2008A01 An 80 year old woman is undergoing major emergency surgery.
Describe the maintenance inhaled concentration of sevoflurane you would choose
and the factors that might influence this.

 

List:

·      Intro

·      PD: age and illness

·      Effect of adjuvant opioid

·      Volatile kinetics: age and illness

·      Anaesthetic and surgical factors

·      The answer

 

Intro:

Aims

·  Ensure amnesia, hypnosis, immobility

·  Prevent awareness (lower risk)

·  Minimise CVS toxicity (higher risk)

Problems

·  Elderly

·  Acutely unwell

·  Major surgery

Maintenance inhaled concentration

·  After 15 mins, close to equilibration between inhaled, alveolar exhaled partial pressure

·  Hence consider equivalent to MAC

 

Pharmacodynamics: age and illness

Values

·  MAC-amnesia 0.25

·  MAC-awake 0.33

·  MAC (immobility) 1.0

Factors reducing all MAC values

·  Age: ↓6% per decade after 40yrs (at 80: ↓24% ≈ 1.4%)

·  Severe illness: mAP<40mmHg, PaO2 <40mmHg, PaO2 >80mmHg, ↓pH, ↓temp

·  Adjuvant drugs: opioids, benzodiazepines

 

Effect of adjuvant opioid:

 

Sevo alone

Sevo + fentanyl 3ng/mL

MACBAR

4.15%

↓80%

MAC

1.85%

↓60%

MACawake (0.3)

0.6%

↓25%

Standard deviation

0.1

<0.1

 

Volatile pharmacokinetics kinetics: age and illness

Absorption

·  ↓Cardiac output -> ↑rate of rise FA/FI

Distribution

·  ↓Cardiac output (↑age) -> ↑rate of rise FA/FI

·  ↓Muscle mass ↑rate of rise FA/FI

Metabolism

·  ↓Liver enzymatic activity -> ↓rate of consumption -> ↑rate of rise FA/FI

Excretion

·  N/A

 

Anaesthetic and surgical factors:

e.g. fractured NOF

·  Spinal anaesthetic-> ↓nociceptive activation of ARAS -> ↓↓MAC requirement

e.g. laparotomy, thoracotomy

·  ↑↑Stimulation -> ↑MAC requirement

 

Answer:

Considerations

·  Very low risk of awareness at 0.8 MAC = 1.1%

·  Significant reduction in MAC with age, illness, adjuvants

·  Recommend EEG monitoring e.g. BIS, aim 40-60

·  Likely rapid ↑FA/FI

Final answer

Inspired concentration 1.5%

(I don’t know if this is the correct answer)

 

 

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