Posolog
In this blog we are discussing about the what is the posology, what are the factor affecting posology, what is the importance of posology and Various rules to calculate paediatric dosage.
What are Factors affecting the dose of drug :
a. Age :
Human beings can be categorized into the following age groups :
1. Neonate : From birth up to 30 days
2. Infant : Up to 1 year age
3. Children between 1 to 4 years
4. Children between 5 to 12 years
5. Adult
6. Geriatric (elderly) patient
In children the enzyme system in the liver and renal excretion remain less developed. Show all the dose should be less than that of an adult. In elderly patients renal functions decline. Metabolism rate in the liver also decreases. Drug absorption from the intestine becomes slower in elderly patients. So in geriatric patients the dosege is less and should be judiciously administered.
b. Sex :
• in case of treatment with sex hormones, female adults generally require smaller doses then males due to the presence of more body fat.
• special care should be taken while administering any drug to a women during menstruation, pregnancy and lactation. strong purgatives should not be given in in menstruation and pregnancy.
• antimalarials, ergot alkaloids should not be taken during pregnancy to avoid deformation of foetus.
• Antihistaminic and sedative drugs are not taken during lactation because These drugs are secreted in the milk.
c. Body weight :
The usual doses for drugs are mentioned generally for 70 kg adult. the drug concentration at site of action is based on the ratio between the amount of drug administered and size of the body. the dose calculation for abnormally thin or obese patient required to calculate on the basis of body weight.
• also the moment and distribution of drug depends upon the proportion of muscles and adipose tissue. Because of high metabolic rate children requires higher dose per kg then adults.
d. Body surface area :
Many physiologic factor such as plasma volume, oxygen consumption and requirement of body fluid electrolytes, calories and glomerular filtration are proportional to the surface area. the surface area is also used to calculate dose. we for example anticancer drug, methotrexate administered according to body surface area.
e. Severity of disease :
• it is a common experience that Dull headache may be necessitate administration of 2-3 tablets of the same drug. But this is no true in all cases. For example in case iron deficiency anemia, the dose of iron salts administered orally remains the same irrespective of severity because there is a limit to which iron can be absorbed from the intestine daily and incorporated in haemoglobin.
f. Routs of administration:
In general, the rapidity of absorption of a drug decrease with route of administaration in the following order :
Intravenous > Intramuscular > Subcutaneous > oral
Thus in general, intravenous dose of a drug is smaller than it's Intramuscular or subconscious or oral dose.
Example : Dose of ergotamine for various routes are as follows.
a. Oral : 2 to 5 mg
b. Intramuscular : 1 mg (about to 1/2 of oral dose)
c. Intravenous : 0.25 mg (about to 1/8 of oral dose and % of 1M dose)
g. Time of administaration :
The drugs are most quickly absorbed from empty stomach. The presence of food in the stomach delays the absorption of Drugs. Hence a potent drug is given before meal. An irritation drug is given after meal so that the drug is diluted with food and thus produce less irritation. Biological half-life of a drug i.e. the time required for the main factor governing frequency of drug administration. For example, if the biological half-life of sulphadiazine is 4 hours, 1 g of the drug has to be given every 4 hours after initial dose of 2 g. But in certain instances e.g., reserpine as a tranquilliser, biological half-life of a drug has no relation to frequency of administaration.
h. Frequency of drug administration :
Biological half-life of a drug i.e., the time required for the blood level to drop down to 50% of the initial peak level is the main factor governing frequency of drug administration. For example, if the biological half-life of sulphadiazine is 4 hours, 1 g of the drug has to be given every 4 hours after initial dose of 2 g. But in certain instances e.g., reserpine as a tranquilliser, biological half-life of a drug has no relation to frequency of administaration.
i. Health and nutrition :
Debilitated and anaemic patients are, in general, more sensitive to the toxic effect of drugs and hence they are given smaller doses. Person with severe anaemia associated with hookworm information are more susceptible to the toxic effect of tetrachloroethylene. Myxoedematous patients are known to show less response to drugs like amphetamine because of low cellular metabolism.
