Chapter 33 Noninvasive Mechanical Ventilation
Noninvasive ventilation (NIV) is mechanical ventilatory support delivered to the airways in the absence of invasive devices such as an endotracheal tube or tracheotomy cannula. The most common devices used to establish the ventilator-patient interface are nasal, oronasal, and full face masks, along with nasal pillows and helmets. The role of NIV has grown steadily in recent years, and in expert hands, it now represents an extremely valuable tool for the management of acute respiratory failure (ARF). The key factors in successful use of NIV are proper patient selection and a high level of competence and experience in both medical and adjunctive personnel on the respiratory care team. It has been shown that in the appropriate setting, NIV is effective in improving gas exchange and alveolar ventilation, as well as in avoiding intubation in patients with ARF. A role for this ventilatory mode also has been proposed in the management of chronic hypercapnic respiratory failure in chronic obstructive pulmonary disease (COPD), although its use is associated with more controversial results.
In general, the main goal of NIV is to provide adequate ventilatory support while avoiding the risks related to tracheal intubation (laryngeal and tracheal damage, lower-airway infections, accidental extubation, higher risk of barotrauma, and volutrauma), as well as sparing the patient the level of pharmacologic sedation commonly required for intubation. In addition, NIV can be performed outside the intensive care unit (ICU)—for example, in step-down units or the emergency department, or on respiratory care wards. On the other hand, it is obvious that NIV cannot guarantee the level of airway control provided by invasive ventilation in terms of leak avoidance, effective delivery of flow and pressure to the airways, and control of minute ventilation. Accurate positioning of the interface device is crucial to minimize air leaks, and patients should be constantly monitored for timely detection of signs of worsening that may necessitate a prompt switch to invasive ventilation.
These signs indicate a combination of increased work of breathing and decline in respiratory pump efficiency and point to the need for ventilatory support to relieve the rapidly worsening inspiratory muscle fatigue and to restore acceptable levels of gas exchange and alveolar ventilation. In the presence of these conditions, NIV should be initiated as soon as possible.
It also is important to bear in mind the conditions that preclude use of NIV and dictate prompt intubation with no delay—severely impaired neurologic state as evidence by a Kelly score (a scale specifically devised to assess patient responsiveness) higher than 4, respiratory arrest, shock, severe cardiovascular instability, and presence of excessive airway secretions. Facial lesions that prevent the fitting of nasal or facial masks also will prevent the use of NIV.
This chapter focuses exclusively on positive-pressure ventilation, which can be broadly defined as the intermittent delivery of pressure to the airways by means of a machine connected to the airway opening. The modes more frequently used for noninvasive ventilation are pressure support ventilation (PSV) and proportional assist ventilation (PAV), often with the addition of extrinsic positive end-expiratory pressure (PEEP); the use of continuous positive airway pressure (CPAP) in the acute setting is confined mainly to the treatment of hypoxemic respiratory failure, especially if the cause is related to cardiogenic pulmonary edema.
1. Typical ICU ventilators, powered by compressed gas, usually from wall outlets, and interfaced to the patient exclusively by means of a double circuit, with separate inspiratory and expiratory limbs. These systems are equipped with a screen to allow complete monitoring of ventilatory parameters and graphic display of flow, volume, and pressure curves. They typically are used for invasive ventilation. When set for pressure ventilation, they function on the PSV/PEEP algorithm, in which, as mentioned, the PSV level is superimposed on the PEEP, with total inspiratory pressure thus resulting from the sum of PSV and PEEP.
2. Portable home ventilators (bilevel ventilators), electrically powered and providing only a single circuit for both inspiration and expiration. These machines are used exclusively for noninvasive ventilatory support. They function on the IPAP/EPAP algorithm, in which the inspiratory positive airway pressure (IPAP) is not superimposed on the expiratory pressure (EPAP).
3. “Hybrid” ventilators, usually powered by electricity, allowing both single- and double-circuit options. They use the PSV/PEEP algorithm. They can be used for both invasive as well as noninvasive ventilation.
Patient-circuit interfaces for NIV can be nasal (nasal mask or nasal pillows or plugs), oral (mouthpiece), or facial (oronasal mask, full face mask, helmet). The choice of the appropriate interface is crucial to ensure the success of the treatment and patient compliance. In general, the nasal mask is better tolerated by patients, because it allows expectoration and creates less overall discomfort and is associated with less subjective claustrophobia. In acutely ill patients, face masks usually are preferred over nasal masks by most clinicians, because they provide better control of air leaks, especially in view of the difficulty of breathing exclusively through the nose for patients with acute respiratory failure. In fact, leak control is a key determinant of success for NIV, because it has been shown that air leaks represent the single most important cause of patient-ventilator asynchrony, even when such leaks are relatively modest. On the other hand, placement of nasal or oronasal masks should never be too tight, to prevent development of severe pressure skin lesions, especially frequent across the bridge of the nose, and to improve tolerability. The marked difference in internal volume associated with different interfaces does not usually affect NIV outcome in terms of clinical response and gas exchange variables, when the treatment is delivered by an experienced staff, as shown by a randomized prospective study that compared full face masks, oronasal masks of various sizes, and mouthpieces.
