Noninvasive Positive-Pressure Ventilation

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51 Noninvasive Positive-Pressure Ventilation

Noninvasive ventilation is defined as the provision of ventilatory assistance to the lungs without an invasive artificial airway. Noninvasive ventilators consist of a variety of devices, including negative- and positive-pressure ventilators. Until the early 1960s, negative-pressure ventilation in the form of tank ventilators was the most common type of mechanical ventilation outside the anesthesia suite.¹ However, during the Copenhagen polio epidemic of 1952, it was observed that the survival rate improved when patients with respiratory paralysis were treated with invasive positive-pressure anesthesia devices. After that, invasive positive-pressure mechanical ventilation gradually became the preferred means of treating acute respiratory failure.² Negative-pressure and other so-called body ventilators were the mainstay of ventilatory support for patients with chronic respiratory failure until the mid-1980s.¹

With improving mask and ventilator technology and the many advantages over negative-pressure ventilation,¹ noninvasive positive-pressure ventilation (NIPPV) displaced negative-pressure ventilation as the treatment of choice for chronic respiratory failure in patients with neuromuscular and chest wall deformities.³ Over the past 15 years, noninvasive ventilation has moved from the outpatient to the inpatient setting, where it is used to treat acute respiratory failure. A 1997 survey of medical intensive care units (ICUs) in France, Switzerland, and Spain demonstrated that noninvasive ventilation was used in 16% of cases in which mechanical ventilation was required for respiratory failure, and a follow-up survey found that this rate was up to 23% in 2001.⁴ More recent surveys suggest that rates continue to increase over these levels.⁵ This chapter discusses the rationale for the increasing use of NIPPV in critical care, as well as appropriate indications, practical applications, and monitoring.

Rationale

The most important advantage of noninvasive ventilation is the avoidance of complications associated with invasive mechanical ventilation. These include complications related to direct upper-airway trauma, bypass of the upper-airway defense mechanisms, increased risk of nosocomial pneumonia, and interference with upper-airway functions, including the ability to eat and communicate normally.⁶ By averting airway intubation, noninvasive ventilation leaves the upper airway intact, preserves airway defenses, and allows patients to eat orally, vocalize normally, and expectorate secretions. Compared with invasive mechanical ventilation, noninvasive ventilation reduces infectious complications including pneumonia, sinusitis, and sepsis.⁷^–⁹ Strengthening the rationale for its use is evidence accumulated over the past decade that noninvasive ventilation lowers morbidity and mortality rates of selected patients with acute respiratory failure and may shorten hospital length of stay or avoid hospitalization altogether,¹⁰ thus reducing costs.

The main indication for mechanical ventilatory assistance is to treat respiratory failure, either type 1 (hypoxemic), type 2 (hypercapnic), or both. Figure 51-1 shows that airspace collapse, surfactant abnormalities, and airway narrowing and closure contribute to ventilation-perfusion abnormalities and shunt, which cause hypoxemia. By opening collapsed airspaces and narrowed airways, positive airway pressure reduces shunt and improves ventilation-perfusion relationships, ameliorating hypoxemia. In addition, positive airway pressure can reduce the work of breathing by improving lung compliance as a consequence of opening collapsed airspaces. Another potential benefit of positive airway pressure is enhanced cardiovascular function via the afterload-reducing effect of increased intrathoracic pressure. Conversely, deleterious cardiovascular effects may occur if the preload-reducing effect outweighs the afterload-reducing effect, as may be seen in patients with reduced intravascular fluid volume.

Figure 51-1 Pathophysiology of acute hypoxemic respiratory failure and points where positive-pressure and oxygen supplementation interrupt the process. Low ventilation-perfusion ratios, shunt, and alveolar hypoventilation cause hypoxemia. Hypoxemia is treated by increasing inspired oxygen fraction (FIO₂) (limited benefit with shunt) and applying positive pressure (continuous positive airway pressure [CPAP] or positive end-expiratory pressure [PEEP]) to increase functional residual capacity, open collapsed alveoli and narrowed airways, and enhance compliance. An additional beneficial effect of CPAP may occur in patients with cardiogenic pulmonary edema, because it reduces both venous return and left ventricular afterload, which may enhance cardiovascular performance in patients with dilated, hypocontractile left ventricles.

