Summary
Preoperatively, the patient will transition from different depths of anesthesia, including the levels of sedation, to general anesthesia (GA). Sedation is a continuum of symptoms that range from minimal symptoms of anxiolysis to symptoms of moderate and deep sedation. Moderate sedation is defined by the patient remaining asleep, but being easily arousable. Deep sedation is achieved when the patient is only arousable to painful stimulation. GA refers to medically induced loss of consciousness with concurrent loss of protective reflexes and skeletal muscle relaxation. GA is most commonly achieved via induction with intravenous sedatives and analgesics, followed by maintenance of volatile anesthetics [1]. Table 9.1 lists the depths of anesthesia and associated characteristics.
Background
Preoperatively, the patient will transition from different depths of anesthesia, including the levels of sedation, to general anesthesia (GA). Sedation is a continuum of symptoms that range from minimal symptoms of anxiolysis to symptoms of moderate and deep sedation. Moderate sedation is defined by the patient remaining asleep but being easily arousable. Deep sedation is achieved when the patient is only arousable to painful stimulation. GA refers to medically induced loss of consciousness with concurrent loss of protective reflexes and skeletal muscle relaxation. GA is most commonly achieved via induction with intravenous sedatives and analgesics, followed by maintenance of volatile anesthetics [Reference El-Orbany and Woehlck1]. Table 9.1 lists the depths of anesthesia and associated characteristics.
Table 9.1 Definitions of different anesthetic depths and associated characteristics
| Minimal sedation/anxiolysis | Moderate sedation/analgesia (“conscious sedation”) | Deep sedation/analgesia | General anesthesia | |
|---|---|---|---|---|
| Responsiveness | Normal response to verbal stimulation | Purposeful response to verbal or tactile stimulation | Purposeful response following repeated or painful stimulation | Unarousable even with painful stimulus |
| Airway | Unaffected | No intervention required | Intervention may be required | Intervention often required |
| Spontaneous ventilation | Unaffected | Adequate | May be inadequate | Frequently inadequate |
| Cardiovascular function | Unaffected | Usually maintained | Usually maintained | May be impaired |
The stages of anesthesia can be based on Guedel’s classification, which includes four stages:
Stage I – analgesia or disorientation: The patient is not yet unconscious and may have just begun to feel the effects of anesthesia. Patients typically will remain conversational, and breathing will often be slow and regular.
Stage II – excitement or delirium: There are many possible patient reactions while beginning this phase. Disinhibition, delirium, uncontrolled movements, hypertension, and tachycardia are commonly associated. The airway will remain intact and will have a higher sensitivity to stimulation. Airway manipulation must be avoided during this phase.
Stage III – surgical anesthesia: Surgical anesthesia is achieved once the patient has reached the appropriate anesthetic level for procedures requiring GA. At this phase, it is safe to manipulate the airway. The patient will begin to show signs of respiratory depression, and eye movements will no longer be present.
Stage IV – overdose: If the patient has received too much of an anesthetic agent, the patient is at higher risk of worsening brain function. The stage will begin with respiratory cessation and ends with potential death. The patient can become hypotensive, with decreased cardiac output and peripheral vasodilation. The patient can also present with weak and thready pulses [Reference Gordon, Cooper and Parotto2].
Preoperative Evaluation
The preoperative evaluation of the patient before surgery requires a detailed history and physical examination, ordering necessary laboratory workup, identifying the ASA physical status classification, and taking appropriate fasting precautions. The ASA physical status categorizes patients by their preanesthesia medical comorbidities. Classification is a clinical decision based on multiple factors, with the final assessment made on the day of anesthesia care. Table 9.2 describes the parameters of each ASA class.
