General Anesthesia 

Updated: Jun 07, 2018
  • Author: Adam C Adler, MD, MS, FAAP, FASE; Chief Editor: Meda Raghavendra (Raghu), MD  more...
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General Considerations

General anesthesia (GA) is the state produced when a patient receives medications to produce amnesia and analgesia with or without reversible muscle paralysis. An anesthetized patient can be thought of as being in a controlled, reversible state of unconsciousness. Anesthesia enables a patient to tolerate surgical procedures that would otherwise inflict unbearable pain, potentiate extreme physiologic exacerbations, and result in unpleasant memories.

The combination of anesthetic agents used for general anesthesia often leaves a patient with the following clinical constellation: [1]

  1. Unarousable to painful stimuli
  2. Unable to remember what happened (amnesia)
  3. Unable to maintain adequate airway protection and/or spontaneous ventilation as a result of muscle paralysis
  4. Cardiovascular changes secondary to stimulant/depressant effects of anesthetic agents

General anesthesia

General anesthesia is induced and maintained using a combination of intravenous and inhaled agents. A point worth noting is that general anesthesia may not always be the best choice; depending on a patient’s clinical presentation, local or regional anesthesia may be more appropriate. [2, 3, 4]

Anesthesia providers are responsible for assessing all factors that influence a patient's medical condition and selecting the optimal anesthetic technique accordingly.

Advantages of general anesthesia include the following:

  • Reduces intraoperative patient awareness and recall [5]
  • Allows use of muscle relaxants
  • Facilitates complete control of the airway, breathing, and circulation
  • Can be used in cases of sensitivity to local anesthetic agent
  • Can be administered without moving the patient from the supine position
  • Can be adapted easily to procedures of unpredictable duration or extent
  • Can be administered rapidly and is reversible

Disadvantages of general anesthesia include the following:

  • Requires, at minimum, some degree of preoperative patient preparation
  • Requires increased complexity of care and associated costs
  • Requires some degree of preoperative patient preparation
  • May induce physiologic fluctuations that require active intervention
  • Associated with less serious complications such as nausea and vomiting, sore throat, headache, and shivering
  • Associated with malignant hyperthermia, an exceedingly rare, inherited muscular condition in which exposure to some (but not all) general anesthetic agents results in acute and potentially lethal temperature rise, hypercarbia, metabolic acidosis, and hyperkalemia

With modern advances in medications, monitoring technology, and safety systems, as well as highly educated anesthesia providers, the risk caused by anesthesia to a patient undergoing routine surgery is extremely remote. Mortality attributable to general anesthesia is said to occur at rates of less than 1:100,000. Minor adverse events occur at more frequent rates, even in previously healthy patients. The more commonly encountered adverse events related to anesthesia include:

  • Vomiting
  • Nausea
  • Sore throat
  • Incisional site pain
  • Emergence delirium (in children)
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Preoperative Period

A general anesthetic can be broken down into three distinct phases, the pre-, intra-, and post-operative periods.  

The pre-operative phase involves patient preparation from the time surgery is scheduled until the patient enters the operating room.

Preparation for general anesthesia

Safe and efficient anesthetic practices require certified personnel, appropriate medications and equipment, and an optimized patient.

Minimum requirements for general anesthesia

Minimum infrastructure requirements for general anesthesia include a well-lit space of adequate size; a source of pressurized oxygen (most commonly piped in); an effective suction device; standard ASA (American Society of Anesthesiologists) monitors, continuous EKG monitoring, blood pressure (minimum 5-minute intervals), continuous pulse oximetry, capnography, temperature; and inspired and exhaled concentrations of oxygen and applicable anesthetic agents. [6]

Beyond this, some equipment is needed to deliver the anesthetic agent. This may be as simple as needles and syringes, if the drugs are to be administered entirely intravenously. In most circumstances, this means the availability of a properly serviced and maintained anesthetic gas delivery machine.

An array of routine and emergency drugs, advanced airway management equipment, a cardiac defibrillator, and a recovery room staffed by properly trained individuals is required when general anesthesia is delivered. When agents known to trigger malignant hyperthermia are used, medications to treat this condition, including dantrolene sodium, must be immediately available.