Mechanisms of Action

Figure 51-2 shows the pathophysiologic mechanisms that contribute to ventilatory failure. Increased airway resistance, reduced respiratory system compliance, and intrinsic positive end-expiratory pressure (PEEP) contribute to increased work of breathing, predisposing to respiratory muscle fatigue. In patients with chronic obstructive pulmonary disease (COPD), the increased radius of the diaphragmatic curvature, which increases muscle tension and thereby increases impedance to blood flow, exacerbates the situation. By counterbalancing intrinsic PEEP with extrinsic PEEP and by augmenting tidal volume with intermittent positive-pressure ventilation, NIPPV reduces the work of breathing and averts the vicious circle leading to respiratory failure. Work of breathing measurements, including transdiaphragmatic pressure, diaphragmatic pressure-time product, and diaphragmatic electromyographic amplitude, are all decreased when NIPPV is delivered to patients with exacerbations of COPD. In such patients, continuous positive airway pressure (CPAP) and pressure-support ventilation (PSV) both reduce the work of breathing, but the combination of the two (PSV + PEEP) is more effective than either alone.¹¹

Figure 51-2 Pathophysiology of acute hypercapnia and points where continuous positive airway pressure (CPAP), positive end-expiratory pressure (PEEP), and pressure support (PS) interrupt the process (large arrows). Hypercapnia (increased partial pressure of carbon dioxide in arterial blood [PaCO₂]) occurs when respiratory muscles fail to adequately ventilate alveoli to maintain homeostasis with carbon dioxide production. Respiratory muscle failure occurs when work of breathing is normal (e.g., acute or chronic neuromuscular disease) or increased (e.g., patients with chronic obstructive pulmonary disease, asthma, or obesity hypoventilation syndrome), and presumably because of inadequate oxygen delivery to respiratory muscles (e.g., ~ a third of patients presenting with cardiogenic pulmonary edema). Strategies to counter these pathophysiologic mechanisms include applying CPAP or PEEP to counterbalance intrinsic PEEP (PEEPi), increasing alveolar ventilation by augmenting tidal volume (VT), using intermittent positive-pressure ventilation (IPPV), and reducing CO₂ production by decreasing the work of breathing.

Indications

A number of causes of acute respiratory failure are now considered appropriate for noninvasive ventilation therapy and are listed in Box 51-1. Evidence supporting these indications is rated and briefly discussed here; guidelines for patient selection are discussed later.

Box 51-1

Indications for Use of Noninvasive Ventilation in the Acute Care Setting

^* Letters in parentheses indicate the level of evidence supporting use of noninvasive ventilation: A, multiple randomized, controlled trials—recommended; B, at least one randomized, controlled trial—weaker recommendation; C, case series or reports—can be tried, but with close monitoring.

Airway Obstruction

Chronic Obstructive Pulmonary Disease

A number of randomized controlled trials ^12,¹³ and meta-analyses¹⁴ have consistently shown that compared with conventional therapy, NIPPV improves vital signs, gas exchange, and dyspnea scores; reduces the rates of intubation, morbidity, and mortality; and shortens hospital length of stay in patients with moderate to severe exacerbations of COPD. Thus NIPPV is considered the ventilatory mode of choice in selected patients with acute exacerbations of COPD. Some studies suggest that the addition of heliox to NIPPV further improves the work of breathing and gas exchange during COPD exacerbations,¹⁵ but a subsequent multicenter trial found no improvement in other outcomes compared with noninvasive ventilation alone.¹⁶

Asthma

Uncontrolled studies have reported improvements in gas exchange and low rates of intubation after the initiation of NIPPV in patients with severe asthma attacks. Two controlled trials have demonstrated a more rapid improvement in expiratory flow rates with NIPPV,^17,¹⁸ and one showed a decreased hospitalization rate in acute asthma patients treated with noninvasive ventilation compared with a sham mask.¹⁸ Neither study was powered adequately to assess intubation or mortality rates. Nonetheless, these data support a trial of NIPPV in asthmatics responding poorly to initial bronchodilator therapy. Noninvasive ventilation can be combined with continuous nebulization and heliox, although the added value of these latter therapies has not been established in controlled trials.

