Monitoring the Airway and Pulmonary Function

Monitoring the Native (Nonintubated) Airway

Assessing a patient’s airway is critically important to ensure that it is patent, to assess any limitation to gas flow, and to evaluate vocal cord function and ability of the patient to protect his or her airway. This assessment is a routine part of perioperative care. However, although anesthesiologists are aware of the importance of airway assessment before tracheal intubation and use a variety of approaches to evaluate the airway itself and ease of intubation, assessment is equally important for patients who may require analgesics or sedatives, each of which has the potential to affect airway patency or gas exchange. Outside of the operating room (OR) environment, most clinicians assume that the patient can maintain a normal airway, so in most cases the assessment is cursory. For selected patient populations, however, whether scheduled to undergo a procedure or not, it is important to obtain a more detailed history to assess for evidence of upper airway obstruction, particularly during sleep. To do so requires specific questioning of patients (and/or their family members) about sleeping patterns, snoring, daytime somnolence, or sleep deprivation. In addition, patients should be asked about their use of continuous positive airway pressure (CPAP) or bi-level positive airway pressure (BiPAP) and, if used, whether the use has improved their sleep pattern.

For most patients without a history of upper airway obstruction during sleep, assessment of the airway is relatively straightforward. If a patient is breathing comfortably, has a normal voice, and is handling oral secretions without difficulty, then the upper airway is generally intact. However, in some situations, the clinical assessment can be challenging and may underestimate the degree of airway compromise and its implications. For example, if a patient is breathing with low inspiratory flow and low respiratory rate, an assessment may not capture the magnitude of change in airway diameter or vocal cord function that might be present because of a mass or other abnormality. As a patient’s respiratory rate or flows increase (e.g., with exercise, fever, or agitation), airway resistance increases markedly, causing stridor or obstruction. As a result, if there is any concern about vocal cord function or large airway narrowing based on history or comorbidities, the patient should be assessed during rapid breathing or while performing mild exercise (e.g., during brisk walking).

Evaluation of the patient who is scheduled for anesthesia and surgery or who may require tracheal intubation in other settings requires a more thorough assessment of the airway, including evaluation of mouth and neck mobility, ease of visualization of the airway, and identification of airway abnormalities that could influence the approach to securing the airway. ^, In addition, for patients who are being intubated because they have respiratory failure requiring ventilatory support, assessment of the upper airway before instrumentation helps not only to define how best to access the airway but also to clarify the extent to which upper airway compromise might be contributing to respiratory failure.

Under elective circumstances, assessment of the airway should include a review of the patient’s medical history and comorbidities that might influence airway management decisions, a review of past experiences with airway management (if available), and any previous episodes of respiratory failure for which the patient required supplemental oxygen or ventilatory support. As previously noted, the patient (or family) should be questioned about snoring or episodes of airway obstruction during sleep; use of CPAP or BiPAP; previous experiences with tracheal intubation, including difficult intubation; hoarseness after airway manipulation or with exercise; history of neck radiation; and known airway or tracheal abnormalities, including stenosis, tracheomalacia, or neck masses (e.g., goiter). Patients with rheumatoid arthritis should be questioned about upper airway problems, particularly those related to potential arthritic changes in the cricoarytenoid joints, as well as any history of associated pulmonary disease. Patients who have had previous neck or mediastinal surgery should be carefully evaluated for evidence of unilateral or bilateral vocal cord dysfunction. In emergency situations, the assessment may not be as thorough as in elective situations but should include some focused elements of the history and a rapid physical assessment, which can be very useful in identifying potential problems with airway management or tracheal intubation. For patients unable to provide a history, discussion with family members or the nurse caring for the patient, a review of the medical record, and direct observation of the airway and ventilatory pattern while preparing equipment for airway intervention will also provide useful information to guide management decisions.

