Fig. 3.1 Comparison of simple and balanced anesthesia techniques.

3.2.1 Simple Anesthesia

In simple anesthesia, a single anesthetic agent is given in sufficient dosage to provide unconsciousness (sleep), lack of movement in response to surgical stimulation, and attenuation of the cardiovascular responses to such stimulation. Traditionally, the anesthetic agent was given in the form of a vapor. The patient would inhale this; it would take a minute or two for consciousness to be lost (induction of anesthesia). The vapor would continue to be inhaled for the duration of the surgical procedure (maintenance of anesthesia), and then the vapor would be discontinued and the patient would wake up.

Sometimes the process of inducing anesthesia could be speeded up so as to take only seconds by giving an intravenous (IV) anesthetic agent at the start and before continuing with the maintenance vapor. The idea of following the bolus of IV anesthetic with an IV infusion of the same drug, replacing the anesthetic vapor altogether, is relatively recent.

The child breathes for himself/herself throughout (spontaneous ventilation). If the child is breathing an anesthetic vapor and there is an unnoticed disconnection of the breathing circuit, then the child will continue to breathe, but will now be breathing room air and so start to awaken. Generally, this will produce some movement and thus alert both the surgeon and the mortified anesthetist before full consciousness returns.

3.2.2 Balanced Anesthesia

In the second method, balanced anesthesia, separate drugs are used to produce unconsciousness, paralysis, cardiovascular control and analgesia. These drugs are typically an anesthetic agent, a muscle relaxant, and an opiate, respectively. The patient is given sufficient anesthetic agent to induce unconsciousness, but not enough to stop muscle contraction or movement in response to surgical stimulation. Instead, lack of movement is now produced by the action of the muscle relaxant—a drug that directly but reversibly paralyzes all skeletal muscles. Thus, the patient can no longer breathe for himself/herself, so the anesthetist must ventilate the patient’s lungs, either by hand or by using a mechanical ventilator. Unnoticed disconnection of the breathing tubing in this circumstance will now cause hypoxia with potentially disastrous consequences—neurologic damage or even death—rather than just causing the child to wake up and the surgeon to complain. More sophisticated monitoring to alert the anesthetist to the early signs of hypoxia is therefore essential. Balanced anesthesia also introduces the possibility of accidental awareness if insufficient anesthetic drug is given while the completely paralyzed patient cannot move to convey his/her wakefulness.

There are, of course, many refinements to this basic pattern of anesthesia and so we will examine how these apply to children’s ENT surgery in more detail.

3.3 Induction of Anesthesia

Inhalational induction continues to be widely used, but nowadays IV induction is more popular. This is because it is rapid and safe and avoids having to get the child to cooperate with the use of a face mask. Patient (or parental) preference, an obvious lack of easily cannulated veins, and poor patient tolerance of IV injection (despite use of effective topical anesthesia to the site of injection) are the usual indications for inhalational induction. However, many authorities would consider that the most important indication for inhalational induction in a child to be the presence of a partially obstructed or potentially difficult airway, and this is of obvious relevance to ENT surgery.

3.3.1 Intravenous Induction

IV anesthetic induction agents include thiopentone and midazolam, but propofol has become the ubiquitous agent of modern times because it gives a very rapid and reliable onset of unconsciousness and suppresses laryngeal and upper airway reflexes.

In children, IV induction is facilitated by applying local anesthetic cream (Ametop [Smith & Nephew], EMLA [AstraZeneca]) over a suitable vein beforehand, a small cannula (24 gauge), oral sedative premedication, some distraction for the child, and a good anesthetic assistant (usually in that order).

IV induction takes seconds. One drawback of IV induction is that it usually produces a near-immediate 10 to 30 seconds of apnea. This acute severe reduction of respiratory drive may be used to advantage most of the time, allowing easier insertion of a laryngeal mask or even an endotracheal tube (ETT), but this may be severely disadvantageous in cases of partial airway obstruction where there may be loss of the airway.

3.3.2 Inhalational Induction

Unconsciousness can also be induced using an inhalational anesthetic agent. Worldwide, common inhaled vapors used for anesthesia include halothane, isoflurane, and desflurane, but the most popular agent for inhalational induction in children is sevoflurane. This gives a smooth, cough-free induction within a minute or so (often claimed to be faster still if nitrous oxide is used in the carrier gas, though there is little or no evidence for this ²). After induction, a simple anesthetic may continue (maintenance anesthesia) with either sevoflurane or another inhalational agent, or an infusion of IV agent (propofol) may be used once IV access has been secured. Alternatively, a combination of drugs may be needed for a balanced anesthetic.

3.4 Methods of Control of the Airway

It is essential that the anesthetist has good control of the child’s airway and that the child is protected from the risks of airway obstruction and aspiration. Anesthetists use a variety of methods for this purpose.