Cystic Fibrosis

Uncontrolled studies indicate that noninvasive ventilation is useful to stabilize gas exchange in the treatment of acute episodes of respiratory failure in end-stage cystic fibrosis patients and can serve as a bridge to transplantation.¹⁹

Upper Airway Obstruction

Anecdotally, noninvasive ventilation can be used to treat patients with upper airway obstruction such as that caused by glottic edema following extubation. In this situation, noninvasive ventilation can be combined with aerosolized medications or heliox, but no controlled trials have demonstrated the efficacy of this approach. If therapy with noninvasive ventilation is considered, patients should be selected with great caution and monitored closely, because upper airway obstruction can lead to precipitous deterioration. The use of noninvasive ventilation in patients with tight, fixed upper-airway obstruction is inappropriate because it delays the institution of definitive therapy.

Hypoxemic Respiratory Failure

Hypoxemic respiratory failure is defined as severe hypoxemia (arterial oxygen partial pressure-inspired oxygen fraction ratio <200) combined with a respiratory rate above 35 breaths per minute and a non-COPD diagnosis including acute pneumonia, acute lung injury (ALI), acute respiratory distress syndrome (ARDS), pulmonary edema, or trauma. Controlled trials of noninvasive ventilation to treat patients with acute hypoxemic respiratory failure have shown statistically significant reductions in the rate of intubation, length of hospital stay, incidence of infectious complications,^8,²⁰ and in one study, ICU mortality.²⁰ However, because of the heterogeneity of causes, these studies fail to demonstrate that all patient subgroups with hypoxemic respiratory failure benefit equally from noninvasive ventilation. Further, when patients are stratified according to acuity of illness, patients with a simplified acute physiologic score (SAPS II) less than 35 fare considerably better with NIPPV than do those with higher scores.²¹ Thus the selection of patients with less severe disease is likely to enhance the success of NIPPV in treating hypoxemic respiratory failure, and studies that examine individual subgroups within the larger category are likely to be more useful clinically.

Pneumonia

One controlled trial showed that noninvasive ventilation in patients with severe community-acquired pneumonia lowers the rate of endotracheal intubation and shortens the length of ICU stay compared with conventional therapy; however, a subgroup analysis revealed that the benefits occurred only in patients with underlying COPD.²² No benefit was apparent in the non-COPD patients with severe pneumonia. A subsequent uncontrolled trial in non-COPD patients with severe pneumonia found that two-thirds of such patients treated with noninvasive ventilation eventually required intubation.²³ Although the latter authors deemed a trial of noninvasive ventilation in non-COPD patients with severe pneumonia to be a reasonable approach, controlled data to support such a recommendation are currently lacking.

Immunocompromised States

The dismal prognosis of invasively ventilated immunocompromised patients makes noninvasive ventilation an appealing ventilatory mode, with its demonstrated ability to decrease the rate of nosocomial infection.⁷ In a study of 51 patients undergoing solid organ transplantation who developed acute hypoxemic respiratory failure within 3 weeks, noninvasive ventilation reduced the rate of intubation, frequency of invasive procedures, rate of nosocomial infection, duration of ICU stay, and ICU mortality (but not hospital mortality) compared with conventional therapy.²⁴ In a subsequent randomized trial of neutropenic patients with pulmonary infiltrates and acute hypoxemic respiratory failure (most of whom had hematologic malignancies), noninvasive ventilation lowered the intubation rate, occurrence of nosocomial infections, and ICU and hospital mortality rates (the latter from 80% to 46%).²⁵ More recently, noninvasive ventilation has been reported to yield similar benefits in acquired immunodeficiency syndrome (AIDS) patients with Pneumocystis carinii pneumonia versus invasive mechanical ventilation in physiologically and demographically matched patients.²⁶ Thus, whenever possible, noninvasive ventilation should be tried first in immunocompromised patients with hypoxemic respiratory failure because of the potential to avoid the high morbidity and mortality rates associated with invasive mechanical ventilation in these patients.

