1512 SECTION XIV Pediatric Critical Care Anesthesia Principles in the Pediatric Intensive Care Unit Basic Airway Management Optimal airway management is dependent on a brisk assessment of the patient’[.]
1512 S E C T I O N X I V Pediatric Critical Care: Anesthesia Principles in the Pediatric Intensive Care Unit Basic Airway Management Optimal airway management is dependent on a brisk assessment of the patient’s breathing and prompt determination of the level of intervention required, if any In virtually every setting in which respiratory difficulty is suspected, oxygen should be administered until the specific abnormality can be identified and adequately treated Although extreme hypercarbia is usually well tolerated, hypoxia is routinely catastrophic and not always obvious on initial examination The alveolar air equation clearly demonstrates that hypercarbia produces hypoxia at low fractions of inspired oxygen (Fio2; Table 127.1) If the patient is breathing spontaneously, attention should be directed first to signs of upper airway obstruction, including inspiratory and/or expiratory stridor, noisy breathing, lack of audible or palpable air flow, or a rocking chest and abdominal motion rather than the normal, smooth rise and fall that should occur with inspiration and expiration An alert child with normal neuromuscular function usually assumes a position that minimizes upper airway obstruction (tripod position) instinctively However, a child with an altered level of consciousness or severe neuromuscular weakness may be unable to maintain a patent airway because of the inability to either alter the position or maintain adequate glossopharyngeal muscle tone Nasopharyngeal Airway A nasopharyngeal airway that extends through nasal passages to the posterior pharynx and beyond the base of the tongue is often adequate to relieve obstruction It is tolerated by most patients, even those who are conscious (Fig 127.2) The largest airway that TABLE Impact of Providing Supplemental Oxygen 127.1 During Hypercarbia on Alveolar Oxygen Tension Alveolar gas equation Pao2 Fio2 (PB – PH2O) – Paco2/0.8 Room air, normocarbia Pao2 0.21 (760 – 47) – 40/0.8 99 mm H2O Room air, hypercarbia Pao2 0.21 (760 – 47) – 80/0.8 50 mm H2O Supplemental O2, hypercarbia Pao2 0.4 (760 – 47) – 80/0.8 185 mm H2O Fio2, Fraction of inspired oxygen; Paco2, partial pressure of carbon dioxide; Pao2, partial pressure of oxygen, alveolar; PH2O, partial pressure of water; PB, barometric pressure • Fig 127.2 Nasopharyngeal good position (left) and oropharyngeal (right) airways in is able to pass through nasal passages without causing blanching of the skin surrounding the nares is extended from the nares to the tragus of the ear The airway tube should be well lubricated before placement Risks of nasopharyngeal airways include nasal ulceration, bleeding, laceration of friable lymphoid tissue, rupture of a pharyngeal abscess, laryngospasm, and potential passage through the cribriform plate in patients with basilar skull fractures Topical vasoconstricting agents reduce but not eliminate the risk of bleeding Like other nasal tubes, use of nasal airways increases the risk of sinusitis; therefore, contraindications to their use include severe coagulopathy, cerebrospinal fluid (CSF) leaks, and basilar skull fractures Oropharyngeal Airways Oropharyngeal airways displace the base of the tongue from the posterior pharyngeal wall and break contact between the tongue and palate (see Fig 127.2) Oropharyngeal airways can play an important role during bag-valve-mask ventilation if chest rise is absent or ineffective by relieving upper airway obstruction, which is the most likely culprit Size selection is important An excessively long airway may encroach on the larynx and cause laryngospasm An airway that is too short may actually push the tongue posteriorly and exacerbate obstruction In approximating ideal sizing, if the airway is held at the side of the face with the flange just anterior to the incisors, the tip should be at or near the angle of the mandible The oropharyngeal airway should be positioned following the curve of the tongue while the tongue is held down and forward with a tongue depressor Inserting the airway with its concave side facing the palate and then rotating it may traumatize the oral mucosa or damage teeth Oral airways are poorly tolerated in any patient with a functional gag reflex and may induce vomiting As a consequence, they are of little more than temporary value in the critically ill child However, they are an important adjunct to help support a patent airway for bag-valve-mask ventilation in preparation for intubation Oxygen Delivery Devices Nasal Cannulas Nasal cannulas consist of two hollow prongs projecting from a hollow face piece Humidified oxygen (100%) flows from a standard source, effectively delivering a pharyngeal concentration of 25% to 40% after mixing with variable amounts of room air The cannulas are easy to use, often readily tolerated, lightweight, economical, and disposable and take advantage of the humidifying properties of the nasopharynx Flow typically is limited to only to L/min because of the