Anesthesia

Contents :

Introduction to anesthesia.

Definition of anesthesia.

Classification of anesthesia.

General Anesthesia.

Purpose & Biochemical M/A of general anesthesia.

Stages of general anesthesia.

Risks of general anesthesia.

Complication of General anesthesia.

Side effects of general anesthesia.

Advantages & disadvantages of general anesthesia.

Local & regional anesthesia.

M/A of local anesthesia.

Classification of local anesthesia.

Methods of administration.

Side-effect/Complication of local anesthesia.

Advantages & disadvantages of local anesthesia

Commonly used medications.

Conclusion.

Introduction to anesthesia

The word anesthesia is coined from two Greek words: "an" meaning "without" and "aesthesis"

meaning "sensation". There are various types of anesthesia. Throughout their lives, most people

will undergo anesthesia either during the birth of their baby or for a surgical procedure, which

could range from relatively short, simple surgery on a day-stay basis through to major surgery

requiring complex, rapid decisions to keep them safe. Many of today's operations are made

possible as a result of developments in anesthesia and training of specialist anesthetists. Patients

having anesthesia will have an anesthetist with them all the way from the preoperative

assessment of their medical conditions and planning of their medical care, to closely monitoring

their health and wellbeing throughout their procedure to ensure a smooth and comfortable

recovery. Relief of pain and suffering is central to the practice of anesthesia. Despite an increase

in the complexity of surgical operations, modern anesthesia is relatively safe due to high

standards of training that emphasis quality and safety. In addition, there have been improvements

in drugs and equipment. Increased support for research to improve anesthesia has resulted in best

patient safety records in the world.

Anesthesia

Anesthesia refers to the practice of administering medications either by injection or by inhalation

(breathing in) that block the feeling of pain and other sensations, or that produce a deep state of

unconsciousness that eliminates all sensations, which allows medical and surgical procedures to

be undertaken without causing undue distress or discomfort.

Classification of anesthesia

Classified into 2 major types. They are :

1. General anesthesia.

2. Local & regional anesthesia.

1. General Anesthesia

General anesthesia is a medically induced state of unconsciousness with loss of protective reflexes,

resulting from the administration of one or more general anesthetic agents. It is carried out to allow

medical procedures that would otherwise be intolerably painful for the patient or where the nature of the

procedure itself precludes the patient being awake. A variety of medications may be administered, with

the overall aim of ensuring unconsciousness, amnesia, analgesia, loss of reflexes of the autonomic

nervous system, and in some cases paralysis of skeletal muscles. The optimal combination of drugs for

any given patient and procedure is typically selected by an anesthetist, or another provider such as a

physician assistant or nurse anesthetist (depending on local practice), in consultation with the patient and

the surgeon, dentist or other practitioner performing the operative procedure.

Purpose

Purpose of General anesthesia has includes:

1. Analgesia (loss of response to pain)2. Amnesia (loss of memory)3. Immobility (loss of motor reflexes)4. Hypnosis (unconsciousness)5. Paralysis (skeletal muscle relaxation)

Biochemical mechanism of action

Biochemical mechanism of action of general anesthetics is not well understood. To induce unconsciousness, anesthetics have myriad sites of action and affect the CNS at multiple levels. Common areas of the central nervous system whose functions are interrupted or changed during general anesthesia include the-

Cerebral cortex, Thalamus, Reticular activating system & Spinal cord.

Current theories on the anaesthetized state identify not only target sites in the CNS but also neural networks and loops whose interruption is linked with unconsciousness. Potential pharmacologic targets of general anesthetics are-

GABA -(Halothane has been found to be a GABA agonist) Glutamate-activated ion channels, NMDA receptor families -(ketamine is an NMDA receptor antagonist) Voltage-gated ion channels, Glycine & Serotonin receptors.

Stages of general anesthesia

Guedel's classification, introduced by Arthur Ernest Guedel in 1937 describes four stages of

anesthesia. Despite newer anesthetic agents and delivery techniques, which have led to more

rapid onset of – and recovery from – anesthesia (in some cases bypassing some of the stages

entirely), the principles remain.

