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

 

List:

·      Airway

·      Breathing

·      Pulmonary circulation

·      Other: including Guedel’s stage 2

 

Airway effects:

Pungent/irritant

·   Isoflurane, desflurane -> laryngospasm, coughing, breath holding, secretions

Pleasant smell

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

Bronchodilatation

·   ↓Airway resistance, ↓work of breathing; use in status asthmaticus

·   ↑Anatomical dead space (not clinically significant)

↓Airway reflexes

·   Tolerate LMA

·   ↑Risk aspiration

↓Pharyngeal dilator tone

·   ↑Risk airway obstruction if native airway

·   (especially if OSA)

↓Ciliary activity

·   ?↑Risk pneumonia

 

Breathing effects:

 

Table of ventilatory effects:

 

Halothane

Isoflurane

Enflurane

Desflurane

Sevoflurane

RR

↑↑

↑↑

↑↑

↑↑

TV

↓↓

↓↓↓

↓↓

↓↓

PaCO2

-

↑↑

↑↑↑

↑↑

 

Causes of hypoventilation:

Skeletal muscle relaxation

Affects intercostals > diaphragm:

·   ↑Glycine, ↑GABA: ↓Spinal cord α-motor neuron output

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

·   Inhibition of post-synaptic nAChR at NMJ

·   Inhibition of pre-synaptic VDCC at NMJ

Chemoreceptor depression

·   ? Mechanism

·   ↓↓Response to ↓PaO2 (50% at 0.1MAC, minimal at 1MAC)

Respiratory centre depression

·   ↑GABA, ↑glycine in brainstem

Response curves

 

At 1MAC: PaCO2 50mmHg without stimulation

 

Pulmonary circulatory effects:

↓HPV

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

·   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: high FGF rate, high temp, exhausted absorber, baralyme > sodalime

 

Other:

↓Resistance

·   Bronchodilatation -> ↑airway radius

↓Compliance

·   ↓FRC 15% on induction -> ↓lung compliance

·   Diaphragm and intercostal relaxation -> ↓chest wall compliance

Guedel’s stage 2 effects

·   ↑↑RR (and irregular)

·   ↑Risk laryngospasm, coughing, breath-holding

 

 

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