Acute Respiratory Distress Syndrome

A small retrospective study reported that NIPPV averted intubation in 50% of patients during the early phase of acute lung injury or ARDS.²⁷ However, for ARDS patients with severe oxygenation defects and multiple organ system dysfunction, invasive ventilation remains the preferred modality. A prospective cohort study²⁸ using noninvasive ventilation as a “first-line” intervention for ARDS found that ventilator associated pneumonia and mortality were much reduced when patients succeeded rather than failed noninvasive ventilation, and a simplified acute physiology score of 34 or less and PAO₂/FIO₂ above 175 within the first hour predicted noninvasive ventilation success. Thus, noninvasive ventilation could be considered in ARDS patients meeting these criteria, but such patients must be monitored closely to avoid any delay in intubation if deterioration occurs.

Acute Cardiogenic Pulmonary Edema

Meta-analyses of randomized, controlled trials demonstrated that compared with oxygen therapy, CPAP (though not a true mode of ventilatory support) is highly effective at relieving respiratory distress, improving gas exchange, and averting intubation when used to treat patients with acute cardiogenic edema.^29,³⁰ Inspiratory assistance combined with expiratory pressure can reduce the work of breathing and alleviate respiratory distress more effectively than CPAP alone, and several uncontrolled trials and two controlled trials found that noninvasive ventilation and CPAP are equally effective in improving vital signs and avoiding intubation. The current recommendation is to use CPAP alone or noninvasive ventilation as initial therapy; if CPAP is used initially, inspiratory pressure support should be added if the patient has persistent hypercapnia or dyspnea.³¹

Postoperative Respiratory Failure

NIPPV and CPAP alone have been studied in postoperative patients who develop respiratory failure after various kinds of surgery. It reduces extravascular lung water and improves lung mechanics and gas exchange after coronary artery bypass surgery.³² Controlled trials show that CPAP averts postoperative complications compared to oxygen supplementation after high risk procedures like thoracoabdominal aortic procedures.³³ Noninvasive ventilation improves oxygenation, reduces the need for re-intubation, lowers the mortality rate after lung resectional surgery,³⁴ and enhances pulmonary function after gastroplasty.³⁵ Thus noninvasive ventilation should be considered in selected postoperative patients at high risk of pulmonary complications or with frank respiratory failure, especially in the setting of underlying COPD or pulmonary edema.

Trauma and Burns

Trauma patients develop respiratory failure for a multitude of reasons, but some have chest wall injuries such as flail chest or mild acute lung injury that might respond favorably to NIPPV. In a retrospective survey of 46 trauma patients with respiratory insufficiency that had been treated with NIPPV, Beltrame and coworkers found rapid improvements in gas exchange and a 72% success rate; however, patients with burns responded poorly.³⁶ More recently, a randomized trial of NIPPV versus high-flow oxygen in thoracic trauma patients with PAO₂/FIO₂ less than 200 was stopped early after enrollment of 50 patients because of significant reductions in intubation rate (12% versus 40%) and hospital length of stay (14 versus 21 days) in the NIPPV group.³⁷ These promising results justify a cautious trial of NIPPV in carefully selected and monitored thoracic trauma patients, but data are too limited to draw firm conclusions.

Restrictive Lung Disease

The use of noninvasive ventilation in patients with underlying restrictive disease and acute deterioration of respiratory status has not been studied extensively because they constitute only a small portion of patients admitted to acute care hospitals. Patients with restriction related to an underlying neuromuscular disease and superimposed acute respiratory failure may benefit from a trial of NIPPV. Small case series have reported that using NIPPV in patients with myasthenic crises may avoid intubation.³⁸ In contrast, patients with end-stage pulmonary fibrosis in respiratory extremis have been reported to do poorly with mechanical ventilation.³⁹

Do-Not-Intubate Patients

Although controversial, noninvasive ventilation may be a useful tool in patients with acute respiratory failure who do not wish to be intubated. There are several reports of good outcomes (>50% survival to discharge) with noninvasive ventilation in this subset of patients, especially those with COPD and congestive heart failure.⁴⁰ Noninvasive ventilation may also be used as a palliative technique to reduce dyspnea, preserve patient autonomy, and provide time for finalization of affairs for some terminal patients.⁴¹ However, there is concern that this may merely prolong the dying process, and patients and their families must be informed that noninvasive ventilation is being used as a form of life support in this setting and should be given the option to refuse it.

