What is the management approach for a 34-week pregnant woman with acute onset of dyspnea, considering the respiratory changes in pregnancy and their impact on arterial blood gas (ABG) interpretation?

Management of Acute Dyspnea in a 34-Week Pregnant Woman

For a 34-week pregnant woman presenting with acute onset dyspnea, immediate oxygen supplementation should be provided to maintain target saturation of 94-98%, followed by prompt assessment for life-threatening causes including pulmonary embolism, respiratory failure, and cardiac conditions. 1, 2

Understanding Respiratory Changes in Pregnancy

Normal Physiological Changes

• Respiratory rate remains unchanged throughout pregnancy 2
• Tidal volume increases by 20-40% by term 2
• Minute ventilation increases by 20-40% above pre-pregnancy levels 2
• Oxygen consumption increases by 20-33% by third trimester 2
• Functional residual capacity decreases by 10-25% due to diaphragm elevation 2
• Mild compensated respiratory alkalosis with:
  • PaCO₂: 28-32 mmHg (lower than non-pregnant)
  • HCO₃: 18-21 mEq/L (decreased to compensate) 2

Impact on ABG Interpretation

• Normal PaO₂ in pregnancy is 100-105 mmHg (sitting position)
• PaO₂ may be up to 2 kPa lower in supine position during third trimester 1
• Expect mild respiratory alkalosis (low PaCO₂, low HCO₃) in normal pregnancy
• Absence of respiratory alkalosis suggests respiratory compromise

Immediate Management Algorithm

Step 1: Initial Assessment and Stabilization

1. Position patient in left lateral tilt to prevent aortocaval compression 1, 2
2. Administer supplemental oxygen to maintain SpO₂ 94-98% 1
3. Establish IV access
4. Continuous monitoring of maternal vital signs and fetal heart rate

Step 2: Rapid Evaluation for Life-Threatening Causes

• Pulmonary embolism (PE): Common in pregnancy with 4x higher risk 1
• Severe respiratory disorders: May cause significant maternal/fetal hypoxia 1
• Cardiac conditions: Heart failure, cardiomyopathy, valvular disease
• Amniotic fluid embolism: Rare but catastrophic
• Severe preeclampsia/eclampsia: Can present with pulmonary edema

Step 3: Diagnostic Approach

1. Immediate bedside assessment:

   • Bedside transthoracic echocardiography if hemodynamic instability is present 1
   • Assess for signs of DVT, PE, heart failure

2. Laboratory studies:

   • Arterial blood gas (drawn in upright position if possible) 1
   • D-dimer (note: physiologically elevated in pregnancy but still useful) 1
   • CBC, electrolytes, BNP, troponin

3. Imaging studies:

   • Chest X-ray (minimal radiation risk to fetus - <10 μGy) 1
   • If PE suspected:
     • Compression ultrasound of lower extremities first 1
     • If negative and suspicion remains high, proceed to CTPA or V/Q scan 1
     • CTPA delivers 13-300 μGy to fetus depending on gestational age 1
     • V/Q scan delivers 10-350 μGy to fetus 1

Management Based on Diagnosis

Pulmonary Embolism

• Initiate therapeutic anticoagulation with LMWH while diagnostic workup is ongoing if clinical suspicion is high 1
• Avoid NOACs (contraindicated in pregnancy) 1
• Consider thrombolysis only for massive PE with hemodynamic instability 1

Respiratory Failure

• For severe respiratory disorders, monitor oxygen saturation closely 1
• Consider supplemental oxygen to maintain maternal SpO₂ >94% 1
• Position patient in left lateral position to optimize ventilation 1
• If intubation becomes necessary:
  • Recognize increased risk of difficult airway in pregnancy 1
  • Pre-oxygenate thoroughly (pregnant patients desaturate faster) 1
  • Consider early involvement of anesthesia team 1

Cardiac Causes

• Early cardiology consultation
• Optimize preload, afterload, and contractility
• Consider early delivery if maternal condition deteriorates despite medical management

Special Considerations

Anesthetic Considerations

• Early epidural analgesia with local anesthetics is preferred for pain relief if needed 1
• Avoid systemic opioids when possible as they suppress ventilation 1

Delivery Planning

• Multidisciplinary approach involving obstetrics, anesthesia, and critical care
• Modification of birth plan may be required for women with significant respiratory disease 1
• Consider elective delivery if maternal condition warrants it 1

Pitfalls to Avoid

1. Failing to position properly: Always maintain left lateral tilt in third trimester to prevent aortocaval compression 1, 2
2. Delayed diagnosis of PE: Maintain high index of suspicion as PE is a leading cause of maternal mortality 1
3. Misinterpreting ABGs: Remember normal pregnancy shows respiratory alkalosis; absence suggests pathology 2
4. Inadequate oxygenation: Pregnant women develop hypoxemia rapidly due to decreased functional residual capacity 1
5. Delayed escalation of care: Have a contingency plan ready if the patient's condition deteriorates 1

Remember that acute dyspnea in late pregnancy requires prompt, systematic evaluation and management to ensure optimal outcomes for both mother and fetus.