Noninvasive Respiratory Support

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Chapter 28 Noninvasive Respiratory Support

Ubbo F. Wiersema

In this chapter, supplemental oxygen therapy, humidification systems, adjunctive respiratory therapy, and noninvasive ventilation (NIV) are discussed. NIV encompasses various methods of positive pressure ventilation, including continuous positive airway pressure (CPAP), delivered without an endotracheal tube.1

OXYGEN THERAPY

Oxygen delivery to the tissues depends on arterial oxygen content, cardiac output, and individual organ perfusion. Arterial oxygen content depends on the hemoglobin concentration and arterial oxygen saturation (Sao2; see Equation 1-13). The latter is, in turn, dependent on the oxyhemoglobin dissociation curve (see Fig. 1-14) and the arterial partial pressure of oxygen (Pao2) (see Chapter 1). Supplemental oxygen therapy is indicated to maintain the Pao2 above 8 κPa (60 mmHg) or the Sao2 above 90%. Only low concentrations of supplemental oxygen are required to treat hypoxemia due to hypoventilation. In contrast, high concentrations of oxygen may be required to treat hypoxemia due to ventilation/perfusion mismatch. Oxygen is ineffective if hypoxemia is due to pure shunt. High-concentration oxygen therapy in the setting of hypoventilation may mask the development of severe hypercarbia and respiratory acidosis (see Chapter 1).

Impaired gas exchange is almost universally present following cardiac and thoracic surgery, so oxygen is indicated for the duration of a patient’s stay in the intensive care unit (ICU). It should also be continued following discharge from the ICU if a patient is receiving opioid analgesia because that predisposes the patient to nocturnal desaturation.2 Supplemental oxygen therapy during the perioperative period has also been shown to reduce the incidence of surgical wound infections.3

Excessive oxygen therapy is not without risk. High-concentration oxygen promotes absorption atelectasis and, in patients with severe chronic obstructive pulmonary disease (COPD), it may exacerbate hypercarbia (see Chapter 27). Administration of high oxygen concentrations (FIo2 >0.6) for longer than several days may potentially cause direct pulmonary toxicity and should be avoided if possible.4 However, supplemental oxygen should not be withheld in the presence of clinically significant hypoxemia (Sao2 <85% to 90%).

Oxygen Delivery Systems

Noninvasive methods of oxygen delivery can be divided into variable-performance and fixed-performance systems. In order to provide a fixed concentration of oxygen, the system must deliver gas at a flow rate that matches the patient’s peak inspiratory flow rate. This is approximately 25 to 35 l/min at rest but can rise to over 60 l/min with respiratory distress. If delivered flow is less than peak inspiratory flow, entrainment of room air dilutes the inspired gas and results in a reduced FIo2. Piped and bottled oxygen and air sources are dry and must be humidified if administered at high flow rates (see later material).

Variable-Performance Systems

Variable-performance systems have a flow rate or a reservoir size that is not sufficient to prevent entrainment of air at high-peak inspiratory flow rates.

Nasal Cannulas.

Nasal cannulas provide a low level of supplemental oxygen. They are well tolerated and allow the patient to eat and expectorate without having to remove the cannulas. The oxygen flow fills the nasopharyngeal and oropharyngeal reservoirs at end-expiration and is then inspired into the lungs, even when the mouth is open. Flow is usually set at 2 to 4 l/min, which is equivalent to an FIo2 of less than 0.3. Higher flow rates require humidification and may be uncomfortable.

Semirigid Face Masks.

Semirigid face masks increase the effective size of the airway reservoir and thus provide a higher oxygen concentration than do nasal cannulas, but they are less practical because they must be removed when eating. The simple face mask has an oxygen inlet at the base and holes at the sides for expiration and air entrainment. The FIo2 depends on the oxygen inflow rate and the patient’s tidal volume, respiratory rate, and respiratory pattern. A minimum flow rate of 5 l/min is required to prevent the rebreathing of expired gas. The partial rebreather mask and nonrebreather mask allow delivery of higher FIo2 levels but are designed for prehospital use and are not recommended in the ICU, where high FIo2 therapy is best given through a humidified fixed-performance system.