Facilitation of Weaning and Extubation

Patients who require invasive mechanical ventilation initially and fail to wean promptly are potential candidates for noninvasive ventilation to facilitate extubation, thus reducing the complications related to prolonged intubation. Several randomized controlled trials have demonstrated that noninvasive ventilation significantly shortens the duration of invasive mechanical ventilation, reduces the length of ICU stay, and improves survival compared with patients weaned in the routine fashion.⁴²^–⁴⁴ Another potential application of noninvasive ventilation in the weaning process is to avoid reintubation in patients with extubation failure, a complication of invasive mechanical ventilation associated with a high mortality rate. Earlier studies looking at the role of NIPPV in this situation showed promise, but one randomized trial found that NIPPV may delay needed intubation in this setting, resulting in an increased ICU mortality rate.⁴⁵ More recent studies have demonstrated that patients at high risk for extubation failure,⁴⁶ especially those with hypercapnia,⁴⁷ have reduced need for intubation and mortality if treated with noninvasive ventilation as opposed to oxygen supplementation alone. Thus, although the use of noninvasive ventilation to facilitate weaning and extubation appears to benefit hypercapnic patients with COPD or congestive heart failure, its overzealous application could lead to increased extubation failure rates and other adverse consequences.

Bronchoscopy

Both CPAP and NIPPV have been studied as ways of supporting oxygenation and ventilation during bronchoscopy. Using a specially designed open CPAP system during bronchoscopy in patients with marginal oxygenation, Maitre et al. observed maintenance of adequate gas exchange and avoidance of respiratory failure.⁴⁸ In a controlled trial, Antonelli et al. demonstrated equivalent oxygenation and complication rates in patients undergoing bronchoscopy and supported with either noninvasive or invasive mechanical ventilation.⁴⁷ Thus NIPPV is an effective way of providing ventilatory support in patients undergoing bronchoscopy.⁴⁹

Practical Application

Patient Selection

Noninvasive ventilation should be viewed as a “crutch” that assists patients through a period of acute respiratory failure while reversible factors are being treated, helping them avoid invasive mechanical ventilation and its attendant complications. To optimize the chance of success, noninvasive ventilation should be used early, when patients first develop signs of incipient respiratory failure. In addition, predictors of success are useful in identifying patients most likely to benefit (Box 51-2). The selection process might be viewed as taking advantage of a “window of opportunity”: the window opens when the patient first needs ventilatory assistance and closes when the patient becomes too unstable.

Box 51-2

Predictors of Noninvasive Ventilation Success in Patients with Acute Respiratory Failure

Lower acuity of illness (Acute Physiology and Chronic Health Evaluation [APACHE] score)

Ability to cooperate; better neurologic score

Ability to coordinate breathing with ventilator

Less air leakage; intact dentition

Hypercarbia, but not too severe (PaCO₂ between 45 and 92 mm Hg)

Acidemia, but not too severe (pH between 7.1 and 7.35)

Improvements in gas exchange and heart and respiratory rates within first 2 hours

Based on the predictors of success and criteria used in prior controlled trials, we recommend the following three-step selection process. First, the patient should have an etiology of respiratory failure likely to respond favorably to noninvasive ventilation. The second step is to identify patients in need of ventilatory assistance by using clinical and blood gas criteria. Patients with mild respiratory distress and no more than mild gas exchange derangement are likely to do well without ventilatory assistance and should not be considered. Good candidates are those with moderate to severe dyspnea, tachypnea, and impending respiratory muscle fatigue, as indicated by the use of accessory muscles of breathing or abdominal paradox. The level of tachypnea used as a criterion depends on the underlying diagnosis. Those with COPD are considered candidates for noninvasive ventilation when the respiratory rate exceeds 24 breaths per minute; with hypoxemic respiratory failure, higher respiratory rates are used, in the range of 30 to 35 breaths per minute. The third step is to exclude patients for whom noninvasive ventilation would be unsafe. Those with frank or imminent respiratory arrest should be promptly intubated because the successful initiation of noninvasive ventilation requires some time for adaptation. Patients who are medically unstable with hypotensive shock, uncontrolled upper gastrointestinal bleeding, unstable arrhythmias, or life-threatening ischemia are better managed with invasive mechanical ventilation. Additionally, noninvasive ventilation should not be used for patients who are uncooperative, unable to adequately protect their upper airway or clear secretions, or intolerant of masks, or for recipients of recent upper gastrointestinal or airway surgery.

