Lecture
At present there are more than 5 million chemical substances, and their number keeps growing steadily. More than 5000 thousand new chemical substances are introduced every year into medicine, agriculture, the food and other industries, and into everyday life. But their poisonous effects do not always manifest themselves, since certain conditions are required for this.
A poison is a substance which, on entering the human body in small doses under certain conditions, is capable of causing pathological changes leading to illness or death (poisoning).
Forensic medicine, in turn, studies and develops methods of proof, in forensic medical examination, of poisoning of a living person and of a corpse, including proof that death resulted from poisoning.
Poisoning is a disorder of health and death caused by the action of poisonous or highly active substances that have entered the body from outside. Forensic medical toxicology divides all poisonings into four types:
a) accidents;
b) poisoning of another person with intent to kill;
c) poisoning with intent to commit suicide;
d) habitual poisonings.
Most often, forensic medical practice encounters acute poisonings resulting from an accident (at home, at work) and food poisonings arising from various causes related to the consumption of food. Occupational poisonings caused by violations of safety procedures at work occur significantly less often. Murder by poisoning is fairly rare.
Types of habitual poisoning include alcoholism, drug addiction and substance abuse. There are known cases of certain technical liquids being used for this purpose: dichloroethane, methyl alcohol, benzene and others. For example, persons suffering from alcoholism add a small amount of dichloroethane to alcoholic beverages, which, in their opinion, «enhances» the effect of alcohol.
The largest number of poisonings is caused by the following main groups of chemical substances:
a) functional poisons (ethyl alcohol, methyl alcohol, cyanide compounds, narcotic and other substances);
b) food poisons (of bacterial and non-bacterial origin);
c) caustic poisons (acetic acid essence, inorganic acids, alkalis, phenol);
d) destructive poisons (mercury and its compounds, arsenic and its compounds);
e) blood poisons (acting on the hemoglobin of the blood);
f) agricultural chemicals (pesticides);
g) bites of poisonous animals.
There is no single generally accepted classification of poisons in medicine. In forensic medical practice, the following classification of poisons is accepted:
1) a group of poisons capable of causing sharp lesions in the area of their primary interaction (local). Many substances that differ in their properties (phenol and its derivatives, caustic acids and alkalis, etc.) have a pronounced local effect (irritating, necrotizing, cauterizing). The action of such poisons may also be accompanied by a number of functional disorders;
2) a group of poisons based on the ability to exert a toxic effect only after absorption (resorptive). These poisons, depending on the clinical and morphological manifestations they cause, are subdivided into:
a) destructive poisons, which lead to sharply pronounced morphological changes in internal organs (compounds of mercury, arsenic, etc.);
b) functional poisons, which affect mainly the central nervous system, causing functional disorders but without serious morphological damage (morphine, atropine, strychnine, etc.);
c) blood poisons, acting on the hemoglobin of the blood (carbon monoxide). The toxicity of a poison is understood as its property of causing pathological changes leading to illness, disability or death of the poisoned person. A toxic dose is the minimum amount of a substance causing poisoning, i.e. a reliable change in some function beyond the limits of physiological fluctuation. Several categories of toxicity are usually distinguished:
a) lethal dose (the amount of a chemical substance that causes the death of a person who has not received treatment). This dose, in turn, is subdivided into absolute, average and minimal;
b) minimum toxic dose (the largest amount of poison capable of causing the picture of acute poisoning without a fatal outcome);
c) minimum effective dose.
Doses of a poison taken by mouth are usually expressed in milligrams per 1 kg of the victim's body weight. Doses of gaseous, vapour or aerosol poisons are expressed in milligrams per 1 L. The toxicity of a poisonous substance that has come into contact with the skin is determined in milligrams per 1 cm2 of skin surface. The toxic action of poisonous chemical substances can also be determined by the rate of their entry into and elimination from the body.
With local action of a poison, a general toxic reaction is observed (to a greater or lesser extent) as a result of the poison entering the body and circulating in the blood. Some poisons produce practically no local action when they enter the body (carbon monoxide, cyanides, sedatives, alcohol and its surrogates, etc.), but when introduced by another route, local phenomena may also occur (accidental subcutaneous injection of cyanides will lead to a local inflammatory-necrotic process).
Regardless of the method of application (entry) of the poison into the body, both reversible and irreversible processes may occur. Reversible processes are more common, when a physiological function of an organ or tissue that has been disrupted by poisoning is restored or compensated as a result of the reverse development of the intoxication.
In irreversible processes, pathological changes occur that, to one degree or another, cause the organ (tissue) to lose its physiological function.
The action of a poisonous substance on the body depends on the following conditions, which must be taken into account during forensic medical examination:
a) the physical state of the substance and its solubility. Substances introduced may be liquid, gaseous or solid, and therefore they differ in the speed and degree of absorption. Liquid substances begin to be absorbed into the body immediately after being introduced internally or upon contact with the skin (if they are readily soluble in fats), and poisoning occurs quickly. Gaseous substances, when inhaled, are absorbed into the blood, as a result of which poisoning occurs even faster. Solid substances begin to act only after they dissolve in the body's fluids and are absorbed into the blood in the gastrointestinal tract.
