The bulk and lack of portability of the iron lung, along with the difficulty in providing routine care for patient , led to the development of smaller, portable negative-pressure devices. The chest cuirass, or shell ventilator, gained wide popularity during the 1950s. Two versions of this device were primarily used to apply negative pressure only to the thorax and upper abdomen. In one version, the patient’s chest was covered by a metal shell with an air-filled rubber edge that sealed the thorax. In later models, the shell was made of plastic, which made it easier to mold and fit to a patient’s chest. The other type of chest ventilator was a wraparound piece of plastic over a shell that was powered by a vacuum-cleaner motor (Fig. 19-1).

The use of positive-pressure ventilation can be traced as far back as 1780, when the first bag-mask apparatus was designed for resuscitative efforts. Positive-pressure ventilation with a mask was first used clinically in the mid-1940s, when Motley et al8 treated patients with acute respiratory failure (ARF) caused by a variety of clini-cal conditions. This intermittent positive-pressure ventilation (IPPV), which used a pressure-targeted ventilator and a mask, later was used primarily to treat ARF complicated by chronic obstructive pulmonary disease (COPD) and asthma. Volume-targeted ventilators and invasive airways were developed in the 1960s and became the standard for providing positive pressure ventilation in the treatment of respiratory failure. Intermittent positive-pressure breathing (IPPB), which used a mask or mouthpiece, became a means of simply delivering aerosolized medication periodically with positive-pressure breaths.

In 1980 clinical evidence began to show that the benefits of IPPB were often overstated and could be accomplished using other simpler and more cost-effective therapies.⁹ As IPPB lost favor in clinical practice, nasal-mask continuous positive-airway pressure (CPAP) began to emerge as a highly effective therapy in the treatment of obstructive sleep apnea (OSA).^10–12 Researchers found that application of low levels of continuous airway pressure through a mask interface created a pneumatic splint that prevented airway collapse during sleep.¹⁰

The use of positive-pressure ventilation via mask soon was reported to be successful in the treatment of chronic ventilatory insufficiency and muscle weakness in patients with various neuromuscular illnesses.^13–15 In 1989 Meduri et al¹⁶ successfully treated a small sample of patients with ARF using pressure-support ventilation through a facemask.

These successes stimulated the production of variable interfaces and small pressure- and volume-targeted ventilators that were lightweight, easy to operate, and ideal for home use. Over the past decade, the use of NIV has increased dramatically, and NIV is now used to treat both acute and chronic respiratory failure in a variety of clinical settings.

In acute respiratory failure, NIV is considered a lifesaving application that offers a number of benefits over invasive positive-pressure ventilation (Box 19-1). The most significant benefit is the avoidance of intubation. Endotracheal intubation is associated with complications such as airway trauma, increased risk of aspiration, nosocomial pneumonia, and considerable patient discomfort, requiring the use of sedatives. Such complications can lead to a longer hospital stay, higher mortality rate, and increased health care costs. Evidence has established that NIV can safely and successfully support ventilation, without endotracheal intubation, until the condition leading to the ARF has been reversed. In addition, evidence strongly indicates that NIV reduces the mortality rate, reduces the duration of ventilator use, and shortens the hospital stay in appropriately selected patients. The avoidance of intubation and invasive ventilation, therefore, is the primary goal of NIV in the acute-care setting.

The physiological goal of NIV in ARF is to improve gas exchange by resting the respiratory muscles and increasing alveolar ventilation. NIV reduces diaphragmatic pressure swings, which suggests that the respiratory muscles are being rested. In addition, when positive end-expiratory pressure (PEEP) is applied during pressure-supported ventilation (PSV), PEEP helps offset auto-PEEP, thereby reducing the work required to initiate inspiration.17 Likewise, pressure support (PS) facilitates inspiration, thus increasing the tidal volume (V_T). Resting of the respiratory muscles and improved V_T lead to a lower arterial partial pressure of CO₂ (PaCO₂), better oxygenation, and decreased respiratory rates.

In chronic respiratory failure, NIV is a supportive therapy rather than a lifesaving treatment. Most of the clinical disorders that require this level of support are characterized by chronic hypo-ventilation, nocturnal desaturation, respiratory muscle fatigue, and poor sleep quality. As the disease process progresses, daytime gas exchange worsens and patients often show classic symptoms of chronic hypoventilation (Box 19-2).

