Tell me about physiological changes in pregnancy from anaesthesia pov esp respitatory.. explain me. The changes withbreason
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I now have extensive, authoritative data from both Barash's Clinical Anesthesia and Morgan & Mikhail's Clinical Anesthesiology. Let me compile a comprehensive, well-structured answer.
Physiological Changes in Pregnancy — Anaesthesia Perspective
Overview
Pregnancy causes profound changes in nearly every organ system, driven by two main forces:
- Hormonal shifts — primarily progesterone and oestrogen
- Mechanical effects — the enlarging gravid uterus compressing structures
These changes are cumulative, reaching a maximum near term, and have direct, critical implications for safe anaesthetic management.
🫁 RESPIRATORY CHANGES (Most Critical for Anaesthesia)
Upper Airway
Capillary engorgement of nasal, oropharyngeal, and laryngeal mucosa occurs throughout pregnancy — worsening at term due to:
- Increased extracellular fluid
- Hormonal changes (oestrogen)
- Preeclampsia, prolonged pushing in second stage, or tocolytic therapy
Implications:
- Avoid nasal routes (nasopharyngeal airways, nasotracheal tubes, NGT) — high risk of bleeding
- Mallampati scores increase progressively through pregnancy and labour
- Breast engorgement + airway oedema = difficult intubation
- Use a short-handled laryngoscope; always have a video laryngoscope available
Lung Volumes — The Core Changes
| Parameter | Change | Reason |
|---|---|---|
| Tidal Volume (TV) | ↑ 40–45% | Progesterone → increased respiratory drive |
| Respiratory Rate | ↑ ~15% | Minor contribution |
| Minute Ventilation | ↑ 50% | Primarily TV increase |
| FRC (Functional Residual Capacity) | ↓ 20–30% | Diaphragm pushed cephalad by uterus |
| Residual Volume (RV) | ↓ ~25% | Diaphragm elevation |
| Expiratory Reserve Volume (ERV) | ↓ ~20% | Diaphragm elevation |
| Total Lung Capacity (TLC) | Minimally ↓ | Compensated by rib flaring |
| Airway Resistance | ↓ 35% | Progesterone → bronchodilation |
| Oxygen Consumption | ↑ 20–50% | Fetal/uterine/placental metabolic demands |
The FRC Problem — Central to Anaesthesia Safety
Why FRC matters so much:
- FRC is the buffer of oxygen available during apnoea (e.g., during intubation)
- In pregnancy: FRC ↓ 20–30% + O₂ consumption ↑ 50% = dramatically shortened time to desaturation
- Closing volume does NOT change in pregnancy — but because FRC falls, the closing volume may exceed FRC, especially in:
- Supine position
- Obese parturients
- On induction of general anaesthesia
This causes airway closure during normal tidal breathing → V/Q mismatch → hypoxaemia
Anaesthetic implication: Preoxygenation (denitrogenation) is MANDATORY before induction. Even with optimal preoxygenation, a pregnant patient will desaturate within 2–3 minutes vs. 8–10 minutes in a non-pregnant adult.