Initiation of Noninvasive Ventilation

Once an appropriate candidate for noninvasive ventilation has been selected, a ventilator and interface must be chosen, initial settings must be selected, and the patient must be monitored closely in an appropriate location until stabilized. The roles of physicians, respiratory therapists, and nurses are of paramount importance in explaining the process to and gaining the confidence of the patient. Noninvasive ventilation can be initiated wherever the patient presents with acute respiratory distress, but he or she should be transferred to a location with sufficient monitoring (usually an ICU or step-down unit) until stabilized. During transfers, ventilatory assistance and monitoring should be continued.

Ventilator Selection

Selection of a ventilator is based largely on availability, practitioner experience, and patient comfort. Pressure-limited modes, including pressure support and pressure control, are available on most critical care ventilators. Pressure-control ventilation delivers time-cycled, preset inspiratory and expiratory pressures with adjustable inspiratory/expiratory ratios at a controlled rate. Most such modes also permit patient triggering and selection of a backup rate. PSV delivers preset inspiratory and expiratory pressures to assist spontaneous breathing efforts. Nomenclature and the specific characteristics of these modes may differ among ventilators, and this must be taken into account to avoid errors. For example, with some ventilators, pressure support is the amount of inspiratory assistance added to the preset expiratory pressure. Others require independent selection of inspiratory and expiratory positive airway pressures, with the difference between the two determining the level of pressure support.

PSV is a flow-triggered and flow-cycled mode, and patient effort determines tidal volume and duration of inspiration. Pressure-support modes have the potential to match breathing pattern quite closely, and they have been rated by patients as more comfortable for NIPPV than volume-limited ventilation.⁵⁰ However, leaks during noninvasive ventilation can interfere with the detection of reduced inspiratory flow at the termination of inspiration, causing expiratory asynchrony. Noninvasive pressure-limited modes of ventilation are usually administered using either standard critical care ventilators or portable bilevel ventilators.

Traditional bilevel devices designed for home use have limited pressure-generating capability (≤30 cm H₂O) and lack oxygen blenders or sophisticated alarm or battery backup systems, precluding their use in patients who require high oxygen concentrations or inflation pressures. Newer versions designed for the acute setting are equipped with sophisticated alarm and monitoring capabilities, graphic displays, and oxygen blenders. These devices are capable of enhancing synchrony by offering ways to limit inspiratory duration and an adjustable “rise time”—the time to reach the targeted inspiratory pressure. Many critical care ventilators now include an “NIV” mode that enhances leak compensation capabilities and silences “nuisance” alarms, but many of these have difficulty maintaining performance in the face of variable air leaks.⁵¹ If desired, volume-limited ventilation can be delivered using critical care ventilators, but a higher tidal volume than that commonly used for invasive mechanical ventilation is recommended to compensate for air leakage.

Initial ventilator pressure settings are usually low to facilitate patient acceptance, but they can be set higher if necessary to alleviate respiratory distress. Typical starting pressures are an inspiratory positive airway pressure of 10 to 12 cm H₂O and a PEEP (or expiratory positive airway pressure) of 4 to 5 cm H₂O. L’Her et al.⁵² demonstrated that increases in inspiratory pressure are helpful to alleviate dyspnea, whereas increases in expiratory pressure are better to improve oxygenation. For volume ventilation, initial tidal volumes range from 6 to 7 mL/kg. The ventilator is set in a spontaneously triggered mode, with or without a backup rate. Pressures commonly used to deliver CPAP in patients with acute respiratory distress range from 5 to 12.5 cm H₂O. CPAP can be applied using compressed air with a regulator system, blower-based CPAP devices, bilevel devices, or critical care ventilators.

Interfaces

The major difference between invasive and noninvasive ventilation is that with the latter, pressurized gas is delivered to the airway via a mask rather than via an invasive conduit. The open breathing circuit of noninvasive ventilation permits air leaks around the mask or through the mouth, rendering the success of noninvasive ventilation dependent on ventilators designed to deal effectively with air leaks and to optimize patient comfort and acceptance. Interfaces—the devices that connect the ventilator tubing to the nose, mouth, or both—enable pressurized gas to enter the upper airway during noninvasive ventilation. Commonly used interfaces in the acute setting include nasal masks and full face (or oronasal) masks.