Stage 1

Stage 1, also known as induction, is the period between the administration of induction agents

and loss of consciousness. During this stage, the patient progresses from analgesia without

amnesia to analgesia with amnesia. Patients can carry on a conversation at this time.

Stage 2

Stage 2, also known as the excitement stage, is the period following loss of consciousness and

marked by excited and delirious activity. During this stage, the patient's respiration and heart rate

may become irregular. In addition, there may be uncontrolled movements, vomiting, suspension

of breathing, and pupillary dilation. Because the combination of spastic movements, vomiting,

and irregular respiration may compromise the patient's airway, rapidly acting drugs are used to

minimize time in this stage and reach Stage 3 as fast as possible.

Stage 3

In Stage 3, also known as surgical anesthesia, the skeletal muscles relax, vomiting stops,

respiratory depression occurs, and eye movements slow and then stop. The patient is

unconscious and ready for surgery. This stage is divided into 4 planes:

1. The eyes roll, then become fixed.

2. Corneal and laryngeal reflexes are lost.

3. The pupils dilate and light reflex is lost.

4. Intercostal paralysis and shallow abdominal respiration occur.

Stage 4

Stage 4, also known as overdose, occurs when too much anesthetic medication is given relative

to the amount of surgical stimulation and the patient has severe brainstem or medullary

depression, resulting in a cessation of respiration and potential cardiovascular collapse. This

stage is lethal without cardiovascular and respiratory support.

Risks of general anesthesia

Overall, general anesthesia is very safe. Even particularly ill patients can be safely anesthetized,

it is the surgical procedure itself which offers the most risk.

Modern general anesthesia is an incredibly safe intervention. However, older adults and those

undergoing lengthy procedures are most at risk of negative outcomes. These outcomes can

include postoperative confusion, heart attack, pneumonia and stroke.

Some specific conditions increase the risk to the patient undergoing general anesthetic:

Obstructive sleep apnea - a condition where individuals stop breathing while asleep

Seizures

Existing heart, kidney or lung conditions

High blood pressure

Alcoholism

Smoking

History of reactions to anesthesia

Medications that can increase bleeding - aspirin, for example

Drug allergies

Diabetes

Obesity or overweight.

Death due to general anesthetic does occur, but only very rarely - roughly 1 in every 100,000-

200,000.

Complication of General anesthesia

Unintended intra-operative awareness

Unintended intra-operative awareness refers to rare cases where patients report a state of

awareness during an operation, after the point at which the anesthetic should have removed all

sensation. Some patients are conscious of the procedure itself and some can even feel pain.

Unintended intra-operative awareness is incredibly rare, affecting an estimated 1 in every 19,000

patients undergoing general anesthetic. Because of the muscle relaxants given alongside

anesthesia, patients are unable to signal to their surgeon or anesthetist that they are still aware of

what is happening. Unintended intra-operative awareness is more likely during emergency

surgery. Patients that experience unintended intra-operative awareness can suffer long-term

psychological problems. Most often, the awareness is short-lived.

According to a recent large-scale investigation of the phenomenon, patients experienced

"tugging, stitching, pain, paralysis and choking," among other sensations.

Because unintended intra-operative awareness is so infrequent, it is not clear exactly why it

occurs. The following are considered to be potential risk factors:

Heart or lung problems

Daily alcohol use

Emergency surgery

Cesarean section

Anesthesiologist error

Use of some additional medications

Depression.

Pre-surgical evaluation

Side effects of general anesthesia

Side effects of general anesthesia include -

Confusion and memory loss - (more common in the elderly)

Dizziness

Difficulty passing urine.

Bruising or soreness from the IV drip.

Nausea and vomiting (Most common)

Shivering and feeling cold.

Sore throat (due to the breathing tube).

Advantages of general anesthesia

Reduces intra-operative patient awareness and recall.

Allows proper muscle relaxation for prolonged periods of time.

Facilitates complete control of the airway, breathing, and circulation.

Can be used in cases of sensitivity to local anesthetic agent.

Can be administered without moving the patient from the supine position.

Can be adapted easily to procedures of unpredictable duration or extent.

Can be administered rapidly and is reversible.