Clinical examination should include a thorough assessment of the upper airway, including evaluation of dentition; mobility of the jaw, chin, and neck; and assessment of the anticipated ease or difficulty of tracheal intubation based on the size of the mandible and visualization of the airway (i.e., Mallampati classification). ^, Most often, this evaluation can be completed expeditiously, at least to assess anticipated relative ease or difficulty of intubation. For patients with obvious signs of upper airway compromise or obstruction, the evaluation may be limited by the emergent need for intervention. Although a lateral neck radiograph can provide useful information about the upper airway, presence of masses in the airway, or epiglottic edema, for most patients a radiologic evaluation is of limited value ^, or cannot be completed without putting the patient at significant risk by delaying access to the airway. However, evaluation of the airway can often be performed while managing the airway with bag-mask ventilation, assuming the airway is not completely obstructed. In all cases, if a patient is not ventilating adequately or has significant hypoxemia, the patient should be provided with supplemental oxygen or positive airway pressure by mask, while preparing for the tracheal intubation. For patients with abnormalities of the upper airway that may make routine laryngoscopy difficult, alternative methods to secure the airway must be considered and the appropriate equipment brought to the patient’s bedside to allow rapid control of the airway. Most hospitals have developed “difficult airway carts” that provide the needed equipment to facilitate tracheal intubation or other access to the airway, including supraglottic airways (SGAs) of varying styles and sizes, standard laryngoscope, a videolaryngoscope, a flexible intubation scope (FIS), intubating stylets, and cricothyroidotomy kits. For patients whose airways are anticipated to be difficult to manage, a surgeon who can perform emergency tracheostomy should be notified and be available at the bedside to gain control of the airway if noninvasive techniques fail.

Monitoring the Airway During Tracheal Instrumentation

For the patient who requires tracheal intubation, although several monitors are available and useful, the clinical judgment of the clinician provides the most important assessment of the airway and respiratory status. An evaluation of a patient’s pattern of ventilation, chest wall and diaphragmatic movement, and mental status can help guide decision-making. Monitoring of oxygenation is also critically important before and during manipulation of the airway because of the risk of hypoxemia, particularly in patients with reduced functional residual capacity (FRC) or high oxygen consumption. During airway management, administration of supplemental oxygen is important, even if the delivery system is not optimal or the delivery of supplemental oxygen is only intermittent, as the airway is being secured. Pulse oximetry is a useful noninvasive monitor for ensuring that the oxygen saturation remains satisfactory during airway manipulation and for guiding delivery of supplemental oxygen and bag-mask ventilation, as clinically appropriate. Although oxygen saturation by pulse oximetry (Sp o ₂ ) is an important monitor during tracheal intubation, it is also important to emphasize that pulse oximetry is not a monitor of ventilation. During delivery of supplemental oxygen, even the apneic patient may maintain a satisfactory Sp o ₂ despite progressively rising arterial carbon dioxide tension (Pa co ₂ ) and respiratory acidosis. ^,

Monitoring ventilation and carbon dioxide (CO ₂ ) levels based on clinical assessment is challenging. Using the electrocardiogram (ECG) leads to monitor chest wall movement (impedance pneumography) is not reliable in upper airway obstruction because there may be chest wall movement despite inadequate ventilation. Other techniques for monitoring air movement are more useful for monitoring ventilation when upper airway obstruction is of concern, such as nasal cannulae that allow monitoring of CO ₂ in expired gas or thermistors in the airway to document changes in temperature of inspiratory and expiratory gases. When using a noninvasive monitor of expired CO ₂ , the measured CO ₂ may not accurately reflect the end-tidal CO ₂ (EtCO ₂ ) or adequacy of ventilation but will confirm air flow if CO ₂ is present during exhalation.

SGAs may provide a satisfactory airway, either for short-term airway management or to facilitate tracheal intubation. For patients undergoing a surgical procedure for which positive-pressure ventilation is not required, an SGA may be used to secure the airway and allows initiation of positive-pressure ventilation using low airway pressures if clinically required. For any patient who has an SGA inserted, proper positioning of the airway must be verified. Direct visualization of SGA placement is usually not required; positioning can be confirmed using clinical signs. If the patient is breathing comfortably without evidence of obstruction, the SGA is usually in a good position. For the spontaneously breathing patient, this clinical assessment is usually sufficient. In selected clinical cases, when positive-pressure ventilation may be required, better confirmation of the correct position is desirable because of the potential risks associated with improper positioning of the SGA. If the SGA is not correctly positioned, positive-pressure ventilation may cause a leak around the SGA, compromising the ability to provide an adequate tidal volume (V t ). In addition, ventilation through an SGA does not prevent entrainment of gas into the stomach, and the risk of regurgitation and aspiration must be considered. In this situation, correct positioning of the SGA must be confirmed, and if there is any question about the appropriate placement, its position must be verified by direct visualization or must be repositioned.