extent to which relatively dry gas flow cools and dries the nasal airway The use of nasal cannulas is limited by the relatively low oxygen concentration that can be delivered Heated high-flow nasal cannulas (HHFNCs) can deliver up to 40 to 80 L/min of warmed, humidified gas Flow is dependent on the size of the HHFNC, and this modality of oxygen delivery is usually well tolerated The oxygen concentration delivered is higher than that with simple nasal cannulas High-flow nasal cannulas generate positive distending pressure similar to that provided by nasal continuous positive airway pressure (CPAP) The pressure generated is dependent on the interaction among the flow rate, patient size, and anatomy of the patient’s airway, but it is probably limited to to cm H2O.9,10 At least in infants, positive-pressure generation requires a closed mouth.11 Recently, the use of nasal cannula oxygen, particularly HHFNC as part of CHAPTER 127 Airway Management the preoxygenation plan to prepare critically ill patients for intubation, has been shown to prevent desaturation during the apneic phase of induction (this will be discussed at greater length later in this chapter) Masks A variety of masks are available for delivering oxygen Simple masks fit loosely and the oxygen concentration delivered varies depending on the patient’s inspiratory flow rate and the oxygen flow into the system Partial rebreathing masks incorporate some sort of reservoir, usually a bag below the chin Provided that flow into the system exceeds the patient’s minute ventilation and the bag does not collapse on inspiration, little carbon dioxide (CO2) is inhaled and concentrations of oxygen up to about 60% can be achieved Nonrebreathing masks must fit snugly They incorporate a mask, reservoir, and one-way valves that vent expired gas but not permit inspiration of room air As a result, they can deliver close to 100% oxygen Noninvasive Positive-Pressure Ventilation Noninvasive positive-pressure ventilation is defined as the delivery of mechanical ventilatory support without the use of an endotracheal or tracheostomy tube (invasive devices) It includes CPAP and bilevel positive airway pressure (BiPAP) CPAP delivers high concentrations of oxygen and maintains positive airway pressure in the spontaneously breathing patient CPAP is applied with an oxygen source connected to either a tight-fitting nasal or full face mask or helmet in children or via nasal prongs in the neonate and older infant CPAP offers the benefit of maintaining alveolar expansion and decreases work of breathing for many patients, particularly those with pulmonary parenchymal disease and some patients with airway obstruction related to poor upper airway tone or laryngeal-, tracheal-, or bronchomalacia Like CPAP, BiPAP can be provided by mask, but it requires a ventilator to assist with flow delivery The patient’s inspiratory effort triggers the BiPAP machine to deliver decelerating flow in order to reach a preset pressure, defined as inspiratory positive airway pressure When a patient’s own inspiratory flow falls below a preset amount, ventilatory assistance ceases and maintains expiratory airway pressure at a predetermined value (typically between and 10 mm Hg) Uses in the pediatric intensive care unit (ICU) include upper airway obstruction, atelectasis, exacerbations of neuromuscular disorders, support for mild to moderate respiratory failure, and as an assist in weaning patients from invasive mechanical ventilation Both CPAP and BiPAP offer the advantage of providing respiratory support without endotracheal intubation, but they require that the child tolerate a close-fitting mask A more extensive discussion of CPAP and BiPAP is available in Chapter 55 Establishing a Functional Airway A patient who is apneic or in severe respiratory distress requires ventilation assistance initially with a bag and mask No skill is more essential for the intensivist than the ability to provide effective manual bag-mask ventilation Bag-mask ventilation can be lifesaving while preparation for endotracheal intubation proceeds or when intubation cannot be accomplished Effective bag-mask ventilation technique requires positioning the patient adequately to open the upper airway, achieving a tight mask-face seal, inserting an oral or nasal airway if needed, and generating an adequate tidal volume while coordinating manual breaths with patient 1513 efforts when they are present Poor technique results in ineffective oxygenation and ventilation, gastric insufflation and distention, and increased risk of aspiration If the child is too weak or obtunded to maintain pharyngeal tone independently, the head should be placed on a thin cushion to cause slight cervical spine flexion and gentle extension at the atlanto-occipital joint In infants, the large occipitofrontal diameter makes the cushion unnecessary and potentially disadvantageous, although a thin pad under the shoulders may be useful It appears that aligning the external auditory meatus with the sternal notch is a reasonable guide to appropriate positioning Current recommendations are to avoid overextending the baby’s flexible cervical