10.1 High-Risk Obstetric Assessment & Hypertensive Disorders of Pregnancy

10.1 High-Risk Obstetric Assessment & Hypertensive Disorders of Pregnancy

Maternal Anatomical & Physiological Adaptations in Pregnancy

Managing the critically ill pregnant patient requires an intimate understanding of normal maternal physiological adaptations. During pregnancy, nearly every organ system undergoes dramatic functional changes designed to support fetal growth and prepare for labor. Failure to recognize these altered baselines can lead to misdiagnosis or inappropriate resuscitation in critical care transport.

Hematologic and Cardiovascular Adaptations

• Blood Volume Expansion: Maternal intravascular blood volume increases by 45% above pre-pregnancy levels (typically an increase of $1500 \text{ mL}$ to $1800 \text{ mL}$). Plasma volume expands disproportionately by 40–50%, whereas red blood cell (RBC) volume increases by only 20–30%.
• Physiological Anemia of Pregnancy: Because plasma volume expansion exceeds RBC mass expansion, hemodilution occurs. Normal third-trimester hemoglobin drops to $11.0–12.0 \text{ g/dL}$ and hematocrit falls to $32–34%$.
• Hemodynamic Parameters: Cardiac output (CO) increases by 30–50% (peaking at 28–32 weeks), driven by an increased stroke volume (+30%) and an elevated baseline resting heart rate (+10 to 15 beats/min). Simultaneously, Systemic Vascular Resistance (SVR) decreases by 20–30% due to smooth muscle relaxation induced by elevated circulating progesterone, prostacyclin, and nitric oxide.
• Blood Pressure Trends: Systolic and diastolic blood pressures fall during the first and second trimesters (reaching a nadir at 24–28 weeks, typically $5–10 \text{ mmHg}$ below baseline) before gradually returning to pre-pregnancy baselines in the third trimester.
• Hypercoagulable State: Pregnancy induces a hypercoagulable state characterized by increased clotting factors (factors VII, VIII, X, and fibrinogen) and decreased fibrinolytic activity, increasing the risk of deep vein thrombosis (DVT) and pulmonary embolism (PE) by 5-fold.

Respiratory Adaptations & Airway Risk

As the gravid uterus expands, it elevates the diaphragm by approximately $4 \text{ cm}$, decreasing Functional Residual Capacity (FRC) by 20–25%. Concurrently, maternal oxygen consumption increases by 20% to meet fetal and metabolic demands.

• Rapid Desaturation: During induction for rapid sequence intubation (RSI) or periods of apnea, pregnant patients desaturate catastrophically fast. Pre-oxygenation with $100% \text{ FiO}_2$ for 3–5 minutes is critical.
• Progesterone-Driven Hyperventilation: Elevated progesterone stimulates the respiratory center, increasing tidal volume by $30–40%$ and minute ventilation by $40–50%$. This causes a physiological chronic respiratory alkalosis ($\text{PaCO}_2 = 27–32 \text{ mmHg}$) with renal metabolic compensation ($\text{HCO}_3^- = 18–22 \text{ mEq/L}$). A "normal" non-pregnant $\text{PaCO}_2$ of $40 \text{ mmHg}$ in a late-gestation patient indicates impending respiratory failure and severe hypercapnia!
• Airway Edema: Upper airway mucosal hypervascularity and edema make the airway narrow, friable, and prone to bleeding. Use an endotracheal tube 0.5–1.0 mm smaller than standard (e.g., 6.0–6.5 mm ETT).

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Aortocaval Compression & Positioning Management

After 20 weeks gestation, a patient placed in the supine position experiences aortocaval compression. The heavy gravid uterus compresses both the Inferior Vena Cava (IVC) against the lumbar spine and the abdominal aorta.

       [ Gravid Uterus ]
              │
              ▼ (Compresses in Supine Position)
     ┌─────────────────┴─────────────────┐
     ▼                                   ▼
 [ Inferior Vena Cava ]          [ Abdominal Aorta ]
     │                                   │
     ▼                                   ▼
 Decreased Venous Return            Decreased Uteroplacental
     │                              & Lower Body Perfusion
     ▼                                   │
 Reduced Preload & Stroke Vol.           │
     │                                   │
     ▼                                   ▼
 Drop in Cardiac Output (30-40%) ──► [ MATERNAL HYPOTENSION & FETAL DISTRESS ]

Pathophysiology & Clinical Impact

• Compression of the IVC severely impedes venous return to the heart, reducing maternal cardiac output by up to 30–40%.
• This produces Supine Hypotensive Syndrome of Pregnancy, characterized by maternal tachycardia, dizziness, diaphoresis, nausea, and profound hypotension, leading directly to uteroplacental hypoperfusion and acute fetal bradycardia/distress.

