Seminar Guide: Physiological Changes in Pregnancy - A Critical Care Perspective

THE ANCHOR CASE

"Mrs. K, 28 years old, G2P1, 34 weeks gestation. She collapses in the antenatal clinic. No pulse, no respirations. The team activates a code."

SYSTEM 1 - CARDIOVASCULAR

What changed and why it matters in the code

The aortocaval compression problem - your first code modification

• At ≥20 weeks, the uterus compresses the IVC when the patient is supine, reducing venous return and cardiac output dramatically.
• Standard CPR in a supine pregnant patient is partially futile if aortocaval compression is not relieved.

• One dedicated team member places hands on the right side of the uterus and displaces it leftward.
• This is preferred over tilting the patient (tilting impairs compression quality, pad placement, and airway management, and it is unknown how tilt changes intrathoracic heart position).
• Chest compressions are placed on the lower half of the sternum (same as non-pregnant), not higher. The uterus does not displace the heart cranially enough to change hand placement.

SYSTEM 2 - RESPIRATORY

What changed and why it matters in the code

• O2 consumption: +20-50% (fetal demands + increased maternal metabolic rate)
• Minute ventilation: +50% (driven by tidal volume +40%, RR +15%)
• FRC: -20% (diaphragm pushed up 4 cm by the gravid uterus)
• PaCO2: 28-32 mmHg (normal in pregnancy - do not overcorrect)
• HCO3: -15% (compensatory metabolic acidosis - base ~20 mEq/L is normal)
• PaO2: +10%

1. Difficult intubation: Upper airway mucosal engorgement and friability from estrogen-driven hyperemia. Mallampati scores are higher. Use a video laryngoscope as first-line. Use a smaller ETT (6.0-6.5 mm).
2. Aspiration risk: Gastric emptying is delayed, lower esophageal sphincter tone is reduced, and intra-abdominal pressure is increased by the gravid uterus. She is never "nil by mouth" enough. Rapid sequence induction (RSI) is mandatory.
3. Fast desaturation: She has minimal apnea tolerance. Pre-oxygenate aggressively. You have less time than you think.

• Target PaCO2 of 28-32 mmHg (her normal - not 40)
• If you ventilate to PaCO2 of 40, you are relatively hypercapnic for her; this impairs fetal CO2 elimination via the placenta.

SYSTEM 3 - HEMATOLOGICAL

What changed and why it matters in the code

• Hemoglobin: -20% (dilutional anemia - plasma volume rises more than RBC mass)
• Platelets: -10% (dilutional thrombocytopenia - a platelet count of 100-120k can still be normal)
• Clotting factors II, VII, VIII, IX, X, XII, fibrinogen, vWF: +30 to +250% - pregnancy is a profoundly hypercoagulable state
• Protein S and antithrombin III: decrease
• Venous thrombosis risk: 5-6 times that of non-pregnant women

• A normal fibrinogen in your lab range (200-400) can mask DIC in pregnancy, where the baseline is 400-600 mg/dL. A "normal" level may actually represent a fall.
• Markedly elevated clotting factors mean pregnancy-related causes of arrest (amniotic fluid embolism, massive PE) must always be on the differential.

SYSTEM 4 - RENAL & METABOLIC

• GFR: +50% (renal blood flow increases substantially)
• Serum creatinine drops to ~0.4-0.5 mg/dL at term (normal non-pregnant ~0.8-1.0)
• BUN also falls

SYSTEM 5 - NEUROLOGICAL / PHARMACOLOGICAL

• MAC (minimum alveolar concentration) for volatile anesthetics: -40% by term
• Sensitivity to local anesthetics markedly increased
• Epidural venous plexus is engorged (IVC compression pushes blood into epidural veins), reducing volume of epidural space and CSF volume, causing exaggerated cephalad spread

• ACLS medications are given at standard doses - do not withhold epinephrine, amiodarone, or other drugs because of pregnancy. The life of the mother takes priority.
• IV access must be established above the diaphragm (upper extremity or central venous). IVC compression means anything given below the diaphragm may not reach the central circulation effectively.

SYSTEM 6 - GASTROINTESTINAL

• Delayed gastric emptying
• Reduced lower esophageal sphincter tone
• Elevated intragastric pressure from uterus

THE CODE ALGORITHM - Integrating all systems

Immediately upon recognizing arrest:

1. Call for help - activate OB and neonatal teams immediately
2. Start high-quality CPR - same standard compressions (lower half of sternum, 2 inches depth, 100-120/min)
3. Apply manual left uterine displacement - assign one dedicated person
4. Do not tilt the patient - this impairs compression quality

Airway (expect difficult):

• Pre-oxygenate maximally - she will desaturate faster than you expect
• Video laryngoscope first line
• RSI mandatory (aspiration risk)
• Smaller ETT (6.0-6.5 mm)
• Ventilate to PaCO2 28-32 mmHg, not 40

Circulation:

• IV access above the diaphragm
• Standard ACLS drugs at standard doses - do not withhold
• Defibrillate at standard energy for shockable rhythms - remove fetal monitors, no dose adjustment needed

The "4-minute rule" - Resuscitative Hysterotomy:

• If no ROSC within 4 minutes of arrest, begin resuscitative hysterotomy
• Target delivery by 5 minutes from collapse
• Gestational age ≥23-24 weeks (viable fetus)
• This is no longer called "perimortem cesarean" - the preferred term is resuscitative hysterotomy because the procedure benefits the mother as much as the fetus (relieves aortocaval compression, dramatically improves CPR effectiveness)
• Do NOT wait for ROSC to perform it
• Do NOT delay CPR to transport to OR - this can be done at bedside

CAUSES OF ARREST IN PREGNANCY - The Obstetric H's

KEY TEACHING TAKE-AWAYS

1. Two patients, one priority. The best fetal resuscitation is maternal resuscitation. Never withhold a maternal intervention for fear of fetal harm.
2. The uterus is your enemy during CPR. Manual left uterine displacement is non-negotiable at ≥20 weeks. Without it, you are doing partial CPR.
3. Her airway will be harder than you expect. Hyperemic mucosa, weight gain, aspiration risk, and fast desaturation all conspire against you. Prepare the video laryngoscope before you need it.
4. Her normal labs are not your normal labs. Hgb 10 can be fine. Creatinine 0.8 may be AKI. "Normal" fibrinogen may be coagulopathy.
5. 4 minutes. 5 minutes. Resuscitative hysterotomy. Know this number. Burn it in. It saves the mother AND the baby.
6. Do not withhold ACLS medications. Give epinephrine. Give the shock. Give the drug. Pregnancy is not a contraindication to life-saving interventions.

SEMINAR FLOW SUGGESTION (60-90 min)