Continuing Education Activity

The primary goal of general anesthesia is to render a patient unconscious and unable to feel painful stimuli while controlling autonomic reflexes. There are five main classes of anesthetic agents: intravenous (IV) anesthetics, inhalational anesthetics, IV sedatives, synthetic opioids, and neuromuscular blocking drugs. Each class has particular strengths and weaknesses, and being familiar with these characteristics, as well as crucial side effects, can prove beneficial for the surgical team. The activity reviews the principles of general anesthesia, focusing on the common medications and equipment utilized and the detection of emergencies in patients who have undergone anesthesia. This activity highlights the role of the interprofessional team in providing general anesthesia, which requires working knowledge, clear communication, and participation of all operating room personnel.

Objectives:

• Contrast the five main classes of anesthetic agents.
• Outline relative contraindications to general anesthesia.
• Describe the equipment commonly required for general anesthesia.
• Summarize the role of effective and clear communication among the interprofessional team in providing safe and effective anesthesia to optimize patient outcomes.

Introduction

The surgeon should have an understanding of basic general anesthesia principles. The primary goal of general anesthesia is to render a patient unconscious and unable to feel painful stimuli while controlling autonomic reflexes. There are five main classes of anesthetic agents: intravenous (IV) anesthetics, inhalational anesthetics, IV sedatives, synthetic opioids, and neuromuscular blocking drugs. Each class has particular strengths and weaknesses in attaining the primary goal of general anesthesia. Knowing these characteristics, as well as crucial side effects, can prove beneficial for surgeons.

Indications

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 cannot be adequately anesthetized with local or regional anesthesia require general anesthesia. Operations 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 more minor procedures. Patient preference can also influence the decision to undergo anesthesia.

Contraindications

There are no absolute contraindications to general anesthesia other than patient refusal. However, there are many relative contraindications. Relative contraindications include patients with medical conditions that are not optimized before elective surgery, patients with a difficult airway or other significant comorbidities (severe aortic stenosis, significant pulmonary disease, CHF, etc.), undergoing procedures that could be done with a regional or neuraxial technique, therefore, avoiding airway manipulation and physiologic changes associated with general anesthesia.  The patients who plan to undergo general anesthesia should undergo preoperative evaluation by the anesthesia provider. This evaluation involves a review of the patient’s prior anesthetic history, 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 before elective surgery.[1].  While not contraindications for general anesthesia, it is imperative to determine if a patient has a personal or family history of malignant hyperthermia and pseudocholinesterase deficiency, as these medical conditions require advanced planning to minimize patient morbidity and mortality if they require general anesthesia. 

Equipment

General anesthesia requires an anesthetic machine that contains a ventilator, gas supply, reducing valves, vaporizers, flow meters, breathing circuits, and suction canisters. The anesthesia machine also contains a monitor to display vital signs. Airway accessories are also required, including a face mask, laryngoscope, endotracheal tubes, styles, and oral/nasal airways. The endotracheal tube size is based on the measurement of the inner diameter of the tube. Size 7.0 ETT and size 8.0 ETT are standard starting sizes for the adult female and male respectively. Additional intubation equipment should be available for patients with difficult airways, including a video laryngoscope, flexible fiber-optic scope, Eschmann catheter, laryngeal mask airway (LMA), and a cricothyrotomy kit for emergent situations.[2]

Personnel

A qualified anesthesia provider must evaluate and monitor patients undergoing general anesthesia. This may include board-licensed anesthesiologists, trainees, and certified registered nurse anesthetists.

Preparation

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 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 before undergoing general anesthesia.

The surgeon should discuss the case with the anesthesiologist before surgery. The discussion should include the following: the procedure to be performed, expected case duration, patient positioning, level of anesthesia, expected blood loss, and ability to use paralytics or vasopressors. If the surgeon knows the patient has a history of a difficult airway or significant medical conditions that can impact the anesthetic plan, this should be relayed to the anesthesia team.