j. Pathological state :
If the organs, through which biotransformation or excretion takes place, are diseased then smaller doses is indicated. For example, in case of renal insufficiency, phenobarbitone should be given in smaller dose and in case of patients suffering from liver disease, morphine should be given in smaller dose. Aspirin has no effect on normal body temperature but lowers body temperature in fevered patients. Quinine precipitates black water fever more often with falciparum malaria than otherwise.
k. Psychological state :
Psychological state of mind can affect the response of a drug, e.g. a nervous and anxious patient requires more general anaesthetics. Placebo is an inert substance that does not contain any Drug. Commonly used placebos are lactose tablets and distilled water injections. Sometimes patients get some psychological effects from this placebo, placebo are more often used in clinical trials of drugs.
What is the importance of Posology :
The minimum dose may be defined as a dose that is needed to produce an intended therapeutic response at the same time the maximum dose may be defined as the amount of API / drug that can be easily tolerated by an average individual.
The actual weight, body surface area, patient sex , patients symptoms, his medical history and the factors like tolerance, idiosyncrasy, routes of drug administration, time of drug administration etc.
Posology is an important and the valuable tool that helps to doctors and pharmacist to evaluate and determine the medicine dosages.
Posology is a vital part of medical and pharmaceutical practice and study, not just for humans, but also for animals. It is the science of determining and understanding drug dosage, as based on research into a huge number of factors. To a limited or extensive degree, Posology may be studied by anyone who wants to become a doctor, nurse, pharmacist, veterinarian, or pharmacologist.
Medication are great, and they have been invaluable to the sum of human medicine. They may cure some disease completely, and they may greatly reduce symptoms of others. The issue of exactly what amount to give is one that must be carefully considered. Digitalis, for instance, can improve heart function, and it can kill people easily. Deciding how to give the right dose is thus extremely important, and those administering this drug need to know to how to prescribe it appropriately.
The science of dosage is not very straight forward. Drugs may react differently with different people, populations, ages, genders and if people take other medications at the same time. Posology is a valuable tool. It helps to prevent what could be devastating drug accidents by finding out in advance how much medication can and should be administered. The study of also informs medical professionals exactly how to dose for each patient. Thought the name is not well known of the medical field, those in medicine must respect it as a means of ensuring safety of patients and better quality of care.
Various rules to calculate paediatric dosage :
Most reference manuals for medications focus primarily on the adult dosages. But children can require very different medication dosages compared to adults. In order to calculate. Paediatric Dose, young's rules, and Clark's rules may be used to determine the correct dosege of medication for a pediatric patient.
1. Nomogram method :
The Nomogram method is utilized to determine the correct pediatric medication dosage based on body surface area (BSA). It takes into consideration the surface area of the average adult (150-154 Ibs). Since it is formulated on the patient's height and weight, the Nomogram method is the best method.
Child's dose = Child's BSA in metres/ 1.73 × Adult dosage
It the height and weight of the patient are known, you can calculate the child's BSA are this line: Body surface area calculator.
2. Clark's rules:
Clark's rules is an obsolete rule for an approximate child's dose but is not used clinically.
• It is a favorite dosage calculation formula for paediatric nursing instructors.
• It determines the approximate dose of medicine for a child of two years age or older by dividing the child's weight in pounds by 150 and multiplying the result by the adult dose.
Child's dose = Weight of child (in Ibs) / 150 × Adult dosage
3. Young's Rule :
A rule for calculating the correct dose of medicine for a child by adding 12 to the child's age, dividing the sum by the child's age, then dividing the adult dose by the child's age, then dividing the adult dose by the figure obtained.
Child's dose = Age of child (in years) / (age + 12) × Adult dosage
4. Friends Rule :
A method of estimating the dose of medication for a child by multiplying the adult dose by the child's age in months and dividing the result by 150.
• Friends rule is generally reserved for use in children less than two years of age.
Child's dose = Age of child (in months) / 150 × Adults dosage
Clark's rules is an obsolete rule for approximate child's dose but is not used clinically.
• It is a favorite dosage calculation
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