Table 9.2 Characteristics of American Society of Anesthesiologists’ physical classification
| ASA physical status classification | Definition | Adult examples (including, but not limited to) |
|---|---|---|
| ASA I | A normal healthy patient | Healthy, nonsmoker, no or minimal alcohol use |
| ASA II | A patient with mild systemic disease | Mild diseases without functional limitations, including current smoker, social alcohol drinker, pregnancy, obesity (30 < BMI < 40), well-controlled DM/HTN, mild lung disease |
| ASA III | A patient with severe systemic disease | Functional limitations with one or more moderate to severe diseases. Examples include poorly controlled DM or HTN, COPD, morbid obesity (BMI ≥40), active hepatitis, moderately reduced EF, ESRD regularly undergoing dialysis, premature infant, history of MI, CVA, TIA, or CAD/PCI |
| ASA IV | A patient with severe systemic disease that is a constant threat to life | Examples include recent MI, CVA, TIA, or CAD/PCI (<3 months), ongoing cardiac ischemia or severe valve dysfunction, severely reduced EF, sepsis, DIC, ARF or ESRD not regularly undergoing dialysis |
| ASA V | A moribund patient who is not expected to survive without the operation | Examples included ruptured abdominal/thoracic aneurysm, massive trauma, intracranial bleed with mass effect, ischemic bowel with threat to cardiac pathology or multiple organ/system dysfunction |
| ASA VI | A declared brain-dead patient whose organs are being removed for donor purposes |
BMI, body mass index; DM, diabetes mellitus; HTN, hypertension; COPD, chronic obstructive pulmonary disease; EF, ejection fraction; ESRD, end-stage renal disease; MI, myocardial infarction; CVA, cerebrovascular accident; TIA, transient ischemic attack; CAD, coronary artery disease; PCI, percutaneous coronary intervention; CVA, cerebrovascular accident; DIC, disseminated intravascular coagulopathy; ARF, acute renal failure.
The preanesthesia history will include all the components of the patient’s current and past medical history, surgical history, family history, social history, allergies, current and recent medication regimen, and history of prior anesthetics. It is important to screen for any recent infections, specifically upper and lower respiratory tract infections. Screening for a family history of adverse reactions to anesthetics will help avoid adverse reactions due to hereditary metabolic syndromes. Prenatal and birth history should also be obtained in pediatric cases. A specific assessment of prematurity at birth, perinatal complications, and congenital chromosomal or anatomic abnormalities is also useful.
While obtaining the medication history, it is important to monitor for recent use of medications that may interfere with anesthetic agents. For example, monoamine oxidase inhibitors should be discontinued 2–3 weeks before surgery. Oral contraceptive therapies should also be discontinued at least 6 weeks before elective surgery due to the increased risk of venothromboembolism. Herbal supplements have been considered by the ASA and are recommended to be stopped 2 weeks before surgery.
The preanesthesia physical examination should focus on airway assessment, examination of the heart and lungs, and documentation of the patient’s vital signs. It is important to identify comorbid diseases during history taking and physical examination that would necessitate further workup and consultations. The anesthesia provider should pay special attention to pulmonary, cardiac, renal, central nervous system (CNS), and bleeding disorders, and follow through with appropriate laboratory, study, or imaging [Reference Hernandez, Klock and Ovassapian3]. Table 9.3 describes the indications for specific preoperative tests.
Table 9.3 Indications for specific preoperative tests
Complete blood count
|
International normalized ratio (INR), activated partial thromboplastin time (aPTT)
|
Electrolytes and creatinine
|
Fasting glucose
|
Electrocardiography
|
Chest radiograph
|
Before surgery, recommendations for fasting status of various foods and liquids are provided for GA, regional anesthesia, and procedural sedation and analgesia. The patient may ingest clear liquids for up to 2 hours, breast milk for up to 4 hours, infant formula for up to 6 hours, and solids and nonhuman milk for up to 6 hours. An additional fasting time of up to 8 hours or more is recommended in patients consuming fried foods, fatty foods, or meat [Reference Haas, Eakin, Konkle and Blank4]. Table 9.4 describes the fasting and pharmacologic recommendations for induction of anesthesia.