spine, which may stretch and compress the trachea and potentiate, rather than relieve, obstruction Studies have questioned the existence of this phenomenon but to date have included only a small number of infants, all with normal airways.12 Appropriate head tilt separates the tongue from the posterior pharyngeal wall If airway obstruction persists, the chin can be pulled forward by encircling the mandible behind the lower incisors between the thumb and fingers The most effective means of relieving functional obstruction is the so-called triple airway maneuver (head extension, jaw thrust, and mouth opening) With the fingers behind the vertical ramus of the jaw, the mandible is displaced downward, forward, and, finally, upward again until it and the lower incisors are anterior to the maxilla This action effectively pulls the tongue forward and away from the pharyngeal wall In some patients, establishing a functional airway is sufficient to allow resumption of effective spontaneous ventilation In other patients, steady positive airway pressure is necessary to overcome residual obstruction If breathing remains inadequate, manual ventilation is necessary Effective ventilation requires a good mask fit The mask should sit smoothly on the bridge of the nose and the bony prominence of the chin It is critical to avoid airway occlusion with the mask or hand or pressure on eyes, soft nasal structures, or branches of the trigeminal and facial nerves An optimal mask seal is predictably difficult in a patient without teeth, a flat or prominent nose, or micrognathia Insertion of a nasal or oropharyngeal airway may help maintain an adequate airway Once a tight mask seal is ensured, ventilation may be assisted Two types of bags are in general use for bag-mask ventilation: self-inflating resuscitation bags and standard anesthesia bags Selfinflating bags vary substantially; therefore, specific directions for their use must be followed carefully All bags incorporate an adapter to connect to a mask or endotracheal tube (ETT), a bag, a pressure-relief valve, and a port for fresh gas inflow Most bags made for children have pressure-relief valves designed to pop off at 35 to 45 cm H2O pressure to prevent excessive volume delivery and subsequent barotrauma In patients with poor compliance or increased airway resistance, it may be necessary to bypass this valve temporarily to provide effective ventilation Most systems incorporate valves that prevent rebreathing Fresh gas flows through the valve on spontaneous inspiration (negative pressure) or on creation of positive pressure when the bag is squeezed (Fig 127.3) Exhaled gas is vented to the atmosphere Not all systems allow spontaneous breathing Those that demand that the patient generate at least a little negative pressure; thus, a mask must fit well Holding the mask above the patient’s face provides no supplemental oxygen The percentage of oxygen delivered depends on the percentage of oxygen from the source, the fresh gas flow rate, and the respiratory rate, which determines the time available for the bag to refill Most systems require some sort of reservoir assembly in addition to the self-inflating bag to prevent 1514 S E C T I O N X I V Pediatric Critical Care: Anesthesia Principles in the Pediatric Intensive Care Unit Angular patient valve Exhalation ports From patient Pressure relief valve EXHALATION Bag inlet valve Safety outlet valve (oxygen) Reservoir bag To patient INHALATION One-way leaf valve Oxygen inlet Safety inlet valve (air) • Fig 127.3 Self-inflating manual ventilation bag with tubing as a reservoir (Inset) Function of one type of valve that permits manual positive-pressure breathing, or spontaneous breathing, but requires generation of negative pressure by the patient to open the valve Simply holding the mask over the patient’s face does not provide supplemental oxygen entrainment of room air With a reservoir, 100% oxygen may be delivered; without a reservoir, most deliver less than 50% Anesthesia bags require flow from a source of gas under pressure in order to expand Many variations have been reviewed extensively in the anesthesia literature These circuits depend on the location of the fresh gas inflow and overflow valves, rate of fresh gas flow, the rate, tidal volume, CO2 production, and whether ventilation is spontaneous or controlled Many ICUs use the Mapleson D configuration, with the fresh gas source attached just distal to the patient connection The overflow valve is proximal to the reservoir bag During expiration, the patient’s exhaled tidal volume mixes with fresh gas flowing into the system and accumulates in the tubing and bag With sufficiently high fresh gas flow, alveolar gas is washed to the overflow valve and eliminated from the circuit To prevent rebreathing, the system requires higher fresh gas flow during spontaneous ventilation than during controlled breathing, but a safe rule of thumb is that fresh gas flow be two to three times the minute ventilation During controlled ventilation, a minimum of 100 mL/kg per minute ensures that CO2 elimination is proportional to minute ventilation.13,14 At flows less than 90 mL/kg