General Anesthesia is ...

Article From : http://emedicine.medscape.com/

Author

Christopher D Press, MD Resident Physician, Department of Anesthesiology, Stanford University School of Medicine

Christopher D Press, MD is a member of the following medical societies: American Medical Association,American Society of Anesthesiologists

Disclosure: Nothing to disclose.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Mark E Krugman, MD Assistant Professor of Plastic Surgery, Clinical Professor of Otolaryngology-Head and Neck Surgery, University of California at Irvine School of Medicine

Mark E Krugman, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Surgeons, American Society of Plastic Surgeons, American Academy of Facial Plastic and Reconstructive Surgery, American Society for Aesthetic Plastic Surgery, American Society for Laser Medicine and Surgery

Disclosure: Nothing to disclose.

Chief Editor

Meda Raghavendra (Raghu), MD Associate Professor, Interventional Pain Management, Department of Anesthesiology, Chicago Stritch School of Medicine, Loyola University Medical Center

Meda Raghavendra (Raghu), MD is a member of the following medical societies: American Society of Anesthesiologists, American Society of Regional Anesthesia and Pain Medicine, American Association of Physicians of Indian Origin

Disclosure: Nothing to disclose.

Acknowledgements

Arjun M Desai, MD Resident Physician, Department of Anesthesiology, Stanford University Hospital and Clinics

Disclosure: Nothing to disclose.

Rick Green, MD Consulting Staff, Department of Surgery, Division of Plastic Surgery, St John Medical Center of Longview

Rick Green, MD is a member of the following medical societies: Alpha Omega Alpha

Disclosure: Nothing to disclose.

Alex Macario, MD, MBA Professor of Anesthesia, Program Director, Anesthesia Residency, Professor (Courtesy), Department of Health Research and Policy, Stanford University School of Medicine

Alex Macario, MD, MBA is a member of the following medical societies: American Medical Association,American Society of Anesthesiologists, California Medical Association, and International Anesthesia Research Society

Disclosure: Covidien Consulting fee Consulting; Merck Consulting fee Consulting

Saifee Rashiq, BM BS, MSc (Epid) DA (UK), FRCPC Associate Professor, Director, Division of Pain Medicine, Department of Anesthesiology and Pain Medicine, University of Alberta Faculty of Medicine and Dentistry, Canada

Saifee Rashiq, BM BS, MSc (Epid) DA (UK), FRCPC is a member of the following medical societies: College of Physicians and Surgeons of Alberta and Royal College of Physicians and Surgeons of Canada

Disclosure: Nothing to disclose.

General Considerations
General anesthesia (GA) is the state produced when a patient receives medications for amnesia, analgesia, muscle paralysis, and sedation. 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:

Unarousable even secondary to painful stimuli
Unable to remember what happened (amnesia)
Unable to maintain adequate airway protection and/or spontaneous ventilation as a result of muscle paralysis
Cardiovascular changes secondary to stimulant/depressant effects of anesthetic agents
General anesthesia
General anesthesia uses intravenous and inhaled agents to allow adequate surgical access to the operative site. 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.

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 
Allows proper muscle relaxation for prolonged periods of time
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 increased complexity of care and associated costs
Requires some degree of preoperative patient preparation
Can induce physiologic fluctuations that require active intervention
Associated with less serious complications such as nausea or vomiting, sore throat, headache, shivering, and delayed return to normal mental functioning
Associated with malignant hyperthermia, a 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 very small. Mortality attributable to general anesthesia is said to occur at rates of less than 1:100,000. Minor complications occur at predicable rates, even in previously healthy patients. The frequency of anesthesia-related symptoms during the first 24 hours following ambulatory surgery is as follows:[6]

Vomiting - 10-20%
Nausea - 10-40%
Sore throat - 25%
Incisional pain - 30%

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, including heart rate, blood pressure, ECG, pulse oximetry, capnography, temperature; and inspired and exhaled concentrations of oxygen and applicable anesthetic agents.

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, including Dantrolene sodium (the specific treatment for malignant hyperthermia), airway management equipment, a cardiac defibrillator, and a recovery room staffed by properly trained individuals completes the picture.

Preparing the patient
The patient should be adequately prepared. The most efficient method is for the patient to be reviewed by the person responsible for giving the anesthetic well in advance of the surgery date.

Preoperative evaluation allows for proper laboratory monitoring, attention to any new or ongoing medical conditions, discussion of any previous personal or familial adverse reactions to general anesthetics, assessment of functional cardiac and pulmonary states, and development of an effective and safe anesthetic plan. It also serves to relieve anxiety of the unknown surgical environment for patients and their families. Overall, this process allows for optimization of the patient in the perioperative setting.

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
Prominent maxillary teeth
Short neck
Limited neck extension
Poor dentition
Tumors of the face, mouth, neck, or throat
Facial trauma
Interdental fixation
Hard cervical collar
Halo traction
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.

Mallampati classification. 
Mallampati classification.
High Mallampati scores have been shown to be predictive of difficult intubations. However, no one scoring system is near 100% sensitive or 100% specific. As a result, practitioners rely on several criteria and their experience to assess the 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 nonemergent, noncardiac 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.

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
The need for coming to the operating room with an empty stomach is well known to health professionals and the lay public. The reason for this is to reduce the risk of pulmonary aspiration during general anesthesia when a patient loses his or her ability to voluntarily protect the airway.

