Volatile liquids: halothane
Halothane (Halothane) is a colorless volatile liquid with a sweetish odor. Its boiling point is 50.2 °C. The drug dissolves excellently in fats, is non-explosive, but requires special storage conditions—exclusively in dark glass bottles.
Clinically, it is a very potent anesthetic. Induction of anesthesia takes only 3–4 minutes, while the excitation stage is either absent or extremely mild. Awakening also occurs rapidly. However, due to the high rate of transition between anesthesia stages, there is a serious risk of overdose.
Effect on organs:
- Cardiovascular system: Halothane depresses cardiac activity, causing bradycardia and a drop in blood pressure. The most critical property is that the drug sharply increases myocardial sensitivity to catecholamines. Administration of epinephrine or norepinephrine against the background of such anesthesia is strictly contraindicated due to the risk of severe arrhythmias.
- Respiratory system: Does not irritate the airways and actively dilates the bronchi. This makes it the drug of choice for patients with respiratory diseases.
- Liver: Has proven hepatotoxicity.
Note: Historical drugs such as chloroform, diethyl ether, and cyclopropane are no longer used in modern medicine.
Isoflurane and sevoflurane
Isoflurane (Isoflurane) is a fluorine-containing colorless anesthetic. Unlike halothane, it is photostable and does not decompose under the influence of light. The surgical stage is achieved at a concentration of 1–2.5% in a mixture with oxygen and nitrous oxide. An important feature is that it powerfully potentiates the action of muscle relaxants.
Effect on body systems:
- During spontaneous breathing, it causes dose-dependent depression of ventilation.
- A slight drop in cardiac output and a mild increase in heart rate are observed from the heart. The myocardium is sensitized to catecholamines significantly less.
- Absolutely safe for parenchymal organs: no cases of toxic damage to the liver or kidneys have been recorded even during many-hour surgeries.
- In small doses, it does not affect blood loss volume during cesarean section, which is why it is widely demanded in obstetrics.
Sevoflurane (Sevoflurane) is a relatively new drug for the Russian Ministry of Health (while used for over a decade in the US, Europe, and Japan). Its main clinical advantage lies in its high controllability. It allows for inhalation mask induction, which is extremely convenient when working with children and in outpatient surgery. No toxic reactions have been described for it.
Gaseous substances: nitrous oxide
Nitrous oxide (Nitrous oxide, or "laughing gas") is an inert, colorless, and odorless gas. It is non-explosive, but can support combustion in the presence of oxygen and ether. The drug is stored in liquid form in special gray metal cylinders under a pressure of 50 atmospheres. The gas does not react with body tissues and is excreted through the lungs in an absolutely unchanged form.
Application features:
- It is used exclusively in a mixture with oxygen. In its pure form, it causes hypoxia.
- Permissible ratios of nitrous oxide to oxygen are 1:1, 2:1, 3:1, and 4:1.
- At the maximum ratio of 4:1, the patient receives 80% nitrous oxide and 20% oxygen. Critical rule: reducing the proportion of oxygen in the inhaled mixture below 20% is unacceptable.
The clinical picture is characterized by rapid and calm induction without an excitation stage. Awakening occurs instantly after stopping the gas mixture supply. However, the anesthetic breadth of the drug is very weak: even at the limit concentration of 80%, only superficial anesthesia can be achieved, and muscle relaxation is completely absent. For this reason, it is used exclusively for minimally traumatic and short surgical interventions.
Briefly: answers to topical questions
What are the absolute contraindications to the use of halothane?
The combined use of halothane with catecholamines is contraindicated. Halothane (Halothane) specifically increases myocardial sensitivity to drugs such as epinephrine and norepinephrine. Their administration against the background of halothane anesthesia is strictly contraindicated due to the high risk of developing dangerous heart rhythm disorders (arrhythmias). Other absolute contraindications to the use of halothane itself are not mentioned in the provided materials, but its hepatotoxicity (toxic effect on the liver) is noted.
What stages of classical ether anesthesia are distinguished according to Guedel?
According to Guedel's classification, four stages of classical ether anesthesia are distinguished:
- Stage I — analgesia.
- Stage II — excitation.
- Stage III — surgical, or tolerance; it is subdivided into 4 levels: III-1, III-2, III-3, and III-4.
- Stage IV — awakening.
These stages are especially clearly manifested during ether anesthesia.
What is the mechanism of action of inhalation anesthetics on the central nervous system?
The mechanism of action of inhalation anesthetics on the central nervous system consists in the development of inhibition processes at the physiological and cellular levels.
- Interaction with receptors — sensitization of receptors to inhibitory neurotransmitters (GABA in the brain and glycine in the spinal cord) occurs, as well as inhibition of the activity of excitatory glutamate NMDA receptors.
- Effect on neurotransmitters — inhalation anesthetics reduce the release of acetylcholine, dopamine, serotonin, and noradrenaline in the central nervous system.
At the cellular level, these effects are due to chemical or physical processes, including effects on colloids, cell membranes, and lipid dissolution.
Which inhalation anesthetics are capable of potentiating the action of nondepolarizing muscle relaxants?
Isoflurane has the ability to potentiate the action of muscle relaxants. Isoflurane (Isoflurane) is a fluorine-containing inhalation anesthetic that potentiates the action of absolutely all groups of muscle relaxants, including nondepolarizing ones. The provided materials do not indicate other inhalation drugs possessing a similar specific property of enhancing the effects of muscle relaxants during anesthesia.
To memorize the properties of halothane, use the "Three Fs" rule: Fast to fall asleep, Fast to wake up, Fills bronchi by dilating them. But remember the heart (arrhythmias) and liver (toxicity).
Class and exam questions
Why is it impossible to use nitrous oxide in its pure form?
In its pure form, nitrous oxide is toxic and causes severe hypoxia. It is always combined with oxygen, the concentration of which must not drop below 20%.
Which drug is better to choose for mask anesthesia in a child?
Sevoflurane is the optimal choice. It allows for smooth inhalation mask induction, is easily controlled, and has no described toxic reactions.
Can adrenaline be used for bleeding during halothane anesthesia?
No, this is strictly contraindicated. Halothane increases myocardial sensitivity to any catecholamines, and the administration of epinephrine can provoke fatal heart rhythm disorders.
Is diethyl ether used today?
No, diethyl ether, like chloroform and cyclopropane, is no longer used in modern anesthesiological practice.
What else to study on this topic
- Mechanism of hepatotoxicity development of volatile anesthetics
- Interaction of isoflurane and modern muscle relaxants
- Physicochemical properties of medical gases and safety rules when working with cylinders
- Stages of inhalation anesthesia and signs of overdose
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