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 units. Until the early 1960s, negative-pressure ventilation in the form of tank ventilators was the most common type of mechanical ventilation used outside the anesthesia suite. 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. Subsequently, invasive positive-pressure mechanical ventilation gradually became the preferred means of treating acute respiratory failure. Negative-pressure and other so-called body ventilators remained the mainstay of ventilatory support for patients with chronic respiratory failure until the mid-1980s.

With improving mask and ventilator technology and recognizing the many advantages of positive- over negative-pressure ventilation, noninvasive positive-pressure ventilation (NIV) displaced negative-pressure ventilation as the treatment of choice for chronic respiratory failure in patients with neuromuscular and chest wall deformities. Over the past 30 years, NIV has spread from the outpatient to the inpatient setting, where it is now used to treat certain forms of acute respiratory failure (ARF). Studies involving large clinical databases show that NIV use for patients with ARF resulting from chronic obstructive pulmonary disease (COPD) and non-COPD diagnoses increased several-fold during the first decade of the millennium. A survey in Massachusetts found that NIV is used very frequently in the acute care setting, constituting up to 40% of initial ventilator starts. This chapter discusses the rationale for the increased use of NIV in critical care, in addition to appropriate indications, practical applications, and monitoring.

Rationale

The most important advantage of NIV is the avoidance of complications associated with airway intubation and invasive mechanical ventilation. These hazards include upper airway trauma, the 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. By avoiding airway intubation, NIV leaves the upper airway intact, preserves airway defenses, and, during breaks, allows patients to eat and vocalize normally and to expectorate airway secretions. Compared with invasive mechanical ventilation, NIV reduces infectious complications, including pneumonia, sinusitis, and sepsis. Strengthening the rationale for its use is evidence that NIV lowers the morbidity and mortality rates of well-selected patients with ARF and may shorten hospital length of stay or avoid hospitalization altogether, thus reducing costs.

The main indication for mechanical ventilatory assistance is to treat respiratory failure, either type 1 (hypoxemic), type 2 (hypercapnic), or both. Fig. 55.1 shows that airspace collapse, surfactant abnormalities, and airway narrowing and closure contribute to ventilation-perfusion abnormalities and shunt, which cause hypoxemia. By opening the collapsed air spaces and narrowed airways, sustained 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 air spaces. 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 observed in patients with reduced intravascular fluid volume.

Fig. 55.1, 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 [PaCO 2 ]) occurs when respiratory muscles fail to ventilate alveoli adequately to maintain homeostasis with carbon dioxide production. Respiratory muscle failure occurs when the 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), presumably because of inadequate oxygen delivery to the respiratory muscles (e.g., approximately one-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 (V t ), using intermittent positive-pressure ventilation (IPPV), and reducing CO 2 production by decreasing the work of breathing. FiO 2 , Fraction of inspired oxygen; IPAP , inspiratory positive airway pressure; LV , left ventricular.

Mechanisms of action

Fig. 55.2 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 the increased work of breathing, predisposing patients to respiratory muscle fatigue. In patients with 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 (IPPV), NIV reduces the work of breathing and avoids the vicious cycle leading to respiratory failure. Work of breathing measurements, including transdiaphragmatic pressure, diaphragmatic pressure-time product, and diaphragmatic electromyographic amplitude, are all decreased when NIV 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.

$Fig. 55.2, Pathophysiology of acute hypoxemic respiratory failure and points where positive-pressure and oxygen supplementation interrupt the process. Low ventilation-perfusion ratios, shunt, and alveolar hypoventilation cause hypoxemia. Hypoxemia is treated by increasing the fraction of inspired oxygen (FiO 2 ) (limited benefit with shunt) and applying positive pressure (continuous positive airway pressure [ CPAP ] or positive end-expiratory pressure [ PEEP ]) to increase the residual functional capacity, open collapsed alveoli and narrowed airways, and enhance compliance. An additional beneficial effect of CPAP may occur in patients with cardiogenic pulmonary edema because it reduces both venous return and left ventricular afterload, which may enhance cardiovascular performance in patients with dilated, hypocontractile left ventricles. IPPV, Intermittent positive-pressure ventilation; PaCO 2 , arterial partial pressure of carbon dioxide; PPPi , intrinsic PEEP; VCO 2 , production of carbon dioxide.$

Indications

A number of causes of ARF are now considered appropriate for NIV therapy and are listed in Box 55.1 . Evidence supporting these indications is rated and briefly discussed here; guidelines for patient selection are discussed later. The European Respiratory Society/American Thoracic Society (ERS/ATS) Task Force for NIV offered recommendations and suggestions on clinical applications of NIV in 2017, which are incorporated into the discussion that follows. At the outset, it is imperative to emphasize that the interface used to apply NIV may be crucial to its efficacy in any individual patient.

BOX 55.1

ARDS, Acute respiratory distress syndrome; COPD, chronic obstructive pulmonary disease; CPAP, continuous positive airway pressure.

