2020B Question 07

Describe the effect of pregnancy, at term, on wash-in (not rate of onset) of volatile anaesthetic agents.

Examiner Report

The rate of volatile wash in is determined by delivery of agent to the alveoli and uptake from the alveoli, and can be illustrated via FA/FI curves. The important physiological parameters that change in pregnancy which must be discussed to pass this question are minute ventilation, functional residual capacity and cardiac output.

For each of these parameters the candidate should indicate:

  • Why they change in pregnancy
  • The degree of change at term
  • The effect this change has upon wash in (increased/decreased rate)
  • An explanation as to the mechanism by which the effect upon wash in is mediated

The overall effect of pregnancy would be faster volatile wash in, respiratory effects outweighing cardiac.

Discussion of all physiological changes in pregnancy, or of all factors related to volatile wash in, were not necessary. However, brief points which demonstrated greater understanding would include:

  • Ventilation under GA usually not patient controlled at term - MV controlled by anaesthetist
  • Increased A-V gradient due to greater tissue uptake (uteroplacental unit) slows wash in
  • Nitrous Oxide has a prominent role in obstetric anaesthesia and enhances volatile wash in
  • Aorto-caval compression may lead to reduced rather than elevated cardiac output if supine
  • Kinetics of more soluble agents is more affected by the physiological changes than less soluble agents

Time constants were referred to by some candidates. The time constant representing volatile wash in (FRC/MV) should ideally be accompanied by a brief note explaining its significance that this implies the time needed for wash in will be less when a smaller volume (FRC) is being filled by a higher flow rate (MV). Many candidates misquoted the formula as MV/FRC, and/or suggested that a higher time constant reflected faster wash in. Other common errors included discussion or labeling of FI/FA curves rather than FA/FI, and the terms delivery/uptake/wash in/onset were often used incorrectly or interchangeably.

Model Answer


  • Wash-in curve
  • Introduction
  • Cardiovascular
  • Respiratory

Wash-in Curve


Factor Detail

- Correlates with equilibration of inspired and alveolar (and effect site) partial pressures

- i.e. Rate at which FA/FI approaches 100%



- Resp: ↑↑ Rate (most important)

- CVS: ↓ Rate

- CNS: ↑ Rate of onset but no effect on wash-in


- ↑ Metabolic rate: Due to foetus and placenta, cardiac work and resp work

- Mass effect: Cephalad displacement of diaphragm by gravid uterus

- Hormones: Progesterone, relaxin



- ↑ Wash-in if mechanical ventilation

- ↑ Wash-in if nitrous oxide (insoluble, second gas effect)


Factor Detail

- ↑ RR 10% and ↑ VT 40% (progesterone at chemoreceptors + ↑ BMR)

 - ↑ VD ↔ VD/VT (progesterone bronchodilation)

 - ↓ FRC -20% erect, -30% supine, further under GA (mass effect)

→ VA:FRC 6.1 cf. 2:1

→ ↑ Rate of rise FA/FI

V/Q matching

- ↑ Pulmonary blood flow → Recruitment and distension → ↑ Matching

→ ↑ Rate of drug uptake

→ ↑ Rate of rise FA/FI (especially insoluble agents)


Factor Detail
Cardiac Output


- ↑ HR 25% (↑ 15% by end T1)

 - ↑ SV 40% (↑ 25% by end T1)

 - ↑ CO 45% (↑ 40% by end T1)


- ↑ Metabolic rate → ↑ Preload

 - Oestrogen → ↑ RAAS → ↑ Plasma volume 40% → ↑ Preload

 - Oestrogen → ↑ Epo → ↑ RBC volume 20% → ↑ Preload

 - Progesterone → ↓ SVR, ↓ afterload

→ ↑ Pulmonary Uptake; but

→ ↑ Distribution

→ ↓ Rate of rise of Pv(Gas)

→ ↓ Rate of rise FA/FI

Uteroplacental Flow

- Flow 750mL.min-1 cf. 100mL.min-1

→ ↑ Tissue uptake

→ ↓ Rate of rise of Pv(Gas)

→ ↓ Rate of rise FA/FI

Aortocaval Compression

- ↓↓ Preload → ↓ Cardiac output

→ May offset the above

Last updated 2021-08-23

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