ANESTHETICS-INTRODUCTORY
Anesthesia can be defined as a medical state which is composed of one or more of the following components of states such as analgesia(relief from the pain), state of muscle relaxation(paralysis), and loss of consciousness.
The patient who is given an anesthetic agent is said to be Anesthetized.
A nursing staff who is not a professional if give an anesthetic to a patient is known as an Anesthetist as in U.S.and North America (including Canada)
A professional who gives an anesthetic to a patient is known as Anesthesiologist.
Medical anesthesia can be divided into two types as 1. General and 2.Local.
General anesthetics are usually given by inhalation or through intravenous.
Local anesthetics are generally injected at the operative site to block the nerve conduction.
Stages of General Anesthesia
1.Analgesia or Stage of Induction:-
Loss of senses to feel pain. Consciousness is retained. The patient can talk.
2.Stage of Excitation:-
Delirium, a mental state of excitement, restlessness, incoherence with sweat.
3.Stage of Surgical Anesthesia:-
The patient is unconscious sufficiently to conduct surgery on him. Regular respiration is safely retained, with muscle paralysis and decreased vasomotor responses to painful impulses.
4.Stage of Medullary Paralysis:-
An irreversible stage if unattended, in which respiratory depression followed by coma and death occurs.
The pharmacokinetics of drugs plays an important role in attaining the different stages.
A slow-acting anesthetic like ether can give a well-distinguished stage. A fast-acting anesthetic would lead to the rapid occurring of the stages.
Induction of Anesthesia can be well defined as from the time of administration of the anesthetic
up to the achievement of the state of surgical anesthesia.
The induction goal can be achieved as quickly as the drug reaches the central nervous system (CNS)
The complication of inducing anesthesia can be avoided by first injecting an ultra fast-acting and ultra short-lived anesthetic such as Propofol through I.V. so that the patient will rapidly progress through the first and second stages so that the anesthesia can easily enter into the state of surgical anesthesia.
The recovery from the anesthesia is the reverse process of induction. This depends on how fast the drug is removed from the CNS.
There are five factors influence the inhalation anesthesia as follows:-
1.Solubility
2.Pulmonary ventilation
3.The partial pressure of the inhaled drug
4.Alveolar blood flow.
5.Arteriovenous concentration gradient.
1. Solubility-This factor affects the rate of induction by the blood-gas partition coefficient. A low coefficient implies low solubility and a low soluble agent require fewer molecules to raise the partial pressure of the agent in the blood. Thus the equilibrium between the arterial partial pressure and the alveolar partial pressure is attained rapidly which leads to rapid induction. Recovery is similarly hastened by discontinuing the agent.
2. Pulmonary Ventilation-The rate and depth of lung ventilation (the minute ventilation) affect the rate of increase in the partial pressure of the anesthetic in the blood. An increase in minute ventilation results in an increased amount of agent and this effect is very important for agents with low solubility because they require a higher amount of the agent to attain equilibrium.
3. Partial Pressure-An increased concentration of the drug in the inhaled air mixture leads to higher concentration in the alveoli and thus increases the arterial partial pressure of the drug. Therefore a greater concentration is given initially to speed induction and then it is reduced to a maintenance level.
4. Alveolar Blood Flow-Increased blood flow causes more rapid uptake of the agent which attain quicker effect on the CNS.
5. Arterio-Venous Concentration Gradient-This depend on the tissue uptake of the drug. If more tissue uptake of the agent is there then there is a decrease venous concentration of the agent, which leads to a longer time to achieve an equilibrium between the arteriovenous concentration.
Factors Affecting Tissue Uptake Of the Anesthetics:-
1.Blood Partition Coefficient
2.Rate of blood flow to the tissue
3.Concentration Gradients between artery and vein.
Highly perfused tissues like the brain, heart, liver, and kidneys will exert greater influence on the arteriovenous concentration gradient.
Skin and skeletal muscles are low perfused and hence they exert a low influence.
Mechanism
The mechanism of action of the GA depends upon the availability of the agent in molecules. All anesthetics are assumed by acting on the action potential by nullifying them through increasing the threshold. Also acting by inhibiting the entry of sodium ion by decreasing the membrane permeability to them. The mechanism is not clear but on assumption, only.No receptors are involved inbound up with anesthetics.
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