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Screening of Local Anaesthestics


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Screening of Local Anaesthestics

  1. 1. Screening of Local Anaesthestics Tulasi Raman P.
  2. 2. 1. Conduction Anaesthesia 2. Infiltration Anaesthesia 3. Surface Anaesthesia 4. Epidural Anaesthesia 5. Spinal Anaesthesia
  3. 3. Conduction Anaesthesia in the Sciatic Nerve of the Rat • Animals: Wistar or Sprague Dawley rats • Weight: 125 to 175 g
  4. 4. • Procedure: – The animal is suspended in a prone position by grasping the base of the tail and thoracic cage. – A hind limb is extended to its full length and the depression for needle insertion is located by palpation with the left index finger. – The site of injection is the area under the skin at the junction of the biceps femoris and the gluteus maximus muscles.
  5. 5. – The sciatic nerve is blocked in the midthigh region with 0.2 ml of the drug solution administered by a 24- to 25-gauge needle attached to a 0.25 ml tuberculin syringe. The other leg is used for a control drug. – Immediately after the injection, repeated checks of the digit of the foot and the walking behavior are performed.
  6. 6. – In the normal foot, the digits are wide apart, while in the blocked leg the digits of the foot are close together. – The successful block is evidenced by dragging of the leg and an inability of the animal to use the leg in walking up the inclined wire mesh cover of the cage. – After the time of block for each leg is noted, each animal is examined every 5 to 10 min in order to note the time of recovery.
  7. 7. Infiltration Anaesthesia in Guinea Pig’s Wheals • Animals: Adult Guniea Pigs • Weight: 250 – 300 g
  8. 8. • Procedure: – On the day preceding the experiment two areas of 4–5 cm diameter are shaved. – The sensitivity of the skin is greatest in the midline and slightly more so in the front than in the back area. For this reason each concentration of a local anesthetic must be tested in both areas. – The doses of local anesthetics are injected intracutaneously in 0.1 ml saline. – The size of the wheal is marked with ink. One side is used for the test preparation, the other side for the standard.
  9. 9. • The reaction to pin prick is tested 5 min after injection in the following way. • After observing the animal’s normal reaction to a prick applied outside the wheal, six pricks are applied inside the wheal and the number of pricks is counted to which the guinea pig fails to react. • The pricks are applied at intervals of about 3– 5 s. Six pricks are applied every 5 min for 30 min.
  10. 10. Surface Anaesthesia on the Cornea of Rabbits • Animals: Albino Rabbits • Weight: 2.5 – 3 kg
  11. 11. • Procedure: – The upper and lower eyelashes are carefully clipped. – The conjunctival sac of one eye is held open, thus forming a pocket. – From a 1 ml syringe with a 22-gauge needle, 0.5 ml of a solution of the anesthetic is applied into the conjunctival sac for 30 s. Then the procedure is repeated, so that 1.0 ml is applied within 1 min. One ml of the standard is applied to the other eye.
  12. 12. • Effective local anesthetics extinguish the corneal reflex (blinking) elicited by any touch of the cornea. • The test is started 5 min after application of the drug and repeated every 5 min until anesthesia vanishes and blinking occurs again. • The time between disappearance and reappearance of the corneal reflex is registered.
  13. 13. Epidural Anesthesia in Rabbits • Animals: Adult Rabbit • Weight: 2.5 – 3 kg • A “loss of resistance technique” is employed in caudal epidural injection.
  14. 14. • Procedure: – The rabbit was carefully restrained by an assistant. – The thumb and the middle finger of the left hand were placed on the two crests and the left index finger used to palpate the midline, L7 spine and the depression over the lumbosacral fossa. – With the index finger in position on the L7 spine to serve as a guide, a short bevel 1.5 cm 20 gauge spinal needle was introduced a right angles to the skin in the midline with the bevel aligned longitudinally.
  15. 15. • After passage through the skin, only minor resistance was felt until the ligamentum flavum was reached. When passing through the ligament, a definite “pop” was felt and resistance to advancement of the needle was lost. • The stylet was then withdrawn and the hub inspected for the presence of blood or cerebrospinal fluid. If absent, the needle was rotated through 90° to direct the bevel caudally, a 1.0 ml syringe was attached and 0.1 ml of air injected. • A syringe containing the desired dose of the local anesthetic was attached and the solution injected over a 5– 10 s period. • The pharmacodynamic responses were assessed by (1) sensory loss,(2) loss of weight-bearing ability, and (3) flaccid paresis.
  16. 16. Spinal Anesthesia in Rats • Animals: Sprague-Dawley rats • Weight: 50–75g • The rat is held firmly by the pelvic girdle.
  17. 17. • Procedure: – A 30-gauge needle is attached to a 25µl Hamilton syringe is inserted into the tissue on one side of the L5 or L6 spinous process at an angle of about 20°. – The needle is advanced to the groove between the spinous and transverse processes and then moved forward the intervertebral space at an angle of about 10°. – Correct placement of the needle is indicated by an arching of the tail. – Drugs are dissolved in saline or water and administered in a volume of 5 µl.
  18. 18. • Antinociception is determined in a modification of tail flick assay in rats by placing the tail of the rat under a focused radiant heat source. • The degree of antinociception is defined as the percentage of maximum possible effect. • This percentage is determined for each dose at each time measured allowing to calculate ED50 values.
  19. 19. References: • Drug Discovery and Evaluation: Pharmacological Assays, 3rd Edition; by H. Gerhard Vogel
  20. 20. Thank You