Preparing the patient

A thorough preoperative plan should ensue prior to induction of general anesthesia and, ideally, in advance of the operative day.

Preoperative anesthesia evaluation allows for obtainment of indicated laboratory tests, imaging procedures, or additional medical consultations when warranted. Complete history should be attained with attention to any new, ongoing, or worsening medical conditions, previous personal or familial adverse reactions to general anesthetics, assessment of functional cardiac and pulmonary states, and allergy and medication history. Preoperative evaluation also helps to relieve anxiety of the unknown surgical environment for patients and their families as well as reduce the likelihood of same-day case cancellation. Overall, this process allows for optimization of the patient in the perioperative setting. [7]

Physical examination associated with preoperative evaluations allow anesthesia providers to focus specifically on expected airway conditions, including mouth opening, loose or problematic dentition, limitations in neck range of motion, neck anatomy, and Mallampati presentations (see below). By combining all factors, an appropriate plan for intubation can be outlined and extra steps, if necessary, can be taken to prepare for fiberoptic bronchoscopy, video laryngoscopy, or various other difficult airway interventions.

Airway management

Possible or definite difficulties with airway management include the following:

  • Small or receding jaw
  • Small mouth opening
  • Large overbite
  • History of difficult intubation and/or mask ventilation
  • Extremes of obesity
  • Prominent maxillary teeth
  • Short neck and thick neck
  • Limited neck extension
  • Poor dentition
  • Tumors of the head and neck
  • Facial trauma
  • Interdental fixation
  • Hard cervical collar
  • Unstable neck or atlantoaxial instability
  • Halo traction
  • Microtia or mid-facial hypoplasia

Various scoring systems have been created using orofacial measurements to predict difficult intubation. The most widely used is the Mallampati score, which identifies patients in whom the pharynx is not well visualized through the open mouth.

The Mallampati assessment is ideally performed when the patient is seated with the mouth open and the tongue protruding without phonating. In many patients intubated for emergent indications, this type of assessment is not possible. A crude assessment can be performed with the patient in the supine position to gain an appreciation of the size of the mouth opening and the likelihood that the tongue and oropharynx may be factors in successful intubation (see image below).

Mallampati classification. Mallampati classification.

High Mallampati scores have been shown to be predictive of difficult intubations. [8]  However, no one scoring system is fully predictive and providers should also anticipate the unexpected difficult airway.

In addition to intubation during surgery, some patients may require unanticipated early postoperative intubation. A large-scale study of 109,636 adult patients undergoing non-emergent, non-cardiac surgery identified risk factors for postoperative intubation. Independent predictors include patient comorbidities such as chronic obstructive pulmonary disease, insulin-dependent diabetes, active congestive heart failure, and hypertension. Severity of surgery is also an identified risk factor. Half of unanticipated tracheal intubations occurred within the first 3 days after surgery and were independently associated with a 9-fold increase in mortality. [9]

When suspicion of an adverse event is high but a similar anesthetic technique must be used again, obtaining records and previous anesthetic records from previous operations or from other institutions may be necessary.

Other requirements

For induction of general anesthesia, it is important that the patient is properly fasted to prevent untoward events such as pulmonary aspiration. Of course, emergency cases are not delayed until fasting times are met. Patients should be instructed as to the ASA guidelines on perioperative fasting. Unnecessarily long fasting times should be avoided to reduce dehydration, postinduction hypotension, and patient dissatisfaction.

  • Published guidelines recommend that solid food (including gum or candy) should be avoided for 6 hours prior to the induction of anesthesia. [10]
  • Clear fluids (e.g., water, Pedialyte, Gatorade) should be avoided for 2-4 hours prior to the induction of anesthesia.It is however preferable to consume clear liquids in advance of surgery to reduce dehydration. [11]

Table. (Open Table in a new window)

American Society of Anesthesiology Preoperative NPO Guidelines

Food

NPO requirement

Example

Clear Liquids

2 hours

Apple juice, water (NO orange juice)

Breast Milk

4 hours

Unfortified

Infant Formula

6 hours

Unfortified

Non-Human Milk

6 hours

Almond milk, soy milk, unfortified

Light Meal

6 hours

Tea and toast (no added fats, like butter)

