The Difference Between CPAP & BiPAP

It should be noted that there's disagreement among the literature as to whether CPAP is a form of NPPV. CPAP provides oxygenation but not ventilation, which is the actual movement of air into the lungs. CPAP augments ventilation, which should qualify it as a form of NPPV.

CPAP is a continuous pressure provided above ambient pressure, which helps prevent alveoli from collapsing and augments ventilation. This pressure remains relatively constant through each phase of the respiratory cycle, varying only slightly as the patient breathes.

Because the pressure is the same throughout all phases of the respiratory cycle, the patient must overcome the pressure during expiration.

NOTE: CPAP is limited by the patient's ability to breathe and would be ineffective in those who can't generate a strong enough tidal breath on their own.

BiPAP also provides CPAP, but it also detects the patient's inspiratory effort and delivers greater pressure during inspiration (see Figure 2). This is similar to squeezing the bag while assisting a patient with breathing. At the end of the inspiratory phase, the pressure drops back to the preset level of CPAP.

The term “BiPAP” is actually the trade name for the biphasic positive airway pressure device developed and sold by Respironics. Today, however, it's commonly used to describe biphasic positive airway pressure. Other names include bilevel (or biphasic) airway pressure, bilevel (or biphasic) positive pressure, bilevel (or biphasic) CPAP, bilevel (or biphasic) pressure support, among others.

CPAP machines are generally capable of delivering only CPAP, whereas BiPAP machines can deliver either CPAP or BiPAP. Respiratory care practitioners and emergency physicians report that BiPAP provides a greater level of control than CPAP. So it's not uncommon for a patient to be removed from the prehospital CPAP device and placed on a device capable of BiPAP. Usually the patient is kept on CPAP until blood gases are obtained. Once blood gas measurements are evaluated, BiPAP is initiated at settings that will provide the best gas exchange and optimum cardiopulmonary performance.

A 1997 study suggests that BiPAP improves ventilation and vital signs more quickly than CPAP. So in systems where transport times are long (e.g., rural areas), it may be beneficial for patients to be placed on BiPAP rather than CPAP. Improvements in ventilation and vital signs would occur sooner and can be maintained for the duration of the transport. However, the study also revealed an increase in the incidence of myocardial infarction in those patients placed on BiPAP. Thus, it's necessary for the provider to reassess the patient frequently.

Benefits & Drawbacks

BiPAP and CPAP are beneficial because they decrease the need for endotracheal intubation, thereby lessening the complications associated with intubation. These include pneumonia, upper-airway injury and prolonged ICU and hospital stays. Additionally, although the use of NPPV is recommended for relaxed, non-combative patients, the use of these forms of NPPV doesn't generally require sedation.

Complications of BiPAP and CPAP are typically minor and include injury to tissues where the mask makes contact with the face. This risk is especially true for older patients who typically have thinner skin.

Other complications include gastric distention, aspiration pneumonia, hypotension and pneumothorax. For these reasons, NPPV shouldn't be used on patients who have undergone recent facial surgery, have excessive secretions, are experiencing gastrointestinal bleeding or don't have the ability to protect their own airways.

When treating patients in the Emergency Department it is better to institute BiPAP because patients are generally more likely to need ventilatory assistance.