Manual Resuscitators.

The components of manual resuscitators (such as the Laerdal and the Ambu) are illustrated in Figure 28-1. As with other variableperformance systems, FIo2 depends on the oxygen flow, tidal volume, and respiratory rate, but it can be increased by attaching an oxygen reservoir bag to the air entrainment inlet. The resuscitation bag is designed for short-term emergency use when augmentation of a patient’s respiratory effort by manual ventilation is required. Care should be taken with a patient who has respiratory distress but good respiratory drive because spontaneous breathing through a manual resuscitator increases the work of breathing, potentially worsening the respiratory distress.

image

Figure 28.1 Manual resuscitator. A, During inspiration, when the self-inflating bag is compressed, flow from the oxygen supply is directed into the reservoir. B, If the reservoir becomes full, which may occur if minute ventilation is less than the oxygen flow, a pressure-release valve prevents a dangerous buildup of pressure within the device. C, During expiration, the self-inflating bag refills from both the oxygen supply and the reservoir bag. If the oxygen supply and the reservoir bag provide insufficient gas to refill the device, room air is entrained through the entrainment valve. The “duck-billed valve” at the patient end of the device ensures that minimal rebreathing of respiratory gas occurs and also allows the incorporation of a spring-loaded PEEP valve that is connected to the exhalation port of the breathing circuit if it is required. Modern valves (e.g., the Ambu Mark III) have very low inspiratory and expiratory resistance. PEEP, positive end-expiratory pressure.

Fixed-Performance Systems

Fixed-performance systems provide a constant FIo2 that is independent of a patient’s peak inspiratory flow. This is achieved by having either a high flow rate or a large-capacity reservoir.

Venturi Mask.

With the Venturi mask system, oxygen inflow is connected to a specific color-coded entrainment device at the base of the mask that provides a set FIo2 at a set oxygen inflow rate. Various entrainment devices can provide an FIo2 of 0.24 to 0.5, with an oxygen inflow of 4 to 15 l/min and a total flow delivered to the patient (including entrained air) of 35 to 45 l/min. If the patient’s peak inspiratory flow exceeds this total flow, a lower FIo2 is inspired. The Venturi mask is ideal for a patient with COPD who has a low to moderate oxygen requirement but is at risk for hypercarbia with uncontrolled oxygen therapy.

High-Flow Nonrebreather Mask

High-flow nonrebreather systems can provide an FIo2 up to 1.0. They consist of high-flow oxygen and air sources, a blender that mixes the oxygen and air to the desired concentration, a flow meter capable of delivering flows that can match the patient’s peak inspiratory flow rate, a humidifier, wide-bore tubing, and a close-fitting face mask. Patients who have high FIo2 requirements and are using this system must be closely monitored because any deterioration in clinical status may mandate the rapid institution of invasive ventilation.

HUMIDIFICATION

Humidity is a measure of the amount of water vapor in a gas. It may be expressed as a partial pressure (kPa), an absolute value (g/m3), or a relative value (%). The last is the absolute mass of water divided by the maximum mass of water that could exist in vapor form in the gas at a certain temperature.

During quiet nose breathing, inspired gas is warmed to body temperature and fully saturated with water vapor before it reaches the carina (absolute humidity 43.4 g/m3, relative humidity 100%, partial pressure 6.3 κPa at 37°C at sea level). Humidification occurs primarily in the nose, where inspired gases are exposed to a large surface area of highly vascular mucosa. Mouth breathing, particularly at high peak inspiratory flow rates, results in less humidification by the upper airway.