Nocturnal use of NIV (4-6 hours) can have certain clinical benefits for patients with chronic hypoventilation disorders (see Box 19-1). The most significant of these are improvement of symptoms associated with chronic hypoventilation and an improved quality of life. Although the physiological mechanism underlying these benefits is not well understood, investigators have hypothesized that NIV benefits these patients in one or all of the following ways^38–41:

Clinical weaning reduces the number of patient ventilator days in the ICU.58,59 Reducing the number of days a patient receives invasive mechanical ventilation reduces the risk of infection and other complications, lowers the mortality rate, and reduces health care costs.^58,60 Many respiratory care departments in acute-care facilities have devised weaning protocols for discontinuing ventilation and extubating patients as soon as possible. However, many weaning protocols depend on patient tolerance of daily spontaneous breathing trials to determine the likelihood of successful extubation. (See Chapter 20 for information on weaning and spontaneous breathing trials.) After extubation, the excessive load spontaneous breathing places on the respiratory muscles can lead to fatigue and reintubation.

In addition, NIV can be used after extubation in patients who show fatigue. In these patients NIV reduces the work of breathing (WOB) and maintains adequate gas exchange as effectively as invasive ventilation.⁶¹ NIV can also shorten the duration of invasive ventilation.^62,63 In one study a group of patients for whom 3 days of spontaneous breathing trials had failed was placed on NIV.⁶⁴ In these patients, WOB was reduced, adequate gas exchange was maintained, and the ICU and hospital stays were shortened. These patients were less likely to need a tracheotomy.⁶³ Most patients who seem to benefit from NIV during weaning from invasive ventilation suffer from chronic illness (e.g., COPD). Therefore, it is questionable whether the benefits of NIV would apply to patients with other disease processes. Nonetheless, the evidence is strong enough to warrant consideration of NIV in patients for whom spontaneous breathing trials fail and who meet appropriate NIV selection criteria.

Patients with terminal or advanced disease who develop ARF are not good candidates for endotracheal intubation and mechanical ventilation. NIV may be an alternative therapy for these patients, although its use remains controversial. Previous studies involving mainly end-stage COPD patients in whom endotracheal intubation was contraindicated, found that most of these patients were successfully supported with NIV and weaned.65,66 Many would not have survived the acute process had they not been placed on NIV. Patients with COPD and acute pulmonary edema may also benefit from support with NIV. However, survival was not improved in patients with ARF arising from other causes such as pneumonia or cancer.⁶⁷

The argument in favor of using NIV with a patient having “do not intubate” status is that NIV may relieve severe dyspnea and preserve patient comfort. It may also reverse the acute process in disorders such as COPD or pulmonary edema and allow the patient to live longer.⁶⁸ The argument against NIV is that it could actually prolong the dying process, add to patient discomfort, and consume valuable resources.⁶⁹ When NIV is used in such cases, the patient and family members should be told that NIV is a form of life support that can be uncomfortable but that may be removed at any time.²⁰

In the acute-care setting, the selection process must consider the patient’s diagnosis and clinical characteristics, as well as the risk of failure. If this assessment is viewed as a two-step process, the first step is to establish the need for ventilatory assistance according to clinical and blood gas criteria. For example, NIV may be unnecessary for patients with mild respiratory distress, and NIV for a patient who has already deteriorated to severe respiratory failure may delay life-saving intubation and ventilation. The consensus of studies is that patients who need ventilatory assistance show signs and symptoms of distress, including tachypnea (respiratory rate >24 breaths/min), use of accessory muscles, and paradoxical breathing.45 Blood gas criteria should reveal a moderate to severe respiratory failure (i.e., a pH <7.35 and PaCO₂ >45 mm Hg, or a PaO₂/F_IO₂ <200).

Once the need for ventilatory support is established, the second step is to exclude patients at increased risk of failure and complications (Box 19-3). Such patients include individuals with respiratory arrest, hemodynamic instability, or other major organ involvement; patients with excessive secretions; and patients unable to protect their airway because of impaired cough or swallowing ability. Patients with any of these disorders are at highest risk for aspiration. Finally, agitated and confused patients or those with facial burns or deformities that preclude a good mask fit are excluded.

A final consideration in the selection of patients with ARF is the potential reversibility of the disease process. Overwhelming evidence supports the use of NIV in acute exacerbations of COPD. Supportive ventilatory assistance allows time for conventional therapies (e.g., bronchodilators, oxygen, antibiotics) to reverse the acute process so that intubation may be avoided. Other causes of ARF may not be treated as successfully as COPD, but a trial of NIV may be warranted if the patient meets the selection criteria. All patients should be monitored closely so that intubation, if necessary, is not unduly delayed (Key Point 19-5).