Disadvantages of general anesthesia Requires increased complexity of care and associated costs.

Requires some degree of preoperative patient preparation.

Can induce physiologic fluctuations that require active intervention.

Associated with less serious complications such as nausea or vomiting, sore throat,

headache, shivering, and delayed return to normal mental functioning.

Associated with malignant hyperthermia, a rare, inherited muscular condition in

which exposure to some (but not all) general anesthetic agents results in acute and

potentially lethal temperature rise, hypercarbia, metabolic acidosis, and hyperkalemia.

2. Local & regional anesthesia

Local anesthesia is the reversible loss of sensation in a defined area of the body and is achieved

by the topical application or injection of agents that block the generation and/or journey of nerve

impulses in tissue.

Regional anesthesia is essentially local anesthesia but covering a larger area of subcutaneous

tissue or larger peripheral nerves.

Chemically they are weak bases formed of lipophylic group connected to ionizable hydrophilic

group by an intermediate chain.

Local anesthesia is used in many dermatological procedures and surgical operations. The aim is

to minimise pain and suffering and maximise patient comfort.

Mechanism of action local anesthesia

- They act from inside the nerve & inhibit Na influx (membrane stabilization)

- Fibers are affected in this sequence (Sensory, cold, touch, pressure & lastly motor) &

unmyelinated before myelinated.

- Recovery occurs in the reverse direction.

Classification of local anesthesia

According to their chemical structure they are classified into 2 types,

(i) Amides

Lidocaine (most frequently used)(effective, acts rapidly)

Dibucaine

Prilocaine

Mepivacaine

Bupivacaine

(ii) Esters

Cocaine Procaine Tetracaine Benzocaine

According to their solubility and therapeutic application they are classified into 3 types,

(i) Soluble L.A suitable for injection:

Lidocaine Dibucaine Procaine Tetracaine

All these can produce surface anesthesia except Procaine which is effective only by injection.

(ii) Soluble L.A used only topically:

Cocaine Phenacaine Butacaine

Mainly used to produce topical anesthesia of the Eye.

(iii) Insoluble L.A:

Benzocaine Orthoform

Used as surface anesthetics in the form of powders and ointments for wounds.

Methods of administration

1. Surface anesthesia

- By direct application for skin & mucous membrane

2. Infiltration anesthesia

- By S.C injection to reach fine nerve branches and sensory nerve terminals.

3. Nerve block anesthesia

- By injection close to the appropriate nerve trunks (Brachial plexus) to produce a loss of

sensation peripherally.

4. Sympathetic block

- It is injected around sympathetic ganglion.

5. Para vertebral block

- It is injected around spinal roots as they emerge from the paraverterbal foramina.

6. Epidural anesthesia

- The LA is injected in the epidural space,between the dura & bony spinal canal

containing fat & connective tissue.

- It can be performed in sacral hiatus (Caudal anesthesia)

7. Spinal

- The LA is injected in the subarachnoid space in the lumbar region

- The level of spinal anesthesia depends upon:

i. Posture of the patient. ii. Specific gravity of the injected solution.

Modification

Adrenaline (epinephrine) is sometimes added to local anesthetic formulations. It is used to:

Prolong duration of anesthesia.

Reduce systemic absorption.

Reduce surgical bleeding.

Increase the intensity of blockade.

Can anyone be allergic to local anesthetics?

True allergy to local anesthetics is rare, the estimated rate of allergic reactions caused by these

agents is less than 1%. Often an adverse reaction is the result of the rapid rise in circulating local

anesthetic, or the absorption of adrenaline, or an allergy to the preservative (most local

anesthetics contain parabens preservative).

Aminoesters are more allergenic than aminoamides because of their cross-reactivity to other

drugs of the para-aminobenzoic acid ester type. Patients with a history of allergy to benzocaine,

sulphonamides, paraphenylenediamine or other para-type substances should avoid using

aminoester local anesthetics.

Prior exposure to parabens or para-aminobenzoic acid may sensitize you to local anesthetics

containing these substances. In such cases, preservative-free aminoamide local anesthetics such

as lignocaine (lidocaine) can be used.

