Chapter 18. Noninvasive Airway Management > Anatomy and Pathophysiology



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Tintinalli's Emergency Medicine > Section 3: Resuscitative Problems and Techniques > Chapter 18. Noninvasive Airway Management >

Anatomy and Pathophysiology


Prior to airway management procedures, when there is sufficient time, the physician should:
1. Inspect the patient's mouth for size of teeth and for size and mobility of the jaw.

2. Open the patient's mouth and observe the palate, tongue, and oropharynx.

3. Flex the stable neck (in the absence of trauma), assess mobility, and place in the sniffing position.

4. Examine the size and alignment of the neck.

5. Inspect the nasal openings for patency.

6. Listen for abnormal airway sounds such as stridor, hoarseness, or gurgling.

7. Ask the patient's history, if possible.

8. Be sure to have suction available at all times, especially during any procedures.

The anatomic airway (Figure 18-1) begins at the oral/nasal cavities and continues posteriorly to the tongue/turbinates, the tonsils/adenoids; past the palate; through the oropharynx; across the epiglottis, which protects the glottis (the narrowest portion of the airway); past the false and true vocal cords; and into the larynx. Surrounding the larynx is the thyroid cartilage, cricoid cartilage, and thyroid gland. The upper airway ends here. The lower airway then continues to the trachea and into the lungs. Potential obstruction may develop anywhere along this route. In infants and small children, the anatomy is somewhat different than in the adult. The tongue is relatively larger in relation to the mandible. The glottis is higher and more anterior and the vocal cords are angled more anteriorly and inferiorly. The epiglottis is large and floppy and may lie against the posterior wall of the pharynx.
Fig. 18-1.

The anatomic airway.


Airway Obstruction


Table 18-1 lists potential causes of upper airway obstruction. Basic management of the obstructed airway is discussed in Chap. 12. Most of these entities cause soft tissue swelling or themselves are soft tissue masses that compromise the upper airway, but a few need mentioning. Certain medical diseases, such as respiratory syncytial virus (RSV) and cystic fibrosis, produce copious secretions in the upper airway that can lead to partial or complete occlusion. Angioedema may present with soft tissue swelling sufficient to preclude an oral airway, requiring a nasal pharyngeal airway, nasotracheal intubation, or surgical intervention to reestablish patency. Laryngospasm, the feared complication of any invasive airway technique, needs to be considered in any patient with a compromised airway, especially in children. It is defined as closure of the glottis by the constriction of intrinsic/extrinsic laryngeal muscles, which can completely restrict ventilation. This pathophysiologic state often persists long after the stimulus has ceased. Laryngospasm may occur secondary to contact with the upper airway receptors on the tongue, palate, and oropharynx. Light touch to the upper airway, traction on the pelvic/abdominal viscera, chemical irritation, secretions, blood, water, and vomitus may all cause laryngospasm. Hypoxia and hypercapnia depress the activity of laryngeal adductor neurons, so laryngospasm is somewhat self-limited. Laryngospasm and bronchospasm occur more frequently in children and particularly following a recent respiratory tract infection.
Table 18-1 Causes of Upper Airway Obstruction
Congenital/Genetic Infectious Medical Trauma/Tumor

Large tonsils Tonsillitis Cystic fibrosis Laryngeal trauma

Macroglossia Peritonsilar abscess Angioedema Hematoma/masses

Micrognathia Retropharyngeal abscess Laryngospasm Smoke inhalation

Neck masses Pretracheal abscess Airway muscle relaxation Thermal injuries

Large adenoids Epiglottitis Inflammatory Foreign body

Laryngitis/RSV* Asthma

Ludwig angina

*Respiratory syncytial virus.