Although a variety of masks and other devices are available to facilitate ventilatory support without the need for tracheal instrumentation, many patients require tracheal intubation for both airway protection and to ensure ventilation either using an endotracheal tube (ETT) inserted through the mouth or nose or with a tracheotomy. When tracheal intubation is required, confirmation of correct placement of the ETT is essential. The most reliable method to assess endotracheal placement is direct visualization of the tube passing through the vocal cords at the time of intubation.

Physical examination is also important to ensure that both lungs are being ventilated after placement of the airway. Auscultation over both lung fields (particularly the apices of the lungs) and stomach should routinely be performed to assess ETT placement. When the ETT is within the trachea, equal breath sounds should be heard over both lung fields while listening over the apices. Auscultation over the upper lung fields minimizes the likelihood of hearing sounds transmitted from the stomach. For most adult patients, if the ETT is located within the trachea, no breath sounds should be heard over the stomach. Unfortunately, auscultation can be misleading. Occasionally, particularly in children, breath sounds are transmitted to the stomach even when the ETT is properly positioned. For patients with extensive parenchymal lung disease, effusions, or endobronchial lesions, breath sounds may not be heard equally over both lung fields even when the ETT is properly positioned within the trachea.

Other clinical signs can be useful for confirming tracheal intubation. They include identifying condensation within the lumen of the ETT during exhalation, palpation of the cuff of the ETT in the suprasternal notch, and the normal feel of a reservoir bag during manual ventilation. Despite the clinical usefulness of these methods, none is infallible, and false-positive and false-negative evaluations have been reported.

A more reliable monitor for confirming tracheal intubation is identification of CO ₂ in exhaled gas. If the airway is within the trachea and the patient is ventilating spontaneously or receiving positive-pressure ventilation, CO ₂ should be eliminated by the lungs. The presence of CO ₂ in exhaled gas or direct measurement of CO ₂ concentration can be used to determine the location of the ETT. Several devices are available to monitor CO ₂ in expired gases. In the OR, CO ₂ can be measured using an infrared device, Raman effect scattering, or mass spectrometry. In the intensive care unit (ICU), emergency department (ED), or other settings including out-of-hospital locations, colorimetric techniques are often used to qualitatively estimate the CO ₂ concentration. More commonly, however, infrared capnography is used to directly measure the CO ₂ concentration in expired gases. ^, As a result of the ease of use and widespread availability of this modality, the documentation of the presence of CO ₂ in exhaled gas after placement of an airway device (i.e., capnography) has become a standard of care in anesthesia practice and is routinely used during emergency airway management in many hospitals and emergency settings. A detailed description of capnography is provided in Chapter 30 .

CO ₂ detection can provide misleading information, however, and is not foolproof. For example, when a patient has been ventilated by mask before intubation, CO ₂ -containing gas may remain in the stomach; a capnogram may then indicate the presence of CO ₂ in expired gas that does not reflect CO ₂ from the lungs. This problem can also occur when capnography is used to monitor the patient who has recently received bicarbonate-containing solutions or has been drinking beverages containing CO ₂ before placement of the airway device. In these situations, CO ₂ is eliminated from the stomach during the first few breaths provided through a misplaced ETT in the esophagus. The presence of CO ₂ from exhaled gas should, therefore, be monitored for several breaths. If CO ₂ continues to be eliminated through the ETT after 4 or 5 breaths, endotracheal placement of the tube can be ensured. Another limitation of capnography is that CO ₂ elimination occurs only if a patient has sufficient cardiac output to deliver CO ₂ to the lungs. If the patient has suffered a cardiac arrest and cardiac output is very low or absent, no CO ₂ is delivered to the lung, leading to no detection of CO _2, even when the ETT is within the trachea. During cardiopulmonary resuscitation, chest compressions may be effective at eliminating enough CO ₂ from the lungs to confirm ETT placement, even when cardiac output is inadequate. In addition, the presence of CO ₂ in exhaled gas provides confirmation that the cardiac output has improved and CO ₂ is being eliminated from the lungs.