Establishment of the need for intermittent ventilatory assistance in patients with chronic respiratory failure begins with the recognition of typical symptoms of nocturnal hypoventilation and poor sleep quality. These most commonly include the following:

Objective criteria, such as blood gases, often are variable and depend on the rate of progression of the disease process. For patients with restrictive thoracic or central hypoventilation disorders, institution of NIV is recommended when PaCO₂ is 45 mm Hg or higher or when sustained nocturnal desaturation occurs, as evidenced by an oxygen saturation by pulse oximeter (SpO₂) under 88% for longer than 5 consecutive minutes.44 NIV also may be indicated if the patient with restrictive thoracic disease is symptomatic and has severe pulmonary dysfunction (vital capacity [VC] <50% of the predicted level), even if CO₂ retention is absent.⁴⁴ Patients with nocturnal hypoventilation or OSA may require only nocturnal CPAP for splinting the airway open to overcome hypoventilation. However, NIV should be initiated if patients with moderate to severe OSA do not respond favorably to CPAP. Patients who recover from episodes of ARF or who are hospitalized repeatedly for exacerbations of their condition also should be considered for noninvasive ventilatory assistance (Key Point 19-6).

The conflicting findings of studies on the use of NIV for severe stable COPD make it difficult to develop evidenced-based selection guidelines. A review of studies with favorable outcomes found that COPD patients with severe hypercapnia were most likely to benefit from NIV.^50,70 Therefore a consensus of medical experts recommends the use of nocturnal NIV in symptomatic yet medically stable patients with COPD whose daytime PaCO₂ is 55 mm Hg or higher.⁴⁴ The term medically stable in this instance means that the patient is being optimally treated, and OSA and CPAP therapy have been considered and ruled out.⁴⁴ These experts also recommend a trial of NIV if the PaCO₂ is 50 to 54 mm Hg in COPD patients receiving long-term oxygen administration (2 L/min or more) who still have evidence of frequent hypopnea episodes and sustained nocturnal desaturation. In addition, a history of frequent hospitalizations for exacerbations helps to justify the use of NIV in COPD patients.

As for any patient requiring NIV, the ability to protect the airway is crucial. This is especially important for patients with chronic respiratory failure because most of these individuals live at home or in an extended-care setting and may not be monitored closely. Patients with neuromuscular disorders may present an even greater challenge as their disease progresses and they begin to lose oropharyngeal muscle strength and the ability to generate an effective cough. Cough-assisting devices or techniques may help these individuals clear secretions and maintain a patent airway.

Patient motivation must be considered with NIV in the chronic-care setting. Very few, if any, therapeutic results are realized if the patient does not comply with the prescribed therapy. Among patients undergoing long-term treatment with NIV, those with COPD and little CO₂ retention were least compliant.⁷¹ This emphasizes the importance of selecting patients who are symptomatic and who are motivated by the desire for relief of those symptoms. Table 19-2 summarizes the symptoms and selection criteria for chronic respiratory failure disorders.

TABLE 19-2

Indications, Symptoms, and Selection Criteria for NIV in Chronic Disorders

Indication	Symptoms	Physiologic Criteria
Restrictive thoracic disorders Muscular dystrophy Multiple sclerosis Amyotrophic lateral sclerosis Kyphoscoliosis Postpolio syndrome Stable spinal cord injuries	Fatigue Dyspnea Morning headache Hypersomnolence Cognitive dysfunction	PaCO2 ≥45 mm Hg Nocturnal SpO2 ≤88% for 5 consecutive minutes MIP <60 cm H2O FVC <50% predicted
Severe stable COPD	Following optimal therapy with bronchodilators, O2, and other therapy, COPD patients must demonstrate: Fatigue Dyspnea Morning headache Hypersomnolence	PaCO2 >55 mmHg PaCO2 50-54 mm Hg + SpO2 <88% for 5 consecutive min PaCO2 50-54 mm Hg + recurrent hospitalizations for hypercapneic respiratory failure (>2 hospitalizations in a 12-mo period)
Nocturnal hypoventilation Obstructive sleep apnea Obesity hypoventilation Idiopathic hypoventilation	Fatigue Morning headache Hypersomnolence	Polysomnographic evidence of OSA not responsive to CPAP Only gold members can continue reading. Log In or Register to continue Share this: Click to share on Twitter (Opens in new window) Click to share on Facebook (Opens in new window) Related Related posts: Ventilator-Associated Pneumonia Final Considerations in Ventilator Setup Weaning and Discontinuation from Mechanical Ventilation Methods to Improve Ventilation in Patient-Ventilator Management Stay updated, free articles. Join our Telegram channel Join Tags: Pilbeams Mechanical Ventilation Physiological and Clinical Appli Aug 7, 2016 | Posted by admin in RESPIRATORY | Comments Off on Basic Concepts of Noninvasive Positive-Pressure Ventilation Full access? Get Clinical Tree Get Clinical Tree app for offline access Get Clinical Tree app for offline access