Altered mental status, somnolence, or even sleep can depress the intrinsic and extrinsic muscle tone of the airway and produce obstruction. Some authors question the long-standing belief that the tongue falling back and occluding the lower pharynx is the major cause of airway obstruction in the somnolent or comatose patient. Recently, it was shown that during anesthesia in the supine patient, the tongue does, in fact, displace posteriorly, but it does not appear to occlude the pharynx. Upper airway obstruction in the unconscious patient occurs primarily because the epiglottis occludes the laryngeal inlet because of intrinsic muscle relaxation, which can be relieved simply by extending the neck. Extension of the neck and anterior displacement of the mandible moves the hyoid bone anteriorly and, in turn, lifts the epiglottis away from the laryngeal inlet. In a supine individual, the degree of extension of the head required to open the airway depends on elevation of the occiput above the horizontal plane. Relative to the neck, the more the occiput is elevated (to a degree), the less extension is required to open the airway, which explains why patients with airway compromise need to have their heads in the "sniffing" position. One can place a folded towel (not rolled) or foam rubber device underneath the patient's occiput (not neck) to create this position. Flexion of the neck has a marked effect of closing the airway, specifically the oropharynx. Recent magnetic resonance imaging (MRI) studies show that the soft palate also relaxes significantly during sedation, partially occluding the nasopharynx and causing complete obstruction when the patient is fully anesthetized. Moreover, extension of the anterior tongue does not appear to relieve this obstruction.1


Esophageal foreign bodies can also obstruct the airway. They can impinge upon the larynx or trachea, causing either acute or subacute obstruction. Some foreign bodies, such as large fruit pits, may have been present for some time; thus, there may not be a history of swallowing or choking on an object.

Oral/Nasal Airways


The oral airway (Figure 18-2) is an "S"-shaped, rigid instrument that is used to prevent the base of the tongue from occluding the hypopharynx. It should be used to maintain the airway only in a patient with an absent gag reflex. The operator places the oral airway over the tongue, being careful not to push it further into the hypopharynx. A tongue blade can be used to aid insertion. The concave portion is placed cephalad, rotated 180 degrees, or aimed toward the ear and rotated 90 degrees inferiorly to hold the tongue away from the pharyngeal wall. It can also be used as a bite block during orotracheal intubation.
Fig. 18-2.

An oral airway.

A nasal airway (nasopharyngeal tube) (Figure 18-3) is made of a pliable material that allows it to be placed into the nostril of a somnolent patient with an intact gag reflex. The nasal airway is a wonderful tool that can be quickly placed in a sonorous patient who may have decreased pharyngeal muscle tone and an obstructing soft palate and tongue. It allows air to bypass such obstructions, and if topical anesthesia is used as a lubricant, may ease subsequent passage of a nasogastric tube. The nasopharyngeal tube should be inserted into the most patent nostril (with the tip lubricated, ideally with a topical anesthetic such as lidocaine jelly) horizontal to the palate, and advanced until maximal airflow is heard. It is important to use the correct size tube and to avoid inserting it far enough to stimulate the gag reflex.
Fig. 18-3.

Nasal airways.


The Bag-Valve-Mask Unit


The bag-valve-mask (BVM) unit (Figure 18-4) is a self-inflating bag with a nonrebreathing valve that can be attached to a face mask. This design allows room air or oxygenated air to be manually delivered into the victim's lungs after any obstruction has been eliminated. This apparatus can be used initially while preparing for definitive airway maintenance. After the mask is placed, the handler clamps it snugly to the face. The thumb and index finger grasp the mask while the other fingers grasp the chin and pull it forward to hyperextend the stable neck. The other hand compresses the bag, expelling air into the patient's respiratory tree. This procedure can be used to manage respiratory failure temporarily, to assist poor inspiratory effort, or to temporize respiratory fatigue. The most common problem with a one-person operation is air leaks around the mask. A two-person operation employs two hands to hold the mask flush and has been shown to result in more effective ventilation.2 Placement of an oral or nasal airway further facilitates airflow. The BVM unit may also be used prior to rapid-sequence intubation (RSI) to quickly assess the ease of BVM ventilation in cases where oral intubation fails. After an intubation, the BVM unit can be attached to the proximal end of the endotracheal tube.
Fig. 18-4.

Bag-valve-mask unit.


