The doses given in the section on pharmaceutical preparations are the ordinary average amounts for an adult of normal weight and size. These doses are considered safe and ordinarily efficient, but they may be increased or diminished as the exigencies of the case require.
Conditions Modifying the Size of the Dose.
3. The existing conditions to be combatted, or the object to be attained.
4. The frequency of the dose.
5. The duration of the action of the drug - i. e., the length of time it will take for it to be excreted.
1. Age. - This is of most importance in children, and late, in life the dose must again become smaller than the adult dose.
Various rules have been formulated for computing the dose proper for a child. Young's formula, perhaps one of the best, is that the fraction of the adult dose proper for a given child shall be that obtained by dividing the age of the child by the age plus 12. Thus, the dose for a child three years old would be 3/3+12=1/5 of the adult dose.
The following table will simplify for the student the computation of the proper dose for a child. If a child's age is between the ages given in the table,. a little less or a little more than the dose computed can be very quickly decided upon. The table will be found perfectly safe, except in the case of narcotics for young children.
The fact should be emphasized that however careful we are in calculating the dose for a child according to its age, it is the weight of the child that decides the dose more than the actual number of years the child has lived. Thus, a child five years old, weighing no more than an ordinary child of three, should receive the proper dose for its weight and not for its age.
Author's Table for Computing the Dose According to Age.
At 20 years, the adult dose.
At 10 years, 1/2 the age, 1/2 the dose.
At 5 years, 1/4 the age, 1/4 the dose.
At 2 1/2 years, 1/8 the age, 1/8 the dose.
At 1 year, 1/12 the dose. 2. Weight. - The actual weight of the body is really the only scientific factor in determining the exact dose. However, in the physician's ordinary daily routine the determination of the exact weight is impracticable. It must be remembered, however, as above emphasized, that in determining the dose for a child its weight is more important than its age. And it is important to remember that the dose for a small woman at the age of thirty would be much less than for a large, robust man at the age of 25. Therefore, the size of the individual should always be taken into consideration when administering a drug for a specific immediate physiologic action.
One exception, however, to this rule should be made: that is, an over-fat individual of 200 pounds and more would take no larger dose than a normal individual of 150 pounds, his weight being largely in fat and not in blood and muscle.
Normallv, it will be remembered that the blood of an adult constitutes about one-thirteenth of his body weight, but this ratio is not true of the over-fat.
A normal baby five months old weighs about 15 pounds, at the end of a year about 20 pounds, at the end of two years about 30 pounds, and from then on the child gains between four and six pounds a year, until at 15 years of age the child normally weighs not far from 100 pounds. Up to this time the girl and boy weigh about the same.
An average man of five feet six inches in height at the age of 20 weighs about 135 pounds; at 30, 145; at 40, about 150; while the average woman of 20, of five feet five inehes in height, weighs about 125; at 30 she weighs about 135; at 40 about 140. These figures are simply suggestive and are the average weights; however, an individual may be perfectly normal and weigh more or less than these figures.
3. The Existing Condition to be Combatted or the Object to be Attained. - This causes the dose of a given drug to vary from its minimum to its maximum. The dose of a drug for its continued tonic or systemic effect would be very different from the dose of the same drug to procure an immediate full physiologic action - as for instance, the dose of quinine to increase the appetite would be very small, but to combat malarial germs must be distinctly larger, and to ward off a congestive chill must be enormous. Also, a patient weakened by fever can take large doses of alcohol without its having any intoxicating effect, particularly true in pneumonia. On the other hand, any brain irritation will allow but small doses of a cerebral excitant. Also diseased conditions of the kidneys or liver make large doses of many drugs unsafe.
4. The Frequency of the Dose. - The frequency with which a drug or mixture is to be given modifies greatly the size of the dose. If a drug is to be given for immediate physiologic effect, and to be given but once, the dose could be very much larger than if it was to be given every hour or two. Therefore, the size of the dose selected should be governed by the decision as to which will have the best effect in a given case - a large dose, once or infrequently, or a small dose frequently repeated. This may be well instanced by the use of calomel as a cathartic, whether it will be best to give one large dose or minute doses every hour.
5. The Duration of the Action of a Drug. - This perhaps more decides the frequency of the dose than the size of it, and really means the rate of excretion of the drug. It is one of the most important points to remember and is the one most frequently forgotten.
Some drugs are quick to act and are quickly excreted or destroyed. Others are slow to act and slow of excretion, and their physiologic effects are slowly recovered from. There is no better instance of this modification of the dose than by the action of drugs on the heart. The dose of alcohol or ammonia for cardiac stimulation should be small and frequently repeated; whereas it would be bad physiologic treatment to give digitalis in the same frequency, as it should be given, in a dose thought proper, not oftener than two or three times in twenty-four hours. Alcohol and ammonia quickly act and are quickly destroyed in the system, while digitalis acts slowly and is excreted slowly.