Tisdale: General Anesthetics Flashcards
General Anesthesia
Description:
Reversible CNS depression resulting in a collection of “component” changes
Unconsciousness, analgesia, amnesia, immobility and attenuation of autonomic responses
Classes: (2)
Inhalation
Intravenous
Balanced Anesthesia:
Balanced Anesthesia: use of combinations of intravenous and inhaled drugs; takes advantage of favorable properties of each while minimizing adverse effects
Minimum Alveolar Anesthetic Concentration (MAC):
MAC: alveolar concentration required to eliminate the response to a standardized painful stimulus in 50% of patients (1 MAC=prevents response in 50% of patients)
Measure of INHALED anesthetic potency
Potency= ?
Potency= 1/MAC (inversely related); therefore, lower MAC= higher potency
Each anesthetic has a defined MAC, but may vary among different patients depending on the following factors
Increase MAC: (6)
- Young age
- Hyperthermia
- CNS hypo-osmolality
- Habituation to alcohol
- CNS stimulants (dextroamphetamine, cocaine)
- Physostigmine
Decrease MAC: (10)
- Old age
- Hypothermia
- CNS hyperosmolality
- Acute effects of alcohol
- CNS depression (benzodiazepines, barbiturates, propofol)
- Tranquilizers (chlorpromazine)
- CNS effects of local anesthetics
- Narcotics
- Pregnancy
- Alpha 2 adrenergic agonists (clonidine, dexmedetomidine)
Effect on height, weight or sex on MAC:
NOT effected by height, weight or sex
What is the main determinant of the potency of an anesthetic
Oil:Gas Partition Coefficient (Lipid Solubility): main determinant of the potency of an anesthetic
Minimum Alveolar Anesthetic Concentration (MAC)
Oil:Gas Partition Coefficient (Lipid Solubility):
High lipid solubility on recovery from anesthesia:
High lipid solubility delays recovery from anesthesia (agent accumulates gradually in body fat and produces a prolonged Hangover)
High lipid solubility = high oil:gas partition coefficient = low MAC = high potency
Minimum Alveolar Anesthetic Concentration (MAC)
Potency of an Intravenous Agent:
Free plasma concentration that produces loss of painful stimulus in 50% of patients
Mechanism of Action
Current Theory:
Unitary Theory of Anesthesia:
Protein Theory of Anesthesia:
Current Theory: current research indicates the mechanism of action is a combination of two historical theories
Unitary Theory of Anesthesia: change in membrane dimension and/or change in membrane physical state
Protein Theory of Anesthesia: GAs directly interact with proteins
Dual Process Model of Anesthesia
GAs POTENTIATE:
GAs POTENTIATE the action of endogenous agonists at INHIBITORY receptors (GABA, glycine; stabilize OPEN state)
- Decrease the EC50 and increase the maximum response
Dual Process Model of Anesthesia
GAs INHIBIT:
GAs INHIBIT the action of endogenous agonists at EXCITATORY receptors (no depolarization of post-synaptic membrane, no AP; non-competitive inhibitor)
- Decrease the maximum response while leaving the EC50 unchanged
Inhaled Anesthetics
Effects on the Cardiovascular System:
Effects on the Cardiovascular System: most prominent effect is DECREASE in systemic arterial BP
Inhaled Anesthetics
Effects on the Respiratory System:
Exception:
Effects on the Respiratory System: reduction/elimination of ventilatory drive and reflexes to maintain patent airway
o Exception: nitrous oxide
Inhaled Anesthetics
Effects on the Brain:
Effects on the Brain: INCREASE cerebral blood flow (increases cerebral blood volume and ICP)
Inhaled Anesthetics
Effects on the Kidney:
Filtration Fraction= ?
Effects on the Kidney: DECREASE GFR and RBF and INCREASE filtration fraction (effects concentration dependent)
Filtration Fraction= GFR/RPF (renal plasma flow)
Inhaled Anesthetics
Effects on the Liver:
Effects on the Liver: DECREASE in hepatic blood (15-45% below baseline; concentration dependent decrease)
Malignant Hyperthermia
Description:
Description: autosomal dominant genetic disorder of skeletal muscles occurring in susceptible individuals undergoing general anesthesia with volatile agents and muscle relaxants (ie. succinylcholine)
o Rare but important cause of anesthetic morbidity and mortality
o Due to increase in free Ca++ concentration in skeletal muscle cells
Malignant Hyperthermia
Symptoms: (6)
Symptoms: rapid onset of o Tachycardia o HTN o Severe muscle rigidity o Hyperthermia o Hyperkalemia o Acid-base imbalance (acidosis)
Malignant Hyperthermia
Treatment:
Treatment: dantrolene
Nitrous Oxide
B:G:
O:G/MAC:
Nitrous Oxide: not used alone except for in dental procedures; only one that provides analgesia
o Low B:G: rapid induction/recovery
o Low O:G/MAC: low potency