What should be verified with the anesthesiologist prior to start of the surgery?

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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