Last updated date: 12-May-2023
Originally Written in English
General anesthesia's main purpose is to make a patient comatose and numb to painful stimuli while suppressing autonomic responses. Intravenous (IV) anesthetics, inhalational anesthetics, IV sedatives, opioid drugs, and neuromuscular blocking medications are the five primary types of anesthetics. Each class has its own set of strengths and shortcomings when it comes to achieving the primary objective of general anesthesia. Doctors can benefit from knowing these qualities as well as major adverse effects.
What is General Anesthesia?
Due to anesthetic drugs, general anesthesia is a medically induced loss of consciousness with accompanying loss of defensive reflexes. To produce unconsciousness, forgetfulness, analgesia, muscle tissue relaxation, and the absence of autonomic system responses, a variety of drugs may be administered. The patient is unresponsive to verbal, physical, or painful stimuli when in this state. During general anesthesia, an upper airway obstruction usually demands the use of a laryngeal mask airway or an endotracheal tube to maintain a patent airway. In addition, the patient's spontaneous breathing is frequently insufficient, necessitating partial or complete mechanical support using positive pressure ventilation. It's also possible that the patient's cardiovascular condition will deteriorate.
General Anesthesia Staging
Anesthetic overdose by an untrained anesthetist was common in the past when the physical examination was the only way to determine a patient's level of anesthesia. The anesthetic community did not create a fully systematic strategy for monitoring until the twentieth century. Dr. Arthur Guedel developed one of the earliest anesthesiology safety systems in the1930s, using a chart that defined the stages of anesthesia in increasing depth from stage one to four. Guedel's categorization is still utilized, despite newer anesthetic substances and administration procedures that have resulted in speedier onset and recovery from general anesthesia.
- Stage 1 (Analgesia or Disorientation). This stage can begin in a preoperative anesthesiology holding area when the patient is given medicine and may begin to feel its effects but has not yet been asleep. The "induction stage" is how most people refer to this stage. The patients are asleep yet talkative. Breathing is slow and consistent. The patient advances from analgesia without amnesia to analgesia with concomitant amnesia at this point. With the loss of consciousness, this stage comes to a close.
- Stage 2 (Excitement or Delirium). Disinhibition, delirium, uncontrolled movements, lack of eyelid reflex, hypertension, and tachycardia are all signs of Stage 2. During this time, airway reflexes remain intact and are generally hypersensitive to stimuli. Airway manipulation, such as the insertion and removal of endotracheal tubes and deep suctioning procedures, should be minimized during this stage of anesthesia. At this point, there is a greater risk of laryngospasm (involuntary tonic contraction of the vocal cords), which can be exacerbated by airway manipulation. As a result, the patient's airway may be compromised by the combination of spastic motions, vomiting, and rapid, irregular breathing. Fast-acting medications aid in reducing the amount of time spent in stage 2 and facilitating progression to stage 3.
- Stage 3 (Surgical Anesthesia). For operations necessitating general anesthesia, this is the targeted anesthetic threshold. The characteristics of this stage are slowed eye movements and respiratory depression. At this level, airway manipulation is safe. For this step, there are four "planes" defined. There is still periodic spontaneous breathing, constricted pupils, and a centered focus during plane 1. Eyelid, conjunctival, and swallow reflexes, on the other hand, frequently cease in this plane. There are occasional cessations of breathing in plane 2 as well as a loss of corneal and laryngeal reflexes. Ocular movements may be slowed and lacrimation may intensify. Complete relaxation of the intercostal and abdominal musculature, as well as the lack of the pupillary light response, characterize Plane 3. Because it is ideal for most surgeries, this plane is known as true surgical anesthesia. Finally, irregular breathing, paradoxical rib cage motion, and complete diaphragmatic paralysis leading to apnea characterize Plane 4.
