NEET MDS Lessons
Pharmacology
Laxatives and cathartics (purgatives)
Constipation is a common problem in older adults and laxatives are often used or overused. Non drug measures to prevent constipation (e.g. increasing intake of fluid and high–fiber foods, exercise) are much preferred to laxatives.
Laxatives and cathartics are drugs used orally to evacuate the bowels or to promote bowel elimination (defecation). Both terms are used interchangeably because it is the dose that determines the effects rather than a particular drug. For example, Castor oil laxative effect = 4ml while Cathartic effect = 15-60ml
The term laxative implies mild effects, and eliminative of soft formed stool. The term cathartic implies strong effects and elimination of liquid or semi liquid stool.
Laxatives are randomly classified depending on mode of action as:
1. Bulk-forming laxatives: are substances that are largely unabsorbed from the intestine.
They include psyllium, bran, methylcellulose, etc. When water is added, the substances swell and become gel-like which increases the bulk of the faecal mass that stimulates peristalsis and defecation.
2. Osmotic laxatives such as magnesium sulphate, magnesium hydroxide, sodium phosphate, etc. These substances are not efficiently absorbed and cause water retention in the colon. The latter causes increase in volume and pressure which stimulates peristalsis and defecation.
Lactulose is a semisynthetic disaccharide sugar that also acts as an osmotic laxative.
Electrolyte solutions containing polyethylene glycol(PEG) are used as colonic lavage solutions to prepare the gut for radiologic or endoscopic procedures
3. Stimulant (irritant) laxatives: these are irritant that stimulate elimination of large bowel contents. Individual drugs are castor oil, bisacodyl, phenolphthalein, cascara sagrada, glycerine, etc. The faeces are moved too rapidly and watery stool is eliminated. Glycerine can be administered rectally as suppositories.
4. Faecal softeners: they decrease the surface tension of the faecal mass to allow water to penetrate into the stool. They have detergent– like property e.g. docusate(docusate sodium, docusate calcium, and docusate spotassium. )
5. Lubricant laxatives e.g. liquid paraffin (mineral oil). It lubricates the intestine and is thought to soften stool by preventing colonic absorption of faecal water. They are used as retention enema.
6. Chloride channel activators
Lubiprostone works by activating chloride channels to increase fluid secretion in the intestinal lumen. This eases the passage of stools and causes little change in electrolyte balances. Nausea is a relatively common side effect with lubiprostone.
Clinical indications of laxatives
1. To relieve constipation.
2. To prevent straining.
3. To empty the bowel in preparation for bowel surgery or diagnostic procedures.
4. To accelerate elimination of potentially toxic substances from the GI tract.
5. To accelerate excretion of parasite after anti-helmintic drugs have been administered.
Dental implications of these drugs:
1. Adverse effects: gingival hyperplasia (phenytoin), osteomalacia (phenytoin, Phenobarbital), blood dyscrasias (all but rare)
2. Drug interactions: additive CNS depression (anesthetics, anxiolytics, opioid analgesics), induction of hepatic microsomal enzymes (phenytoin, Phenobarbital, carbamazepine), plasma protein binding (phenytoin and valproic acid)
3. Seizure susceptibility: stress can → seizures
FUNDAMENTALS OF INJECTION TECHNIQUE
There are 6 basic techniques for achieving local anesthesia of the structures of the oral cavity:
1. Nerve block
2. Field block
3. Infiltration/Supraperiosteal
4. Topical
5. Periodontal ligament (PDL)
6. Intraosseous
Nerve block- Nerve block anesthesia requires local anesthetic to be deposited in close proximity to a nerve trunk. This results in the blockade of nerve impulses distal to this point. It is also important to note that arteries and veins accompany these nerves and can be damaged. To be effective, the local anesthetic needs to pass only through the nerve membrane to block nerve conduction Field block/Infiltration/Supraperiosteal - Field block, infiltration and supraperiosteal injection techniques, rely on the ability of local anesthetics to diffuse through numerous structures to reach the nerve or nerves to be anesthetized:
- Periosteum
- Cortical bone
- Cancellous bone
- Nerve membrane
Topical - Topical anesthetic to be effective requires diffusion through mucous membranes and nerve membrane of the nerve endings near the tissue surface
PDL/Intraosseous - The PDL and intraosseous injection techniques require diffusion of local anesthetic solution through the cancellous bone (spongy) to reach the dental plexus of nerves innervating the tooth or teeth in the immediate area of the injection. The local anesthetic then diffuses through the nerve membrane
