General Anesthetics Flashcards
What are the physicochemical properties of inhaled general anesthetics that determine anesthetic potency?
Oil:Water partition coefficient. This is the ratio of concentration of a compound in the two phases of a mixture of two immiscible solvents in equilibrium. The higher the oil:water partition coefficient, the higher the potency of the inhaled general anesthetic
Minium alveolar concentration (MAC): This is defined as the concentration of a vapor in the lungs that is required to prevent movement/motor response in 50% of subjects in response to surgical (pain) stimulus. The lower the MAC, the higher the potency of an inhaled general anesthetic
What is the current thinking regarding the mechanism of action of inhaled anesthetics in producing anesthesia?
Protein Theory: Volatile anesthetics act via interactions with hydrophobic pockets in membrane proteins. The partitioning of volatile anesthetics into the hydrophobic regions of cell membrane perturbs the normal function of integral membrane proteins, particularly ion channels responsible for membrane excitability.
GABA-a Receptors: General anesthetics depress neuronal excitability → best-described effect is through interactions with GABAA. Potentiation of GABAA receptor activity by clinically relevant doses of either volatile or IV anesthetics increases the duration of inhibitory postsynaptic potentials. The overall effect is greater inhibition of the CNS leading to decreased neuronal excitability.
What are the ideal characteristics of a general anesthetic?
- Rapid and smooth onset of action
- Rapid recovery from anesthesia
- Wide margin for safe use
What are the signs in the development of general anesthesia?
- Loss of fine motor function/coordination
- Alteration of consciousness/analgesia
- Loss of temperature regulation
- Unconsciousness
- Changes in eye motion
- Loss of muscle tone
- Respiratory failure
- CV failure
- Coma and death
What are the fundamental physical principles that determine uptake and elimination of inhaled anesthetics?
x
What are the differences between tissue groups that are important in determining uptake of general anesthetics?
Vessel-rich group: Brain, heart, kidney, liver, endocrine. Very high uptake rate
Muscle group: (lean tissue) Muscle and skin. Slower uptake (2-4 hours)
Fat group: Very slow uptake due to high solubility of anesthetic in fat and low perfusion. Unloading of anesthetic is also slow
What is the rationale for use of a combination of pharmacological agents to achieve effective surgical anesthesia?
- Increase rate of induction phase
- Relax skeletal muscle (allows for decreasing anesthetic dose) – neuromuscular blocking agents. Relax skeletal muscle of abdominal wall to facilitate surgical manipulation of the torso. Also allows you to lower the amount of volatile anesthetic dose to reduce the dangers of overdose.
- Reduce patient anxiety, pain and nausea
- Counter vagal effects
- Anti-emetics
- Multiple anesthetics to increase potency (MACs are additive)
What are the methods of application of inhaled anesthetics?
- Anesthetic machines are used to measure and control mixture
- Vaporizers add anesthetic to inspired gas
- Breathing circuit allows for CO2 removal
- Machine allows for measurement of gas composition to estimate anesthetic depth
- Small therapeutic indece – doce producing circulatory failure is 2-4x the dose for adepquate anesthesia.
What are the stages in the development of general anesthesia?
Stage I: analgesia (inability to feel pain)
Stage II: excitement, delirium
Stage III: surgical anesthesia
Plane 1: regular respirations
Plane 2: muscular relaxation, fixed pupils
Plane 3: good muscular relaxation, depressed intercostals with respiration
Plane 4: diaphragmatic breathing and dilated pupils
Stage IV: medullary paralysis
What are the advantages, disadvantages and problems associated with Nitrous Oxide
Advantages: Low potency. Useful as adjunctive agent. Rapid onset and recovery
Disadvantages: MAC is 105% so it can’t be used as a sole anesthetic agent. Contraindicated in Respiratory obstruction, COPD and pregnancy
What are the advantages, disadvantages and problems associated with Desflurane
Advantages: High potency, Low blood and fat solubility, No hepatotoxicity
Disadvantages: Smells bad (airway irritation and cough), DO NOT use in pts predisposed to Malignant hypothermia
What are the advantages, disadvantages and problems associated with Enflurane
Advantages: Excellent analgesic, good muscle relaxant. Moderately fast induction and recovery. Used in maintenance of anesthesia in adults. Decreased hepatotoxicity
Disadvantages: Seizure induction
What are the advantages, disadvantages and problems associated with Halothane
Advantages: Moderate to high potency. Low blood:gas partition coefficient
Disadvantages: MAC = .75%. Not a good analgesic. Easily produces respiratory failure and arrythmias. High rate of hepatotoxicity. Malignant hypothermia
What are the advantages, disadvantages and problems associated with Isoflurane
Advantages: Most widely used agent! Like enflurane with higher potency. No seizure risk. Rapid and smooth induction. Good muscle relaxant. Less hepatotoxicity or renal toxicity
Disadvantages: Stinky
What are the advantages, disadvantages and problems associated with Sevoflurane
Advantages: High potency, smells nice. Low blood:gas coefficient
Disadvantages: Chemical instability that may be toxic to kidneys