Full Meal

8 hours

Fatty meal

Patients should continue to take regularly scheduled medications up to and including the morning of surgery. Exceptions may include the following:

  • Anticoagulants to avoid increased surgical bleeding
  • Oral hypoglycemics (e.g., metformin is an oral hypoglycemic agent that is associated with the development of metabolic acidosis under general anesthesia)
  • Monoamine oxidase inhibitors
  • Certain anti-hypertensive agents (e.g., ACE inhibitors) that may promote anesthesia induced hypotension
  • Beta blocker therapy should be continued perioperatively for high-risk patients undergoing major noncardiac surgery [12]  
  • Many non-prescription medications, herbal supplements, vitamins can interfere with anesthesia and surgery, especially those with anticoagulant properties (e.g., garlic)

Recent catastrophes under anesthesia have focused attention on the interaction between non-prescribed medications and anesthetic drugs, including interactions with vitamins, herbal preparations, traditional remedies, and food supplements. Good information on the exact content of these supplement preparations is often hard to obtain. [13]

Premedication

This stage, which is usually conducted in the surgical ward or in a preoperative holding area, originated in the early days of anesthesia, when morphine and scopolamine were routinely administered to make the inhalation of highly pungent ether and chloroform vapors more tolerable.

The goal of premedication is to have the patient arrive in the operating room in a calm, relaxed frame of mind. The most commonly used premedication is midazolam, a short-acting benzodiazepine that has the benefit of providing antegrade amnesia. Oral midazolam syrup is often given to children to facilitate calm separation from their parents prior to anesthesia.

In anticipation of surgical pain, nonsteroidal anti-inflammatory drugs or acetaminophen can be administered preemptively. When a history of gastroesophageal reflux exists, H2 blockers and antacids may be administered.

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Intraoperative Period

Induction

Induction of anesthesia can be accomplished by inhalation of anesthetic gases or by use of intravenous agents, or both.

For the most part, contemporary practice dictates that adult patients and most children aged at least 10 years be induced with intravenous drugs, this being a rapid and minimally unpleasant experience for the patient. However, sevoflurane, a well-tolerated anesthetic vapor, allows for elective inhalation induction of anesthesia in adults. In addition to the induction drug, most patients receive an injection of an opioid analgesic. Induction agents and opioids work synergistically to induce anesthesia. In addition, anticipation of events that are about to occur, such as endotracheal intubation and incision of the skin, generally raises the blood pressure and heart rate of the patient. Opioid analgesia helps reduce this undesirable response, which can prove catastrophic in patients with severe cardiac disease.

The next step of the induction process is securing the airway. This may be a simple matter of manually holding the patient's jaw such that his or her natural breathing is unimpeded by the tongue, or it may demand the insertion of a prosthetic airway device such as a laryngeal mask airway or endotracheal tube. [14]  Various factors are considered when making this decision. The major decision is whether the patient requires placement of an endotracheal tube. Potential indications for endotracheal intubation under general anesthesia may include the following:

  • Potential for airway contamination (full stomach, gastroesophageal [GE] reflux, gastrointestinal [GI] or pharyngeal bleeding)
  • Surgical need for muscle relaxation
  • Predictable difficulty with endotracheal intubation or airway access (eg, lateral or prone patient position)
  • Surgery of the mouth or face
  • Prolonged surgical procedure

If surgery is taking place in the abdomen or thorax, an intermediate or long-acting muscle relaxant drug is administered in addition to the induction agent and opioid. This paralyzes muscles indiscriminately, including the muscles of breathing. Therefore, the patient's lungs must be ventilated under pressure, necessitating an endotracheal tube.

Persons who, for anatomic reasons, are likely to be difficult to intubate are usually intubated electively at the beginning of the procedure, using a flexible or rigid videoscope or another advanced airway tool.

Maintenance of anesthesia

For the duration of the procedure, a plane of anesthesia is maintained using either continuous inhalation or intravenous agents, either alone or in combination. For certain cases, it is preferable to use a total intravenous anesthetic (e.g., scoliosis surgery).

