The surgeon should have an understanding of basic general anesthesia principles. The primary goal of general anesthesia is rendering a patient unconscious and unable to feel painful stimuli while controlling autonomic reflexes. There are 5 main classes of anesthetic agents: intravenous (IV) anesthetics, inhalational anesthetics, IV sedatives, synthetic narcotics, and neuromuscular blocking drugs. Each of the classes has particular strengths and weaknesses in attaining the primary goal of general anesthesia. Knowing these characteristics as well as key side effects can prove beneficial for surgeons.
Patients undergoing surgical procedures that require deep relaxation for long periods of time are best suited for general anesthesia as long as there are no contraindications. Surgeries that are unable to be adequately anesthetized with local or regional anesthesia require general anesthesia. Operations that are likely to result in significant blood loss or in which breathing will be affected necessitate general anesthesia. Uncooperative patients are also better treated with general anesthesia even for smaller procedures. Medications that provide deep relaxation usually cause at least some degree of respiratory depression. Patient preference can also influence the decision to undergo anesthesia.
There are no absolute contraindications to general anesthesia. However, there are many relative contraindications. The patient’s who plan to undergo general anesthesia should undergo preoperative evaluation by the anesthesia provider. This evaluation involves a review of the patient’s medical comorbidities, heart/lung/kidney function, and pregnancy/smoking status. The patient’s medical condition is maximized preoperatively if at all possible. For example, a patient with unstable angina should undergo cardiac catheterization or bypass prior to any elective surgery.
General anesthesia requires an anesthetic machine which contains a gas supply, reducing valves, vaporizers, flow meters and breathing circuits. Accessories are also required including a face mask, laryngoscope, endotracheal tubes, stylets and oral/nasal airways. Endotracheal tube size is based on the measurement of the inner diameter of the tube. Size 6.0 ETT and size 8.0 ETT are standard starting size for the adult female and male respectively. Additional intubation equipment should be available for patients with difficult airways including Eschmann catheter, C-mac or flexible fiber-optic scope.
Patients undergoing general anesthesia must be evaluated and monitored by a qualified anesthesia provider. This may include board licensed anesthesiologists, trainees, and certified registered nurse anesthetists.
Surgeries can be classified as elective, semi-elective, urgent, and emergent. Elective surgeries do not involve a medical emergency and can be scheduled in advance. Semi-elective surgeries are done to preserve a patient’s life but do not need to be performed immediately. Urgent surgeries should be done within 1 to 2 days but can wait a short amount of time to stabilize the patient medically. Emergency surgery must be done without delay to decrease the risk of permanent disability or death. Stratifying surgeries based on urgency can help apply the relative contraindications to the specific surgery. If possible, patients should be medically maximized prior to undergoing general anesthesia. Workup should include recent pulmonary function test in patients with known chronic obstructive pulmonary disease (COPD) and an echocardiogram in patients with significant cardiac history.
The surgeon should discuss the case with anesthesia prior to surgery. The discussion should include the following: the position of the patient (supine vs. prone) and bed (90 degrees versus 180degrees), case duration, level of anesthesia, and ability to use paralytics or vasopressors. If the patient has a known history of difficult airway or risk factors concerning for a difficult airway, this should be relayed to the anesthesia team.
Side effects are common with the administration of general anesthesia. These can include transient confusion or memory loss, dizziness, urinary retention, nausea, vomiting, chills, and sore throat. Older, sicker patients undergoing lengthy procedures are at increased risk of serious complications including persistent confusion, memory loss, heart attack, pneumonia, thromboembolism and cerebrovascular accident. Death as a result of general anesthetic is rare and estimated to be approximately one in 150,000.
The first step in inducing general anesthesia in most surgical procedures is the administration of a hypnotic drug which is then followed by maintenance with inhalational anesthetic. Patients better tolerate intravenous (IV) induction, but inhalational induction is often used in children or where IV access is problematic. All IV anesthetics produce rapid unconsciousness, and redistribution from the brain to muscle and adipose tissue leads to awakening. Barbiturates are lipid soluble, alkalotic compounds which produce deep unconsciousness. Providers should exercise caution using these agents if the airway is difficult to maintain as this can cause apnea. Barbiturates also cause pronounced myocardial depression and vasodilation. Phenobarbital is a long-acting barbiturate which begins working within five minutes and its effects last 4 to 48 hours. Etomidate is an IV anesthetic related to antifungal drug ketoconazole. Use of etomidate is usually limited to induction, and repeated doses or infusions should not be used. Pain and phlebitis are common side effects which can be reduced with prior IV injection of lidocaine. Risk of nausea or vomiting makes etomidate a less ideal drug for use in an ambulatory setting. Propofol is a phenol agent with rapid onset and short duration of action and can be used for induction and maintenance of anesthesia. Profound respiratory depression can be caused by an induction dose, similar to barbiturates. Propofol offers the advantage of effortless awakening with minimal residual sedation even with prolonged infusion. Additionally, it has antiemetic properties making it popular for outpatient procedures. Ketamine is a dissociative anesthetic meaning it distorts perception of sight and sounds as well as producing feelings of detachment from environment and self. Unique among IV anesthetics, ketamine produces intense analgesia. Important side effects of ketamine include increased secretions, the risk of laryngospasm and hallucinations. Dexmedetomidine is a selective alpha-2 receptor agonist with sedative, sympatholytic and analgesic properties. Advantages of dexmedetomidine include better patient tolerance, hemodynamic stability, and preservation of patent airway. These qualities make it a preferred agent for conscious fiberoptic intubation.
