2020B07 Describe the effect of pregnancy, at term, on wash-in (not rate of onset)
of volatile anaesthetic agents.

List:

· Wash-in curve

· Intro

· Cardiovascular

· Respiratory

Wash-in curve

Intro:

Wash-in	· Correlates with equilibration of inspired and alveolar (and effect site) partial pressures · i.e. rate at which F_A/F_I approaches 100% · ∝ delivery rate / uptake rate
Effects	· Resp: ↑↑rate (most important) · CVS: ↓rate · CNS: ↑rate of onset but no effect on wash-in
Causes	· ↑Metabolic rate: due to foetus and placenta, cardiac work and resp work · Mass effect: cephalad displacement of diaphragm by gravid uterus · Hormones: progesterone, relaxin
Considerations	If GA LUSCS: · ↑Wash-in if mechanical ventilation · ↑Wash-in if nitrous oxide (insoluble, second gas effect)

Respiratory:

V_A : FRC

· ↑RR 10% and ↑V_T 40% (progesterone at chemoreceptors + ↑BMR)

· ↑V_D ↔V_D/V_T (progesterone bronchodilation)

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

->> V_A:FRC 6.1 cf. 2:1

->> ↑Rate of rise F_A/F_I

V/Q matching

· ↑Pulmonary blood flow -> recruitment and distension -> ↑matching

->> ↑Rate of drug uptake

->> ↑Rate of rise F_A/F_I (especially insoluble agents)

Cardiovascular:

Cardiac output

Changes:

· ↑HR 25% (↑15% by end T1)

· ↑SV 40% (↑25% by end T1)

· ↑CO 45% (↑40% by end T1)

Causes:

· ↑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 F_A/F_I

Uteroplacental flow

· Flow 750mL/min cf. 100mL/min

->> ↑Tissue uptake

->> ↓Rate of rise of Pv(Gas)

->> ↓Rate of rise F_A/F_I

Aortocaval compression

· ↓↓Preload -> ↓cardiac output

->> May offset the above