Most commonly, maintenance of anesthesia is performed by continuous inhalation of anesthetic gases. These may be inhaled as the patient breathes spontaneously or delivered under pressure by each mechanical breath of a ventilator.

The maintenance phase is usually the most stable part of the anesthesia. However, understanding that anesthesia is a continuum of different depths is important. A level of anesthesia that is satisfactory for surgery to the skin of an extremity, for example, would be inadequate for major abdominal surgery.

Appropriate levels of anesthesia must be chosen both for the planned procedure and for its various stages. In complex plastic surgery, for example, a considerable period of time may elapse between the completion of the induction of anesthetic and the incision of the skin. During the period of skin preparation, urinary catheter insertion, and marking incision lines with a pen, the patient is not receiving any noxious stimulus. This requires a very light level of anesthesia, which must be converted rapidly to a deeper level just before the incision is made. As the procedure progresses, the level of anesthesia is adjusted to provide the minimum amount of anesthesia that is necessary to ensure adequate anesthetic depth. This requires experience and judgment. The specialty of anesthesiology is working to develop reliable methods to avoid cases of awareness under anesthesia.

Excessive anesthetic depth, on the other hand, is associated with decreased heart rate and blood pressure, and, if carried to extremes, can jeopardize perfusion to vital organs. Short of these serious misadventures, excessive depth results in slower awakening and more adverse effects.

As the surgical procedure draws to a close, the patient's emergence from anesthesia is planned. Experience and close communication with the surgeon enable the anesthesia provider to predict the time when the operation will be complete.

Excess muscle relaxation is reversed using specific drugs and an adequate long-acting opioid analgesic for continued analgesia in the post-operative period. Removal of a placed airway management device is performed only after the patient has met a long list of extubation criteria.

Thermoregulation may also prove challenging during general anesthesia as the normal shivering thermogenesis is blunted in addition to drug-induced vasodilation. Use of forced air warmers in addition to warming the external environment can be helpful. In cases with neonates, the room should be pre-warmed in addition to using forced air warmers and external heat lamps. Severe hypothermia may result in coagulopathy, delayed awakening, or arrhythmia.

Commonly Used Anesthetic Drugs

Numerous choices exist for every aspect of anesthetic care; the way in which they are sequenced depends partially on the personal preference of the person administering them.

Induction agents

Propofol, a non-barbiturate intravenous anesthetic, has displaced barbiturates in many anesthesia practices.

  • The use of propofol is associated with less postoperative nausea and vomiting and a more rapid return of cognition.
  • In addition to being an excellent induction agent, propofol can be administered by slow intravenous infusion instead of inhaled anesthetic agents to maintain the anesthesia.
  • Among its disadvantages are the facts that it often causes pain on injection and that it is prepared in a lipid emulsion, which, if not handled using meticulous aseptic precautions, can be a medium for rapid bacterial growth.

Anesthetic inhalation agents (gases)

These are highly potent chlorofluorocarbons, which are delivered with precision from vaporizers and directly into the patient's inhaled gas stream. They may be mixed with nitrous oxide, a much weaker but nonetheless useful anesthetic gas.

In the late 1990s, desflurane and sevoflurane came into use. These inhaled anesthetics are much more maneuverable than their predecessors and are associated with a more rapid emergence from anesthesia.

Anesthesia can also be induced by inhalation of a vapor. Based on its chemical profile, sevoflurane is most commonly used for this purpose. Inhalation agents are delivered by a vaporizer that converts liquid anesthetic to gas for inhalation. Each gas requires its own unique vaporizer to deliver a predetermined concentration that varies based on the chemical property of the agent in use. The required concentration (dose) of anesthesia gases varies based mostly by patient age and to a lesser degree on other physiological patient factors.  

Traditional opioid analgesics

Morphine, meperidine, and hydromorphone are widely used in anesthesia as well as in emergency departments, surgical wards, and obstetric suites.

In addition, anesthesia providers have at their disposal a range of synthetic opioids, which, in general, cause less fluctuation in blood pressure and are shorter acting. These include fentanyl, sufentanil, and remifentanil.

Muscle relaxants

Succinylcholine, a rapid-onset, short-acting depolarizing muscle relaxant, has traditionally been the drug of choice when rapid muscle relaxation is needed.