Other techniques can be used to confirm tracheal intubation. The use of a self-inflating bulb has been advocated as a simple way to confirm the proper positioning of an ETT in out-of-hospital intubations. The technique uses a bulb that is applied to the ETT; self-inflation of the bulb within 4 seconds confirms that the ETT is in the proper position. Although the technique has some proponents, most studies are unable to demonstrate that this is a reliable method to verify ETT placement. More recently, the use of ultrasound has been demonstrated to be helpful for confirming tracheal intubation. ^,

Once tracheal intubation has been confirmed, assess the exact location of the ETT within the trachea to avoid placement that is too proximal (increasing the risk of accidental extubation) or too distal (endobronchial). Incorrect positioning of the ETT has been associated with several complications, including pneumothorax and death. The position of the ETT should be confirmed at the time of placement and should be regularly assessed while it remains in place because the position can change even after it is secured. Flexion of the neck moves the ETT toward the carina, whereas extension moves the tube up toward the vocal cords. In adult patients, flexion and extension of the head change the position of the ETT tip by as much as 2 cm. ^, Additionally, ETT position can change because of softening of the plastic as it warms or as a result of the patient manipulating the ETT with the tongue. These changes in ETT position place patients at risk for self-extubation, even when the ETT is properly secured and the extremities are restrained.

Several techniques can be used to assess proper positioning of the ETT within the trachea. For example, securing the ETT at a predetermined depth has been advocated as a way to minimize the likelihood of endobronchial intubation. At least one study has suggested that endobronchial placement of the ETT could be avoided if the tube was at a depth of 21 cm in women and 23 cm in men when referenced to the anterior alveolar ridge or the front teeth. However, subsequent studies have not confirmed that this technique prevents endobronchial intubation in critically ill adults or that it is predictive of the relationship between the position of the ETT at the teeth and the tube’s position relative to the carina (see Chapter 30 ).

Flexible laryngoscopy/bronchoscopy is now commonly used to confirm ETT placement if there are clinical concerns about the location of the tip of the ETT. This technique is useful, although not without some risk. Insertion of an FIS reduces the effective cross-sectional area of the ETT, potentially compromising ventilation and oxygenation. Peak inspiratory pressure increases during the visualization. Partial obstruction of the ETT results in an increase in airway resistance, which may lead to the unrecognized development of elevated end-expiratory pressure, increasing the risk of pneumothorax or hemodynamic compromise. Despite these limitations, in experienced hands the assessment can be completed rapidly and without complications. It is a particularly useful way of documenting the location of the ETT within the trachea in the patient for whom the specific location of the tube is critically important, such as one with abnormal tracheal anatomy, the patient at risk for obstruction of the right upper lobe bronchus, or one with specific needs related to the planned surgical procedure.

Capnography can also be a useful tool for identifying endobronchial migration of an ETT in some patients. With distal migration of the ETT, the EtCO ₂ falls; this is usually associated with an increase in peak inspiratory pressure. These changes, although not always reliable, can provide early evidence of ETT migration because the EtCO ₂ changes precede a change in arterial blood gases (ABGs) or other signs of displacement.

Probably the most used method to assess the positioning of the ETT within the trachea is the routine postintubation chest radiograph. The distance of the ETT from the carina can be measured from a portable anteroposterior radiograph obtained at the bedside. Although many clinicians have questioned whether the cost of chest radiography warrants its routine use for documentation of ETT placement, it remains the most useful and reliable method to determine the appropriate depth of the ETT within the trachea. ^, Alternative methods, such as ultrasonography (USG), are becoming more commonly used to assess positioning of the ETT, ^, although the chest x-ray continues to be used, not only for assessment of the ETT but also to confirm placement of catheters, to assess lung fields, and to identify potential complications of mechanical ventilatory support.