Normal mucociliary function of the large airways is dependent on adequate humidification of the inspired gas. Abnormal mucociliary function results in cytologic damage and leads to the accumulation of tenacious secretions and microatelectasis. At a relative humidity of 75% (32.5 g/m3 at 37°C), mucociliary function is impaired; at 50%, it fails entirely. Changes in mucociliary function occur within minutes to a few hours if low-humidity gases are inspired. This is the rationale for providing humidification of inspiratory gas of at least 32.5 g/m3 to all patients who are receiving high flows of inspiratory gases and to those whose upper airways have been bypassed using an endotracheal tube.

Humidification Devices

Heat and Moisture Exchangers

Microbiologic filters have been developed that conserve the heat and moisture of the gas expired by a patient; they use them to heat and humidify the inspired gas delivered during the next breath. Modern heat and moisture exchangers are light in weight, low in resistance, and low in volume. They provide variable degrees of humidification (approximately 20 to 30 g/m3), that decrease with duration of use and high minute ventilation. Some may be effective for several days of continuous use. Heat and moisture exchangers should not be used with noninvasive ventilation because their resistance, although low, imposes increased work of breathing.

Hot-Water Humidifiers

Hot-water humidifiers can deliver up to 100% relative humidity at 37°C by actively heating both the water bath and the gas as it passes to the patient. Gas is either blown over (a blow-over system) or through (a bubble-through system) a heated water bath, and its temperature is maintained by heating wires placed within the delivery tubing. A thermistor senses the temperature of the delivered gas near the patient, and the temperature of the water bath and heating wires is varied accordingly. Blow-over systems are preferred to bubble-through systems because the latter may produce aerosolized water, which facilitates bacterial transmission. For patients who are not intubated, the humidifier temperature is usually set at about 32°C for comfort. Cold-water humidifiers are ineffective, supplying a maximum of only 10 g/m3 of water content.

ADJUNCTIVE RESPIRATORY THERAPIES

Patients unable to generate high expiratory flow rates because of respiratory muscle weakness or obstructive airway disease may be unable to cough effectively. This leads to the accumulation of excessive secretions and entry of foreign material into the lungs, with the development of atelectasis, impaired gas exchange, and pneumonia. When cough is poor, lung-expansion techniques and maintenance of normal mucociliary clearance become important.

Physical Therapy

Lung-expansion techniques duplicate the normal sigh breath (periodic voluntary hyperinflation), increasing end-inspiratory lung volume and reversing atelectasis. The most commonly used technique is incentive spirometry. This requires proper patient instruction and encouragement and should be done several times per hour, with a target tidal volume greater than 14 ml/kg.

Excessive mucus production and impaired mucociliary clearance normally occur following general anesthesia, particularly when mechanical ventilation is prolonged. Respiratory therapy techniques that improve mucociliary clearance include postural drainage, chest percussion, and chest vibration. These techniques are helpful if there is a high sputum load or lobar atelectasis, but they may be limited by pain in patients who have recently undergone cardiothoracic surgery. Attempted tracheal suctioning in patients who are not intubated is potentially hazardous and should be avoided. Pharyngeal suctioning is commonly practiced in response to the accumulation of secretions in the upper airway, but it has no proven benefit.

Bronchodilators

Bronchodilators include β2-adrenergic agents (e.g., albuterol) and anticholinergic drugs (e.g., ipratropium bromide). In addition to acting as bronchodilators, they improve mucociliary function by stimulating ciliary activity and reducing mucus production. Delivery is achieved by nebulizer or metered-dose inhaler, both of which are designed to aerosolize the drugs such that the particle size is small enough to reach the distal airways. Both routes of administration have similar efficacy in spontaneously breathing and intubated patients.5

Jet nebulizers use an extrinsic gas flow to draw the bronchodilator drug from a liquid reservoir onto a baffle that reduces the particle size. In mechanically ventilated patients, the extrinsic gas flow can increase tidal volume or impair triggering unless it is compensated for by the ventilator. Ultrasonic nebulizers (for intubated patients) use high-frequency sound waves to create the aerosol above the liquid reservoir. Although tidal volume is not affected, there is a risk of overhydration and increased airway resistance. Metered-dose inhalers should be used with spacer devices in nonintubated patients so as to reduce aerosol deposition in the mouth. In intubated patients, metered-dose inhalers require adaptors that should be placed in-line immediately adjacent to the endotracheal tube.