Complications

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 due to general anesthetic is rare and estimated to be approximately one in 150,000.[3]

Clinical Significance

Intravenous Anesthetics

General anesthesia is most commonly achieved via IV sedatives and analgesics induction, followed by volatile anesthetics maintenance.  Patients better tolerate intravenous (IV) induction, but inhalational induction is often used in children or where IV access is problematic.  All IV anesthetics can produce rapid unconsciousness depending on dosage and rate of administration.  Redistribution from the brain to muscle and adipose tissue and metabolism leads to awakening.  Propofol is a phenol agent with rapid onset and a short duration of action and can be used for induction and maintenance of anesthesia.  An induction dose can cause profound respiratory depression. However, Propofol offers the advantage of effortless awakening with minimal residual sedation, even with a prolonged infusion.

Additionally, it has antiemetic properties making it popular for outpatient procedures.  Etomidate is an IV anesthetic related to the antifungal drug ketoconazole.  Etomidate is usually limited to induction; repeated doses or infusions should not be used.  Pain and phlebitis are common side effects that can be reduced with prior IV lidocaine injection.  The risk of nausea or vomiting makes etomidate a less ideal drug for use in an ambulatory setting.  Ketamine is a dissociative anesthetic that distorts the perception of sight and sounds and produces feelings of detachment from the environment and self.  Unique among IV anesthetics, ketamine produces intense analgesia. Significant 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.  The advantages of dexmedetomidine include better patient tolerance, hemodynamic stability, and preservation of the patent airway.  These qualities make it a preferred agent for conscious fiberoptic intubation.

Inhalational Anesthetics

Inhalational anesthetics are liquids at ambient temperature and pressure.  These liquids are transformed by vaporization into gas for rapid absorption and elimination by the pulmonary circulation.  These medications are absorbed in the alveoli, and the anesthetic concentration in the brain is directly related to alveolar concentration.  Inhalational agents are commonly used for the 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%), which 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, primarily 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.  Sevoflurane and desflurane are non-flammable, volatile halogenated agents which are completely fluorinated analogs 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 can notably cause coughing or laryngospasm.  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.  Halogenated volatile anesthetics are potent triggers of malignant hyperthermia (MH) and must be avoided in patients with a personal or family history of MH due to the high risk of morbidity and mortality associated with MH.  Malignant hyperthermia is an inherited genetic condition resulting from an abnormal ryanodine receptor in muscle tissue.  MH is triggered by volatile anesthetics and succinylcholine and results in muscle rigidity, rhabdomyolysis, high temperatures, acidosis, organ failure, and possibly death.  MH is treated with dantrolene.

Intravenous Sedatives

Benzodiazepines are often used as a premedication for general anesthesia or anxiolysis in patients 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 with patients with COPD or respiratory insufficiency.

Synthetic Opioids

Synthetic opioids are particularly potent opioids, which restricts their routine use to the operating room, where ventilatory support is readily available.  As with other opioids, these drugs can cause meiosis, respiratory depression, bradycardia, constipation, and urinary retention. Synthetic opioids include alfentanil, sufentanil, remifentanil, and fentanyl. Semi-synthetic opioids include hydromorphone, hydrocodone, and oxycodone.  Synthetic opioids in IV form produce rapid and intense analgesia.  Fentanyl is one hundred times more, and sufentanil is 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 procedures requiring rapid anesthesia emergence; however, rapid tolerance can occur, resulting in increased opioid requirements postoperatively.  All opioids 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 non-competitive (depolarizing).  Succinylcholine is a noncompetitive NMBD that 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 an intermittent bolus or infusion.  It carries a risk of malignant hyperthermia in susceptible patients.  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 at the neuromuscular junction.  These drugs include the following: atracurium, cisatracurium, pancuronium, vecuronium, and rocuronium.  The maximum block is reached in 1 to 3 minutes, and the duration of action is greater than 40 minutes with each of these medications, depending on the dose and drug used.  [4]

Enhancing Healthcare Team Outcomes

When choosing among the various available anesthetic agents, the anesthesiologist and nurse anesthetist should consider the specific clinical scenario. Factors to consider include patient characteristics (age, cooperativity, medical comorbidities), surgeon/anesthesiologist preference, and type of surgery being performed. Knowing the advantages and disadvantages of the previously discussed agents can improve communication between the surgery and anesthesia teams.