Table 9.4 Fasting and pharmacologic recommendations
| Fasting recommendations | |
|---|---|
| Ingested material | Minimum fasting period (hours) |
| Clear liquids | 2 |
| Breast milk | 4 |
| Infant formula | 6 |
| Nonhuman milk | 6 |
| Light meal | 6 |
| Fried foods, fatty foods, or meat | 8 or more |
| Pharmacologic recommendations | |
|---|---|
| Medication type and common examples | Recommendation |
Gastrointestinal stimulants:
| May be used/no routine use |
Gastric acid secretion blockers:
| May be used/no routine use |
Antacids:
| May be used/no routine use |
Antiemetics:
| May be used/no routine use |
Anticholinergics:
| No use |
Induction of Anesthesia
Inhalational Induction
Inhalation anesthesia began with the introduction of diethyl ether in 1846. Modern halothane was produced in 1956, leading to a new method of rapid and safe inhalational induction. Inhalational induction with halothane had been primarily utilized in pediatric populations and patients with difficult airways. Sevoflurane was later introduced in the 1990s and gained popularity due to minimal airway irritation. Inhalational induction has been shown to have a perceived advantage over propofol through its avoidance of suppression of airway reflexes.
Sevoflurane can also provide relatively stable cardiac and hemodynamic conditions in patients at risk of hypotension, hypertension, and tachycardia. Studies have also shown similar outcomes when measuring cardiovascular stability during tracheal intubation in hypertensive patients compared to propofol. There are also documented case studies utilizing rapid sequence induction with sevoflurane in cesarean section without any reported adverse advents. Due to changes in the respiratory physiology in pregnant patients, induction is accelerated. It will have a decreased minimum alveolar concentration (MAC) due to increased ventilation with a reduced functional reserve capacity [Reference El-Orbany and Woehlck1].
Intravenous Induction
Inhalational induction was initially the only practiced technique before the introduction of thiopental in 1934. Intravenous induction became a popular method, allowing the care provider more options to utilize safe induction agents. Compared to inhalation anesthesia, specific intravenous anesthetic medications decreased overall side effects. Intravenous induction agents include barbiturates, phenols, imidazoles, phencyclidines, and benzodiazepines [Reference El-Orbany and Woehlck1].
Thiopental is a barbiturate, producing a smooth onset of hypnosis and rapid recovery. There have been reportedly low incidences of restlessness, nausea, and vomiting. The liver slowly metabolizes thiopental via first-order kinetics. Elimination will remain constant with increases in doses. It also acts as a negative inotrope depressing contractility of the heart. This will have a subsequent reduction in cardiac output and blood pressure. It is also common to have respiratory depression and can be seen following a bolus dose.
Propofol is a short-acting general anesthetic medication and the onset of action is approximately 30 seconds. Induction will typically be smooth, and patients generally experience a rapid recovery from anesthesia due to its short half-life of 2–4 minutes. Induction with propofol can be associated with drops in blood pressure, secondary to systemic vasodilation, and can have associated reflex tachycardia.
Etomidate is an imidazole ester and is associated with the least amount of cardiovascular side effects of intravenous anesthetics. It can, however, have cardiovascular effects and can cause a mild reduction in cardiac output and blood pressure. It is rapidly metabolized by the liver, yielding inactive metabolites, and is eliminated via the urine, with a half-life of 1–5 hours. Standard recovery is rapid due to redistribution to muscle and fat. Etomidate was used in shocked, elderly, and cardiovascular-compromised patients but has recently become less commonly used.
Ketamine is a derivative of phencyclidine, which was a formerly used anesthetic agent. It is metabolized in the liver and excreted in the urine. Common associations on induction include tachycardia, hypertension, and increased cardiac output. Due to these effects, it is also chosen as induction agent in shocked and unwell patients. Airway reflexes remain intact and it produces minimal effects on the respiratory drive.
Benzodiazepines, including midazolam and diazepam, are the most commonly used agents for anxiolysis, and α2 inhibitors and melatonin are also used, albeit much less often. Controlling anxiety has demonstrated decreased complications in perioperative care. More specifically, it has decreased hemodynamic instability, decreased anesthetic consumption during anesthesia, and improved postoperative pain, recovery time, and hospital stay [Reference Malhotra, Malhotra, Kumra, Radhakrishnan and Basu5].
