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Flashcards in Volatiles Deck (17)
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1

Colour codes

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

2

MAC - definition

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

3

MAC of different volatiles

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

4

Age-specific MAC of Sevoflurane

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

5

Factors Decreasing MAC

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

6

Factors increasing MAC

  • Hyperthermia
  • Chronic alcohol abuse
  • MAO inhibitors
  • Hypernatraemia

7

Blood Gas Coefficients

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

8

Boiling Point

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

9

Vapour Pressure at 20 degrees Celcius

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

10

Physiological Effects - CNS

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

11

Physiological Effects - CVS

  • ↓ 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?)

12

Physiological Effects - Respiratory

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

13

Physiological Effects - Other

  • 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

14

Metabolism

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

15

Nitrous Oxide - Pharmacological Properties

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

16

Nitrous Oxide - Physiological Effects

  • 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 CO
    • 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

17

Nitrous Oxide - Adverse Effects

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