- Stage 4 (Overdose). This stage happens when anesthetic agents are administered in excess of the amount of surgical stimulation, causing an already serious brain or medullary depression to exacerbate. This stage begins with the absence of breathing and ends with the possibility of death. At this stage, the skeletal muscles are flaccid, and the pupils are fixed and dilated. Due to the inhibition of the cardiac pumping and vasodilation in the peripheral circulation, blood pressure is often substantially lower than normal, with weak and diffracted pulses. This stage is fatal without circulatory and respiratory support. As a result, the anesthesiologists' goal is to move the patient to stage 3 anesthesia as quickly as possible and keep them there for the remainder of the procedure.
General Anesthesia Drugs
Induction with IV sedatives and analgesics, followed by maintenance with volatile anesthetics, is the most frequent method of achieving general anesthesia. Although patients tolerate intravenous (IV) induction well, inhalational induction is frequently employed in youngsters or in situations where IV access is difficult. Depending on the amount and rate of administration, all IV anesthetics can cause fast unconsciousness. Awakening is caused by a redistribution of energy from the brain to muscle and adipose tissue, as well as metabolic changes. Propofol is a phenol agent with a fast onset and brief duration of action that can be utilized for anesthesia induction and maintenance. An induction dose can result in severe respiratory depression. Propofol has the advantage of allowing you to wake up with little residual drowsiness, even after a long infusion. It also has antiemetic qualities, which makes it a common choice for outpatient treatments. Etomidate is a type of intravenous anesthetic that is similar to the antifungal medication ketoconazole. Etomidate is normally only used for induction, and it should not be given in several doses or infusions. Pain and thrombophlebitis are common adverse effects that can be alleviated with a lidocaine IV injection. Because of the risk of nausea and vomiting, etomidate is not recommended for usage in an outpatient setting. Ketamine is a dissociative anesthetic, which means it alters the perception of sight and sound while also causing emotions of detachment from one's surroundings and self. Ketamine is an IV anesthetic that provides significant analgesia. Increased secretions, the possibility of laryngospasm, and hallucinations are also common ketamine adverse effects. Dexmedetomidine is a hypnotic, sympatholytic, and analgesic selective alpha-2 receptor agonist. Dexmedetomidine has several advantages, including improved patient tolerance, hemodynamic stability, and the preservation of a patent airway. Because of these characteristics, it is the ideal agent for conscious fiberoptic intubation.
At room temperature and pressure, inhalational anesthetics are liquids. These liquids are vaporized into gas, which is quickly absorbed and eliminated via the pulmonary circulation. These drugs are absorbed in the alveoli, and the concentration of anesthetics in the brain is proportional to alveolar concentration. Anesthesia is frequently maintained with inhalational gases. The minimum alveolar concentration (MAC) of these drugs is defined as the concentration that prevents movement in 50% of individuals in response to a painful stimulus such as a surgical cut. Importantly, because nitrous oxides' MAC is so high, it's unlikely to be used alone to achieve general anesthesia. Nitrous oxide (NO) is an odorless non-halogenated anesthetic that can be used in conjunction with a halogenated anesthetic to speed up induction and emergence. Because NO can cause combustion, especially when combined with a high oxygen concentration, it should be avoided during laser endoscopy. Halothane was once a popular anesthetic, but it has been phased out in favor of alternative halogenated drugs like sevoflurane, which has a smoother mask induction, faster emergence, and less cardiac depression and arrhythmogenic propensity than halothane. Allergic hepatitis to halothane is also a possibility. Sevoflurane and desflurane are non-flammable, unstable halogenated analogs of isoflurane that are totally fluorinated. In comparison to isoflurane, fluorinated drugs cause quick awakening, especially in obese patients who have undergone a prolonged operation. The fluoride in isoflurane is not totally fluorinated. Coughing or laryngospasm are common side effects of desflurane. Patients should be continuously monitored during transit to the post-anesthetic care facility since small quantities of inhalational drugs can substantially impair ventilatory responsiveness to acute hypoxia. Because of the significant risk of morbidity and death associated with malignant hyperthermia (MH), halogenated volatile anesthetics should be contraindicated in patients with a personal or family history of malignant hyperthermia. Malignant hyperthermia is a hereditary condition caused by a malfunctioning ryanodine receptor in muscle tissue. Muscle stiffness, rhabdomyolysis, high temperatures, acidosis, multiple organ failure, and possibly death are all symptoms of malignant hyperthermia, which is provoked by volatile anesthetics and succinylcholine. Dantrolene is used to treat malignant hyperthermia.