Blood Gases & Acid-Base
| Parameter | Change | Reason |
|---|---|---|
| PaCO₂ | ↓ 15% (to ~32 mmHg) | Hyperventilation from progesterone |
| PaO₂ | ↑ 10% (~105 mmHg) | Increased alveolar ventilation |
| HCO₃⁻ | ↓ 15% (~20 mEq/L) | Renal compensation for respiratory alkalosis |
| pH | Slightly ↑ (7.44) | Partially compensated respiratory alkalosis |
Why this matters:
- The mild respiratory alkalosis is physiological — do NOT aggressively ventilate to a "normal" PaCO₂ of 40 mmHg during anaesthesia; this creates relative hypercapnia for the pregnant patient and can cause uterine vasoconstriction and fetal acidosis
- Target PaCO₂ ~32 mmHg in ventilated pregnant patients
Inhalational Anaesthetic Uptake
- ↑ Minute ventilation → faster alveolar delivery of volatile agents → faster induction
- ↓ FRC → smaller reservoir → faster rise in alveolar concentration
- Combined effect: dramatically faster onset of inhalational anaesthetics
- Also, faster elimination on discontinuation — faster emergence
❤️ CARDIOVASCULAR CHANGES
| Parameter | Change |
|---|---|
| Blood volume | ↑ 35% |
| Plasma volume | ↑ 55% (> RBC mass → dilutional anaemia) |
| Cardiac output | ↑ 40% |
| Stroke volume | ↑ 30% |
| Heart rate | ↑ 20% |
| Systolic BP | ↓ 5% |
| Diastolic BP | ↓ 15% |
| Peripheral vascular resistance | ↓ 15% |
Aortocaval compression: From ~20 weeks, the gravid uterus compresses the IVC and aorta in the supine position → ↓ venous return → ↓ CO by up to 30%. Treat with left lateral uterine displacement (15° wedge under right hip).
🧠 CNS / NEUROLOGICAL CHANGES
| Parameter | Change | Reason |
|---|---|---|
| MAC (Minimum Alveolar Concentration) | ↓ 40% | Progesterone (sedating), β-endorphin surge |
| Sensitivity to neuraxial/local anaesthetics | ↑ | Engorged epidural veins → smaller epidural space; increased neural sensitivity |
- Requires less volatile agent to maintain anaesthesia — risk of overdose if normal adult doses used
- Epidural vein engorgement → reduced epidural space volume → local anaesthetic spreads higher with same dose → risk of high block
🩸 HAEMATOLOGICAL / GI / RENAL (Brief, Anaesthesia-Relevant)
Haematological:
- Dilutional anaemia (Hb ↓ ~20%) — large plasma volume expansion
- Clotting factors ↑ 30–250% → hypercoagulable state → DVT/PE risk
- Platelets ↓ ~10% (mild gestational thrombocytopenia in ~8%)
Gastrointestinal (CRITICAL):
- Progesterone reduces lower oesophageal sphincter tone
- Upward displacement of stomach by uterus
- Increased gastric acid production (gastrin from placenta)
- Delayed gastric emptying (especially in labour)
- Result: Full stomach from 18–20 weeks → high aspiration risk
- All pregnant patients beyond 20 weeks requiring general anaesthesia need rapid sequence induction (RSI) with cricoid pressure and a cuffed endotracheal tube
Renal:
- GFR ↑ 50% → lower serum creatinine and urea (normal creatinine in pregnancy is ~0.5 mg/dL; a "normal" non-pregnant value may actually indicate impaired function)
Summary Table: Key Anaesthetic Implications
| Physiological Change | Anaesthetic Consequence | Management |
|---|---|---|
| ↓ FRC + ↑ O₂ consumption | Rapid desaturation on apnoea | Mandatory preoxygenation |
| Airway oedema, ↑ Mallampati | Difficult intubation | Video laryngoscope, smaller ETT (6.5–7.0 mm), short-handle laryngoscope |
| Full stomach (↓ LES tone, ↑ gastric acid) | Aspiration risk | RSI + cricoid pressure from 18–20 weeks |
| Faster volatile uptake | Faster induction, risk of overdose | Careful titration |
| ↓ MAC (40%) | Less inhalational agent required | Reduce doses |
| ↓ PaCO₂ (32 mmHg baseline) | Risk of fetal acidosis if over-ventilated | Target PaCO₂ 32 mmHg on ventilator |
| Enhanced LA sensitivity | High spinal/epidural block | Reduce neuraxial doses; use divided dosing |
| Aortocaval compression | Maternal hypotension, fetal distress | Left lateral tilt ≥15° |
Sources: Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e; Morgan and Mikhail's Clinical Anesthesiology, 7e
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