In the past decade, the role of NIV has steadily grown, so that it has now become a first-line intervention in a number of clinical conditions. In particular, on the basis of solid evidence gathered over the past 2 decades, NIV has been established as a first-choice intervention in the treatment of acute respiratory failure in the setting of COPD exacerbations and cardiogenic pulmonary edema. The role of NIV also appears to be well established in the management of respiratory failure developing in immunocompromised patients. Finally, NIV has been used as a weaning strategy and to reduce extubation failure.
Acute respiratory failure frequently complicates COPD exacerbations for a number of reasons deeply rooted in the pathophysiology of the disease. In particular, a combination of airway disease and loss of elastic recoil contributes to the generation of airflow limitation, which in turn can lead to air trapping and lung hyperinflation. As a consequence, the diaphragm is flattened, with loss of optimal length of its muscle fibers and diminished contractile efficiency. Moreover, intrinsic positive end-expiratory pressure (PEEPi) often is present, as a result of the nonreversible airflow limitation. PEEPi places a further burden on inspiratory muscles, because it has to be overcome before a negative alveolar pressure, necessary to the initiation of inspiratory airflow, can be generated.
The contraction of the diaphragm initially is isometric, and only after PEEPi has been counterbalanced can chest expansion begin. The combined effect of increased work of breathing and loss of contractile efficiency places the diaphragm at risk for development of fatigue, a precursor to acute alveolar hypoventilation with hypercapnia and respiratory acidosis. The first studies on the role of noninvasive ventilation in patients with COPD experiencing acute hypercapnic respiratory failure were conducted in the late 1980s. Since then, several controlled randomized studies have shown that NIV added to standard medical treatment is effective in reducing mortality, avoiding intubation, relieving dyspnea, and reducing length of hospital stay in patients with COPD with acute respiratory failure when compared with medical management plus oxygen therapy alone.
A randomized controlled study conducted on 236 patients showed that the rate of success with NIV was especially high in subjects with mild acidosis (pH greater than 7.30), whereas patients with more severe acidosis did not fare equally well and were more likely to require intubation. These findings point to the need for an early and accurate differentiation between patient subgroups on the basis of severity of illness, to allow prompt initiation of the most effective treatment. Patients with mild to moderate acidosis can receive NIV in hospital units outside the ICU, so long as they are staffed with trained personnel. More severely affected patients can still undergo a NIV trial, but only under strict management in an ICU setting, where they can be intubated with no delay in case signs of failure become apparent. Patients treated with NIV, irrespective of their initial severity status, had lower rates of infectious complications (ventilator-associated pneumonia, sepsis). A recent prospective study conducted in the United Kingdom in 9716 inpatients with COPD exacerbation and ARF managed in general clinical practice showed an overall mortality rate of 25% for patients receiving NIV—significantly higher than rates reported in the randomized controlled trials (RCTs). The study pointed to several potential explanations, including inaccurate selection of candidate patients for NIV, including, in some cases, patients with mixed acidosis or prevailing metabolic acidosis; the use of NIV as a “ceiling” of treatment in subjects with very severe disease; and substantial delays in initiating the ventilatory treatment. In addition, patients with mild acidosis (those in which the effectiveness of NIV is higher) appeared to be a minority among the overall group and often do not receive NIV at all. These results highlight the need for better implementation, in general clinical practice, of proper use of NIV in accordance with the evidence gathered from the RCTs.
Cardiogenic pulmonary edema is a consequence of left ventricular failure. It frequently leads to a reduction in lung compliance, with decreased functional residual capacity, regional atelectasis, ventilation-perfusion mismatch and poor gas exchange, resulting in lung failure with hypoxemic respiratory failure. The latter condition is characterized by PaO2/FIO2 of less than 300. Treatment with CPAP has long been known to improve survival rate and to lower the need for intubation in patients with cardiogenic pulmonary edema, compared with those receiving conventional medical treatment plus oxygen therapy.
In hypoxemic patients, conventional NIV has not produced significant improvements over those achieved with CPAP, although it can be effective in patients with cardiogenic pulmonary edema exhibiting hypercapnia. These data were recently questioned by a multicenter trial comparing oxygen therapy alone, CPAP, and NIV. The investigators concluded that with NIV, the physiologic improvements were faster than with oxygen alone, but without any significant effect on intubation or mortality rates. However, the very low intubation rate in this study (less than 3%) raises questions regarding whether the patient population was comparable to that of other studies.