2001A12 Briefly describe the respiratory effects of the volatile agents.

 

List:

·     A: airway reflexes

·     B: respiratory depression

·     C: pulmonary circulation

·     D: excitation

 

A: airway:

Physicochemical

·      Isoflurane, desflurane: irritation -> laryngospasm, coughing, secretions

·      Sevoflurane, halothane, enflurane -> gas induction well tolerated

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 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

·      Enflurane > isoflurane/desflurane > sevoflurane > halothane

Response curves

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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

·      Inhibition of L-Ca2+ channel, ↑NO release

·      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

Carbon monoxide poisoning

·      Desflurane > isoflurane = enflurane

·      Reaction with CO2 absorber

·      HbCO can rise to 35%

·      Risk factors: ↓FGF, ↑temp, exhausted absorber, baralyme > sodalime

 

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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