Transport Positioning Protocols

• Left Lateral Tilt: Position all pregnant patients past 20 weeks gestation in a $15–30^\circ$ left lateral tilt using a wedge, pillow, or rolled blanket placed under the right hip and spine.
• Manual Left Uterine Displacement (LUD): If the patient is spinal-immobilized or undergoing cardiopulmonary resuscitation (CPR), perform manual LUD by using one or two hands to manually displace the uterus to the patient's left side away from the IVC. Manual LUD is essential during ACLS to restore effective CPR stroke volume!

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Hypertensive Disorders of Pregnancy Spectrum

Hypertensive disorders complicate up to 10% of pregnancies worldwide and remain a leading cause of maternal and fetal mortality.

                   [ MATERNAL HYPERTENSION (SBP ≥ 140 or DBP ≥ 90 after 20 wks) ]
                                                │
                 ┌──────────────────────────────┴──────────────────────────────┐
                 ▼                                                             ▼
        [ NO Proteinuria & NO ]                                  [ Proteinuria OR End-Organ ]
        [ End-Organ Dysfunction ]                                [      Dysfunction         ]
                 │                                                             │
                 ▼                                                             ▼
       Gestational Hypertension                                          Preeclampsia
                                                                               │
                                                      ┌────────────────────────┴────────────────────────┐
                                                      ▼                                                 ▼
                                            [ Without Severe Features ]                       [ With Severe Features ]
                                            (SBP 140-159 / DBP 90-109)                        (SBP ≥ 160 or DBP ≥ 110,
                                                                                              Severe Headache, RUQ Pain,
                                                                                               Plts < 100k, Cr > 1.1)
                                                                                                        │
                                                                                       ┌────────────────┴────────────────┐
                                                                                       ▼                                 ▼
                                                                                   Eclampsia                       HELLP Syndrome
                                                                             (New-Onset Seizures)            (Hemolysis, Elevated LFTs,
                                                                                                              Low Platelets < 100k)

Diagnostic Definitions

1. Gestational Hypertension:
   • Onset of SBP $\ge 140 \text{ mmHg}$ or DBP $\ge 90 \text{ mmHg}$ measured on two occasions at least 4 hours apart after 20 weeks gestation in a previously normotensive woman.
   • Absence of proteinuria and systemic end-organ signs.
2. Preeclampsia:
   • New-onset hypertension (SBP $\ge 140 \text{ mmHg}$ or DBP $\ge 90 \text{ mmHg}$ after 20 weeks) AND Proteinuria ($\ge 300 \text{ mg}$ in a 24-hour urine collection or urine protein/creatinine ratio $\ge 0.3$).
   • OR in the absence of proteinuria, new-onset hypertension accompanied by any of the following End-Organ Dysfunction indicators:
     • Thrombocytopenia: Platelet count $< 100,000 / \mu\text{L}$.
     • Renal Insufficiency: Serum creatinine $> 1.1 \text{ mg/dL}$ or doubling of baseline.
     • Impaired Liver Function: Serum transaminases (AST/ALT) elevated to twice the upper limit of normal.
     • Pulmonary Edema.
     • New-Onset Cerebral or Visual Disturbances (severe persistent headache, scotomata, cortical blindness).
3. Preeclampsia with Severe Features:
   • SBP $\ge 160 \text{ mmHg}$ or DBP $\ge 110 \text{ mmHg}$ (confirmed 15 minutes apart).
   • Severe, persistent headache or visual disturbances refractory to analgesics.
   • Persistent severe epigastric or Right Upper Quadrant (RUQ) abdominal pain.
   • Progressive renal failure or severe thrombocytopenia.
4. Eclampsia:
   • Manifestation of new-onset generalized tonic-clonic seizures or unexplained coma in a pregnant or postpartum patient with preeclampsia, not attributable to underlying CNS pathology (e.g., stroke, intracranial hemorrhage).

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HELLP Syndrome

HELLP syndrome is a life-threatening variant of severe preeclampsia present in 10–20% of severe preeclampsia cases.

• H (Hemolysis): Microangiopathic hemolytic anemia. Endothelial disruption leads to fibrin deposition in microvessels, shredding red blood cells. Characterized by schistocytes on peripheral blood smear, elevated total bilirubin ($\ge 1.2 \text{ mg/dL}$), and significantly elevated Lactate Dehydrogenase (LDH $> 600 \text{ U/L}$).
• EL (Elevated Liver Enzymes): Microvascular thrombosis in liver sinusoids causes hepatic tissue ischemia, elevation of AST and ALT ($> 2\times$ upper limit of normal), and stretch of Glisson's capsule presenting as severe epigastric or RUQ pain.
• LP (Low Platelets): Consumptive thrombocytopenia with platelet counts $< 100,000 / \mu\text{L}$ due to widespread platelet activation and aggregation at damaged vascular endothelium.