To deliver 100 percent oxygen, there must be a reservoir with the same volume as the bag and an oxygen flow rate equal to the respiratory minute volume of the patient. By using a 2.5-L reservoir bag with an oxygen flow of 15 L per min, 100 percent oxygen can theoretically be delivered, although most nonrebreathers deliver about 75 percent oxygen. Similarly, a demand valve attached to the reservoir port of the ventilation bag will deliver a high concentration of oxygen.

Esophageal Airways


Esophageal obturator airways (EOAs) (Figure 18-5)—the pharyngotracheal lumen airway and the esophageal tracheal Combitube—are all devices used in the prehospital setting when oral endotracheal intubation is not a viable option. These devices are designed to be placed in apneic, unconscious adults only.
Fig. 18-5.

A. Pharyngotracheal lumen airway. B. Esophageal tracheal Combitube. C. Tracheoesophageal airway (used with permission).


Esophageal Obturator Airway/Esophageal Gastric Tube Airway
EOAs/esophageal gastric tube airways (EGTAs) are mentioned here only for historic reasons. They are rarely used now. The original primary benefit, placement without direct laryngeal visualization, has been supplanted by other more efficacious devices. The EGTA is a modification of the EOA and has an open distal tube containing a valve that allows passage of a nasogastric tube. Compared with the endotracheal tube (ETT), ventilation and oxygenation studies reveal varying results but suggest that the EOAs are adequate during cardiac arrest.2 One study showed that some physicians had never seen an EOA, and unfamiliarity is reason enough to avoid using this tool.
Pharyngotracheal Lumen Airway
The pharyngotracheal lumen airway (PTLA) (Figure 18-5A) is another two-tubed, cuffed airway that seals the oropharynx proximally and occludes the esophagus distally, allowing for ventilation through the short tube. There is no need for a face-to-mask seal. If the trachea is intubated by the long tube, ventilation can occur through the lumen, similar to an ETT.
Esophageal Tracheal Combitube
The esophageal tracheal Combitube (ETC) (Sheridan Catheter Corp.) (Figure 18-5B) is a plastic twin-lumen tube with a proximal low-pressure cuff that seals the pharyngeal area and a distal cuff that seals the esophagus, allowing ventilation between the cuffs. The proximal seal also removes the need for a facemask and, as compared with the PTLA, minimizes dental damage to the cuff. The distal cuff is similar to an ETT and serves to seal either the esophagus or the trachea when inflated. If the distal tube enters the esophagus, perforations in the esophageal lumen serve to ventilate the patient. If the trachea is intubated, the patient is ventilated directly, as with the cuffed ETT.
Tracheoesophageal Airway
The tracheoesophageal airway (TEA) (Figure 18-5C) is a standard ETT attached to a ventilation mask with two ports, one for the ETT and the other for oropharyngeal mask ventilation. It is designed to function equally well if inserted into the trachea or the esophagus. Tracheal intubation is facilitated by using cricoid pressure and extending the neck. While the tube is in the trachea, the cuff is inflated and the patient is ventilated normally. While the tube is in the esophagus, the patient is ventilated through the mask and the ETT allows for gastric venting or decompression.
Laryngeal Mask Airway
The laryngeal mask airway (LMA) (Intavent, Ltd.) (Figure 18-6) was developed by Brain, in 1983, as another artificial airway that can be placed blindly yet can provide a positive-pressure airway. The LMA consists of a tubular oropharyngeal airway similar to the ETT, but it is shorter and has a distal silicone laryngeal mask (balloon-type bulb) that inflates and provides a seal around the larynx. The LMA, when placed, is similar to other esophageal airways in that it can be inserted without manipulation of the patient's head. Because of its large diameter and short length, intubation of the bronchi or esophagus is circumvented. The hypopharynx, which is adapted to the passage of food, is less sensitive to a foreign body than the larynx and vocal cords, which have sensitive, protective reflexes.
Fig. 18-6.

