2019A Question 03
Describe the effects of pregnancy on the respiratory system that are relevant to the practice of anaesthesia. Include in your answer how these effects influence the conduct of anaesthesia.
Examiner Report
This question covered three topics from the primary examination syllabus: The respiratory system, physiological changes in pregnancy, and anaesthetic implications of these. To pass this question, candidates were expected to cover the respiratory changes in pregnancy with good understanding of the wider concepts demonstrated. Part of this was demonstrating understanding of the role of progesterone in driving these respiratory changes.
Candidates who scored well covered the respiratory changes in pregnancy completely and with adequate detail. They included values or percentage changes of the various lung volumes affected. Discussion about the maternal foetal oxygen transfer was not required for a pass, but was often discussed in the higher scoring papers.
For a question primarily about physiology, there were a wide range of non-physiology topics covered. Detailed plans for difficult airway, reflux and awareness management, with little demonstration of the primary physiology behind the respiratory changes in pregnancy, did not score well.
There were some common areas of physiology which were misunderstood. Many discussed the reduction in FRC, but not to grasp that this was worsened in a supine patient or in a patient undergoing a general anaesthetic. The reasons given for upper airway oedema, should it occur, were often incorrect. The relationship between the FRC and the closing capacity in a term pregnant woman was often neglected or incorrectly described.
Model Answer
Structure:
- Introduction
- Anatomical changes
- Lung volumes
- Breathing
- Gas exchange
- Labour
Introduction
| Factor | Detail |
|---|---|
| Objectives | - Support increased metabolic demand - Augment foetal gas exchange |
| Most important change | - Hyperventilation |
| Causes | - Increased metabolic rate (MR) +20% at term, +60% in labour - Mass effect (ME) of gravid uterus (later in pregnancy) - Hormones: Oestrogen (O), progesterone (P), relaxin (R) |
Anatomical Changes
| Region | Detail |
|---|---|
| Upper airway | - Mucosal swelling: Due to ↑ RAAS activity (O) and vasodilation (P) - Breast enlargement (O) (P)? - Implication: ↑ Risk difficult intubation (relative risk 7-10x), ↑ Risk difficult bag-mask ventilation |
| Lower airway | - Bronchodilation → ↓ Airway resistance, ↑ VD 35% (P) - But ↔VD/VT due to hyperventilation |
| Chest wall | - Lax ligaments (P) → ↓ Intercostal effect - Diaphragm displacement (ME) → ↑ Diaphragm excursion - ↑ AP and transverse diameters |
Lung Volumes
| Volume | Detail |
|---|---|
| ↓ Static volumes | - TLC: 4.1L (cf. 4.2L) - FRC: 1.35L (cf 1.7) (↓ 20% upright, ↓ 30% supine, ↓ further under GA) - RV: 0.8L (cf 1L) - Cause: ME - Implication: Rapid desaturation if apnoeic; ↑ Rate of rise FA/FI of inhalational agent |
| ↑ Dynamic volumes | - TV 0.6L (cf. 0.45L (↑ 30%) - FVC: 3.3L (cf. 3.2L) - IC: 2.75L (cf. 2.5L) - ERV 0.55L (cf. 0.7L) - Cause: (P) - Implication: Ventilatory capacity preserved |
Breathing
| Variable | Detail |
|---|---|
| Ventilation | - ↑ RR 10% (from early T1) - ↑ VT 35% (from early T1) - ↑ VD 35% - ↑ MV 50% - ↑ VA 50% |
| Control of ventilation | - PaCO2 vs. minute ventilation curve: Left shift and 3x increase in slope - PaO2 vs. minute ventilation curve: 2x increase in response - Cause: Sensitization of central chemoreceptors (P); also ↑ BMR → ↑ VCO2 - Implication: ↓ PaCO2; ↑ respiratory work; dyspnoea common |
| Mechanics | - ↓ Resistance: Due to bronchodilation (P) - ↑ Lung compliance: Due to ↓ lung volume (ME) (not sure!) - ↓ Chest wall compliance: Due to lax ligaments (P), chest wall splaying (ME) |
Gas Exchange
| Variable | Detail |
|---|---|
| V/Q matching | - ↑ Cardiac output - > ↑ Recruitment and distension → Improved matching - Implication: ↑ PaO2 by 7.5mmHg, ↓ A-a gradient |
| Oxyhaemoglobin dissociation curve | - ↓ PaCO2 and ↑ pH → Left shift (P) - ↓ Hb → ↑ [2,3-DPG] 30% → R shift (O) - Net effect minimal change |
| ABG | - Compensated resp alkalosis (P) → ↑ Partial pressure gradient for CO2 - pH 7.45 - PaCO2 30mmHg (note ↓ EtCO2 target during mechanical ventilation) - HCO3- 20mmol.L-1 - PaO2 ~105mmHg → ↑ Partial pressure gradient |
| Oxygen flux | - CaO2 16mL.min-1 (cf. 20) – due to anaemia - Cardiac output = 7.5L/min (cf. 5) – mainly due to ↑ MR and ↑ blood volume → ↑ Preload - DO2 = 1.2L/min (cf. 1) - VO2: 300mL.min-1 (cf. 250) - EO2 = 300/1200 = 0.25 (i.e. unchanged!) |
Labour
| Time | Detail |
|---|---|
| During contractions | - ↑ Minute ventilation (pain) |
| Between contractions | - ↓ PaCO2 below apnoeic threshold → Mild desaturation |