Transport Risks & Management

• High risk for spontaneous subcapsular hepatic hematoma formation and rupture, resulting in catastrophic intra-abdominal hemorrhage and rapid hypovolemic shock. Avoid palpating the liver vigorously.
• Immediate transport to a tertiary perinatal facility for prompt C-section delivery, fluid/platelet resuscitation, and seizure prophylaxis.

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Magnesium Sulfate Therapy & Toxicity Management

Magnesium sulfate ($\text{MgSO}_4$) is the primary neuroprotective and anticonvulsant agent used to prevent eclamptic seizures in preeclampsia and terminate seizures in eclampsia. It acts as a central NMDA receptor blocker, cerebral vasodilator, and competitive calcium antagonist at the neuromuscular junction.

Dosing Protocols

• Loading Dose: $4–6 \text{ g}$ IV diluted in $100 \text{ mL}$ $D_5W$ or Lactated Ringer's, administered slowly over 15–20 minutes IV.
• Maintenance Infusion: $1–2 \text{ g/hr}$ continuous IV infusion (diluted as $20 \text{ g}$ in $500 \text{ mL}$ LR or $40 \text{ g}$ in $1000 \text{ mL}$ LR).
• Recurrent Seizure Protocol: If a seizure occurs while on magnesium, administer an additional $2 \text{ g}$ IV bolus over 3–5 minutes.
• Therapeutic Serum Level: $4.0–7.0 \text{ mEq/L}$ ($4.8–8.4 \text{ mg/dL}$ or $2.0–3.5 \text{ mmol/L}$).

Toxicity Monitoring Protocol

Critical care transport clinicians must assess deep tendon reflexes (patellar reflex), respiratory rate, and urine output every 15–30 minutes during transport.

 Serum Mg Level
   (mEq/L)
     ▲
  >15│ ─────────────────────────────────────── [ CARDIAC ARREST / ASYSTOLE ]
     │
10-12│ ─────────────────────────────────────── [ RESPIRATORY DEPRESSION (<12/min) / SOMNOLENCE ]
     │
 8-10│ ─────────────────────────────────────── [ LOSS OF DEEP TENDON REFLEXES (PATELLAR) ]
     │
  4-7│ ═══════════════════════════════════════ [ THERAPEUTIC RANGE (Seizure Prophylaxis) ]
     │
     └────────────────────────────────────────► Progressive Toxicity

1. Loss of Deep Tendon Reflexes (Patellar DTRs): Occurs at serum levels of $8–10 \text{ mEq/L}$ ($9.6–12.0 \text{ mg/dL}$). This is the earliest reliable clinical warning sign of magnesium overdose!
2. Respiratory Depression: Respiratory rate drops below 12 breaths/min at $10–12 \text{ mEq/L}$ ($12.0–14.4 \text{ mg/dL}$), accompanied by progressive somnolence and slurred speech.
3. Cardiac Conduction Defects & Arrest: Conduction delays, PR/QRS prolongation, severe heart block, and cardiac arrest occur at levels $> 15 \text{ mEq/L}$ ($> 18.0 \text{ mg/dL}$).
4. Oliguria Warning: Because magnesium is excreted 100% by the kidneys, urine output must remain $> 30 \text{ mL/hr}$. Decreased renal clearance rapidly causes fatal magnesium accumulation.

Antidote Administration Protocol

If loss of reflexes or respiratory depression occurs:

1. Immediately STOP the magnesium sulfate infusion!
2. Administer Calcium Gluconate: $1 \text{ g}$ ($10 \text{ mL}$ of a 10% solution) IV slow push over 3 minutes.
3. Alternative (if Calcium Gluconate unavailable): Calcium Chloride $500–1000 \text{ mg}$ ($5–10 \text{ mL}$ of 10% solution) IV push slowly (preferably via central line to prevent severe tissue necrosis if extravasation occurs).
4. Provide bag-valve-mask (BVM) ventilation or endotracheal intubation as needed.

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Acute Antihypertensive Therapy in Preeclampsia

Severe acute hypertension (SBP $\ge 160 \text{ mmHg}$ or DBP $\ge 110 \text{ mmHg}$) requires urgent pharmacologic reduction to prevent maternal hemorrhagic stroke and encephalopathy. The goal is to lower SBP to $140–150 \text{ mmHg}$ and DBP to $90–100 \text{ mmHg}$ without precipitous drops that compromise placental blood flow.