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The interface distinguishes NIV from invasive ventilation. Unlike invasive ventilation, where the airway is sealed, leaks of variable degree occur with NIV. A variety of interfaces are available and these have improved in variety and quality in recent years. The patient interface has a major impact on patient's comfort and compliance during NIV. Common interfaces for NIV in patients with acute respiratory failure are the oronasal mask, nasal mask, and total facemask (Figure 11-1). The nasal pillows and mouthpieces are more commonly used during NIV for chronic respiratory failure and continuous positive airway pressure (CPAP) used to treat obstructive sleep apnea. The oronasal mask and total facemask are more commonly used for acute respiratory failure. Outside of North America, the helmet is used for NIV and CPAP. There are advantages and disadvantages of each type of interface (Table 11-4).
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Selecting the correct mask size is critical. The nasal mask should fit just above the junction of the nasal bone and cartilage, directly at the sides of both nares, and just below the nose above the upper lip. The oronasal mask should fit from just above the junction of the nasal bone and cartilage to just below the lower lip. A common mistake is to choose a mask that is too large. This results in leaks, decreased effectiveness, and patient's discomfort. Leaks through the mouth are not uncommon when using a nasal mask. When mouth leak interferes with the effectiveness of ventilation, an oronasal mask can be used. For acute respiratory failure, an oronasal mask or total facemask is better tolerated and more effective than a nasal interface. Upper airway dryness is greater with use of a nasal mask because of mouth leak, which can be addressed by using heated humidification or an oronasal mask. A humidifier should be used for NIV and set to patient's comfort.
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A common mistake is to fit the headgear too tightly. It should be possible to pass one or two fingers between the headgear and the face. Fitting the headgear too tightly usually will not improve the fit and always decreases patient's comfort and compliance. The design of most masks for NIV is such that the top of the mask is secured on the forehead rather than at the bridge of the nose. Forehead spacers and an adjustable bridge on the mask are important to fill the gap between the forehead and the mask, thus reducing pressure on the bridge of the nose. This improves comfort and decreases the likelihood of pressure sores.
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Aerophagia commonly occurs with noninvasive ventilation, but this is usually benign because the airway pressures are less than the esophageal opening pressure. Thus, a gastric tube is not routinely necessary for mask ventilation. In fact, a gastric tube may interfere with the effectiveness of mask ventilation in several ways. It may be more difficult to achieve a mask seal if a gastric tube is present. The gastric tube forced against the face by the mask cushion increases the likelihood of facial skin breakdown. A nasogastric tube also increases resistance to gas flow through the nose, which may decrease the effectiveness of mask ventilation—particularly nasal ventilation.
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Pressure sores on the bridge of the nose can occur during NIV. Fortunately, ulceration and skin breakdown can be avoided in most patients. Correct mask fit and size should be reassessed. The tension of the headgear should be reduced. A different mask style may be tried. A hydrocolloid dressing or commercially available nasal pad may also be helpful.
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Leaks during NIV can be a significant contributor to patient-ventilator asynchrony. Large leaks can also compromise inspiratory and expiratory pressures, and tidal volume delivery. Thus, an important consideration in the selection of a ventilator for NIV is leak compensation.
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Three categories of ventilators can be used for NIV: critical care ventilators, bilevel ventilators, and intermediate ventilators. Bilevel ventilators use a single limb circuit with a passive exhalation port. Critical care ventilators have separate inspiratory and expiratory limbs, with an active exhalation valve. Intermediate ventilators are typically used for patient transport or home ventilation; they may have a passive exhalation port or an active exhalation valve. Ventilators that use an active exhalation valve have traditionally been leak-intolerant. However, the newer-generation of critical care ventilators features NIV modes that compensate for leaks. With bilevel ventilators, leak is composed of the intentional leak through the passive exhalation port as well as unintentional leaks that may be present in the circuit or at the interface.
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Bilevel ventilators compensate well for leaks. They are blower devices that vary inspiratory and expiratory pressures in response to patient's demand. These ventilators provide pressure-controlled or pressure support ventilation. None provides volume-controlled ventilation, although some provide volume-targeted adaptive pressure ventilation. Some bilevel ventilators automatically adjust the inspiratory trigger and expiratory cycle by tracking the patient's inspiratory and expiratory flows. Others allow the clinician to adjust the trigger and/or cycle. Rise time can be adjusted on some bilevel ventilators to improve patient-ventilator synchrony. To minimize CO2 rebreathing, bilevel ventilators cannot be used without positive end-expiratory pressure ([PEEP] ≥ 4 cm H2O). Modern bilevel ventilators use a blender to provide a precise Fio2.
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With a critical care ventilator, the level of pressure support is the pressure above the baseline level of PEEP. The approach is different with bilevel ventilators, where an inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP) are set. Here the difference between the IPAP and EPAP is the level of pressure support (Figure 11-2).
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