A. Laryngeal mask airway (LMA). B. LMA diagram showing placement at the larynx (used with permission).


Many published cases show the LMA to be an effective alternative when the ETT fails because of nonvisualization of the cords secondary to ETT difficulty, airway masses, or cervical pathology.3 One study of LMA use by nonphysician emergency personnel in fasting patients found it easier to place than the ETT. In this study, there was no failure in LMA placement, whereas 21 percent of ETT placement attempts failed. Furthermore, the LMA required only half as many attempts and one-fifth the time to perform, and was rated equal to the ETT as an airway by anesthesiologists.4 Complications of the LMA include partial or complete respiratory obstruction (approximately 3 percent) and general failure to protect against aspiration of gastric contents. The LMA is also inadequate in severe chronic obstructive pulmonary disease (COPD) because of the high pressure requirement.5 Applying cricoid pressure in the acute setting almost always impedes insertion of the LMA and therefore reduces the chance of successful ventilation of the patient.6 Therefore the LMA seems an effective alternative to the ETT when endotracheal intubation fails or when cervical pathology exists.

Noninvasive Positive-Pressure Ventilation


The widespread use of noninvasive positive-pressure ventilation (NIPPV) for chronic sleep apnea in the 1980s has prompted investigators to look at NIPPV in the acute setting today. NIPPV can be described as an application of a preset volume/pressure of inspiratory air through a face or nasal mask.
Inspiratory muscle fatigue is the final phase of ventilatory failure in patients with severe reactive airway disease, COPD, and end-state pulmonary edema/pneumonia. The airway resistance overcomes the patient's muscular ability to ventilate. An effective alternative to the traditional ETT, with its potential complications, is noninvasive, mechanically assisted ventilation with continuous positive airway pressure (CPAP) or bilevel positive airway pressure (BiPAP).
Continuous Positive Airway Pressure
CPAP is one method of NIPPV where positive pressure is held constant throughout the respiratory cycle and applied through a face or nasal mask. It recently received renewed application in the treatment of patients with acute hypoxemic respiratory failure.7 CPAP improves pulmonary function by reducing the work of breathing, maintaining inflation of atelectatic alveoli, and improving pulmonary compliance. CPAP also improves hemodynamics by reducing preload and afterload, therefore improving patient's cardiac output in left ventricular failure.
NIPPV has been used to support patients with acute respiratory failure but has been primarily studied in the intensive care setting. Only in the last decade have patient diagnostic categories individually been studied in the emergency department (ED) using CPAP. Those diseases include COPD, status asthmaticus, adult respiratory distress syndrome (ARDS), acute cardiogenic pulmonary edema (ACPE), pneumonia, or traumatic respiratory insufficiency. CPAP can be adjusted according to the patient's clinical response. Pressures of 5 to 10 cm of H2O are most commonly used, and pressures above 15 are rare.
In early published and unpublished studies, CPAP at 10 cm of H2O used in the hospital and prehospital settings showed more rapid improvement of vital signs (heart rate, respiratory rate, blood pressure) and oxygen saturation versus standard medical therapy alone in ACPE patients.8 In other studies, using CPAP up to 10 to 12.5 cm H2O, there was a significant improvement in Pao2, stroke volume index, lower rate-pressure product, intrapulmonary shunt fraction, alveolar-arterial oxygen gradient, and, more importantly, lower rate of tracheal intubation.9 After meta-analysis on the above studies, there was found a statistically significant reduction in the need for tracheal intubation and possibly a decrease in mortality.10
Bilevel Positive Airway Pressure
BiPAP is a method of NIPPV where positive airway pressure is used to assist the patient's spontaneous ventilation at a "bilevel." The positive airway pressure increases during inspiration and a positive expiratory pressure provides the physiological positive end-expiratory pressure known as PEEP. BiPAP machines respond to the patient's respiratory cycle by alternating between a higher flow rate during the inhalation phase of respiration and a lower flow rate during the exhalation phase.