Classification
I) Esters
1. Formed from an aromatic acid and an amino alcohol.
2. Examples of ester type local anesthetics:
Procaine
Chloroprocaine
Tetracaine
Cocaine
Benzocaine- topical applications only
2) Amides
1. Formed from an aromatic amine and an amino acid.
2. Examples of amide type local anesthetics:
Articaine
Mepivacaine
Bupivacaine
Prilocaine
Etidocaine
Ropivacaine
Lidocaine
Structure of the CNS
The CNS is a highly complex tissue that controls all of the body activities and serves as a processing center that links the body to the outside world.
It is an assembly of interrelated “parts”and “systems”that regulate their own and each other’s activity.
1-Brain
2-Spinal cord
The brain is formed of 3 main parts:
I. The forebrain
• cerebrum
• thalamus
• hypothalamus
II. The midbrain
III. The hindbrain
• cerebellum
• pons
• medulla oblongata
Different Parts of the Different Parts of the CNS & their functions CNS & their functions
The cerebrum(cerebral hemispheres):
It constitutes the largest division of the brain.
The outer layer of the cerebrum is known as the “cerebral cortex”.
The cerebral cortex is divided into different functional areas:
1.Motorareas(voluntary movements)
2.Sensoryareas(sensation)
3.Associationareas(higher mental activities as consciousness, memory, and behavior).
Deep in the cerebral hemispheres are located the “basal ganglia” which include the “corpus striatum”& “substantianigra”.
The basal gangliaplay an important role in the control of “motor”activities
The thalamus:
It functions as a sensory integrating center for well-being and malaise.
It receives the sensory impulses from all parts of the body and relays them to specific areas of the cerebral cortex.
The hypothalamus:
It serves as a control center for the entire autonomic nervous system.
It regulates blood pressure, body temperature, water balance, metabolism, and secretions of the anterior pituitary gland.
The mid-brain:
It serves as a “bridge”area which connects the cerebrum to the cerebellum and pons.
It is concerned with “motor coordination”.
The cerebellum:
It plays an important role in maintaining the appropriate bodyposture& equilibrium.
The pons:
It bridges the cerebellum to the medulla oblongata.
The “locus ceruleus”is one of the important areas of the pons.
The medulla oblongata:
It serves as an organ of conduction for the passage of impulses between the brain and spinal cord.
It contains important centers:
• cardioinhibitory
• vasomotor
• respiratory
• vomiting(chemoreceptor trigger zone, CTZ).
The spinal cord:
It is a cylindrical mass of nerve cells that extends from the end of the medulla oblongata to the lower lumbar vertebrae.
Impulses flow from and to the brain through descending and ascending tracts of the spinal cord.
Properties of inhalation anesthetics
The lower the solubility, the faster the onset and the faster the recoverability.
All general anesthetics:
1. inhibit the brain from responding to sensory stimulation.
2. block the sensory impulses from being recorded in memory.
3. prevent the sensory impulses from evoking “affect”.
Most general anesthetic agents act in part by interacting with the neuronal membranes to affect ion channels and membrane excitability.
· If the concentration given is too low:
1. Movement may occur
2. Reflex activity present (laryngeal spasm)
3. Hypertension
4. Awareness
Premedication of analgesic drugs and muscle relaxants are designed to minimise these effects
· If the concentration given is too high:
1. Myocardial depression
2. Respiratory depression
3. Delayed recovery
Factors affecting onset and duration of action of local anesthetics
pH of tissue
pKa of drug
Time of diffusion from needle tip to nerve
Time of diffusion away from nerve
Nerve morphology
Concentration of drug
Lipid solubility of drug