ANESTHESIA SPINAL

Neuroaxial Block

Spinal and Epidural Anesthesia

Spinal anesthesia is produced by injecting very low doses of LA (e.g., 50mg Lidocaine) via lumbar puncture into the cerebrospinal fluid below the termination of the spinal cord.

The LA diffuses in the CSF and produces dense anesthesia extending cephalic from the sacral dermatomes to as high as the lower cervical dermatomes.

A single dose of LA through fine-gauge (22- to 29-gauge) needle is usually injected, which results in a limited duration of anesthesia, but catheters can be placed in the subarachnoid space for prolonged anesthesia.

Spinal LA blocks somatic and visceral sensory afferent nerves, as well as efferent motor and autonomic (sympathetic) fibers. Sensory anesthesia, muscle relaxation and sympathetic block are the result of this technique

Spinal anesthesia is usually chosen for procedures performed in the dermatomes at or below the midabdonan.

Epidural anesthesia is produced by injection of LA into the epidural space outside the dura mater.

An epidural injection requires significantly more LA in both volume and dose than a spinal injection, but the injection can be made anywhere from the cervical epidural space to the sacral hiatus (caudal anesthesia).

The spinal nerves are blocked most intensely at the site of injection and less intensely both above and below that site.

Compared to spinal anesthesia, epidural anesthesia is often performed through a catheter introduced into the epidural space through a large (16- to 18-gauge) needle.

The catheter provides flexibility for prolonged anesthesia as well as prolonged analgesia for days into the postoperative period with infusion of dilute LA.

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ANESTHESIA 5

5. For intravenous regional anaesthesia

Perpheral Nerve Block
Upper Extremity
The most common LA's employed are 1% to 1.25% lidocaine or mepivacaine epinephrine for 2- to 4-hour duration of surgical block. Longer blockade (but slower onset) is achieved with 0.5% bupivacaine, levobupivacaine, or ropivacaine with epinephrine, which provide 4 to 12 hours of surgical block.

The technique of IVRA.
An intravenous catheter is placed in the hand, a tourniquet (usually a double cuff) is applied to the arm, the limb is exsanguinated by elevation and wrapping with an elastic bandage, and the proximal tourniquet is inflated well above arterial pressure. The bandage is removed and a dilute LA solution, usually 40 to 50ml of 0.5% lidocaine, is injected into the catheter. The LA gains access to nerves through the blood vessels supplying those nerves, and anesthesia occurs in 10 to 15 minutes and remains as long as the tourniquet is inflated.

Substances used are for local anesthesia:
a) Cocaine is used essentially for topical anaesthesia and thus
for operations in the nasal cavity in a concentration ranging
from 4-20%.
b) Procaine (Novocaine) the substance was introduced in 1905 and
is not effective topically but is much less toxic than
cocaine.
It can be used for subcutaneous infiltration in a 0.5%
solution; 2% solution is required for nerve blocks.
c) Lignocaine (Xylocaine) 0.5% solution is used for local
infiltration. 1-2% for nerve block. The relative advantages
and disadvantages of procaine and lignocaine can be
appreciated.
Lignocaine it is apparent, provides longer duration of
anaesthesia, however, has a lower maximum dose.

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REGIONAL ANESTHESIA LOWER LIMB

LOWER LIMB

Lumber plexus block (LPB) is performed from a posterior approach with a translumber, paravertebral needle placed to stimulate the plexus. A volume of 25 to 30 ml of LA provides adequate block.

An anterior approach to LPB is described at the groin with placement of the needle into the femoral nerve sheath and injection of 25 to 30 ml of LA.

The surgical requirements and ability to position a patient in the lateral position determine which approach is chosen. One or both halves of the nerve are identified, and 20 to 40 ml of LA injected to achieve blockade. The classic approach deep to the gluteal muscles is easily made at the same time.

Ankle block is a reliable technique that can used as sole anesthetic or for postoperative analgesia for procedures on the foot. The nerves to the foot diverge below the knee, and five separate nerve branches must be blocked for complete anesthesia as shown in. the entire ankle block requires 20 to 25 ml of LA.

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ANESTHESIA LOCAL REGIONAL

5.4 REGIONAL ANAESTHESIA
Technique used for the interruption of sensory pathways involves
the use of local anaesthetic agents at different sites in the
nervous system and can be used -

1. On mucosal surfaces
2. for subcutaneous infiltration
3. for nerve blocks

UPPER LIMB

Brachial plexus blocks above the clavicle include inter-scalene (ISB) and supraclavicular is performed at the level of the C6 transverse process, and 30 ml of LA is injected between the fascia of the anterior and middle scalene muscles after identification of the C5-C6 nerve roots. Rapid anesthesia of the superficial cervical plexus and upper toots of the brachial plexus ensures, so the technique is particularly well suited for shoulder procedures.

Brachial plexus blocks below the clavicle include infraclavicular (IFCB) and axillary (AXB) blocks.