Indications for Use of Noninvasive Ventilation in the Acute Care Setting

Airway obstruction

COPD (A) *
Asthma (B)
Cystic fibrosis (C)
Obstructive sleep apnea or obesity hypoventilation (B)
Upper airway obstruction (C)
Facilitation of weaning in COPD (A)
Extubation failure in COPD (B)

Hypoxemic respiratory failure

ARDS (C)
Pneumonia (C)
Trauma or burns (B)
Acute pulmonary edema (use of CPAP) (A)
Immunocompromised patients (A)
Restrictive thoracic disorders (C)
Postoperative patients (B)
Do-not-intubate patients (C)
During bronchoscopy (C)

* Letters in parentheses indicate the level of evidence supporting the 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 attempted, but with close monitoring.

Airway obstruction

Chronic obstructive pulmonary disease

Randomized controlled trials ^, and meta-analyses have consistently shown that compared with conventional therapy, NIV 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 NIV is considered the ventilatory mode of choice in selected patients with acute exacerbations of COPD. Some studies suggest that the addition of heliox to NIV further improves the work of breathing and gas exchange during COPD exacerbations, but a subsequent multicenter trial found no improvement in other outcomes compared with NIV alone. The ERS/ATS Task Force gave a strong recommendation for use of NIV as the ventilatory modality of first choice for patients with hypercapnic respiratory failure caused by COPD exacerbations. More recent clinical practice guidelines suggest the use of nocturnal NIV in the home in addition to usual care for patients with chronic stable hypercapnic COPD (arterial partial pressure of carbon dioxide [PaCO ₂ ] ≥52 mm Hg) given several desirable effects of NIV, including possible reductions in mortality and hospital admissions, improved quality of life (QOL), reduced dyspnea, and produced improvements in functional capacity, awake blood gases, and exertion tolerance. ^,

Asthma

Uncontrolled studies have reported improvements in gas exchange and low rates of intubation after the initiation of NIV in patients with severe asthma attacks. Two controlled trials have demonstrated a more rapid improvement in expiratory flow rates with NIV, ^, and one showed a decreased hospitalization rate in acute asthma patients treated with NIV compared with a sham mask. Neither study was powered adequately to assess intubation or mortality rates. Nonetheless, these data support a trial of NIV in asthmatics responding poorly to initial bronchodilator therapy. NIV can be combined with continuous nebulization and heliox, although the added value of these latter therapies has not been established in controlled trials. In a more recent study using a large medical record dataset, 13,588 admissions for acute asthma were identified in 58 hospitals. Use of NIV for acute asthma was widely variable between hospitals, with 36% not using it at all, and one hospital using it in 16.3% of such patients. Overall, NIV was used in 4.0% of patients with acute asthma and invasive mechanical ventilation (IMV) in another 5.7%. NIV failure rate was only 4.9%, and the case-fatality rate of patients using NIV was only 2.3%. Despite these low rates, use of NIV was not associated statistically with lower mortality rate than IMV after risk standardization, but hospital length of stay was slightly shorter. Based on the tentativeness of the evidence, the ERS/ATS Task Force made no recommendation regarding the use of NIV for acute asthma.

Cystic fibrosis

Uncontrolled studies indicate that NIV 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.

Obesity hypoventilation syndrome

Acute hypercapnic respiratory failure related to obesity hypoventilation (OHS) is becoming more prevalent given the high and increasing obesity rates in the general population. A single-center prospective observational study was conducted to examine the use of NIV in these patients. Using COPD patients with acute hypercapnic respiratory failure for comparison, the authors found no change in the rate of NIV failure between the two groups and found lower rates of late NIV failure, readmission to the intensive care unit (ICU), and ICU and hospital mortality in the OHS group. Their conclusion was that NIV can be used safely and efficaciously in acute hypercapnic respiratory failure related to OHS in the ICU. Furthermore, a recent systematic review found that hospital discharge with positive airway pressure devices reduces mortality risk in OHS patients admitted with acute-on-chronic hypercapnic respiratory failure compared with those not discharged on such devices.

Upper airway obstruction

Anecdotally, NIV can be used effectively to treat patients with reversible upper airway obstruction, such as that caused by glottic edema after extubation. In this situation, NIV can be combined with aerosolized medications or heliox, but no controlled trials have demonstrated the efficacy of this approach. If therapy with NIV is considered, patients should be selected with great caution and monitored closely, because upper airway obstruction can lead to precipitous deterioration. The use of NIV 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 to fraction of inspired oxygen (PaO ₂ /FiO ₂ ) ratio less than 200) combined with an unassisted respiratory rate >30 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 NIV 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, ^, and in one study, ICU mortality. However, because of the heterogeneity of causes, these studies fail to demonstrate that all patient subgroups with hypoxemic respiratory failure benefit equally from NIV. Furthermore, when patients are stratified according to the acuity of illness, those with a Simplified Acute Physiologic Score (SAPS II) less than 35 fare considerably better with NIV than do those with higher scores. Thus the selection of patients with less severe disease is likely to enhance the success of NIV in treating hypoxemic respiratory failure, and studies that examine individual subgroups within the larger category are likely to be more useful clinically.

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