General Anesthesia Adjuncts
Premedication with benzodiazepines for general anesthesia or anxiolytic properties in patients having regional anesthesia is common. The most often used preoperative sedative is midazolam, which can cause anxiolysis, sleepiness, and amnesia. In contrast to midazolam, which is painless, diazepam induces veno-irritation when injected. In comparison to lorazepam, midazolam has a faster onset and shorter duration of effect. Lorazepam is a sedative-hypnotic with a long half-life that isn't usually used for anesthesia. The ventilatory response to hypercapnia is suppressed by all benzodiazepines. As a result, in patients with COPD or respiratory impairment, clinicians must be cautious.
Synthetic opioids are extremely strong opioids; thus, they are only used in operating rooms where ventilatory support is easily available. These medications can induce meiosis, respiratory depression, bradycardia, constipation, and urine retention, just like other opioids. Alfentanil, sufentanil, remifentanil, and fentanyl are examples of synthetic opioids. Hydromorphone, hydrocodone, and oxycodone are semi-synthetic opioids. Injections of synthetic opioids generate quick and strong analgesia. Fentanyl is 100 times stronger than morphine, while sufentanil is 1,000 times more potent. Remifentanil is a costly, ultra-short-acting opioid with no residual analgesia and negligible drug hangover. These characteristics can be advantageous in surgeries that require a speedy recovery from anesthesia; nonetheless, rapid tolerance can evolve, resulting in greater opioid doses following surgery. All opioids have the potential to cause severe respiratory depression and chest tightness.
Neuromuscular Blocking Drugs
Neuromuscular blocking drugs (NMBDs) inhibit nicotinic cholinergic receptors' postsynaptic membrane. These can be divided into two categories: competitive (non-depolarizing) and non-competitive (depolarizing). Succinylcholine is a noncompetitive NMBD that binds firmly to the receptor site and causes fasciculations by mimicking the actions of acetylcholine. If taken as an intermittent bolus or infusion, it can cause persistent paralysis or bradycardia. In vulnerable persons, it can cause malignant hyperthermia. Because it can cause rhabdomyolysis, hyperkalemia, and cardiac arrest in individuals with undiagnosed myopathy, it should only be administered in children with a clear indication. Succinylcholine has a brief period of action and reaches a maximum block in less than a minute. As a result, succinylcholine is frequently employed in rapid sequence induction. At the neuromuscular junction, competitive NMBDs attach weakly to nicotinic cholinergic receptors and compete with acetylcholine. Atracurium, cisatracurium, pancuronium, vecuronium, and rocuronium are some of these medications. With each of these drugs, the maximum block is obtained in 1 to 3 minutes, and the duration of effect is longer than 40 minutes, depending on the dose and substance administered.
General Anesthesia Indications
As long as there are no limitations, patients having surgical procedures that require a relaxed state for long periods of time are especially suitable for general anesthesia. General anesthesia is used for surgeries that cannot be effectively sedated with a local or regional anesthetic. General anesthesia is required for operations that are anticipated to cause severe blood loss or compromise breathing. Even for minor procedures, general anesthesia is preferable for uncooperative individuals. The determination to undergo anesthesia might also be influenced by the patient's preferences.
General Anesthesia Contraindications
Except for patient refusal, there are no definite contraindications to general anesthesia. There are, however, a number of relative contraindications. Patients with medical problems that have not been optimized before elective surgery, patients with a difficult airway, or patients with other substantial comorbidities (severe aortic stenosis, significant pulmonary disease, CHF) currently conducting procedures that could be performed with a regional or neuraxial technique, thus avoiding airway manipulation and physiologic alterations associated with general anesthesia, are relative contraindications. Patients who are about to get general anesthesia should have their anesthesia practitioner evaluate them before the procedure. The patient's previous anesthetic history, medical comorbidities, heart/lung/kidney function, and pregnancy/smoking history are all reviewed during this assessment. If at all possible, the patient's medical state is improved prior to surgery. A patient with unstable angina, for example, should have a heart catheterization or bypass before undergoing any elective surgery. While malignant hyperthermia and pseudocholinesterase deficiency are not contraindications to general anesthesia, they do necessitate extensive planning to reduce patient morbidity and mortality if patients require general anesthesia.