per minute, increasing ventilation may only increase CO2 rebreathing Endotracheal Intubation The pediatric intensivist is frequently called on to intubate critically ill patients when brief periods of ventilation with a bag and mask are inadequate to address the underlying disorder Few of these intubations are performed under the optimal conditions commonly attainable in the operating room—that is, relatively healthy children with empty stomachs who have previously been sedated and are intubated in a controlled environment with all members of the team experienced in and prepared for airway management Instead, patients are often critically unstable and require intubation suddenly, commonly in settings where the procedure is not routine Intubation often is viewed only as a means to an end, namely, mechanical ventilation However, it is associated with profound physiologic consequences that may dramatically affect the patient The intensivist’s appreciation of these factors and ability to minimize the adverse physiologic consequence of airway manipulation may determine patient outcome as decisively as the intensivist’s skill in providing the intensive care that follows Indications Respiratory Failure Respiratory failure may result from dysfunction at any point along the ventilatory pathway To provide appropriate support and to avoid hazards specific to the individual disorder, airway intervention must be tailored to the underlying cause Outside the operating room, the need for intubation is most commonly associated with respiratory failure resulting from upper or lower airway or pulmonary parenchymal disorders that require mechanical ventilation Respiratory failure is defined in terms of excessive work of breathing or inadequate oxygenation (in the absence of cyanotic congenital heart disease) or CO2 elimination Box 127.1 contains a set of criteria for intubation Hemodynamic Instability Patients with hemodynamic instability often benefit from assisted ventilation Positive-pressure ventilation decreases left ventricular CHAPTER 127 Airway Management • BOX 127.1 Indications for Intubation • Pao2 ,60 mm Hg with fraction of inspired oxygen 0.6 (in absence of cyanotic congenital heart disease) • Paco2 50 mm Hg (acute and unresponsive to other intervention) • Upper airway obstruction, actual or impending • Neuromuscular weakness • Maximum negative inspiratory pressure 20 cm H2O • Vital capacity ,12 to 15 mL/kg • Absent protective airway reflexes (cough, gag) • Hemodynamic instability (cardiopulmonary resuscitation, shock) • Controlled therapeutic (hyper)ventilation • Intracranial hypertension • Pulmonary hypertension • Metabolic acidosis • Pulmonary toilet • Emergency drug administration transmural pressure and left ventricular wall stress, leading to afterload reduction.15 Controlled ventilation is often a critical component of cardiopulmonary resuscitation Additionally, early intubation in anticipation of impending cardiovascular collapse may prevent catastrophic tissue hypoxia Redistribution of blood flow away from respiratory muscles, especially the diaphragm, in patients with marginal cardiac output may improve perfusion of other vital organs, including the heart, and help prevent cardiac arrest.16–20 Neuromuscular Dysfunction For additional information on neuromuscular dysfunction, see Chapter 46 Neuromuscular dysfunction or severe chest wall instability (or deformity) may cause failure of the bellows apparatus for breathing.21 Initially, tidal volume remains normal or at least sufficient to maintain normal blood gas tensions, but vital capacity and maximal inspiratory and expiratory pressures decrease Inability to take a deep breath or cough forcefully risks progressive segmental or lobar atelectasis, inability to clear secretions, bronchial obstruction, and possible major airway obstruction with sudden severe hypoxia or CO2 retention Increasing weakness results in progressively smaller tidal volumes, loss of upper airway tone, and, ultimately, inadequate minute ventilation Bulbar dysfunction may lead to aspiration as a result of impaired swallowing and inadequate cough Measurement of ventilatory reserve provides a better assessment of the patient’s need for ventilatory assistance than does arterial blood gas tension alone Maximum negative inspiratory pressure and vital capacity are two simple tests commonly used for this purpose A variety of other measures also help assess respiratory “strength,” but most are difficult to perform in sick, uncooperative infants and children Patients with diffuse neuromuscular weakness of any cause, spinal cord dysfunction above the level of T6, or loss of phrenic nerve or diaphragm function are particularly prone to respiratory failure.22 Infants younger than approximately months tolerate diaphragmatic paralysis poorly because of the high compliance of their chest walls and relative ineffectiveness of intercostal muscles.23–27 Many patients with neuromuscular weakness respond well to noninvasive forms of ventilatory support.21,28 Decisions about the best approach to airway management should be based on the 1515 nature and likely progression of the