3.4.1 Face Mask

The simplest method of controlling the airway in an anesthetized child is to hold a face mask over the mouth and nose. The airway is held open by pulling the angle of the jaw forward, thus pulling the base of the tongue away from the posterior pharyngeal wall. A well-fitting face mask allows spontaneous ventilation to continue or intermittent positive pressure ventilation (IPPV) to be given (“bag-and-mask” ventilation), but it provides no separation of airway and gastrointestinal tract, so the stomach can be distended by gas during bag-and-mask ventilation and there is an ever-present risk of aspiration.

3.4.2 Oropharyngeal and Nasopharyngeal Airways

An oral Guedel airway or a nasopharyngeal airway (usually, in pediatrics, a cut-down ETT) may be inserted to help keep the airway patent.

3.4.3 The Laryngeal Mask Airway

The laryngeal mask airway (LMA; ▶ Fig. 3.2, ▶ Fig. 3.3) has revolutionized anesthesia in the past 20 years. It has all but replaced the use of the face mask for everything other than “bag-and-mask” ventilation prior to intubation and has greatly reduced the numbers of patients undergoing endotracheal intubation. It has popularized a return to “simple” (spontaneously breathing) anesthesia for short- and medium-duration surgery, and in pediatrics, the LMA may also be used as a conduit for flexible fiberoptic scopes, both in cases of difficult airway and for fiberoptic tracheobronchoscopy.

The LMA consists of a small (usually inflatable) collar, which is inserted into the mouth and pushed backward and downward until it comes to rest on the ridge of mucosa overlying cricopharyngeus muscle. In this position, the orifice of the mask should sit behind the inferior surface of the epiglottis, facing the glottic opening. The tubular part of the airway then passes up through the pharynx and mouth to connect up to the anesthetic breathing circuit.

This provides an excellent “anesthetist’s hands-free” airway for spontaneous ventilation and also provides a good enough seal to allow IPPV in most circumstances.

Indeed, it could be argued that by occupying the laryngopharynx, the LMA will direct such liquid into the airway. Newer variants of the laryngeal mask (ProSeal [Teleflex Health]) have a second channel to allow easy passage of a nasogastric tube to try to reduce this problem. The LMA can be readily inserted at a relatively light plane of anesthesia and is particularly easy to insert if propofol is used as an IV induction agent as propofol diminishes the gag and cough reflexes. ³

3.4.4 Endotracheal Tubes

An ETT ( ▶ Fig. 3.4) not only provides a safe and easily accessible airway but also brings about definitive separation of the airway and gastrointestinal tract to reduce the risk of aspiration. In adults, the narrowest part of the airway is the inverted-V-shaped glottis, so in order to provide a leak-free connection between ETT and the airway, a thin cuff about the tube had to be inflated to form a seal.

3.4.5 Cuffed or Uncuffed Endotracheal Tube?

It was argued from cadaveric studies that since the narrowest part of the infant airway was the cricoid, and since this was all but circular in cross-section, then a well-chosen plain (uncuffed) ETT could provide an adequate seal both to allow inflation of the lungs and to protect the airway from overt aspiration of blood or regurgitated gastric contents. ⁴ Later, this view that a cuff was unnecessary developed into the idea that a cuff was positively contraindicated. This came about because there was recognition that following intubation, a proportion of patients suffered damage to the tracheal mucosa in the region of the subglottis, which could, on healing, form a constricting scar which decreased the size of the tracheal lumen, subglottic stenosis. It is thought that the development of this subglottic stenosis relates to the degree of radial force exerted by either a “too large” tube being forced into the trachea or by an overinflated cuff pressing on the subglottic mucosa (see ▶ 21). Thus, cuffed ETTs fell out of use within pediatrics, and the smaller the child, the truer this was. This view has been challenged over recent years. ⁵

There has been a marked increase in the use of cuffed ETTs in children and infants. Reasons for this center on the fact that cuffed tubes provide a better seal of the airway, which, in turn, allows use of low-flow anesthesia with inherently better humidification of gases, better use of mechanical ventilation, more accurate monitoring of end-tidal carbon dioxide, and less environmental pollution. “Sizing” of the tube is easier; if the tube chosen always errs on the side of being too small, then the leak can always be compensated for by inflating the cuff, avoiding the scenario of trying several tubes to get the best tracheal fit. The problem of the radial pressure of the cuff has been addressed in several ways. The cuffs themselves are of higher volume and more cylindrical than “olive shaped” ( ▶ Fig. 3.5) and there is greater emphasis on care with their use, for example, intermittent deflation and continuous monitoring of cuff pressure.

Oral or Nasal Intubation?

In the context of children’s ENT surgery, endotracheal intubation is usually through the oral route, as this is quicker and easier to establish. Endotracheal intubation through the nose allows easier and more secure fixation and is more readily tolerated by lightly sedated patients, and for these reasons, nasal intubation is the preferred route for medium-term intubation (days to weeks) in the intensive care unit.