The use of face-mask positive-pressure ventilation with acute exacerbation of COPD avoids intubation up to 76 percent of the time.11 The benefits of BiPAP have been proven most effectively in the setting of severe COPD and is now being evaluated for ACPE. With the positive inspiratory pressure decreasing the work of breathing, and the positive expiratory pressure providing the physiologic CPAP, BiPAP has a theoretical advantage over CPAP alone. BiPAP has shown to reduce respiratory rate, dyspnea, and allow a more rapid improvement in both oxygenation and ventilation versus CPAP. Most recent studies using BiPAP with ACPE has shown to be an effective treatment in acute respiratory failure when compared to conventional oxygen therapy alone.12–14
NIPPV has been shown to decrease need for endotracheal intubation (ETI), thus decreasing the risks of ETI in the immunocompromised patient.15 A major goal in the management of respiratory failure in the immunocompromised patient is avoiding ETI. These patients are at high risk of pneumonia, bronchitis, sinusitis, and ETI increases the risk of acquiring these diseases. In patients with pulmonary infiltrates, fever, and acute respiratory failure, early NIPPV has shown to decrease rates of ETI and the serious associated complications. Early NIPPV improves the likelihood of survival to hospital discharge.15 One intensive care unit study showed that irrespective of the severity of the patient's illness, the use of NIPPV reduced the risk of ventilation-associated pneumonia and nosocomial infections.16
NIPPV decreases the requirement for mechanical support and lowers the average stay in the intensive care unit, among hemodynamically stable patients with impending respiratory failure.17 Studies also demonstrate arterial blood gas improvement, intubation avoidance, and decreased respiratory rates with low complication risk.7,18
In the elderly, where the decision to intubate is more complex (because of age, illness, or cancer), nasal-mask ventilation yielded improvements in PaO2 similar to those of other studies (60 percent), but hypercarbia improved more slowly.19 NIPPV should be considered for any patient where invasive respiratory support presents significant risk of sequelae.
In the pediatric patient population, BiPAP appears to improve respiratory rate, heart rate, PaCo2, and O2 saturation. It also decreases the need for intubation in obstructive apnea.
Trauma patients frequently have a significant loss of functional residual capacity (FRC) that often leads to mild to moderate respiratory insufficiency. In such instances, CPAP has been used to improve respiratory function and reduce hypoxemia.20 CPAP is helpful in decreasing the work of breathing and improving FRC, thus preventing hypoxemia, hypocarbia, and tachypnea. Criteria for NIPPV have included spontaneous respirations, absence of respiratory acidemia/hypercarbia, intact mental status, a PaO2 above 65 mm Hg, presence of a functioning nasogastric tube, and absence of severe maxillofacial injury. Improvement may be seen starting at 5 cm H2O. Studies show that a mean CPAP of 8.6 cm H2O meets therapeutic goals. Duration of therapy may range from a few hours to 2 days. Trauma conditions that have been studied include pulmonary contusion, flail chest, pneumothorax, hemopneumothorax, and multiple chest and abdominal gunshot/stab wounds. In this setting, a functioning nasogastric tube and respective chest tube placement, when indicated, are extremely important. Patients suffering high esophageal or tracheal injuries should not be supported with a NIPPV. Maxillofacial and basilar skull fractures are also contraindications to NIPPV by face mask. Because many traumatized patients exhibit a remarkable capacity to breathe spontaneously and improvements in hemodynamics are one of the benefits of using spontaneous ventilation versus mechanical ventilation, CPAP/BiPAP is an appropriate adjunct for managing the airway in the trauma patient assuming non-airway related indications for ETI do not exist.
Technique
Nasal-mask or facial-mask ventilation employs a tight-fitting mask that allows for a CPAP or BiPAP support system. The patient with impending respiratory failure receives either continuous pressure or inspiratory/expiratory (bilevel) support, thus allowing a decrease in inspiratory effort, rest for respiratory and accessory muscles, improvement of gas exchange, avoidance of intubation, and improved comfort.11,21 A nasal-mask protocol with BiPAP appears to be the most advanced protocol and appears to allow more sensitive changes during the course of treatment (Figure 18-7). The nasal mask allows the patient to eat, drink, and converse with the emergency staff. However, the nasal positive-pressure ventilation (NPPV) (distinct from NIPPV) does allow for air leaks through the mouth.
Fig. 18-7.

A patient with severe COPD on nasal BiPAP (used with permission).