illness, the child’s maturity and level of consciousness, and the timing of the onset of respiratory insufficiency In an emergency, endotracheal intubation is likely to be safest, with transition to noninvasive support when careful planning allows.28 Failure of Central Nervous System to Regulate Ventilatory Drive and Airway Reflexes Failure of the central nervous system to regulate ventilatory drive may prompt intubation Centrally mediated hypoventilation is manifested as CO2 retention, usually in the absence of increased work of breathing On occasion, the decision to support ventilation may be based on observing abnormal ventilatory patterns in anticipation of neurologic deterioration Loss of protective airway reflexes, including the cough and gag reflexes, can result from central nervous system depression, cranial nerve abnormalities, or severe motor weakness In such patients, intubation is indicated to prevent aspiration Intubation may be appropriate when the practitioner anticipates a need to protect the airway and support ventilation during deep sedation for procedures or diagnostic studies Other Indications Intubation is indicated as a step toward therapeutic, controlled (hyper)ventilation (e.g., in patients with increased intracranial pressure [ICP] or pulmonary hypertension) or to support spontaneous hyperpnea in patients with metabolic acidosis and other conditions Patients with profuse, thick, or tenacious secretions (e.g., as a result of bacterial pneumonitis or smoke inhalation) may benefit from having an artificial airway placed as a channel for effective suction Mucociliary clearance is impaired in patients exposed to high oxygen concentrations or other airway irritants (including particulate and gaseous components of smoke), those experiencing severe hypoxia or hypercarbia, and, paradoxically, those who have airway trauma induced by endotracheal intubation and suctioning Endotracheal intubation also provides an effective means of delivering drugs during cardiopulmonary resuscitation when venous or intraosseous access is not available Finally, intubation may be warranted for nonemergent indications, such as bedside bronchoscopy or imaging studies, during which general anesthesia is needed to optimize image quality Physiologic Effects of Intubation Laryngoscopy is a potent physiologic stimulus (Box 127.2).29,30 At the very least, it is a noxious stimulus that causes significant pain and severe anxiety, especially in children who cannot understand or accept the need for it Laryngoscopy causes an increase • BOX 127.2 Potential Physiologic Effects of Laryngoscopy and Intubation Pain Tachycardia Anxiety Bradycardia Hypoxia Systemic hypertension Hypercarbia Decreased systemic venous return Increased intraocular pressure Decreased jugular venous pressure return Increased intragastric pressure Increased intracranial pressure Laryngospasm Bronchoconstriction Pulmonary hypertension 1516 S E C T I O N X I V Pediatric Critical Care: Anesthesia Principles in the Pediatric Intensive Care Unit in systemic blood pressure and heart rate initiated by pressure on the back of the tongue or lifting the epiglottis.29,31 This effect is augmented by endotracheal intubation and suction.32 Nodal or ventricular dysrhythmias may occur Sensory impulses that trigger this reflex probably are carried along the vagus nerve supplying the base of the tongue, epiglottis, and trachea The efferent limb is less well defined but most likely the product of enhanced sympathetic activity Infants respond more variably than older patients Hypertension develops in most patients, but a few become hypotensive, especially if they are hypoxic.33 Infants may demonstrate moderate to severe bradycardia rather than tachycardia, perhaps as a consequence of their greater parasympathetic tone Sedation and light anesthesia decrease but not eliminate the hypertension and tachycardia; topical anesthesia and deeper general anesthesia are more effective.34 Children with previous hypertension display an exaggerated vasopressor response Sedation and neuromuscular blockade during airway manipulation in infants minimize the associated bradycardia and systemic hypertension.35–39 The impact of positive-pressure ventilation on cardiac performance depends on the underlying disorder (discussed in Chapter 32) and should be carefully considered in preparation for intubation Laryngoscopy and intubation are potent stimulators of laryngospasm and may cause bronchoconstriction, especially in patients with a history of reactive airway disease Increased airway resistance probably results from parasympathetic stimulation, with release of acetylcholine and stimulation of muscarinic receptors on airway smooth muscle, especially large central airways Therefore, it is imperative to use adequate sedation and analgesia before laryngoscopy to prevent these complications During the intubation process, oxygen delivery to the patient is interrupted Ineffective breathing or apnea increases the likelihood of hypoxia, especially in children, with their relatively low functional residual volume and higher basal metabolic rate Patients with severe pulmonary