3.5 Muscle Relaxation (Paralysis) during Anesthesia and Reversal

3.5.1 Paralysis

All movement associated with intubation can be abolished by administering a muscle relaxant (paralyzing agent) before intubation. This gives the best possible conditions for intubation, but commits the anesthetist, first, to knowing that he/she will be able to intubate the patient and, second, to ventilating the patient for the duration of action of the muscle relaxant.

Suxamethonium is unique in that it is a depolarizing muscle relaxant, causing an initial contraction of muscle fibers before inducing paralysis. It is all but no longer used in elective surgery because of the problem of postoperative myalgia, though it retains a use in the emergency situation of the patient with a full stomach at risk of aspiration, where the rapid onset of paralysis is of overriding importance. All the other muscle relaxants are nondepolarizing agents. Some relevant properties of the commonly used muscle relaxants are given in ▶ Table 3.1.

3.5.2 Reversal

In balanced anesthesia for elective surgery, nondepolarizing neuromuscular blocking agents (NMBs) are used to paralyze the patient.

• 
  

  Acetylcholine released into the neuromuscular cleft by the nerve impulse causes the muscle to contract. A nondepolarizer works by binding to an acetylcholine receptor on the muscle end plate, thereby blocking the effect of acetylcholine and inhibiting muscle contraction.

  
  
• 
  

  The binding of NMB to acetylcholine receptors is not permanent and after 40 to 50 minutes there are sufficient “free” acetylcholine receptors for neuromuscular transmission to be reestablished.

  
  
• 
  

  Thus, if a muscle relaxant is used to facilitate intubation, then the anesthetist is usually committed to giving at least 40 to 50 minutes of anesthesia if he/she wishes to allow paralysis to wear off spontaneously.

  
  
• 
  

  This time may be reduced to 15 to 20 minutes if the anesthetist reverses the neuromuscular blockade using neostigmine. This neostigmine is an acetylcholinesterase enzyme inhibitor; it increases the amount of acetylcholine within the neuromuscular cleft, favoring acetylcholine in the competition with the NMB drug for binding with the muscle end plate acetylcholine receptors and so speeding up the restoration of neuromuscular transmission.

Reversal with neostigmine increases acetylcholine concentrations at nerve endings throughout the peripheral nervous system, producing unwanted muscarinic autonomic effects (bronchoconstriction, bradycardia, increased salivation, increased gastrointestinal peristalsis), and so neostigmine needs to be given with an antimuscarinic drug (atropine or glycopyrrolate) to attenuate these effects.

This is often inconvenient in, say, a list of tonsillectomies that may take on average 15 to 20 minutes. If the surgeon is quick, then this necessitates prolonging the anesthetic while the surgeon prowls about room and indulges in rhetorical requests to “send for the next.” If the surgeon is slow and paralysis wears off, the anesthetist has to judge whether to give a second dose of muscle relaxant, risking a rapid conclusion of surgery and further demonstrating surgical impatience, or to eke out inadequate muscle relaxation with deeper anesthesia and loss of the “light plane of anesthesia” advantage of the balanced technique.

Newer Muscle Relaxants and Reversing Agents

There is now an elegant, if presently expensive, solution to the problem of using muscle relaxants with a surgeon of unpredictable speed: use rocuronium as the muscle relaxant and then reverse its action using sugammadex rather than neostigmine.

• 
  

  In using neostigmine to restore neuromuscular transmission, there need to be sufficient free acetylcholine receptors on the muscle end plate with which acetylcholine can interact. Immediately after a dose of nondepolarizing muscle relaxant is given, virtually all of the acetylcholine receptors are occupied with molecules of nondepolarizing drug, and so giving neostigmine will be entirely ineffective. It needs some time to elapse (usually at least 20 minutes) before sufficient acetylcholine receptors on the muscle end plate are free of their occupation by molecules of nondepolarizing agent.

  
  
• 
  

  Sugammadex has an entirely different mechanism of action. It is a rocuronium-chelating drug that “removes” the muscle relaxant from the neuromuscular junction. Thus, it does not require any time to have elapsed before it can be used, allowing earlier and complete reversal of neuromuscular blockade. ⁶

3.6 Duration of Surgery

Difficulty in predicting duration of surgery has given rise to another approach: trying to combine the good intubating conditions of the relaxant technique with the flexibility of duration of the simple anesthetic approach, avoiding the use of muscle relaxants completely.

• 
  

  The patient is induced with a generous dose of propofol.

  
  
• 
  

  Then the laryngeal and cough reflexes are further suppressed with a dose of short-acting opiate (fentanyl, alfentanil, or the ultrashort-acting remifentanil). This is usually sufficient to allow smooth intubation (by a smooth anesthetist).

  
  
• 
  

  Following intubation, the patient’s lungs are mechanically ventilated using deeper anesthesia rather than muscle relaxants to suppress coughing and bucking.

  
  
  
  

  
  

  Skilled exponents of this technique can combine tranquil operating conditions with a turnover sufficiently rapid as to leave the surgeon with several cold half-consumed cups of coffee at the end of a well-run list.

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