NONINVASIVE VENTILATION

Positive-pressure ventilation increases functional residual capacity, improves oxygenation and lung mechanics, and reduces the work of breathing. In patients with auto positive end-expiratory pressure (PEEP), external PEEP (or CPAP) reduces the inspiratory threshold load (see Fig. 27-4), which reduces the work of breathing. In addition, tidal positive pressure ventilation (e.g., bilevel positive airway pressure ventilation) unloads inspiratory muscles, augments tidal volume, and reduces arterial carbon dioxide tension (Paco2). In patients with left ventricular dysfunction, cardiac performance is improved by reduction in left ventricular afterload. The rationale for NIV is to obtain these benefits without the disadvantages of endotracheal intubation, such as the need for sedation and increased risk of nosocomial pneumonia.1

Patients treated with NIV require continuous monitoring of oxygen saturation, heart rate, blood pressure, respiratory rate, respiratory pattern, ability to expectorate, and mental status. Facilities for urgent intubation and initiation of invasive ventilation must be rapidly available. In most cases, an intraarterial catheter for monitoring blood pressure and arterial blood gases is indicated. Inappropriate persistence with NIV in the face of clinical deterioration of a patient may lead to serious adverse events and worse outcome. Patients in whom NIV is most likely to be successful have a low volume of respiratory secretions, intact dentition, and minimal nonrespiratory organ dysfunction.

Indications and Contraindications

NIV is well established for treating acute exacerbations of COPD.6–8 However, of greater relevance to the cardiothoracic ICU is the use of NIV in treating cardiogenic pulmonary edema, postoperative respiratory failure, and thoracic trauma. Patient refusal or failure to cooperate precludes the use of NIV. Other contraindications to NIV are listed in Table 28-1.

Table 28-1 Contraindications to Noninvasive Ventilation

Neurologic Impaired level of consciousness, confusion, or inability to protect airway
Anatomic Trauma or recent surgery to the upper airway, face, or esophagus
  Upper airway obstruction
Respiratory Severe hypoxemia
  Moderate or large quantity of sputum
  Weak cough
  Undrained pneumothorax
Cardiovascular Marked hemodynamic instability
Abdominal Abdominal distension or vomiting

Cardiogenic Pulmonary Edema

In patients with acute cardiogenic pulmonary edema, the addition of face-mask CPAP to standard medical therapy leads to a more rapid improvement in respiratory rate, heart rate, Paco2, and oxygenation than does standard medical therapy (oxygen, morphine, diuretics, and nitrates) alone.9,10 In addition, CPAP reduces the need for endotracheal intubation and may decrease mortality rates.11 Similar physiologic benefits have been demonstrated with bilevel positive airway pressure ventilation.12,13 In a head-to-head comparison of bilevel positive airway pressure and CPAP, an increased rate of myocardial infarction was reported in the former group.14 However, because a greater proportion of patients had chest pain at entry into the bilevel support group, it is possible that this outcome was not related to the mode of ventilation. Until more data are available, CPAP is the preferred mode of NIV.

An improvement in vital signs should be seen within 30 minutes, and many patients can be successfully weaned within a few hours. A patient who requires intubation because of deterioration despite CPAP therapy has a very poor prognosis unless the underlying cause of the cardiogenic pulmonary edema responds to a specific intervention (e.g., thrombolysis, percutaneous coronary intervention, or cardiac surgery).

