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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