Published guidelines recommend that solid food (including gum or candy) should be avoided for 6 hours prior to the induction of anesthesia. 
Clear fluids (ie, water, Pedialyte, or Gatorade ONLY; no other liquids) should be avoided for 2-4 hours prior to the induction of anesthesia. 
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 (For example, metformin is an oral hypoglycemic agent that is associated with the development of metabolic acidosis under general anesthesia.)
Monoamine oxidase inhibitors
Beta blocker therapy (However, beta blocker therapy should be continued perioperatively for high-risk patients undergoing major noncardiac surgery.  )
Recent catastrophes under anesthesia have focused attention on the interaction between nonprescribed 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.

The Process of Anesthesia
Premedication
Premedication is the first stage of a general anesthetic.

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. Most patients do not want to have any recollection of entering the operating room.

The most commonly used premedication is midazolam, a short-acting benzodiazepine. For example, 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.

Drying agents (eg, atropine, scopolamine) are now only administered routinely in anticipation of a fiberoptic endotracheal intubation.

Induction
The patient is now ready for induction of general anesthesia, a critical part of the anesthesia process.

In many ways, induction of general anesthesia is analogous to an airplane taking off. It is the transformation of a waking patient into an anesthetized one. The role of the anesthesia provider is analogous to the role of the pilot, checking all the systems before taking off. The mnemonic DAMMIS can be used to remember what to check ( D  rugs, A  irway equipment, M  achine, M  onitors, I  V, S uction).

This stage can be achieved by intravenous injection of induction agents (drugs that work rapidly, such as propofol), by the slower inhalation of anesthetic vapors delivered into a face mask, or by a combination of 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, such as fentanyl (a synthetic opioid many times more potent than morphine). Many synthetic and naturally occurring opioids with different properties are available. 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 control this undesirable response.

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. 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
Not all surgery requires muscle relaxation.

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 fiberoptic bronchoscope or other advanced airway tool. This prevents a situation in which attempts are made to manage the airway with a lesser device, only for the anesthesia provider to discover that oxygenation and ventilation are inadequate. At that point during a surgical procedure, swift intubation of the patient can be very difficult, if not impossible.

Maintenance phase
At this point, the drugs used to initiate the anesthetic are beginning to wear off, and the patient must be kept anesthetized with a maintenance agent.

For the most part, this refers to the delivery of anesthetic gases (more properly termed vapors) into the patient's lungs. 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 manipulation of the bowel.

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. When the anesthesia provider and surgeon are not accustomed to working together, good communication (eg, warning of the start of new stimuli, such as moving the head of an intubated patient or commencing surgery) facilitates preemptive deepening of the anesthetic. This maximizes patient safety and, ultimately, saves everyone's time.

As the procedure progresses, the level of anesthesia is altered to provide the minimum amount of anesthesia that is necessary to ensure adequate anesthetic depth. Traditionally, this has been a matter of clinical judgment, but new processed EEG machines give the anesthesia provider a simplified output in real time, corresponding to anesthetic depth. These devices have yet to become universally accepted as vital equipment.

If muscle relaxants have not been used, inadequate anesthesia is easy to spot. The patient moves, coughs, or obstructs his airway if the anesthetic is too light for the stimulus being given.

If muscle relaxants have been used, then clearly the patient is unable to demonstrate any of these phenomena. In these patients, the anesthesia provider must rely on careful observation of autonomic phenomena such as hypertension, tachycardia, sweating, and capillary dilation to decide whether the patient requires a deeper anesthetic.

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 of vital organs or be fatal. 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 at which the application of dressings and casts will be complete.

In advance of that time, anesthetic vapors have been decreased or even switched off entirely to allow time for them to be excreted by the lungs.

Excess muscle relaxation is reversed using specific drugs and an adequate long-acting opioid analgesic to keep the patient comfortable in the recovery room.

If a ventilator has been used, the patient is restored to breathing by himself, and, as anesthetic drugs dissipate, the patient emerges to consciousness.

Emergence is not synonymous with removal of the endotracheal tube or other artificial airway device. This is only performed when the patient has regained sufficient control of his or her airway reflexes.

Anesthesia Drugs in Common Use
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
For 50 years, the most commonly used induction agents were rapidly acting, water-soluble barbiturates such as thiopental, methohexital, and thiamylal. These drugs are not commonly in use today.

Propofol, a nonbarbiturate 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, clear-headed recovery.
In addition to being an excellent induction agent, propofol can be administered by slow intravenous infusion instead of vapor 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.
Anesthesia can also be induced by inhalation of a vapor. This is how all anesthetics were once given and is a common and useful technique in uncooperative children. It is reemerging as a choice in adults. Sevoflurane is most commonly used for this purpose.

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.

For decades, anesthesia providers have used it extensively despite numerous predictable and unpredictable adverse effects associated with its use.
The search for a drug that replicates its onset and offset speed without its adverse effects is the holy grail of muscle relaxant research.
Other relaxants have durations of action ranging from 15 minutes to more than 1 hour.

Older drugs in this class, such as pancuronium or curare, were often associated with changes in heart rate or blood pressure. Newer muscle relaxants are devoid of these adverse properties.

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.

Anesthetic vapors
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.

The prototype of modern anesthetic vapors is halothane. It is no longer used in routine clinical practice. In the 1980s, it was displaced by isoflurane and enflurane, agents that were cleared from the lungs faster and thus were associated with more rapid anesthetic emergences.

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.

Intense commercial interest is present in anesthesia drug research, and the continuous introduction of new and better drug products for many years to come seems inevitable.

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, he or she has lost all protective reflexes, 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.

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.

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 (eg, ulnar nerve in the ulnar groove-lateral epicondyle of elbow). Prevent positioning up against hard objects (eg, metal, plastic). Prevent hyperextension/flexion of the spine or neck. Exercise vigilance by checking positioning every 15 minutes during general anesthesia.