Chapter 11: Noninvasive Ventilation

One of the significant advances in respiratory critical care over the past 20 years has been the emergence of noninvasive ventilation (NIV). NIV is used increasingly in patients with acute respiratory failure. In appropriately selected patients, need for intubation is reduced with the use of NIV. Because an artificial airway increases the risk of nosocomial pneumonia, it is not surprising that use of NIV decreases the incidence of ventilator-associated pneumonia. In some clinical settings, such as chronic obstructive pulmonary disease (COPD) exacerbation or acute cardiogenic pulmonary edema, the use of NIV affords a survival benefit. This chapter covers clinical and technical issues related to use of NIV.

Patient Selection

The strength of evidence for the use of NIV for various causes of acute respiratory failure is summarized in Table 11-1. High-level evidence supports the effectiveness of NIV for COPD exacerbation. Equally strong evidence supports the use of NIV for acute cardiogenic pulmonary edema. There is also evidence to support the use of NIV in patients with respiratory failure following solid organ transplantation and those who are immunosuppressed. Although some evidence supports the use of NIV for acute asthma, the evidence in this setting is weak. The use of NIV as an alternative to invasive ventilation in severely hypoxemic patients with acute respiratory distress syndrome is not recommended.

NIV can be used to allow earlier extubation in selected patients who do not successfully complete a spontaneous breathing trial (SBT), but its use in this setting should be restricted to patients who are intubated for COPD exacerbation or patients with neuromuscular disease. In patients who successfully complete an SBT, but are at risk for extubation failure, NIV should be used to prevent extubation failure. These patients are extubated directly to NIV. NIV should be used cautiously in patients who successfully complete an SBT, but develop respiratory failure after extubation. In this setting, NIV is indicated only in patients with hypercapnic respiratory failure.

When to Start and When to Stop

A two-step process is used to identify patients likely to benefit from NIV (Table 11-2). In appropriately selected patients, NIV reduces, but does not eliminate, the need for intubation. Thus, it is important to promptly recognize when patients are failing NIV. A more rapid decrease in Paco₂ occurs when NIV is successful. NIV failure has been associated with greater severity of illness, greater mouth leak, and difficulty acclimating to NIV. NIV success is greater for patients with higher baseline pH levels, perhaps because low pH is considered a marker of more severe illness. A good level of consciousness also has been associated with successful responses to NIV for patients with COPD and acute hypercapnic respiratory failure. Other factors that contribute to NIV failure include poor patient selection, progression of the underlying disease process, clinician's inexperience, and lack of appropriate equipment. If a patient does not improve on NIV within 1 to 2 hours of initiation, alternative therapy such as intubation should be considered. Patients receiving NIV for acute respiratory failure should be transferred to a unit where patients are well monitored, such as an intensive care unit (ICU) (Table 11-3).

Patient Interface

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).

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.

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.

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.

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.

Ventilators for NIV

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.

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.

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.

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).

If NIV is to be successful, those caring for the patient (physicians, nurses, respiratory therapists) must be committed to this approach. This is usually achieved by familiarizing these persons with the accumulated evidence suggesting that NIV improves outcome in selected patients. Physicians must appreciate selection criteria, respiratory therapists must understand the issues related to ventilator management and selection of an appropriate interface, and nursing personnel must appreciate issues related to mask fit and skin care. Some clinicians are reluctant to initiate NIV due to concerns related to time requirements and difficulties encountered when NIV is initiated. Fitting the mask, selection of appropriate ventilator settings, and patient coaching are labor-intensive for the first hours of NIV.

The application of NIV (Figure 11-3) requires caregiver's patience and skills with both the technical aspects of mechanical ventilation and the ability to coach patients to adapt to the mask and ventilator. The primary goal when initiating NIV is patient's comfort and not an improvement in arterial blood gases per se (an improvement in blood gases will usually follow if patient comfort and respiratory muscle unloading is achieved). Important steps in the clinical application of NIV are as follows: (1) choose a ventilator capable of meeting patient's needs (usually pressure ventilation), (2) choose the correct interface and avoid a mask that is too large, (3) explain therapy to the patient, (4) silence alarms and choose low settings, (5) initiate NIV while holding the mask in place, (6) secure the mask, avoiding a tight fit, (7) titrate pressure to patient's comfort, (8) titrate Fio₂ to Spo₂ > 90%, (9) avoid peak pressure > 20 cm H₂O (which increases the risk of gastric insufflation), (10) titrate PEEP per trigger effort and Spo₂, and (11) continue to coach and reassure patient, and make adjustments to improve patient's compliance. Complications of NIV (usually minor) include leaks, mask discomfort, facial skin breakdown, oropharyngeal drying, eye irritation, sinus congestion, patient-ventilator asynchrony, gastric insufflation, and hemodynamic compromise.