Positive airway pressure (PAP) is a form of non-invasive respiratory support that delivers pressurized air through a facial or nasal interface to keep the airways open. It is used primarily to treat sleep-related breathing disorders, especially obstructive sleep apnea, and is also applied in hospital and emergency settings for conditions causing respiratory distress or ventilatory impairment. PAP therapy can reduce airway collapse during sleep, improve oxygenation, and lessen the work of breathing. PAP systems use a device connected to a facial or nasal interface to generate positive-pressure airflow. The principal forms of PAP therapy are continuous positive airway pressure (CPAP), which maintains a constant pressure level; bilevel positive airway pressure (BiPAP or BPAP), which provides different pressures during inhalation and exhalation; and automatic positive airway pressure (APAP), which automatically adjusts pressure in response to breathing patterns. These therapies are used in both home and clinical settings. PAP therapy became widely adopted after the introduction of CPAP for obstructive sleep apnea in 1981. PAP therapy has been associated with improvements in sleep quality, daytime alertness, and quality of life, particularly among people with obstructive sleep apnea. However, its effectiveness often depends on long-term adherence, which may be limited by discomfort and difficulties tolerating the equipment or positive-pressure airflow.
Medical uses
The main indications for positive airway pressure are congestive heart failure and chronic obstructive pulmonary disease. There is some evidence of benefit for those with hypoxia and community-acquired pneumonia. PAP ventilation is often used for patients who have acute type 1 or 2 respiratory failure. Usually, PAP ventilation is reserved for the subset of patients for whom oxygen delivered via a face mask is deemed insufficient or deleterious to health (see CO2 retention). Usually, patients on PAP ventilation will be closely monitored in an intensive care unit, high-dependency unit, coronary care unit or specialist respiratory unit. The most common conditions for which PAP ventilation is used in hospital are congestive cardiac failure and acute exacerbation of obstructive airway disease, most notably exacerbations of COPD and asthma. It is not used in cases where the airway may be compromised, or consciousness is impaired. CPAP is also used to assist premature babies with breathing in the NICU setting. CPAP has been used for pregnant women with preeclampsia. The mask required to deliver CPAP must have an effective seal, and be held on very securely. The "nasal pillow" mask maintains its seal by being inserted slightly into the nostrils and being held in place by various straps around the head. Some full-face masks "float" on the face like a hover-craft, with thin, soft, flexible "curtains" ensuring less skin abrasion, and the possibility of coughing and yawning. Some people may find wearing a CPAP mask uncomfortable or constricting: eyeglass wearers and bearded men may prefer the nasal-pillow type of mask. Breathing out against the positive pressure resistance (the expiratory positive airway pressure component, or EPAP) may also feel unpleasant to some patients. These factors lead to inability to continue treatment due to patient intolerance in about 20% of cases where it is initiated. Some machines have pressure relief technologies that makes sleep therapy more comfortable by reducing pressure at the beginning of exhalation and returning to therapeutic pressure just before inhalation. The level of pressure relief is varied based on the patient's expiratory flow, making it easier to breathe out against the pressure. Those who have an anxiety disorder or claustrophobia are less likely to tolerate PAP treatment. Sometimes medication will be given to assist with the anxiety caused by PAP ventilation. Unlike PAP used at home to splint the tongue and pharynx, PAP is used in hospital to improve the ability of the lungs to exchange oxygen and carbon dioxide, and to decrease the work of breathing (the energy expended moving air into and out of the alveoli). This is because:
During inspiration, the inspiratory positive airway pressure, or IPAP, forces air into the lungs—thus less work is required from the respiratory muscles. The bronchioles and alveoli are prevented from collapsing at the end of expiration. If these small airways and alveoli are allowed to collapse, significant pressures are required to re-expand them. This can be explained using the Young–Laplace equation (which also explains why the hardest part of blowing up a balloon is the first breath). Entire regions of the lung that would otherwise be collapsed are forced and held open. This process is called recruitment. Usually these collapsed regions of lung will have some blood flow (although reduced). Because these areas of lung are not being ventilated, the blood passing through these areas is not able to efficiently exchange oxygen and carbon dioxide. This is called ventilation–perfusion (or V/Q) mismatch. The recruitment reduces ventilation–perfusion mismatch. The amount of air remaining in the lungs at the end of a breath is greater (this is called the functional residual capacity). The chest and lungs are therefore more expanded. From this more expanded resting position, less work is required to inspire. This is due to the non-linear compliance–volume curve of the lung.
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