disease and abnormally low functional reserve capacity are at particular risk.35,38 During apnea, CO2 tension increases at a rate of to mm Hg/min in healthy, sedated adults and probably more rapidly in children, particularly those with severe cardiopulmonary disease or increased metabolic rate resulting from fever, sepsis, or pain.40,41 The process for optimizing oxygenation in preparation for laryngoscopy, also known as denitrogenation, is described in further detail later ICP rises immediately during laryngoscopy, even in patients without intracranial pathology, before changes in blood gas tensions occur.34,36,41–43 Cerebral metabolic rate and blood flow increase Hypoxia, hypercarbia, and diminished jugular venous drainage, particularly in struggling patients, contribute further to increases in cerebral blood volume and increased ICP Although normally transient, such intracranial hypertension may predispose patients with coagulopathies or vascular malformations to intracranial hemorrhage Systemic hypertension in patients with impaired autoregulation of the cerebral circulation (e.g., sick infants or patients with a variety of intracranial disorders) and impedance to jugular venous return by jugular compression, pneumothorax, or coughing and struggling stress both the arterial and venous sides of the cerebral circulation In patients with poor intracranial compliance, this effect is exaggerated and prolonged In infants without primary central nervous system disease, muscle paralysis (even without sedation or analgesia) effectively blocks the rise in ICP associated with intubation.38 The systemic hypertensive response is generally unaffected by neuromuscular blocking agents (NMBAs) but can be modified by analgesia and sedation or intravenous anesthesia • BOX 127.3 Recognizing the Difficult Airway History Difficult intubation Upper airway obstruction, current or past, including snoring and sleep apnea Anatomic Features Gross macrocephaly Severe obesity Facial asymmetry Facial trauma Midface hypoplasia Airway bleeding Small mouth Oropharyngeal mass Glossoptosis Abnormal soft tissue infiltration Midline clefts or high arched palate Limited temporomandibular joint mobility Micrognathia Nasal obstruction Limited neck mobility Laryngotracheal abnormalities (congenital or acquired) Patients with severe pulmonary hypertension are at high risk for adverse effects of laryngoscopy Decreased oxygenation and progressive hypercarbia lead to elevated pulmonary artery pressure The noxious stimulus of visualizing the airway in itself may precipitate life-threatening pulmonary hypertensive crisis Recognition of a Difficult Airway Recognition of a difficult airway is important if potentially lethal surprises in airway management are to be minimized (Box 127.3) Although the intensive care physician is usually focused on the immediate physiologic disturbances affecting the patient, careful preparation and an evaluation of each patient is critical Key components of the history and physical examination, as well as the clinical scenario, can provide insight into potential problem airways A history of difficult intubation in the past or episodes of upper airway obstruction (including snoring or sleep apnea) suggest structural abnormalities that may or may not be evident at the moment Recent tonsillectomy and adenoidectomy, cleft palate repair, or any prolonged surgical procedure resulting in oral edema or bleeding increases the likelihood of difficulty Examining facial structure is essential; inspecting the child’s profile is particularly important because significant abnormalities may not be fully apparent on frontal view alone (Fig 127.4) Certain genetic syndromes are associated with craniofacial anomalies, midline defects, or neuromuscular disorders that may make successful intubation via standard techniques exceptionally difficult.44 Treacher Collins syndrome (mandibulofacial dysostosis); Goldenhar syndrome (oculoauriculovertebral dysplasia); Down syndrome; Pierre Robin syndrome; and the mucopolysaccharidoses, such as Hurler syndrome and Hunter syndrome, are a few of the syndromes that produce characteristic features associated with a high probability of a challenging airway Isolated micrognathia, macroglossia (glossoptosis), facial clefts, midface hypoplasia, prominent upper incisors or maxillary protrusion, facial asymmetry, a high arched narrow palate, small mouth, and short, muscular neck or morbid obesity are features that can interfere with effective bag-mask ventilation or visualization of the larynx ... been shown to prevent desaturation during the apneic phase of induction (this will be discussed at greater length later in this chapter) Masks A variety of masks are available for delivering oxygen... resistance, it may be necessary to bypass this valve temporarily to provide effective ventilation Most systems incorporate valves that prevent rebreathing Fresh gas flows through the valve on... reservoir (Inset) Function of one type of valve that permits manual positive-pressure breathing, or spontaneous breathing, but requires generation of negative pressure by the patient to open the valve