Non-depolarizing muscle relaxants are most commonly used that provide reversible inhibition at the neuromuscular junction. The typical duration of action of single administration is between 30 and 60 minutes but varies by medication and is significantly prolonged in continuous or repeated administration.  

Muscle relaxants generally are excreted by the kidney, but some preparations are broken down by plasma enzymes and can be used safely in patients with partial or complete renal failure.

Newly available is sugammadex, the reversal agent for non-depolarizing muscle relaxants that binds to the active drug resulting in inhibition of action. [15]

Positioning

When inducing general anesthesia, patients can no longer protect their airway, provide effective respiratory effort, or protect themselves from injury. For these reason, ideal positioning for general anesthesia is extremely important and can help prevent potential injuries and devastating consequences.

Positioning for induction of general anesthesia

When inducing general anesthesia, the patient is no longer able to protect their airway or provide an effective respiratory effort. The goal of care is to provide adequate ventilation and oxygenation during general anesthesia. Patients are evaluated in the preoperative period for the signs of difficult mask ventilation and/or intubation. Positioning is especially important in morbidly obese patients. The body habitus of these patients can make them difficult to ventilate and intubate.

Ideal masking and intubating position is called the "sniffing" position. This is obtained by lifting the patients chin upward (when supine) so as to look, from a profile view, that the patient is sniffing the air. Doing this in addition with lifting the mandible forward (to remove the tongue from the oropharynx) facilitates easiest mask ventilation.

In obese patients, it is often difficult to mask ventilate and intubate owing to their body habitus. When mask ventilating, even with perfect technique, there is often excess tissue on the chest wall, which will make it difficult to properly ventilate at low pressures, so as not to inflate the stomach with air during attempted ventilation. Often, obese patients are ramped at a 30° angle to help improve the mask ventilation and intubation.

When attempting intubation, the goal of positioning is to align the tragus of the ear with the level of the sternum. This improves intubating conditions and creates direct visualization of the vocal cords when performing direct laryngoscopy.

Positioning during general anesthesia

When a patient is under general anesthesia, all protective reflexes are lost, so providers must be very careful to position the patient. The primary concerns of positioning are ocular injuries, peripheral nerve injuries, musculoskeletal injuries, and skin injuries. [16]

Initially after induction of anesthesia, eyelids should be gently taped down in a closed position. This helps prevent corneal injury by accidental scratching of the cornea. Another ocular injury that can be made less likely during surgical positioning is to prevent ocular venous congestion, which can cause perioperative vision loss. This is often seen in prone patient who develops increased ocular pressure either through mechanical force on the eye or increased venous congestion, especially prevalent in long surgeries for which there is major blood loss (e.g., scoliosis).

Another concern during general anesthesia is peripheral nerve injuries. The most common peripheral nerve injuries are ulnar nerve, common peroneal nerve, and brachial plexus injuries. These can be prevented with appropriate positioning, padding, and vigilance during general anesthesia. The arms should be at less than 90° in relation to the body. Gel/foam padding should be used for superficial nerves (e.g., ulnar nerve in the ulnar groove-lateral epicondyle of elbow). Prevent positioning up against hard objects (e.g., metal, plastic). Prevent hyperextension/flexion of the spine or neck.

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Postoperative Period

After awakening, patients typically recover in the post anesthesia care unit (PACU). In more critically ill patients, recovery may occur directly in the intensive care unit.

Patients recover in the recovery unit until they have met PACU discharge criteria. The criteria for discharge from phase 1 to phase 2 of PACU are often based on the (modified) Aldrete score, which includes adequate activity, circulation, consciousness oxygen saturation, and maintenance of respiration.

Phase 2 of PACU should be met prior to discharging the patient home. This includes the ability to maintain appropriate surgical site dressings, adequate pain control, normothermia, ambulation ability, absence of nausea, and omitting and stable vital signs.

All patients undergoing a general anesthetic at minimum must have a post-operative note that documents many of these items (institution dependent). Ideally, the patient should be queried after return to baseline cognition when more clandestine issues may be addressed (e.g., corneal abrasions and extremely rarely, awareness under anesthesia).

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