Noninvasive positive-pressure ventilation (NPPV) is the delivery of mechanical ventilatory support without an invasive nasotracheal or endotracheal tube, using a tight-fitting face or nasal mask. Continuous positive airway pressure (CPAP) is occasionally included in discussions of NPPV, although CPAP does not directly increase tidal volume or minute ventilation. In contrast, bilevel positive airway pressure (BiPAP) provides supplemental inspiratory tidal volume.
Both modes are useful in the hospital under a number of circumstances. CPAP or BiPAP sometimes can help stave off intubation for acutely ill patients with respiratory failure from exacerbations of acute pulmonary edema or chronic obstructive pulmonary disease (COPD). Physicians also frequently extubate patients to BiPAP to reduce the risk of reintubation.
How CPAP works
A CPAP machine forces a continuous column of compressed air at a fixed designated pressure against the face and nose of the patient, who is wearing a mask or nasal cap. When the patient's glottis opens to inhale, the pressure is transmitted throughout the airway, helping to open it. There is no inspiratory flow of air from the CPAP machine; whatever additional tidal volume the patient pulls in is from the additional airway volume recruited by the pressure.
When the patient exhales, pressure from the deflating lungs and chest wall pushes air out against the continuous pressure, until the two pressures are equal. The air pressure in the airway at the end of exhalation is equal to the external air pressure of the machine, and this helps “splint” the airway open, allowing better oxygenation and airway recruitment. If the CPAP is set too high for the patient, exhalation becomes very uncomfortable, since the patient has to push against the pressure to exhale. This can increase the work of breathing for fragile COPD patients or weak critically ill patients. Most ICU patients tolerate pressures of 5 to 8 cm H2O. Patients with obstructive sleep apnea may require up to 20 cm H2O to keep their airways open (see Figure) .
How BiPAP works
BiPAP has two levels of continuous airway pressure. When the machine senses the patient's inspiratory flow starting to increase, it increases the inspiratory pressure (IPAP) applied, so that air flow is enhanced and the patient's own inspiratory tidal volume is augmented. When the machine senses flow is slowing or stopped, it reduces the applied airway pressure so the patient has less work upon exhaling, but maintains a continuous positive expiratory pressure (EPAP). This allows the patient to receive higher inspiratory pressure, but not have to work against higher expiratory pressure. In addition, the machine can be set to deliver a fixed respiratory rate. Air flow is generated until the airway pressure as sensed by the machine reaches a set target inspiratory pressure. The tidal volume received by the patient depends upon airway resistance, lung and chest wall compliance, patient synchrony with machine, and the absence of air leakage around the mask. BiPAP can only augment the patient's respiration; it should not be used as a primary form of ventilation. As one might expect, BiPAP machines are much more expensive than CPAP machines ($10,000 vs. $3,000).
When to use BiPAP
While the literature mainly supports CPAP therapy for sleep, BiPAP is an optional therapy when high pressure is needed and a patient experiences difficulty exhaling against a fixed pressure, or when coexisting central hypoventilation is present. This guideline is reasonable for ICU patients or other hospitalized patients with respiratory difficulty.
Research suggests BiPAP can reduce the need for endotracheal intubation, and get mechanically ventilated patients off their machines more quickly. A 1995 study in the New England Journal of Medicine found BiPAP reduced the need for endotracheal intubation, as well as hospital length of stay and mortality, in acutely ill COPD patients with a pO2 less than 45 mm Hg, a potential hydrogen (pH) level less than 7.35, and a respiratory rate (RR) greater than 30 breaths/minute. A 2003 Cochrane review of studies with mostly COPD patients also found that BiPAP decreased mortality, incidence of ventilator-associated pneumonia, ICU and hospital length of stay, total duration of mechanical support and duration of endotracheal mechanical ventilation.
Meanwhile, a 2008 Cochrane review of 21 studies involving patients with acute pulmonary edema found that both CPAP and BiPAP significantly reduced hospital mortality and endotracheal intubation with numbers needed to treat of 13 and 8, respectively, compared to standard medical care. While there was no difference in hospital length of stay or heart attack incidence, ICU stay was reduced by one day for CPAP and BiPAP patients.
When to use CPAP
When to use CPAP instead of BiPAP in non-mechanically ventilated inpatients has not been clearly determined. In the U.K., guidelines call for using CPAP with patients being weaned from ventilation; patients who are hypoxemic following extubation; or patients with a variety of acute conditions “who are hypoxic but not exhausted” (i.e., those who are ventilating themselves adequately).
Mortality risk was lower with CPAP than standard or BiPAP therapy in a 2006 Lancet meta-analysis comparing CPAP, BiPAP or both in patients with acute pulmonary edema. The need for mechanical ventilation was equal between CPAP and BiPAP, however, and lower with both than with standard therapy.