Side effects / Complications of local anesthetic

Local side effects such as bruising and a temporary sensation of stinging or burning are common.

When administered correctly the chances of more serious side effects occurring are minimal.

Signs and symptoms of systemic toxicity include:

Severe numbness or tingling

Dizziness and drowsiness

Tinnitus (ringing in the ears)

Slurred speech

Metallic taste in mouth

Mental status change

Muscle twitching

Which local anesthetic is right for us?

The choice of which anesthetic to use depends on a number of factors:

Patient factors

Age

Pregnancy status

History of allergies

Other medical conditions such as renal or hepatic failure, cardiac problems

Current medications

Procedure being performed

Consider site

Consider area involved

Consider duration of operation

Doctor's own preference and experience.

Advantages of Local anesthetic

During local anesthesia the patient remains conscious.

Patient maintains own airway.

Aspiration of gastric contents unlikely.

Recovery is smooth as it requires less skilled nursing care as compared to other

anesthesia like general anesthesia.

Postoperative analgesia.

There is reduction surgical stress.

Earlier discharge for outpatients.

Expenses are less.

Disadvantages of Local anesthetic Sometimes patient may prefer to be asleep.

It needs a practiced and skilled person for the best results.

Some blocks require up to 30 min or more to be fully effective.

It is also possible that analgesia may not always be totally effective. May be the patient

requires additional analgesics, IV sedation, or a light general anesthetic.

Sometimes toxicity may occur if the local anesthetic is injected intravenously or if the

overdose is given Operation like thoracotomies is not suitable for local anesthetics.

Commonly used medications

Volatile anesthetics

All are bronchodilators, except for desflurane which is irritating and may cause

bronchospasm. Administered alone (i.e., without narcotics), inhaled anesthetics increase

respiratory rate but decrease tidal volume.

Except for halothane, inhaled anesthetics are not metabolized by the body and are

eliminated by ventilation.

All volatile anesthetics (but not nitrous oxide) are capable of triggering malignant

hyperthermia (MH).

While in many cases volatile anesthetics are used for maintenance of anesthesia, in some

circumstances these drugs may be chosen to induce anesthesia such as in pediatrics cases

in which the child may not tolerate IV placement awake.

Halothane

PRO Cheap, nonirritating so can be used for inhalation induction

CON Long time to onset/offset, Significant Myocardial Depression, Sensitizes

myocardium to

catecholamines, Association with Hepatitis

Isoflurane

PRO Cheap, excellent renal, hepatic, coronary, and cerebral blood flow

preservation

CON Long time to onset/offset, irritating so cannot be used for inhalation

induction

Desflurane

PRO Extremely rapid onset/offset

CON Expensive, Stimulates catecholamine release, Possibly increases

postoperative nausea and vomiting,

Requires special active-temperature controlled vaporizer due to high vapor

pressure, Irritating so cannot be used for inhalation induction

Sevoflurane

PRO Nonirritating so can be used for inhalation induction. Extremely rapid

onset/offset.

CON Expensive. Due to risk of “compound A” exposure must be used at flows >

2 L/min. Theoretical potential for renal toxicity from inorganic fluoride

metabolites.

Nitrous Oxide

PRO Decreases volatile anesthetic requirement, Dirt cheap, Less myocardial

depression than volatile agents

CON Diffuses freely into gas filled spaces (bowel, pneumothorax, middle ear,

eye, Decreases Fi02, Increases

pulmonary vascular resistance

Iv anesthetics

Most sedative hypnotics work through the inhibitory gamma-aminobutyric acid (GABA)

neurotransmitter system in which increased chloride conductance leads to neuronal inhibition.

Most IV induction agents bind to a specific site called GABAA for this inhibitory effect, and

they have a rapid onset due to lipophilic properties which allow them to quickly partition into the

highly perfused lipophilic brain and spinal cord. They also have short duration of action, with

their termination of effect due to redistribution into less perfused tissues such as muscle and fat.

Barbiturates (e.g., thiopental)

Decrease ICP by decrease in cerebral oxygen consumption. Since cerebral perfusion is

preserved, desirable drug for neurosurgery cases. Causes respiratory and cardiac depression.