Inhalational anesthetics are liquids at ambient temperature and pressure. These liquids are transformed by vaporization into gas for rapid absorption in and elimination by the pulmonary circulation. These medications are absorbed in alveoli, and the anesthetic concentration in the brain is directly related to alveolar concentration. Inhalational agents are commonly used for maintenance of anesthesia. A key measure of these medications is the minimal alveolar concentration (MAC), which is the concentration that will prevent movement in 50% of patients in response to a painful stimulus like a surgical incision. Importantly, nitrous oxide MAC is very high (104%) meaning it is unlikely to produce general anesthesia as a single agent. Nitrous oxide (NO) is an odorless nonhalogenated agent that can be combined with a halogenated anesthetic to hasten induction and emergence. NO can support combustion especially if delivered with a high oxygen concentration, thus should be avoided in laser endoscopy. Halothane was a commonly used agent historically but has been replaced by other halogenated agents like sevoflurane, which offers smoother mask induction, quicker emergence, and less myocardial depression and arrhythmogenic potential than halothane. Halothane also carries a risk of allergic hepatitis and malignant hyperthermia. Sevoflurane and desflurane are non-flammable, volatile halogenated agents which are completely fluorinated analogues of isoflurane. The fluorinated agents produce rapid awakening compared to isoflurane especially in obese patients following prolonged surgery. Isoflurane which contains fluoride is not completely fluorinated. Desflurane notably can cause coughing or laryngospasm. Inhalational agents are especially advantageous for use in head and neck surgery as they decrease bronchoconstriction by relaxing the bronchial smooth muscle. This provides controlled muscle relaxation without the use of neuromuscular blocking drugs, allowing monitoring of nerve function. Small concentrations of inhalational agents may severely depress ventilatory response to acute hypoxia so patients should be closely monitored during transport to the post-anesthetic care unit.
Benzodiazepines are often used as premedication for general anesthesia or for anxiolysis in patients who are undergoing regional anesthesia. Midazolam (Versed) is the most commonly used preoperative sedative and can provide anxiolysis, sedation, and amnesia. Diazepam (Valium) causes veno-irritation on injection in contrast to midazolam which is painless. Midazolam also offers quicker onset and shorter duration of action than lorazepam. Lorazepam is a long-acting sedative hypnotic not commonly used for anesthesia. All benzodiazepines suppress the ventilatory response to hypercarbia. Therefore, providers must be careful in patients with COPD or respiratory insufficiency.
Synthetic narcotics are particularly potent narcotics, which restricts their routine use to the operating room where ventilatory support is readily available. As with other narcotics, these drugs can cause meiosis, respiratory depression, bradycardia, constipation and urinary retention. Synthetic narcotics include alfentanil, sufentanil, remifentanil, and fentanyl. These medications produce rapid and intense analgesia. Fentanyl is one hundred times, and sufentanil one thousand times more potent than morphine. Remifentanil is an expensive, ultrashort-acting opioid resulting in minimal “drug hangover” and no residual analgesia. These qualities can be beneficial in endoscopic procedures and neurosurgery; however, rapid tolerance can occur resulting in increased opioid requirements postoperatively. These agents can cause profound respiratory depression and chest wall rigidity.
Neuromuscular Blocking Drugs
Neuromuscular blocking drugs (NMBDs) act on the postsynaptic membrane of nicotinic cholinergic receptors. These can be subclassified into competitive (non-depolarizing) and noncompetitive (depolarizing). Succinylcholine is a noncompetitive NMBD which binds strongly to the receptor site and mimics the effects of acetylcholine leading to fasciculations. It can cause prolonged paralysis or bradycardia if used as intermittent bolus or infusion. It carries the highest risk of malignant hyperthermia of all NMBDs. It should only be used in pediatrics with a clear indication as it can cause rhabdomyolysis, hyperkalemia and cardiac arrest in patients with undiagnosed myopathy. Succinylcholine reaches a maximum block in less than one minute and has a short duration of action (less than 10 minutes). This makes succinylcholine a commonly used agent in rapid sequence intubation. Competitive NMBDs bind loosely with nicotinic cholinergic receptors and compete with acetylcholine. These drugs include the following: atracurium, cisatracurium, pancuronium, vecuronium, and rocuronium. The maximum block is reached in 1 to 3 minutes with, and duration of action is greater than 40 minutes with each of these medications.
When choosing among the various anesthetic agents available, the physician should consider the specific clinical scenario at hand. Factors to consider include patient characteristics (age, cooperativity, medical comorbidities), surgeon/anesthesiologist preference, and type of surgery being performed. For example, intravenous induction should be avoided in patients with obstructing glottic mass as these agents produce profound respiratory depression. Knowing the advantages and disadvantages of the previously discussed agents can lead to improved communication between the surgery and anesthesia teams.