised by vasodilation and reduced SVR (hyperdynamic state initially).
• Anaphylactic shock: Reaction to antibiotics (oxytocin, penicillin), blood products, latex, anaesthetic agents.
• Neurogenic shock: High spinal anaesthesia block.

C. Cardiogenic Shock

• Peripartum cardiomyopathy, severe pre-eclampsia/eclampsia with cardiac failure, amniotic fluid embolism (AFE) with ventricular dysfunction, myocardial infarction during pregnancy.
• AFE is one of the most feared obstetric emergencies - presents with sudden cardiovascular collapse during labour or delivery, followed by profuse bleeding and DIC.
• Creasy & Resnik's Maternal-Fetal Medicine, p. 1114

D. Obstructive Shock

• Pulmonary embolism (especially peripartum), amniotic fluid embolism (mixed obstructive/distributive), cardiac tamponade, tension pneumothorax.

4. Stages of Shock

• Rising lactate (>4 mmol/L = severe)
• Worsening base deficit (more negative than -4 mEq/L)
• A combination of worsening base deficit + increasing lactate + falling urine output = persistent/worsening shock
• Rosen's Emergency Medicine, p. 58-59

5. Challenges of Diagnosing Shock in Obstetrics

1. Expanded blood volume: Plasma volume increases by 40-50% and total blood volume by 30-35%. This means a pregnant woman can lose >1000 mL before showing classical signs of hypovolaemia. The textbook signs appear late.
2. Physiological tachycardia and hypotension of pregnancy: Resting HR is already elevated in pregnancy (by ~15-20 bpm) and BP falls in the second trimester, making it difficult to identify pathological changes.
3. Masked hypotension: Peripheral vasoconstriction in haemorrhagic shock can maintain a near-normal BP even with 25-30% blood loss. Diastolic BP may even rise initially as pulse pressure narrows.
4. Variable HR response: As Rosen's notes, "the responses of HR and BP are notoriously variable in haemorrhage; no firm conclusion can be made at the bedside about the presence, absence, or degree of haemorrhagic shock by simple evaluation of HR and BP."
5. Concealed haemorrhage: Placental abruption, broad ligament haematoma, posterior uterine rupture - blood loss may be entirely internal with no visible bleeding.
6. Coexisting conditions: Pre-eclampsia causes relative hypertension, so a woman who is "normotensive" may actually be severely hypovolaemic relative to her baseline. Chronic hypertension similarly masks the degree of shock.
7. Fetal distress as early indicator: Uteroplacental blood flow is compromised early (non-essential circulation), so fetal heart rate abnormalities can precede maternal haemodynamic compromise.
8. Rapid deterioration: The physiological reserve allows compensation until a sudden decompensation occurs, often with rapid, catastrophic decline.
9. Coagulopathy development: Obstetric haemorrhage is particularly prone to early development of DIC (especially abruption, AFE, sepsis, prolonged hypotension), compounding the haemorrhage.

6. Shock Index and Its Importance in Obstetrics

Definition

Obstetric Shock Index (O-SI)

• Normal in pregnancy: approximately 0.7-0.9 (slightly higher than non-pregnant due to physiological tachycardia and lower BP)
• Concerning: SI ≥ 0.9
• Significant haemodynamic compromise: SI ≥ 1.0
• Severe obstetric shock: SI ≥ 1.7

Importance

• Early detection: The SI rises before conventional vital sign thresholds are crossed, making it useful for identifying haemorrhage and impending shock before overt hypotension develops.
• Compensates for pregnancy physiology: Because SI incorporates both HR and SBP, it captures the narrowing pulse pressure and rising HR that characterise early compensated shock even when individual values appear normal.
• Predicts transfusion need: An elevated SI (≥1.0) at triage has been associated with need for blood transfusion and ICU admission.
• Simple bedside tool: Requires no equipment beyond a pulse and BP reading; can be calculated immediately.
• Serial monitoring: Trends in SI are as important as a single value. A rising SI indicates deterioration; a falling SI with resuscitation indicates improvement.
• Limitations: Can be falsely reassuring in women with pre-eclampsia/chronic hypertension (elevated BP masks true haemodynamic compromise); beta-blocker use blunts the tachycardia response.