Postoperative Respiratory Failure

NIV may be used to treat (rescue therapy) or prevent (prophylaxis) postoperative respiratory failure. Prophylaxis refers to the application of NIV immediately following extubation so as to prevent respiratory failure. As rescue therapy, NIV has been shown to reduce the incidence of reintubation and other complications in patients who develop respiratory distress following thoracic surgery15 and solid-organ (liver, kidney, lung) transplantation.16 It has also been shown to be beneficial as rescue therapy following major abdominal surgery.17

In patients deemed at high risk for postextubation respiratory failure, prophylactic NIV has been shown to reduce the need for reintubation,18,19 with a possible survival advantage in hypercarbic patients19; however, in both trials, postoperative patients constituted only a minority of study subjects. Neither rescue nor prophylactic NIV has been specifically studied following cardiac surgery.

CPAP is a highly efficacious treatment for obstructive sleep apnea and, although there are few trial data, prophylactic use during the postoperative period is strongly recommended in patients with this condition who were being treated with NIV preoperatively.20

No clear advantage of either CPAP or bilevel positive airways pressure can be identified from the literature for treating postoperative patients. As a simple rule of thumb, CPAP may be used to treat isolated hypoxemia, and bilevel positive airways pressure can be used to treat hypoxemia associated with hypercarbia.

Thoracic Trauma

NIV may be considered in patients who have experienced respiratory failure due to thoracic trauma (rib fractures, flail chest, pulmonary contusion). In one small, randomized study of patients with flail chest, face-mask CPAP plus patient-controlled opioid analgesia were associated with lower mortality rates than found in those undergoing invasive ventilation.21 There is the potential for any form of positive pressure ventilation to convert a simple pneumothorax into a tension pneumothorax, so pneumothoraces should be drained before instituting NIV.

Patients With Do-Not-Intubate Orders

Among patients with do-not-intubate orders, NIV is used principally to provide relief from dyspnea. It should not be used against a patient’s wishes or if they find it uncomfortable.

Delivery systems and settings

Continuous Positive Airway Pressure Systems

Continuous Fresh-Gas-Flow Systems.

Continuous fresh-gas-flow systems consist of a humidified fixedperformance gas delivery system, an interface (tightfitting mask), and a spring-loaded valve (CPAP valve) that provides constant pressure against expiratory flow (Fig. 28-2). Gas delivery is usually provided by a high-flow system, but gas flow can be reduced and system-imposed work of breathing reduced if a weighted, high-compliance reservoir bag is incorporated into the inspiratory limb of the system.22 High fresh-gas-flow rates are generated by a pressurized gas supply, a gas turbine, or a jet Venturi mechanism. The addition of a pressure-relief valve in the inspiratory limb of the circuit provides safety against barotrauma in the event that the CPAP valve becomes obstructed.

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Figure 28.2 Continuous fresh-gas-flow CPAP system. A reservoir bag may be included in the inspiratory limb of the circuit (see text for details).

Demand Valve Systems.

Demand valve systems are used for the application of CPAP through a mechanical ventilator; they rely on an efficient inspiratory trigger and expiratory valve mechanism. For most modern intensive care ventilators, this is an acceptable alternative to continuous-gas-flow systems, and they have the advantage of providing extra patient monitoring.23 However, in a patient with a high peak inspiratory flow and high respiratory rate, the imposed work of breathing through this type of CPAP system becomes appreciable and is undesirable.

Commonly used initial settings for treating acute cardiogenic pulmonary edema are 10 cm H2O CPAP and FIo2 1.0. With a continuous fresh-gas-flow system, the gas flow must be sufficiently high to ensure that there is flow through the CPAP valve throughout the respiratory cycle (usually 40 l/min for systems that incorporate a reservoir bag); that is, the CPAP valve should always be open. With clinical improvement, FIo2 can be progressively reduced, ensuring that Spo2 is greater than 90%. Once an FIo2 of 0.4 is reached and the patient remains clinically stable, with a respiratory rate of fewer than 30 breaths per minute, a trial without CPAP (on humidified oxygen) or a reduction in the level of CPAP to 7.5 cm H2O (and then 5 cm H2O) should be attempted.

