Volatiles

Question 1

MAC (dose)
- Sevoflurane
- Desflurane
- NO

Answer 1

Always reduce dose for elderly/unwell/sick patients
MAC (dose)

Sevoflurane: end tidal concentration aim = 1.8 - 2.1%
Desflurane: end tidal concentration aim = 6.6%
NO: end tidal concentration aim = 105%

Question 2

Blood gas partition co-efficient (BGPC)
Lower BGPC = faster onset

Answer 2

Lower BGPC = faster onset
Sevoflurane: 0.7
Desflurane: 0.42
NO: 0.47

Question 3

Special features for volatiles
- Sevoflurane
- Desflurane
- NO

Answer 3

Sevoflurane: Fast onset ++, fast offset ++
Desflurane: Fast onset ++, fast offset ++
NO: Fast onset +++, fast offset +++, most cardiostable, analgesia-emergency, labour

Question 4

Main side effects for volatiles

Answer 4

Sevoflurane: Theoretical risk of renal injury when used at low flows

Desflurane: Airway irritant (therefore not used without an ETT), potent greenhouse gas

NO: unable to achieve full anaesthesia by itself, potent greenhouse gas, need high fraction inspired (limits ability to increase O2 fraction), diffusion hypoxaemia, rapid expansion of air filled spaces (do not use if pneumothorax, pneumoperitoneum, intestinal surgery)

Question 5

Side effects for all volatiles

Answer 5

PONV

CVS depression - hypotension

RS depression - less than IV agents

Malignant hyperthermia (not NO)

Variable effects at doses on cerebral blood flow and ICP

Greenhouse gas

Question 6

Special features for all volatiles

Answer 6

More cardiostable during induction with IV agents

Able to accurately measure blood levels to prevent awareness

Able to perform inhalational induction with patient spontaneously breathing

Question 7

Definition of MAC

Answer 7

MAC = minimum alveolar concentration

The percent gas at steady state which would prevent a reflex response to a standard surgical incision (a skin incision) in 50% of volunteers at 1 atmosphere pressure

It is a measure of potency all allows us to know how much anaesthesia is needed as a percent

Question 8

Other derived MAC values

Answer 8

MAC awake = to prevent an appropriate voluntary response (eye opening) to a verbal command (0.3-0.5 MAC)

MAC bar = to block autonomic responses to nociceptive stimuli (1.7-2 MAC)

MAC95 = to prevent 95% of volunteers having a reflex response to a standard surgical incision (~1.3 MAC)

Note that MAC is additive e.g. 0.5MAC of sevo + 0.5 MAC of NO = 1 MAC