7. Massive Transfusion Protocol (MTP)

Definition

• Rosen's Emergency Medicine, p. 2430

Indications for MTP Activation in Obstetrics

• Estimated blood loss >1500 mL with ongoing haemorrhage
• Clinical shock not responding to initial resuscitation (≥2 units PRBCs in 20 min with ongoing bleeding)
• Shock Index ≥ 1.0 to 1.7 with active haemorrhage
• Anticipated massive haemorrhage (placenta accreta spectrum, placenta praevia in labour, uterine rupture)
• Any obstetric haemorrhage with developing coagulopathy (DIC, INR >1.5, fibrinogen <2 g/L)
• Haemorrhage with haemodynamic instability despite uterotonics and initial measures

• Creasy & Resnik's Maternal-Fetal Medicine, p. 1113

Blood Product Ratios

• Rosen's Emergency Medicine, p. 2430

Additional MTP Components

• Tranexamic acid (TXA): 1g IV over 10 minutes, given within 3 hours of onset of PPH - an RCT demonstrated efficacy in reducing maternal death from PPH. Should be integrated into all institutional PPH protocols.
• Creasy & Resnik's Maternal-Fetal Medicine, p. 1114
• Cryoprecipitate: For fibrinogen replacement (target fibrinogen >2 g/L, especially in obstetric haemorrhage where hypofibrinogenaemia is an early and specific marker)
• Calcium: Ionised hypocalcaemia is common with rapid transfusion (citrate chelation); IV calcium chloride or gluconate should be administered
• Recombinant Factor VIIa: Considered in refractory coagulopathic haemorrhage not responding to standard products

8. Investigations

Immediate / Point-of-Care

Additional Investigations

• Henry's Clinical Diagnosis and Management, p. 881

9. Management of Obstetric Shock

A. Immediate Resuscitation (First 15-30 minutes)

• Airway assessment and protection; high-flow oxygen (15 L/min via non-rebreather mask)
• Early intubation if airway at risk, SpO2 <90%, or GCS <9
• Rapid sequence induction if intubation required (full stomach precaution in all obstetric patients)

• Two large-bore IV lines (14-16G) in antecubital fossae; intraosseous if IV access fails
• Consider central venous access early in refractory shock
• Arterial line for continuous BP monitoring and serial ABG sampling

• Left lateral tilt (15-30°) until delivery to relieve aortocaval compression by the gravid uterus - failure to do so further reduces venous return

• Crystalloids: restricted use; avoid large volumes (dilutional coagulopathy, hypothermia, acidosis)
• The goal is permissive hypotension in haemorrhagic shock until surgical control (target SBP 80-90 mmHg), EXCEPT in pre-eclampsia/head injury
• Balanced transfusion (1:1:1 PRBCs:FFP:Platelets) is the cornerstone - replace blood with blood, not crystalloid

• Oxytocin 10 IU IM (or 40 IU in 500 mL normal saline IV infusion)
• Ergometrine 0.5 mg IM (avoid in hypertension)
• Carboprost (15-methyl PGF2α) 0.25 mg IM every 15 min up to 8 doses
• Misoprostol 800-1000 mcg sublingual/rectal if other uterotonics unavailable

B. Source Control / Definitive Treatment

• PPH: Uterine massage, bimanual compression, intrauterine balloon tamponade (Bakri balloon), uterine compression sutures (B-Lynch), uterine/internal iliac artery ligation, interventional radiology (uterine artery embolisation), hysterectomy as last resort
• Ectopic pregnancy: Emergency laparoscopy or laparotomy
• Uterine rupture: Emergency laparotomy and repair/hysterectomy
• Placenta accreta spectrum: Planned delivery at tertiary centre; often requires hysterectomy
• Septic shock: IV antibiotics within 1 hour (broad-spectrum, e.g., piperacillin-tazobactam + metronidazole), source control (evacuation of septic uterus), vasopressors (noradrenaline first-line)
• AFE: Aggressive cardiopulmonary support (intubation, pressors), MTP with emphasis on coagulation factors, early TTE, consider venoarterial ECMO for refractory ventricular dysfunction
• Creasy & Resnik's Maternal-Fetal Medicine, p. 1114

C. MTP Management and Monitoring

• Monitor FBC, coagulation, fibrinogen every 30-60 minutes during active MTP
• Targets:
  • Hb >8 g/dL (≥7 g/dL acceptable when bleeding controlled)
  • Platelets >50 × 10⁹/L (>75-100 × 10⁹/L in active DIC)
  • PT/APTT <1.5x normal
  • Fibrinogen >2 g/L
  • pH >7.35, Temperature >36°C
  • Ionised Ca²⁺ >1.1 mmol/L

D. Complications of Massive Transfusion to Anticipate and Prevent

• Hypothermia: Use blood warmers, warming blankets - hypothermia reduces clotting factor activity
• Hypocalcaemia: Citrate chelation; administer IV calcium routinely
• Metabolic acidosis/alkalosis: Optimise oxygen delivery and ventilation; sodium bicarbonate not routinely recommended
• Dilutional coagulopathy: Prevented by 1:1:1 ratio
• Transfusion-associated circulatory overload (TACO)
• Transfusion-related acute lung injury (TRALI)
• Hyperkalaemia/hypokalaemia: Monitor electrolytes serially

E. Monitoring and End-Points of Resuscitation

• Serial lactate and base deficit (downward trend = adequate resuscitation)
• Urine output >0.5 mL/kg/hr (normal >1 mL/kg/hr)
• Improving HR and BP
• Improving mental status
• TEG/ROTEM-guided component therapy when available - goal-directed approach reduces overall transfusion requirements

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