Bilevel Positive Airway Pressure Ventilation

Theoretically, any mode of ventilation can be used for NIV. Intensive care ventilators provide a wide choice of modes, but they are designed for invasive ventilation and may be unable to compensate for interface leaks, potentially resulting in inadequate tidal ventilation (see subsequent material). Modern noninvasive ventilators are leak tolerant, can provide high inspiratory flow rates and high FIo2, and have appropriate monitors and alarms. In general, pressure-limited modes are better tolerated than volume-limited modes and are less prone to inadequate tidal breath delivery in the presence of a significant leak. Therefore, the majority of NIV is performed with CPAP plus pressure support or bilevel positive airway pressure modes, both of which are pressure-limited modes.

The pressure-support mode is either pressure-triggered or flow-triggered and is flow-cycled (see Chapter 29). The bilevel positive airway pressure mode is also pressure- or flow-triggered and is flow-cycled but has a facility for time triggering at a set respiratory rate (spontaneous/timed mode). Bilevel positive airway pressure has two pressure settings—the expiratory positive airway pressure (EPAP) and the inspiratory positive airway pressure (IPAP). The level of IPAP is inclusive of EPAP. (In contrast, the level of pressure support is in addition to CPAP.) Standard initial settings are an EPAP of 4 to 8 cm H2O and an IPAP of 8 to 12 cm H2O, with an FIo2 sufficient to maintain the SPo2 above 90%. These pressures can subsequently be increased according to the patient’s response and tolerance, but IPAP values over 20 cm H2O are likely to cause unacceptable leak or patient discomfort. Because bilevel positive airway pressure is primarily a spontaneous mode of ventilation, the breath rate is set to a lower rate than the patient’s expected breath rate. A rate of 8 to 12 is appropriate. Bilevel positive airway pressure (BiPAP), which is available in the Vision noninvasive ventilator (Respironics, Murrysville, PA; Fig. 28-3) is an example of a commercially available bilevel positive airway pressure NIV mode.

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Figure 28.3 Vision (Respironics, Murrysville, PA) noninvasive ventilator.

(Image courtesy of Care Medical, Portland, OR.)

Confusion in terminology can arise because BIPAP (with a capital I) refers to biphasic positive airway pressure mode, which is similar to pressure-control ventilation, with the ventilator switching between inspiratory and expiratory pressure levels at set time intervals (i.e., time cycled). It differs from true (invasive) pressure-control ventilation in that additional spontaneous breathing is allowed at both pressure levels because of the presence of an open (active) expiratory valve. In contrast, the invasive pressure-control ventilation mode has a closed (or inactive) expiratory valve during inspiration.

Patient-ventilator dysynchrony

Even with minimal interface leak, patient-ventilator dysynchrony is common with bilevel modes and may be the principal determinant of success or failure of NIV.24 Graphic monitoring of pressure-time and flow-time signals can help to detect specific patterns of dysynchrony.

Inspiratory Dysynchrony

Inspiratory dysynchrony usually occurs during the trigger phase (the signal that initiates the change to the inspiratory pressure level). It manifests as ineffective respiratory efforts, with either failure to trigger or, at the other extreme, autotriggering. Failure to trigger is likely in patients with dynamic lung hyperinflation and auto-PEEP that is much higher than the set EPAP level. Autotriggering occurs with sensitive trigger settings and either a variable leak or water in the tubing. Flow triggering reduces the inspiratory effort and the trigger delay more than pressure triggering does, but all ventilators have unavoidable, intrinsic trigger-activation delays, usually greater than 120 ms.25,26 The theoretic advantage of flow triggering may be less significant in modern ventilators that have pressure sensors closer to the patient, thus reducing trigger delay.