Some chemical substances that are toxic to the body do not have a negative effect when taken, because they are not soluble in gastric juice (barium sulfate, etc.)
b) the manufacturing date and storage conditions of the substance. Substances with
expired shelf life or improperly stored may have significantly lower toxic properties or lose them entirely;
c) the presence in the administered substance of various chemical impurities, which may increase its toxic properties (for example, a large amount of fusel oils in ethyl alcohol);
d) the dose (amount) of the substance. Depending on the dose, the same substance may have a therapeutic effect, while in large doses it may cause poisoning and even death;
e) the concentration of the substance. The same amount of a substance, at different concentrations, may have a different effect on the body (for example, pure alcohol acts faster and causes stronger intoxication than the same amount of alcohol contained in vodka or wine but at a lower concentration);
f) the route of administration of the substance, which determines the speed of its action on the body and the degree of intoxication. Poisonous substances enter the body intravenously, intramuscularly, subcutaneously, by mouth (orally), via the rectum, the vagina, wound surfaces, the mucous membrane of the eyes, the spinal canal, the bladder, etc. The therapeutic, toxic and lethal doses for the same substance differ depending on the route of administration into the body. If, taken orally, it produces a therapeutic effect, then when introduced directly into the blood, the same dose may become lethal.
When introduced into the rectum, poisons are absorbed quickly and act very strongly, since they are carried by the bloodstream into the systemic circulation, bypassing the liver and, consequently, not undergoing detoxification in it.
Poisoning is also caused by poisons entering the body through absorption via the mucous membranes of the eyes and the female genital labia. Some poisons can be absorbed through healthy skin (chlorophos, thiophos and others, dissolved in organic substances – fats, lipoids), while others (organophosphorus compounds, mercury preparations, etc.) can do so through damaged skin and lead to acute poisoning.
The entry of poisonous substances into the vagina, rectum, conjunctiva of the eyes or nose is rare in forensic medical practice and occurs mainly as accidents during injuries, anaesthesia or termination of pregnancy.
When a poisonous substance is taken orally, absorption occurs in the gastrointestinal tract. The rate of absorption here will depend on a number of factors: the concentration and dose of the poison, the food present in the stomach, its composition, the rate of gastric emptying, etc. Poison absorbed into the digestive tract enters the bloodstream through the liver. Vomiting helps remove the poison from the stomach and weakens the intoxication.
When a poison enters the body through the respiratory organs, it passes through the lungs very quickly directly into the blood and leads to intoxication.
With subcutaneous and intramuscular administration of medicinal and poisonous substances, the toxic effect appears after approximately 15–20 minutes, while with intravenous administration it appears at the moment of administration or immediately after it is completed. The strength of the toxic effect with intravenous, intramuscular and subcutaneous administration is, as a rule, several times greater than its effect with oral administration;
g) concomitant substances. When introduced into the body together with toxic substances, they can strengthen or weaken the effect of the poison on the human body;
h) the rate of absorption and elimination of the poison from the body in the development of poisoning. If a poison is absorbed slowly but eliminated quickly, then even a lethal dose of it may not create a concentration in the blood that is dangerous to life and health;
i) the ability of a substance to accumulate in the body and the degree of toxicity of the poison's breakdown products. A number of chemical substances (salts of heavy metals, many medicinal preparations, organophosphorus compounds) are capable of accumulating in the body, i.e. possess cumulative properties. Thus, arsenic and methyl alcohol persist for several days, heavy metals for several months, and gold and silver for several years;
k) individual characteristics of the organism. The different course of intoxication in different people depends on the unequal character of the metabolism of chemical substances. There are many factors that influence the rate of the reaction forming the breakdown products of toxic substances - metabolites (genetic, physiological, and environmental factors). Age, body weight, state of health, type and state of the higher nervous system, and increased sensitivity to certain chemical substances also have a certain significance in the individual reaction of the organism to a chemical substance. Children and the elderly are more sensitive to poisons. The lower a person's weight, the smaller the dose of poison capable of causing poisoning. There are cases of increased sensitivity to specific chemical substances, when intake into the organism of a therapeutic dose of a substance can lead to severe poisoning and even cause death. But at the same time, habituation to a chemical substance (for example, to alcohol), on the contrary, may not lead to severe poisoning even when a lethal dose is taken. For example, drug addicts, owing to habituation to morphine, may take 5-10 g of morphine without pronounced signs of intoxication (the lethal dose of morphine for an adult is 0.2-0.3 g). General state of health also affects the toxic properties of the poison;
l) various environmental factors (overheating, cold, noise, ionizing radiation, repeated exposure to chemical substances, etc.), which to a certain extent can affect the course of intoxication (stimulating or inhibiting the rate of metabolism).
Repeated exposure to chemical substances can lead to the development of dependence and the occurrence of allergic reactions. Chemical dependence manifests itself in the fact that discontinuation of prolonged use of a number of medicinal preparations (morphine, barbiturates, etc.) can cause severe disturbances of physical state (withdrawal syndrome). With prolonged treatment with the same drug (a hypnotic, an analgesic, etc.), habituation may appear, and this often entails an increase in its dose in order to achieve the expected therapeutic effect. In such cases, severe intoxication can suddenly develop. An allergic reaction to a chemical substance (including a medicinal one) can also occur upon its repeated administration. Even the most insignificant amount of a preparation is capable of causing the most severe reaction.
When a poison enters the organism, as a rule, the organs, through which it enters, is excreted, or in which it accumulates are affected. Acute and chronic poisonings are distinguished. Acute poisonings develop rapidly (in the first minutes or hours after taking the poison), while chronic ones appear when the human organism has been exposed for a long time to small doses of poison, each of which did not cause visible symptoms of poisoning.
Excretion of the poison and its breakdown products (metabolites) from the organism is carried out by all organs possessing an external secretory function (the poison and metabolites are excreted in urine and bile, partly in sweat, exhaled air, saliva, and through the stomach and intestines). At the same time, a significant part of them undergoes various changes in the organism, which leads to a complete or partial loss of the substance's toxic properties, or, conversely, to the formation of compounds more poisonous than the precursor («lethal synthesis»).
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