The exact way inhaled anesthetics induce anesthesia is still an area of ongoing research, as their full physiological effects are not completely understood. However, it is generally accepted that these agents primarily act on the central nervous system by affecting the activity of several neurotransmitter receptors and ion channels. Inhaled anesthetics are known to enhance the function of inhibitory receptors and reduce the activity of excitatory receptors. Specifically, they increase signaling through chloride channels associated with gamma-aminobutyric acid (GABA) receptors, one of the brain's primary inhibitory neurotransmitters. By increasing the flow of chloride ions into neurons, these agents make it harder for neurons to fire, thereby reducing neural activity. Additionally, inhaled anesthetics are believed to enhance potassium channel activity, contributing further to the suppression of neural activity.
On the other hand, inhaled anesthetics inhibit excitatory pathways that are essential for maintaining normal consciousness. They decrease activity at acetylcholine receptors (both muscarinic and nicotinic), which are crucial for communication between neurons. These anesthetics also block glutamate receptors, especially N-methyl-D-aspartate (NMDA) receptors, which are key to excitatory neurotransmission. This suppression of excitatory pathways helps create the sedative, amnesic, and immobility effects necessary for anesthesia. While serotonin (5-HT) receptors are also involved, their exact role in the anesthetic process is less clear. Overall, inhaled anesthetics work by enhancing inhibitory signals and reducing excitatory signals, resulting in a depression of the central nervous system.
Inhaled anesthetics can be divided into two main categories: volatile and non-volatile agents. Volatile anesthetics like isoflurane, sevoflurane, and desflurane are liquids at room temperature that must be vaporized before administration. These agents are particularly effective at reducing systemic blood pressure by decreasing vascular resistance, making them useful for a variety of surgeries. Although these anesthetics generally preserve cardiac output, instances of cardiac depression can occur, particularly when combined with intravenous agents or in patients with heart conditions like acute cardiogenic shock. Desflurane is known for causing hypertension and tachycardia when administered rapidly, which requires careful monitoring.
Nitrous oxide, a non-volatile anesthetic, works differently from the volatile agents. While it can cause myocardial depression, this effect is balanced by its ability to increase sympathetic nervous system activity, leading to minimal changes in overall heart function. This balance makes nitrous oxide relatively stable in terms of cardiovascular effects, and it is often used in combination with other anesthetics for its analgesic properties. However, nitrous oxide does not produce the same level of amnesia or immobility as volatile anesthetics, which is why it is often used alongside other agents.
A key aspect of inhaled anesthetics is their impact on breathing. These agents are dose-dependent respiratory depressants: they reduce the depth of each breath (tidal volume) and blunt the ventilatory responses to carbon dioxide and hypoxemia. This reduction is typically paired with an increase in breathing rate, but the increase is not sufficient to fully compensate for the reduced tidal volume, leading to a decrease in overall ventilation. This can cause hypoventilation, which is why patients often require ventilatory support during general anesthesia to ensure proper oxygenation and gas exchange.
The brain responds to volatile anesthetics in a characteristic way. They lower the cerebral metabolic rate for oxygen (CMRO2), but above about 1 MAC they dilate cerebral vessels and increase cerebral blood flow, uncoupling flow from metabolism; in a patient with poor intracranial compliance this can raise intracranial pressure, which is why doses are kept at or below about 1 MAC in neurosurgical cases. Nitrous oxide, in contrast, raises both CMRO2 and cerebral blood flow.