The flow waveform method of triggering (BiPAP Vision, Respironics) results in a transition from expiration to inspiration by means of volume triggering or by the shape signal method (Fig. 28-4). The flow waveform method results in fewer episodes of trigger failure but more frequent autotriggering than standard flow triggering, although this has been studied only in intubated patients.25,27

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Figure 28.4 Pressure-time and flow-time waveforms in a patient ventilated by a BiPAP Vision ventilator (Respironics, Murrysville, PA), illustrating the shape signal method of triggering and cycling. The flow-time curve from the circuit is shown (thick line). From this, the machine generates a flow-shape signal (thin line) by offsetting (by 0.25 l/s) and delaying (by 300 ms) the actual flow-time curve. A mechanical breath is initiated and terminated when the actual flow-time curve crosses the flow shape signal at end-expiration and end-inspiration, respectively. The inspiratory rise time is defined as the time from the onset of inspiration until the set inspiratory pressure level is reached. BiPAP, bilevel positive airway pressure; EPAP, expiratory positive airway pressure; IPAP, inspiratory positive airway pressure.

Inspiratory dysynchrony may also result from an inappropriate rate of pressure rise to the set inspiratory pressure level (rise time; see Fig. 28-4). A rapid rise time reduces inspiratory effort but increases air leak and worsens patient tolerance.28

Expiratory Dysynchrony

Expiratory dysynchrony occurs when the ventilator’s transition from inspiration to expiration occurs before or after the end of the patient’s inspiratory effort. This manifests as “fighting the ventilator” if the patient actively expires when the transition is delayed or as retriggering if termination of inspiratory flow is premature. The transition is determined by the method of cycling. With flow cycling, the ventilator cycles to expiration when the inspiratory flow falls to a certain proportion of the peak inspiratory flow or to a predetermined (low) flow rate. Flow cycling in the presence of a significant leak or long patient respiratory time constant can result in delayed transition to expiration and thus dysynchrony (see Chapter 4); time cycling may provide the most consistent patient-ventilator synchrony.29

The shape signal method described earlier has also been applied to the cycling variable, although in the setting of a large leak or long expiratory time constant, it too can result in a delayed transition to expiration.27 The Vision ventilator also provides the spontaneous expiratory threshold, time, and flow reversal methods of cycling in an effort to reduce this risk.

Interfaces

Appropriate selection and application of the patient interface is essential for the successful management of NIV.

Oronasal Masks

The oronasal mask is the most commonly used interface for NIV in the ICU. When correctly secured, this style of mask should provide a firm seal around the patient’s nose and mouth. It is held in place by an adjustable head harness, and problems attributed to poor mask fit can be solved by diligent adjustment of the harness in conjunction with appropriate support of the NIV system’s tubing. Most masks are made of a clear polycarbonate shell that is surrounded by a silicone-based sealing cuff that may be filled with air. Masks are available in a range of shapes and sizes, and the selection of a correctly fitting mask is imperative so as to minimize leakage, reduce the risk of pressure necrosis, and maximize patient comfort and compliance. Pressure necrosis of the skin most commonly develops over the bridge of the nose. Use of cushioning materials and periods of rest from NIV help to reduce this complication. A large leak may cause eye irritation. Compared to the nasal mask, the oronasal mask can deliver higher ventilation pressures with fewer leaks, requires less patient cooperation, and permits mouth breathing, making it the most suitable type of mask for the acutely dyspneic patient in the ICU.

Nasal Masks

Nasal masks are most commonly used for patients with chronic ventilatory failure. They are better tolerated than oronasal masks and allow both oral intake and communication. The mouth has to be closed to maintain positive pressure, which makes them less suitable for the acutely dyspneic patient. A chinstrap may be used to keep the mouth closed.

Complications

The most common complications are related to mask fit (described earlier). Complications attributable to positive pressure ventilation include gastric insufflation (with risk of aspiration of gastric contents), hypotension, and pneumothorax. A nasogastric tube should not be used unless there is a clear indication to do so because it exacerbates mask leak and may increase the risk for pressure-induced skin necrosis. Delay in the institution of invasive ventilation may also occur.

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