Volatiles

Question 1

Colour codes

Answer 1

• Red - halothane
• Orange - enflurane
• Purple - isoflurane
• Yellow - sevoflurane
• Blue - desflurane

Question 2

MAC - definition

Answer 2

Corresponds to the minimal alveolar concentration that prevents movement in 50% of patients to a standardised noxious stimulus

Question 3

MAC of different volatiles

Answer 3

• Halothane - 0.75%
• Enflurane - 1.69%
• Isoflurane - 1.2%
• Desflurane - 6%
• Sevoflurane - 2%

Question 4

Age-specific MAC of Sevoflurane

Answer 4

• Adult - 2.05
• Elderly (>80) - 1.2
• Neonate - 3.2
• Infant - 2.5

Question 5

Factors Decreasing MAC

Answer 5

• Hypothermia
• Acute alcohol abuse
• Severe hypoxia
• Metabolic acidosis
• Pregnancy
• Medications
  
  • Opioids
  • Pancuronium
  • Propofol infusion
  • Ketamine
  • Benzodiazepines
  • Barbiturates
  • Chlonidine
  • Lignocaine

Question 6

Factors increasing MAC

Answer 6

• Hyperthermia
• Chronic alcohol abuse
• MAO inhibitors
• Hypernatraemia

Question 7

Blood Gas Coefficients

Answer 7

• Halothane - 2.3
• Enflurane - 1.9
• Isoflurane - 1.4
• Desflurane - 0.42
• Sevoflurane - 0.65

Question 8

Boiling Point

Answer 8

• Halothane - 50.2
• Enflurane - 56.5
• Isoflurane - 48.5
• Desflurane - 23.5
• Sevoflurane - 58.6

Question 9

Vapour Pressure at 20 degrees Celcius

Answer 9

• Halothane - 243 mmHg
• Enflurane - 175 mmHg
• Isoflurane - 250 mmHg
• Desflurane - 644 mmHg
• Sevoflurane - 170 mmHg

Question 10

Physiological Effects - CNS

Answer 10

• ↑ CBF causing ↑ ICP (less so for isoflurane and sevoflurane)
• ↓ cerebral metabolic rate for O2
• Seizure activity on EEG (enflurane)

Question 11

Physiological Effects - CVS

Answer 11

• ↓ Myocardial contractility - ↓ CO and BP (enflurane > halothane > others)
• ↓ SVR
• Blunted response to hypotension causing relative bradycardia
• Depresses SA node, increases refractory period of AV node
• Increased catecholamine sensitivity - increased risk for adrenaline-induced arrhythmia (halothane > enflurane - limit adrenaline to 1.5μg/kg and 4.6μg/kg respectively)
• Coronary steal (isoflurane?)

Question 12

Physiological Effects - Respiratory

Answer 12

• Respiratory depression - shallow, rapid respiration (enflurane > isoflurane > halothane)
• Bronchomotor tone reduced
• Salivary and bronchial secretions suppressed
• Airway irritant (enflurane, isoflurane, desflurane)

Question 13

Physiological Effects - Other

Answer 13

• Uterine relaxation
• Decreased renal blood flow, GFR and urine output
• Decreased hepatic blood flow (not for desflurane)
• Hepatitis (halothane - avoid repeat dose within first 28 days)
• Potentiate NDMRs
• May trigger malignant hyperthermia
• Nausea and vomiting

Question 14

Metabolism

Answer 14

• Halothane - 15-20%
• Enflurane - 2-8%
• Isoflurane - 0.2%
• Desflurane - 0.02%
• Sevoflurane - 3-5%

Question 15

Nitrous Oxide - Pharmacological Properties

Answer 15

• MAC - 104%
• Blood:gas coefficient - 0.47

Question 16

Nitrous Oxide - Physiological Effects

Answer 16

• CNS
  
  • ↑ ICP through cerebral vasodilation
  • ↑ Cerebral oxygen requirements
  • Weak anesthetic, moderate analgesic
  • Can inactivate vitamin B12
• CVS
  
  • Direct cardiodepressant
  • Sympathetic stimulation following cardiac depression
• Respiratory
  
  • Decreases response to CO₂
  • Small increase in pulmonary artery pressure
• GIT
  
  • Nausea and vomiting
  • Liver damage (chronic exposure - little evidence)
• Genitourinary: scanty evidence of abortions and congenital abnormality
• Musculoskeletal: may trigger malignant hyperthermia
• Bone marrow suppression
• Impaired neutrophil chemotaxis

Question 17

Nitrous Oxide - Adverse Effects

Answer 17

• Expansion of closed gas spaces
• Potential of administering hypoxic gas mixtures
• Expensive