Positive airway pressure
Positive airway pressure is a mode of respiratory ventilation used in the treatment of sleep apnea. PAP ventilation is also commonly used for those who are critically ill in hospital with respiratory failure, in newborn infants, and for the prevention and treatment of atelectasis in patients with difficulty taking deep breaths. In these patients, PAP ventilation can prevent the need for tracheal intubation, or allow earlier extubation. Sometimes patients with neuromuscular diseases use this variety of ventilation as well.
Variations include continuous positive airway pressure and bi-level positive airway pressure. BPAP has two pressure settings: the prescribed pressure for inhalation, and a lower pressure for exhalation ; the dual settings allow the patient to get more air in and out of their lungs.
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 will be reserved for the subset of patients for whom oxygen delivered via a face mask is deemed insufficient or deleterious to health. 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 females 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 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 breathing out against the pressure less difficult. 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. 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.
- 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. 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 mismatch. The recruitment reduces ventilation–perfusion mismatch.
- The amount of air remaining in the lungs at the end of a breath is greater. 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.
Disadvantages
Prospective PAP candidates are often reluctant to use this therapy, since the nose mask and hose to the machine look uncomfortable and clumsy. Airflow required for some patients can be vigorous. Some patients will develop nasal congestion while others may experience rhinitis or a runny nose. Some patients adjust to the treatment within a few weeks, others struggle for longer periods, and some discontinue treatment entirely. However, studies show that cognitive behavioral therapy at the beginning of therapy dramatically increases adherence—by up to 148%. While common PAP side effects are merely nuisances, serious side effects such as eustachian tube infection, or pressure build-up behind the cochlea are very uncommon. Furthermore, research has shown that PAP side effects are rarely the reason patients stop using PAP. There are reports of dizziness, sinus infections, bronchitis, dry eyes, dry mucosal tissue irritation, ear pain, and nasal congestion secondary to CPAP use.
PAP manufacturers frequently offer different models at different price ranges, and PAP masks have many different sizes and shapes, so that some users need to try several masks before finding a good fit. These different machines may not be comfortable for all users, so proper selection of PAP models may be very important in furthering adherence to therapy.
Beards, mustaches, or facial irregularities may prevent an air-tight seal. Where the mask contacts the skin must be free from dirt and excess chemicals such as skin oils. Shaving before mask-fitting may be necessary in some cases. However, facial irregularities of this nature frequently do not hinder the operation of the device or its positive airflow effects for sleep apnea patients. For many people, the only problem from an incomplete seal is a higher noise level near the face from escaping air.
The CPAP mask can mimic the action of an orthodontic headgear and move the teeth and the upper and/or lower jaw backward. This effect can increase over time and may or may not cause TMJ disorders in some patients. These facial changes have been dubbed "Smashed Face Syndrome".
Mechanism of action
Continuous pressure devices
Fixed-pressure CPAP
A continuous positive airway pressure machine was initially used mainly by patients for the treatment of sleep apnea at home, but now is in widespread use across intensive care units as a form of ventilation. Obstructive sleep apnea occurs when the upper airway becomes narrow as the muscles relax naturally during sleep. This reduces oxygen in the blood and causes arousal from sleep. The CPAP machine stops this phenomenon by delivering a stream of compressed air via a hose to a nasal pillow, nose mask, full-face mask, or hybrid, splinting the airway so that unobstructed breathing becomes possible, therefore reducing and/or preventing apneas and hypopneas. It is important to understand, however, that it is the air pressure, and not the movement of the air, that prevents the apneas. When the machine is turned on, but prior to the mask being placed on the head, a flow of air comes through the mask. After the mask is placed on the head, it is sealed to the face and the air stops flowing. At this point, it is only the air pressure that accomplishes the desired result. This has the additional benefit of reducing or eliminating the extremely loud snoring that sometimes accompanies sleep apnea.The CPAP machine blows air at a prescribed pressure. The necessary pressure is usually determined by a sleep physician after review of a study supervised by a sleep technician during an overnight study in a sleep laboratory. The titrated pressure is the pressure of air at which most apneas and hypopneas have been prevented, and it is usually measured in centimetres of water. The pressure required by most patients with sleep apnea ranges between 6 and 14 cm. A typical CPAP machine can deliver pressures between 4 and 20 cm. More specialised units can deliver pressures up to 25 or 30 cm.
CPAP treatment can be highly effective in treatment of obstructive sleep apnea. For some patients, the improvement in the quality of sleep and quality of life due to CPAP treatment will be noticed after a single night's use. Often, the patient's sleep partner also benefits from markedly improved sleep quality, due to the amelioration of the patient's loud snoring.
Given that sleep apnea is a chronic health issue which commonly doesn't go away, ongoing care is usually needed to maintain CPAP therapy. Based on the study of cognitive behavioral therapy, ongoing chronic care management is the best way to help patients continue therapy by educating them on the health risks of sleep apnea and providing motivation and support.