The median nerve is located just medial to the brachial artery pulse, 2 to 3 cm proximal to the elbow flexion crease, and the radial nerve is located as it courses around the lateral supracondylar ridge of the humerus 3 to 4 cm proximal to the elbow crease. Each blocked with 5 to 10 ml of LA.

Wrist block of the three peripheral nerves is performed with 3 to 5 ml of LA per nerve, as shown. Finally, digital block can be performed for individual fingers with non-epinephrine-containing LA just distal to the metacarpophalangeal joint.

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AIRWAY DELIVERY SYSTEM

5.3 GENERAL ANAESTHESIA
The inhalation agent for anesthesia, is in most instances delivered
through an endotracheal tube

Short term barbiturates (pentothol) injected, into a vein intravenously.

This is referred to as induction, and enables the introduction
of the endotracheal tube with the help of direct laryngoscope


The tube is then connected with an anesthesia machine

5.3.1 TECHNIQUE
The dose of anesthetic to be given to any patient by inhalation
cannot be calculated based on milligrams of anesthetic per
kilogram body weight.

* In addition, there is a problem of maintaining sufficient
ventilation.
* Both these objectives are obtained by frequent clinical
examination of the patient so as to ensure sufficient level
of anaesthesia and adequate ventilation.
* The mixture of the anesthetic agent, and oxygen is adjusted
to provide sufficient anesthesia, without causing excess
depression of the central nervous system, by observing the
effect on reflexes.
* The ideal state to be maintained is a plane of anesthesia
in the 3rd state.
The ideal plane will depend upon the surgical procedure to
be performed.
* In all events, 4th stage, a stage of over dosage is to be
avoided.
These reflexes are listed, and the stages indicated, in the
Table.

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INDUCTION

5. GENERAL ANAESTHESIA
* Intravenous drugs
* Inhalation anesthesia

5.1 The intravenous route is used to provide:

* Unconscious state for induction of anesthesia ie. Thiopental, Etomedate, Ketamine
* Analgesia i.e. morphine, fentenil, alfentanil
* Muscle relaxant to facilitate intubations and condition of
Relaxed muscles for operation

5.2 In most instances, a combination of these two is utilised.
Intravenous routes, is used for inducing the anesthetic state.
The inhalation route is used for maintaining the anesthetic state.
The volatile inhalation anesthetics are:
* Diethyle ether
* Divinyle ether
* Chloroform
* Ethyle chloride
* Trichlorethylene
* Halothane
The gases used in inhalation anesthesia are:
* Nitrous oxide
* Cyclopropane
* Ethylene
Other gases used are:
* Oxygen
* Carbon dioxide

GAS EXCHANGE
These agents rely on the alveoli of the lungs, for exchange to blood for their action.
The final destination for actions is the C.N.S.
The partial pressure of these inhalational agents, has to be adequate in the C.N.S.
This is achieved through the partial pressure maintained
in the arterial blood by exchange in the alveolus.
The alveolar partial pressure is the first step towards achieving the
Anesthetic state.
In the same way on reversal can occur, if the inhalation agent is reduced, or eliminated from the respiratory circuit, the partial pressure in the alveolus fall back leading to a fall in the level, available in the CNS.

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CHECK OUT THE PATIENT

3. Undetected DISEASE AND RISK

3.1 ISCHAEMIC HEART DISEASE:
Patients with ischemic heart disease within three months of an
episode of Myocardial Infarction (MI) have a risk of 30%
possibility of a second MI.
Those between 4 and 6 months, the rate drops to 16%,those with an episode of more than 7 months the risk
drops to 6%.
Main complications are because of heart failure and arrhythmias.

3.2 HYPERTENSION
In cases of diastolic pressure of less than 110 mm Hg, there is
no increased risk in the absence of ischaemia of the myocardium.
Those with a diastolic pressure of more than 120 mm Hg, the risk
increases.

3.3 ENDOCRINE
Patients with a history of diabetes have an increased risk.
Those on steroid therapy, and with hypothyroidism, also carry a
high risk.
Hyperthyroid patients have a 2% chance of storm in the
Post operative period.

3.4 LIVER FUNCTIONS
Those with acute viral hepatitis have a 9% risk of mortality.
Those with acute fulminant hepatic failure have an 85% mortality.

3.5 PULMONARY
Infections, and chronic obstructive airway disease, increase the
risk of post-operative complications.
Thus, risk is increased in patients with asthma and with a
history of heavy smoking.

This information is required to set into motion an action plan
which is directed:

*Correct the effects of the disease process for which he
is undergoing surgery.
*Correct the effect of any undetected disease, so as
to reduce risk.
*Identify the need for intra-operative monitoring
devices.
*Identify correct choice of anesthetic technique to be
employed.
*Plan post-operative care.
*Prepare the patient emotionally for the anaesthetic
experience.

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