Noninvasive Ventilation in Critical Care
Nicholas S. Hill, MD
Tufts University School of Medicine
Tufts-New England Medical Center
Boston, Massachusetts, USA
The increased use of noninvasive ventilation (NIV), a form of respiratory support that avoids airway invasion, has been one of the most important developments in the field of mechanical ventilation in the past 10 to 15 years. Earlier experiences with noninvasive positive pressure ventilation have included mouthpiece ventilation, which was used to treat respiratory failure due to neuromuscular disease at some specialized centers dating back to the polio epidemics, and intermittent positive pressure breathing (IPPB), which was used primarily for bronchodilator aerosol delivery to patients with exacerbations of chronic obstructive pulmonary disease (COPD) or asthma from the 1960s through the 1980s. Modern approaches have built off these early therapies. Assisting patients with acute respiratory failure through noninvasive ventilation first was introduced during the late 1980s and early 1990s. These attempts combined pressure-limited ventilator modes, such as pressure support ventilation, or bilevel positive airway pressure (BiPAP-type modes), with either full face or nasal masks. This offered patients with certain types of acute respiratory disease a safer and usually more tolerable ventilatory assist system that could support breathing during acute respiratory crises, helping them to avoid intubation and its complications.
Applications of NIV to Critical Care
A number of randomized clinical studies have established the efficacy of NIV in improving respiratory distress, vital signs and gas exchange rapidly while reducing morbidity, lengths of intensive care unit and hospital stays, and mortality rates (mainly in patients with respiratory failure due to exacerbations of COPD.)(1) Efficacy also was established in patients with acute cardiogenic pulmonary edema, although it was more difficult to demonstrate reductions in mortality in these patients.
This is likely because their mortality rates for an acute episode are relatively low to begin with and because most studies were powered inadequately. Furthermore, although some studies suggested that respiratory distress, vital signs and gas exchange improve more rapidly with NIV than with continuous positive airway pressure (CPAP), meta-analyses show that CPAP and NIV perform equally well in avoiding intubation and reducing mortality.(2)
Other well-established applications of NIV include patients with respiratory failure due to immunocompromised states.(3) Intubation in this population increases the risk of healthcare-acquired infections and death due to sepsis. Patients with COPD require intubation initially, but they can be extubated early to NIV to avoid the complications of prolonged intubation, despite failure of spontaneous breathing trials.(4) Evidence is accruing to demonstrate that either NIV or CPAP can be used to prevent respiratory complications after certain types of surgery, such as major abdominal procedures, and NIV has been shown to treat respiratory failure effectively in patients following lung resection.(5) NIV also is useful in patients with post-extubation respiratory failure, if it is applied in patients who are hypercapnic and at high risk(6); however, it should not be used as a routine early measure in patients with some risk factors.(7) Do-notintubate patients also often respond well to NIV if they have COPD or congestive heart failure, and they usually survive the hospitalization. Unfortunately, these patients do not see the same results if they have other diagnoses such as cancer or pneumonia.(8) NIV may have a palliative role in the latter population, if the patient is suffering from respiratory distress or wishes to delay death long enough to settle affairs.
Controversial applications for NIV include its use in treating acute asthma; NIV has been associated with more rapid improvement in air flow than sham treatment.(9) However, because of a dearth of controlled clinical studies, it remains unclear whether it improves other clinical outcomes or when it should be applied. Another controversial application is hypoxemic respiratory failure. This is a broad category that includes acute cardiogenic pulmonary edema, for which NIV clearly is effective, and acute respiratory distress syndrome (ARDS) and severe pneumonia, for which reported success rates have been considerably lower. Recent evidence suggests that if NIV is tried in ARDS patients with two or fewer organ system failures (individuals who are good candidates for NIV otherwise) and oxygenation improves substantially within the first hour, the likelihood of success is good.(10)
Optimizing Success
Failure rates of NIV still range from 25% to 40%, and optimization of NIV success rates requires careful patient selection and knowledge of proper application and monitoring techniques. Patients should manifest symptoms and signs of impending need for intubation, such as respiratory distress, tachypnea and excessive accessory muscle use. They should be treated early enough to avoid florid respiratory failure necessitating intubation. Poor candidates for NIV include those with respiratory arrest or unstable hemodynamics and those who are uncooperative or unable to protect the airway. Full face masks are preferred for initiation, but patients with claustrophobia or the need to expectorate frequently may tolerate nasal masks better. Either bilevel-type ventilators or critical care ventilators can be used, but they should be adjusted to optimize comfort and minimize respiratory distress.
Starting with relatively low pressures is useful to facilitate adaptation, but these should be adjusted upward promptly as tolerated to provide sufficient ventilatory assistance to alleviate dyspnea and gas exchange derangement. Patients should be monitored closely as long as they are at risk for NIV failure. It is preferred that staff monitor patients in an intensive care unit or step-down unit, paying attention to patient comfort, breathing effort, respiratory rate and synchrony. Oximetry should be monitored continuously, with blood gases obtained at baseline and within the first two hours. Caregivers must
function as a team with physicians selecting appropriate patients, respiratory therapists initiating the technique, and nurses helping to monitor patients following initiation.
The overall utilization rate of NIV, as determined by proportion of total ventilator initiations, is roughly one quarter, but a U.S. survey demonstrated enormous variability among acute care institutions.(11) Barriers to greater use at low utilizing institutions included lack of education of the staff and lack of appropriate equipment. Educational programs are likely to remove some of these barriers, and more recent surveys suggest that utilization rates are increasing. Part of this increase also is due to advances in ventilator and mask technology, and increasing experience and skill at acute care centers.
Conclusion
NIV can be seen as a respiratory system protective strategy that helps to minimize ventilator-related complications, especially ventilator-associated pneumonias. We can anticipate more widespread use in coming years, with improved outcomes in appropriately selected patients, as a consequence of further refinements in technology and accruing evidence about optimal applications.
References
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