One special clinical situation warrants additional monitoring of the airway. Some patients require placement of a double-lumen tube (DLT) to facilitate a unilateral surgical procedure on the lung, to provide differential lung ventilation, or to protect one lung from contamination with blood or infected secretions from the other lung. In these cases, proper placement of the DLT must be ensured. Physical examination alone and other monitoring techniques are usually insufficient to confirm proper positioning. Flexible bronchoscopic evaluation is required to confirm DLT position after initial placement and to reevaluate placement after the patient is repositioned for a surgical procedure or while requiring differential lung ventilation in the ICU. Direct visualization of the tip of the DLT and the relationship between the tracheal and bronchial lumens ensures that the tube is in the proper position and that the two lungs are isolated. Other techniques can be used to diagnose malpositioning of DLTs, although few studies confirm their value. Capnography, which can be useful in identifying endobronchial migration of a single-lumen ETT, may provide information about the position of a DLT, particularly if only one lung is being ventilated at the time of evaluation. Spirometry, which can be obtained from in-line monitoring devices added to the anesthesia circuit or monitoring modalities provided by critical care ventilators, can also provide early detection of DLT malpositioning. ^, As an ETT migrates, expiratory flow obstruction can be detected as a change in the shape of the expiratory limb of the flow-volume loop; inspiratory obstruction is best diagnosed by a change in the pressure-volume loop.

Monitoring the Airway in the Patient With a Tracheostomy

For patients with a tracheostomy, the patency and positioning of the tracheostomy tube should be assessed regularly. As part of the monitoring and assessment of the airway, it is essential to understand the reason for tracheostomy and the consequences of the tracheostomy tube becoming occluded or dislodged. For the patient who has a permanent tracheotomy tube in place, the management options depend on whether the patient has a patent upper airway or has undergone a laryngectomy. If the latter, the patient cannot be intubated from above under any circumstance, and, in this case, the patient may or may not require a cuffed tube because the risk of aspiration is minimal. For most of these patients, the tracheal stoma is well healed and risk of loss of the stoma is low. Many of these patients can remove the tracheostomy tube for cleaning and replace it without difficulty. On the other hand, for patients with a fresh tracheostomy, monitoring of the airway is critically important. If a freshly placed tracheotomy tube becomes dislodged, replacing the tube can be very difficult, because the stoma may be difficult to cannulate, and the risk of misplacement is high. To minimize the likelihood of loss of the airway, the physician who performed the procedure will usually place stay sutures into the tracheal wall to facilitate replacement of the tube in case of dislodgement. Even when care has been taken to allow easier access to the airway, if a fresh tracheotomy tube becomes dislodged, reinsertion carries significant risk. As a result, whenever caring for a patient with a new tracheotomy tube in place, the clinician should review the patient records to determine whether oral intubation was difficult and, if so, what was done to ensure proper placement and to identify the reason that the tracheostomy was performed. At all times, additional backup airway equipment, including replacement tracheotomy tubes, should be readily available.

Most tracheotomy tubes used to facilitate mechanical ventilation or provide airway protection are disposable, although for those patients with a permanent tracheostomy, an uncuffed metal tube may be used to access the airway. For most other applications, a disposable tracheostomy tube is used; most have inner cannulas, which can be replaced or removed for cleaning. In every case, the clinician caring for the patient should be knowledgeable about the device used to secure the airway and backup equipment should be available. Alternative approaches to the airway may be necessary if the patient develops upper airway obstruction or other clinical complications, such as occlusion of the inner cannula. It is useful to have a clearly defined algorithm for addressing these clinical challenges. Many hospitals have developed signage to clarify the reason for the tracheostomy, the patient’s underlying anatomy, and the options for securing the airway, should it become dislodged. This approach can be very helpful in addressing emergent situations, particularly when those involved in the placement of the original airway are not available to assess the patient and guide clinical decision-making.

