General anesthesia is a state of controlled, temporary unconsciousness achieved through specialized medications. While intravenous agents initiate this state, gases are often used to sustain anesthesia during long surgical procedures. The lungs offer a direct pathway into the bloodstream, allowing the anesthetic concentration to be precisely and rapidly adjusted. This ability to instantly alter the level of unconsciousness makes inhaled agents preferred for maintaining a consistent depth of anesthesia.
Primary Inhaled Anesthetics
The primary agents used in general anesthesia are volatile anesthetic agents, which are halogenated ethers existing as liquids at room temperature. These potent compounds must be converted into a vapor state using specialized instruments called vaporizers. The most common agents in use today are sevoflurane, desflurane, and isoflurane, each having slightly different chemical properties.
These agents achieve their effect by acting on the central nervous system, primarily by depressing the communication pathways that excite the brain and spinal cord. They also enhance the activity of inhibitory receptors, quieting the nervous system to produce unconsciousness, amnesia, and immobility. The concentration required to prevent a patient from moving in response to a surgical incision is a standard measure of an agent’s potency.
A major advantage of these agents is the precision they offer in controlling the depth of anesthesia. Since they are breathed in and out, the anesthesiologist can quickly adjust the administered concentration, and the patient’s body concentration follows suit. This allows for fine-tuning the anesthetic state based on real-time physiological responses. Furthermore, these modern agents are designed for minimal metabolism, meaning they are largely exhaled unchanged, contributing to a faster emergence.
Supplementary Anesthetic Gases
Alongside the primary volatile agents, a non-volatile gas is frequently used to augment the anesthetic plan, specifically Nitrous Oxide (\(\text{N}_2\text{O}\)). Often called “laughing gas,” \(\text{N}_2\text{O}\) is a weak general anesthetic that provides strong pain-relieving effects (analgesia). It is administered as a gas rather than a vaporized liquid.
Nitrous Oxide is often used to speed up the induction of anesthesia. Its high volume uptake, known as the “concentration effect,” helps rapidly increase the concentration of more potent volatile agents in the patient’s lungs and blood. Including \(\text{N}_2\text{O}\) allows the required dose of the primary volatile agent to be reduced, mitigating associated side effects. It is a valuable tool for its analgesic properties and ability to work synergistically with other anesthetic drugs.
Gases for Minimally Invasive Procedures
Not all gases used during surgery are intended to induce unconsciousness; some are used for purely mechanical purposes, most notably Carbon Dioxide (\(\text{CO}_2\)). This gas is the standard agent used in minimally invasive operations, such as laparoscopic or robotic surgery. During these procedures, \(\text{CO}_2\) is insufflated into a body cavity, such as the abdomen.
The purpose of this insufflation is to create a clear, dome-like working space, known as a pneumoperitoneum, by lifting the abdominal wall away from the internal organs. This process gives the surgeon room to manipulate instruments and provides an unobstructed view through a camera, often maintained at an intra-abdominal pressure between 12 and 15 mmHg. \(\text{CO}_2\) is specifically chosen for this task because of its high solubility in blood.
If \(\text{CO}_2\) is inadvertently absorbed into the bloodstream, it is quickly dissolved and efficiently expelled through the patient’s normal respiratory system. This rapid clearance minimizes the risk of a dangerous gas embolism. Furthermore, \(\text{CO}_2\) is non-flammable, making it safe to use with electrosurgical tools and lasers.