Principle of General Anaesthetics Flashcards
What is General Anaesthesia?
Drug-induced REVERSIBLE loss of consciousness and all sensations
Goals of surgical anaesthesia?
-Muscle relaxation(motor reflexes)
-Loss of consciousness(pain and suffering)
-Automatic stabilisation(autonomic reflexes)
-Analgesia(pain stimulus)
Stages of general anaesthesia?
1) Analgesia: anaesthetic inhalation -> analgesia(amnesia) -> loss of consciousness
2) Delirium/Disinhibition: loss of consciousness -> surgical anaesthesia
(associated with excitement-shouting, crying, violent behavior)
3) Surgical anaesthesia(4 planes): patient is unconscious, has no pain reflexes; respiration and BP are regular
4) Medullary paralysis: occur in OD. Patient develops severe respiratory/CVS depression -> death
Why fast-acting GA used?
-to help reduce time spent in Delirium/Disinhibition stage
-facilitate entry to surgical anaesthesia stage
Properties of ideal GA agent?
-non-irritating, non-inflammable and inexpensive
-can be administered easily and in a controllable fashion
-gives a rapid and pleasant anaesthesia induction and recovery
-provides adequate analgesia, immobility and muscle relaxation
-have wide margin of safety, should not affect CVS functions
What is balanced anaesthesia?
-Ideal GA agent is yet to be found
-A combination of drugs is employed at different time points of the GA process to achieve the anaesthesia goals and to reduce the adverse effects:
-anaesthesia
-muscle relaxants
-pre-medications
-analgesics
Example of drugs for anaesthesia induction?
Sevoflurane, Propofol, Ketamine
Example of drugs for anaesthesia maintenance?
Sevoflurane, Nitrous Oxide, Propofol, Ketamine
Time course of inhalational anesthesia?
-Sufficient concentration or partial pressure of GA(effective anaesthetic tension) needs to be found in the brain to induce and maintain anaesthesia
-Depth of anaesthesia depends on potency of GA(MAC) and GA’s partial pressure in the brain
-Induction and recover depends on rate of change of partial pressure in the brain(how fast partial pressure rise and drops)
How to estimate the potency of inhaled anaesthetics?
-Estimated based on MAC
-Minimum Alveolar Concentration(MAC): alveolar concentration required to eliminate the response to a standardised painful stimulation in 50% patients
-more potent, small MAC(eg. halothane)
-less potent, bigger MAC(eg N2O)
Speed of induction of anaesthetic effects depends on?
- Blood solubility of GA agent (blood:gas partition coefficient, lambda) lower lambda, faster induction
-Arterio-venous concentration gradient(Pa-Pv), higher Pa-Pv, slow induction
-Inspired gas partial pressure (Fi), increase Fi, faster induction
-Alveolar ventilation rate(V), increase V, faster induction
-Pulmonary blood flow(Q), slower Q, faster induction
How does the Blood solubility of GA agent influences the speed of induction?
Insoluble GA(small lambda):
-minimally interacts with H2O
-saturate blood circulation or reach equilibrium very fast
-easy to leave blood into brain and fat tissue
-fast accumulation in brain
-faster induction
Soluble GA(big lambda):
-forms hydrogen bonds with H2O
-hard to leave blood into brain
-saturate the blood circulation or reach equilibrium slower
-slower accumulation in brain
-slower induction
How does the Arterio-venous concentration gradient(Pa-Pv) influences the induction of GA?
if Pa-Pv is high:
-more GA distribute into the tissue
-less GA found in venous blood
-less agents enters brain
-slow induction
How does the alveolar ventilation rate(V) affects the induction of GA?
-increase ventilation(V) -> Fa raised faster to the level of Fi
-the faster the GA induction occurs
How does the alveolar ventilation rate(V) affects the induction of GA?
-increase ventilation(V) -> Fa raised faster to the level of Fi
-the faster the GA induction occurs
