2019A03 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.

 

List:

·         Intro

·         Anatomical changes

·         Lung volumes

·         Breathing

·         Gas exchange

·         Labour

 

Intro:

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:

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:

↓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:

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:

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

o pH 7.45

o PaCO2 30mmHg (note ↓etCO2 target during mechanical ventilation)

o HCO3- 20mmol/L

·   PaO2 ~105mmHg -> ↑partial pressure gradient

Oxygen flux

·   CaO2 16mL/min (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 (cf. 250)

·   EO2 = 300/1200 = 0.25 (i.e. unchanged!)

 

Labour:

During contractions

·   ↑Minute ventilation (pain)

Between contractions

·   ↓PaCO2 below apnoeic threshold -> mild desaturation

 

 

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