An anesthetic equipment with a ventilator, gas source, reduction valves, vaporizers, flow meters, breathing circuits, and suction cylinders is required for general anesthesia. A monitor for vital signs is also included in the anesthetic machine. A face mask, laryngoscope, endotracheal tubes, styles, and Oro- or nasopharyngeal airways are all essential airway accessories. The inner diameter of the tube is used to determine the endotracheal tube size. The adult female and male standard starting sizes are 7.0 ETT and 8.0 ETT, respectively. For patients with problematic airways, additional intubation instruments such as a video-assisted laryngoscope, flexible fiber-optic scope, Eschmann catheter, laryngeal mask airway (LMA), and cricothyrotomy kit should be provided.
General Anesthesia Preparation
Surgical procedures are divided into four categories: elective, semi-elective, urgent, and emergency. Elective procedures are not medical emergencies and can be booked ahead of time. Semi-elective procedures are conducted to save a patient's life but do not have to be done right away. Urgent procedures should be completed within 1 to 2 days but can be postponed for a short period of time while the patient's medical condition is stabilized. To avoid lifelong impairment or death, emergency surgery must be performed as soon as possible. The relative contraindications can be applied to the specific surgery by stratifying surgeries depending on urgency. Prior to obtaining general anesthesia, people should be medically optimized if at all possible.
Prior to surgery, the doctor should discuss the case with the anesthesiologist. The procedure to be performed, estimated case duration, patient positioning, level of anesthetic, predicted blood loss, and capability to use paralytics or vasopressors should all be discussed. The doctor should inform the anesthesia team if the patient has a history of a difficult airway or any serious medical issues that could affect the anesthetic plan.
Procedure for General Anesthesia
The brain and body’s nerve signals are interrupted by general anesthesia. It inhibits the brain from processing pain and recalling the events of the surgery.
An anesthesiologist is a professionally trained doctor who administers a general anesthetic and monitors you before, during, and after surgery. Your care may also include a nurse anesthetist and other team members.
An IV line will be inserted into a vein in the arm or hand before the operation to administer the anesthetic. You might also use a mask to inhale the gas. Within a few minutes, you should be completely asleep.
Once you are unconscious, the doctor may insert a tube into the larynx through the mouth. During operation, this tube provides that you get sufficient oxygen. The doctor will initially give you drugs to relax the throat muscles. When the tube is implanted, you will not feel anything.
The anesthesia team will monitor these and other body functions throughout the surgical procedure:
- Breathing and temperature
- Heart rate
- Blood pressure
- Oxygen saturation in the blood
- Fluid status
These measurements will be used by the medical team to alter the medications or give you more fluids or blood if necessary. They will also make certain you are sleepy and pain-free throughout the operation.
The anesthetic medications will be stopped by the doctor following surgery. You will be sent to a recovery area and slowly awaken. The physicians and nurses will make sure you are not in any pain and that the operation and anesthetic haven't caused any complications.
General Anesthesia Side Effects
When general anesthesia is used, it is common to experience side effects. Transient confusion or memory loss, dizziness, urinary retention, nausea, vomiting, shivering, and sore throat are all possible symptoms. Patients who are older and unwell are more likely to experience major problems such as prolonged disorientation, memory loss, myocardial infarction, pneumonia, thromboembolism, and cerebrovascular accident. The risk of dying as a result of general anesthesia is believed to be one in 155,000.
Although general anesthetics are riddled with mystery, they are vital in surgery and medicine as a whole. There are several risks and consequences related to general anesthesia. However, it is a relatively safe medication in general, with the procedure being the greatest danger. Patients will be assessed before any form of anesthesia is delivered to establish the most appropriate combination of medications to use and the doses to use, based on whether they have any possible risks or a family history of anesthetic allergies.