Nasal masks are widely used for the administration of CPAP or NIPPV, particularly for chronic applications. Nasal masks are usually better tolerated than full face masks for long-term applications, because they cause less claustrophobia and discomfort and allow eating, conversation, and expectoration. The standard nasal mask is a triangular or cone-shaped clear plastic device that fits over the nose and uses a soft cuff that forms an air seal over the skin. The mask exerts pressure over the nasal bridge, often causing skin irritation and redness and occasionally ulceration. Many modifications are available to avoid complications, such as the use of forehead spacers or masks with ultrathin silicon seals or heat-sensitive gels that minimize skin trauma.

Full facemasks cover both the nose and the mouth (Figure 51-3) and are preferable to nasal masks in the acute setting. The efficacy of both nasal and oronasal masks in lowering PaCO₂ and avoiding intubation is similar in the acute setting, but a randomized controlled trial⁵³ observed better patient tolerance with full facemasks because of reduced air leakage through the mouth. More recently, a “total” facemask has become available; it seals around the perimeter of the face and resembles a hockey goalie’s mask. Made of optical-grade plastic, it is easy to apply and causes no more claustrophobia than standard facemasks. Mouthpieces are seldom used to administer noninvasive ventilation in the acute setting but are occasionally used during initiation, when the patient holds the mouthpiece in place to adapt to the sensation of positive-pressure ventilation.

Figure 51-3 Full facemask with soft silicon seal to minimize pressure on nasal bridge. A disposable version of this mask is widely used in the acute care setting.

Selection of a comfortable mask that fits properly is key to the success of noninvasive ventilation. The full facemask should be tried first in the acute setting, and if possible, the patient should be allowed to hold the mask in place initially. The mask straps are then tightened with the least tension necessary to avoid excessive air leakage. Some leaking is acceptable and even obligatory with bilevel ventilators, because of the need to flush carbon dioxide from the single-channel ventilator circuit. Bilevel ventilators compensate for air leakage better than critical care ventilators do, but excessive air leakage can lead to noninvasive ventilation failure with any ventilator.

Head straps hold the mask in place and are important for patient comfort. Straps attach at two to five points, depending on the type of mask. More points of attachment add to stability.

Oxygenation and Humidification

Oxygen is titrated to achieve a desired oxygen saturation, usually greater than 90% to 92%, either by using oxygen blenders on critical care and some bilevel ventilators or by adjusting liter flow (up to 15 L/min, as per manufacturer’s recommendations) delivered via oxygen tubing connected directly to the mask or ventilator circuit. Bilevel ventilators have limited oxygenation capabilities (maximal inspired oxygen fraction, 0.45-0.5), so ventilators with oxygen blenders should be used for patients with hypoxemic respiratory failure. A heated humidifier should be used to prevent drying of the nasal passage and oropharynx when the duration of application is anticipated to be more than a few hours.

Monitoring

Once noninvasive ventilation is initiated, patients should be closely monitored in a critical care or step-down unit until they are sufficiently stable to be moved to a regular medical floor. The aim of monitoring is to determine whether the main goals are being achieved, including relief of symptoms, reduced work of breathing, improved or stable gas exchange, good patient-ventilator synchrony, and patient comfort (Box 51-3). A drop in the respiratory rate with improved oxygen saturation or improving pH with a lower PaCO₂ within the first 1 to 2 hours portends a successful outcome.⁵⁴ Abdominal paradox, if present initially, subsides, and the heart rate usually falls. The absence of these propitious signs indicates a poor response to noninvasive ventilation and the need to make further adjustments. Leaks should be sought and corrected, patient-ventilator synchrony should be optimized, and pressures may have to be adjusted upward to relieve respiratory distress and achieve a reduction in PaCO₂. If these adjustments fail to improve the response within a few hours, noninvasive ventilation should be considered a failure, and the patient should be promptly intubated if it is still clinically indicated. Excessive delay in intubation may precipitate a respiratory crisis and add to morbidity and mortality.