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

 

List:

·     A: airway reflexes

·     B: respiratory depression

·     C: pulmonary circulation

·     D: excitation

 

A: airway reflexes

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

 

B: respiratory depression

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

 

C: pulmonary circulation

↓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

 

D: excitation

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

 

 

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