PRO Excellent brain protection, Stops seizures, Cheap

CON Myocardial depression, Vasodilation, Histamine release, Can precipitate

porphyria in susceptible

patients

Propofol

In adults, induction dose 1.5 to 2.5 mg/kg while continuous infusion of 100 to 200

micrograms/kg/min maintains unconsciousness. These values differ for children and for the

elderly.

PRO Prevents nausea/vomiting, Quick recovery if used as solo anesthetic agent

CON Pain on injection, Expensive, Supports bacterial growth, Myocardial

depression (the most of the four),

Vasodilation, cross reactivity in patients with egg allergy.

Etomidate

Minimal depression of cardiovascular and pulmonary function. Ideal for patients with CVD or

hemodynamic instability.

Induction dose of 0.2 to 0.4 mg/kg that causes pain on injection and myoclonus. Suggested that it

may suppress cortisol synthesis.

PRO Least myocardial effect of IV anesthetics

CON Pain on injection, Adrenal suppression (? significance if used only for

induction), Myoclonus, Nausea/Vomiting

Keratin

Works via antagonism of the N-methyl-D-aspartate receptor channel complex. Minimally

depresses the cardiorespiratory system. Induction dose of 1 to 2 mg/kg in adults. Directly

stimulates SNS and increases BP and heart rate. Increasing demand on the heart and is not a

good choice for CAD patients.

PRO Works IV, PO, PR, IM - good choice in uncooperative patient without IV,

Stimulation of SNS → good for hypovolemic trauma patients, often

preserves airway reflexes

CON Dissociative anesthesia with postop dysphoria and hallucinations, Increases

ICP/IOP and CMR02, Stimulation of SNS → bad for patients with

compromised cardiac function, increases airway secretions

Dexmedetomidine

Selective alpha-2 adrenergic agonist, which is used in the operating room as an adjunct to

general anesthesia, or to provide sedation for awake fiberoptic intubation or for regional

anesthesia. It is generally given as a loading dose of 0.5-1 mcg/kg over 10 minutes, followed by

an infusion of 0.2 to 0.7 mcg/kg/hr. It produces sedative-hypnotic and analgesic effects without

causing respiratory depression.

Benzodiazepines (BDZ)

Usually provided as premedication for sedation and anxiolysis before general anesthesia.

Properties include anxiolytic effects to sedation and unconsciousness at higher doses. Midazolam

(Versed) induction dose of 0.1 to 0.2 mg/kg and infusion rates of 0.25 to 1 microgram/kg per

minute. BDZs produce respiratory, cardiovascular, and upper airway reflex depression and in the

presence of hypovolemia, may cause significant hypotension. Reversal of the sedative action of

these compounds with the competitive antagonist, flumazenil.

Opioids

Morphine

Depresses breathing principally by impairing the medullary response to CO2. Also trigger the

chemoreceptor trigger zone (CTZ) which may lead to nausea, and may in turn stimulate the

vomiting center and produce emesis. Also, morphine decreases GI motility and propulsion,

produces urinary retention, and releases histamine by stimulating basophils in the lungs and mast

cells in the skin. In the CVS, morphine may produce vascular dilation, decrease SVR, and

overall hypotension. It is long acting & renally excreted → active metabolite has opiate

properties, therefore beware in renal failure

Demerol

Euphoria, stimulates catecholamine release, so beware in patients using MAOI’s, renally active

metabolite associated with seizure activity, therefore beware in renal failure

Conclusion

In conclusion we can say that for healthy patients undergoing a planned operation, general

anesthesia is about as dangerous as pregnancy in a healthy woman. In other words, general

anesthesia is very safe. However, the poorer the health of a person, the older they are, and the

higher the risk of the operation - the greater the chance of dying as a result of anesthesia and

surgery. There are actually very few conditions where anesthesia is likely to be lethal for a

patient, e.g. extremely severe aorta stenosis, major coronary artery stenosis, someone in deep

shock, etc. Fortunately, these conditions occur very seldom. In general, for nearly all people, as

the discussion above clearly demonstrates, anesthesia is very safe and far less dangerous than the

effects of surgery.

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