Airway Management
Mask Ventilation
Mask ventilation is fundamental to the practice of anesthesia. This basic skill is one of the most important tools in airway management. It is performed before endotracheal intubation and used as a rescue maneuver if intubation is difficult or fails. Obtaining a tight seal with the face mask is an important aspect of successful mask ventilation. The face mask should fit over the bridge of the nose and enclose the mouth, with the bottom positioned between the lower lip and the chin. Leaks may develop if the mask is not the correct size, the cushion is improperly inflated, or the patient has a beard or an abnormal anatomy. Risk factors for difficult mask ventilation include a beard, increased BMI, edentulousness, limited mandibular protrusion, Mallampati III or IV, history of snoring or obstructive sleep apnea (OSA), airway masses or tumors, history of neck radiation, male gender, and age >55. The triple maneuver (jaw thrust, head extension, and chin lift) should be used to maximize pharyngeal patency. Once the triple maneuver is performed, the left hand holds this position, with the mask sealed tightly to the face, while the right hand squeezes the reservoir bag. If maintaining the triple maneuver position or an adequate seal with one hand proves difficult, the two-handed mask ventilation technique should be utilized and a second provider assists ventilation with the bag. An oral or nasal airway may be required to overcome obstruction, resulting from the tongue falling back to the posterior pharyngeal wall. It is important to note that using an oral airway may elicit a gag reflex or cause laryngospasm if the depth of anesthesia is inadequate. A nasal airway is better tolerated in these situations. Ventilating pressure should not exceed 20 cmH2O to avoid gastric distension, regurgitation, and aspiration. If mask ventilation proves difficult or impossible, alternative strategies to ventilate the patient must be attempted such as intubation or placement of a supraglottic airway (SGA) [Reference El-Orbany and Woehlck1, Reference Malhotra, Malhotra, Kumra, Radhakrishnan and Basu5].
Supraglottic Airways
SGA devices are an alternative method to mask ventilation and endotracheal intubation. They have made their way into the difficult airway algorithm with their ease of use and rescue ventilation ability. Quick placement, less sympathetic stimulation, avoidance of neuromuscular blockers, and maintenance of spontaneous ventilation are a few advantages of SGAs over endotracheal tubes (ETTs). SGAs may be used as primary airway devices for selected patients and surgeries, in emergencies in and out of the hospital, to facilitate endotracheal intubation, and most importantly for airway rescue in the unable-to-ventilate-or-intubate situation. Nonfasting status, morbid obesity, and pregnancy are contraindications for laryngeal mask airway (LMA) use as the primary airway. OSA, gastro-esophageal reflux disease (GERD), gastroparesis, and position other than supine are factors that increase the risk of complications when using SGAs [Reference Gordon, Cooper and Parotto2].
The LMA Classic, one of the first SGAs, is a reusable device made of silicone. A disposable, single-use version of the LMA Classic is the LMA Unique. Designed for intraoral procedures, the LMA Flexible has a wire-reinforced shaft to allow flexible positioning away from the surgical site. The LMA Fastrach, an intubating LMA, has a rigid, curved shaft and handle that facilitate placement of an ETT through its ventilating tube with or without the assistance of a fiberoptic scope. The reusable LMA ProSeal was the first LMA designed with a drainage tube to reduce the risk of aspiration. This drainage tube also facilitates placement of an orogastric tube. The LMA ProSeal’s design creates an improved airway seal without adding pressure to the oropharyngeal tissue. A disposable alternative to the LMA ProSeal is the LMA Supreme. In addition to the gastric drainage tube like the LMA ProSeal, the LMA Supreme has a curved shaft like the LMA Fastrach. The LMA Classic Excel is an intubating version of the LMA Classic. The AirQ (disposable) and Intubating Laryngeal Airway (reusable) were designed with unique features to assist endotracheal intubation but may also be used as a primary airway. The Cobra Perilaryngeal Airway (PLA) differs from the previously discussed SGAs with its high-volume, low-pressure pharyngeal cuff that sits just proximal to the cuffless mask. It also allows passage of an ETT. The Esophageal-Tracheal Combitube and the King Laryngeal Tube were designed to achieve ventilation after blind insertion, making them useful for prehospital use or by unskilled operators. Their double cuff design allows ventilation via the larynx by inflating the esophagus’s distal cuff and the hypopharynx’s proximal cuff. The uniquely designed I-GEL device uses a cuffless mask made of a gel material that conforms to the larynx. It has a gastric drainage tube that allows passage of an orogastric tube [Reference Hernandez, Klock and Ovassapian3].
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