Clinical Assessment

Clinical examination remains one of the most important and valuable methods to monitor a patient’s respiratory status. Too often, attention is placed on technologically sophisticated monitoring devices, and the physical examination is cursory, or the clinical findings are undervalued. Nonetheless, much information about actual or potential airway problems and abnormalities in pulmonary mechanical function or gas exchange can be obtained from a carefully performed and thorough examination. Many of the early signs of respiratory failure are apparent on physical assessment before the abnormalities are apparent by other means. For example, the respiratory rate provides important information about respiratory reserve, dead space, and respiratory drive, particularly when interpreted in conjunction with Pa co ₂ . Tachypnea is frequently the earliest sign of impending respiratory failure. The patient’s pattern of breathing should be evaluated. Subtle changes in the respiratory rate, V t , and pattern of breathing may provide an early indication of increased work of breathing (WOB) (as may occur with reduced lung compliance, increased airway resistance, or phrenic nerve dysfunction) or altered ventilatory drive. Although inspiratory flow and minute ventilation (V. e ) are difficult to quantify by clinical examination alone, respiratory distress often manifests as the patient attempts to increase alveolar ventilation by taking larger, more rapid inspirations.

Upper airway obstruction, as may occur after manipulation of the airway or in association with epiglottitis or a mass in or around the airway, can be assessed by careful clinical evaluation. Nasal flaring, stridor, and chest wall movement in the absence of airflow suggest upper airway obstruction. If the patient is making respiratory efforts and has abdominal expansion during inspiration without chest excursions, he or she has upper airway obstruction and may require manipulation of the upper airway, including a jaw thrust, initiation of noninvasive positive-pressure ventilation (NIPPV), or tracheal intubation. When the patient presents with stridor, the physical evaluation is also useful in identifying the location of airway compromise. When the stridor occurs primarily during inspiration, it is caused by extrathoracic obstruction; when it occurs during exhalation, it reflects an intrathoracic obstruction. If the stridor occurs during both inspiration and exhalation, the obstruction is fixed, such as may occur with tracheal stenosis. A fixed obstruction is rarely amenable to conservative treatment, and tracheal intubation is most often required until a more definitive therapy can be provided. In selected patients, helium therapy can be used as a temporizing intervention until a more definitive treatment can be provided.

Respiratory dyssynchrony (when the patient has no evidence of upper airway obstruction) is an early and critical indicator of respiratory muscle fatigue and impending respiratory failure. ^, Respiratory dyssynchrony is identified by assessing chest wall and abdominal movement during normal tidal breathing. A paradoxical respiratory pattern suggests that the patient may have inadequate muscle strength to sustain spontaneous respiration and that positive-pressure ventilation support may be required. Tobin and colleagues found that respiratory muscle dyssynchrony could occur before the development of fatigue, although fatigue of the respiratory muscles does not always result in the development of dyssynchrony.

Clinical observation of the patient should include careful assessment of the respiratory muscles to assess the patient’s respiratory reserve. Use of accessory muscles, including the sternocleidomastoid and scalene muscles, is commonly seen in patients with long-standing respiratory failure associated with chronic obstructive pulmonary disease (COPD). The position of the diaphragm and diaphragmatic motion are also affected in patients with severe COPD. The patient who relies on accessory muscles and has minimal diaphragmatic excursion does not have any respiratory reserve and is at risk for recurrent respiratory failure; these patients present a significant challenge during weaning when mechanical ventilatory support is required.

A routine physical examination of the lungs should be performed as part of the assessment for every patient. The examination can provide evidence of parenchymal lung abnormalities and cardiopulmonary pathology. Auscultation of the lungs can provide useful information about the presence of pleural effusions, pneumothorax, or other extrapulmonary air and can assess the location of the diaphragms. The examination can provide information about potential physiologic abnormalities and guide the selection of other monitoring techniques, including ABGs and chest radiography.

Although the physical examination is useful and should be performed routinely, some of the physical signs and symptoms of respiratory failure are not diagnostic but instead reflect the physiologic manifestations of the underlying problem. The greatest value of the physical examination is that it provides an initial baseline assessment of the patient, and subsequent examinations can clarify the response to clinical interventions. The physical examination combined with other monitoring modalities remains an important monitor of respiratory status.