2021B08 Describe the effects of sevoflurane on the respiratory system.

· A: airway reflexes

· B: respiratory depression

· C: pulmonary circulation

· D: excitation

Physicochemical	· Non-irritant -> suitable for inhalational induction
Bronchodilatation	· ↓L-Ca²⁺, ↑NO activity · ↓Airway resistance, ↓work of breathing; use in status asthmaticus · ↑Anatomical dead space (not clinically significant)
↓Airway reflexes	· Tolerate LMA · Risk of aspiration
↓Pharyngeal dilator tone	· Skeletal muscle relaxation -> airway obstruction (especially if OSA)
↓Ciliary activity	· ?↑Risk pneumonia

Summary

· ↑RR, ↓VT

· ↓MV, ↑PaCO₂ (~50mmHg at 1 MAC)

· ↓Response to ↑PaCO₂

· ↓↓Response to ↓PaO₂

· Respiratory arrest at high concentration

Response curves

Chart, line chart Description automatically generated

Chart, scatter chart Description automatically generated

Mechanisms

· Respiratory centre depression

o ↑GABA, ↑glycine in brainstem

· Chemoreceptor depression

o ? mechanism

· Skeletal muscle relaxation

o Affects intercostals > diaphragm

o ↑Glycine, ↑GABA: ↓spinal cord α-motor neuron output

o Inhibition of L-Ca²⁺ channel: ↓force of contraction

o Inhibition of post-synaptic nAChR at NMJ

o Inhibition of pre-synaptic VDCC at NMJ

↓HPV

· ↓L-Ca²⁺, ↑NO activity

· Pulmonary vasodilatation

· ↓PVR, ↓HPV

· ↑V/Q mismatch -> ↓PaO₂

↑Dead space

· ↓Venous return -> ↓RV output

· ↓PASP -> ↑West zone 1 i.e. alveolar dead space

· ↑PaCO₂ – etCO₂ difference

i.e. Guedel’s stage 2: at concentration just above MAC-awake

Airway

· ↑Sensitivity to stimulation

· Coughing, laryngospasm, bronchospasm, regurgitation/aspiration

Breathing

· ↑↑RR

· Irregular breathing, breath holding