The ideal BiPAP ventilator is small, relatively inexpensive, very mobile, and tolerates some leaks. It is possible to set the inspiratory positive airway pressure (IPAP) and the expiratory positive airway pressure (EPAP/PEEP) independently. Three modes of ventilatory triggering are available: spontaneous, combined spontaneous/timed, and timed. The proper-size mask should be chosen (allowing no mouth coverage) and tight enough to allow a good, comfortable seal. Settings should include spontaneous mode, IPAP set at 10, EPAP set at 3 cm H2O initially and increasing IPAP in 3-cm increments and EPAP slowly. Continuing hypercarbic failure is treated by increasing IPAP alone in 3-cm increments.22 Caution must be applied when using NIPPV at pressures approaching 15 cm H2O. There is evidence of increased risk of acute myocardial infarction with higher NIPPV pressures. BiPAP, CPAP, and NIPPV at high pressures (15 cm H2O), may produce a greater fall in blood pressure because of a higher intrathoracic pressure reducing myocardial perfusion.13,14
Complications
Known complications include difficulty with mask seal requiring multiple readjustments, gastric distention, aspiration (rare), intolerance of the positive pressure, and facial skin breakdown (with long-term use). These complications appear to occur infrequently, but the most common intolerance is excessive respiratory secretions, which, in fact, may be a relative contraindication to NPPV along with life-threatening epistaxis, or pre-existing bullous lung disease. Other contraindications to NPPV are severe maxillofacial trauma and potential basilar skull fracture where pneumocephalus may occur, pneumothorax, pneumomediastinum, or hypotension due to or associated with intravascular volume depletion. Another problem with mask ventilation is that using a conventional ventilator can be difficult or even counterproductive because of the inadvertent triggering of alarms in systems in masks not designed for this use. The BiPAP ventilatory system, which is designed for NIPPV use, has been used with success, but may not be readily available in the ED, and respiratory services may have to be contacted for this setup.
Application of NPPV provides ventilatory support for impending respiratory failure and has been shown to decrease the workload of the respiratory muscles. Oxygen saturation, Pao2, and pH remain stable or improve as compared with unassisted ventilation. Therefore this technique may prove useful in respiratory failure when complication of intubation is high.
This modality may decrease long-term hospital admissions, prevent unwanted intubations in the elderly or severely ill, and circumvent borderline respiratory failure intubations. Each patient must be closely monitored for tolerance of upper airway positive pressure and for instability.
Patients who receive NIPPV need to be cooperative and should not have life-threatening cardiac ischemia, dysrhythmias, or hypotension. NIPPV is inappropriate in patients who have absent or agonal respiratory effort, or who produce excessive airway secretions. Airway management and apparatus associated with NIPPV can be distracting. However, medical treatment, such as in-line nebulized updrafts, anticholinergics, steroids, and respiratory hygiene should proceed as appropriate.

References


1. Nandi PR, Charlesworth CH, Taylor SJ, et al: Effects of general anesthesia on the pharynx. Br J Anesth 66:157, 1990.

2. Hammargren Y, Clinton JE, Ruiz E: A standard comparison of esophageal obturator airway and endotracheal tube ventilation in cardiac arrest. Ann Emerg Med 14:953, 1985. [PMID: 3929654]


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15. Hilbert G, Gruson D, Vargas F, et al: Noninvasive ventilation in immunosuppressed patients with pulmonary infiltrates, fever, and acute respiratory failure. N Engl J Med 344(7):481, 2001.


16. Hill NS: Noninvasive ventilation for immunocompromised patients. N Engl J Med 344(7):522, 2001.


17. Bersten AD, Holt AW, Vedig AE, et al: Treatment of severe cardiogenic pulmonary edema with continuous positive airway pressure delivered by face mask. N Engl J Med 325:1825, 1991. [PMID: 1961221]


18. Pennock BE, Kaplan PD, Carlin BW, et al: Pressure support ventilation with a simplified ventilatory support system administered with a nasal mask in patients with respiratory failure. Chest 100:1371, 1991. [PMID: 1935296]


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