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|Continuous positive airway pressure|
CPAP therapy: flow generator, hose, full face mask
Continuous positive airway pressure (CPAP) is a form of positive airway pressure ventilator, which applies mild air pressure on a continuous basis to keep the airways continuously open in people who are able to breathe spontaneously on their own. It is an alternative to positive end-expiratory pressure (PEEP). Both modalities stent the lungs' alveoli open and thus recruit more of the lung's surface area for ventilation. But while PEEP refers to devices that impose positive pressure only at the end of the exhalation, CPAP devices apply continuous positive airway pressure throughout the breathing cycle. Thus, the ventilator itself does not cycle during CPAP, no additional pressure above the level of CPAP is provided, and patients must initiate all of their breaths.
CPAP typically is used for people who have breathing problems, such as sleep apnea. CPAP also may be used to treat preterm infants whose lungs have not yet fully developed. For example, physicians may use CPAP in infants with respiratory distress syndrome. It is associated with a decrease in the incidence of bronchopulmonary dysplasia. In some preterm infants whose lungs haven't fully developed, CPAP improves survival and decreases the need for steroid treatment for their lungs. In resource-limited settings where CPAP improves respiratory rate and survival in children with primary pulmonary disease, researchers have found that nurses can initiate and manage care with once or twice daily physician rounds.
CPAP therapy utilizes machines specifically designed to deliver a constant flow of pressure. Some CPAP machines have other features as well, such as heated humidifiers. CPAP is the most effective treatment for obstructive sleep apnea, in which the mild pressure from the CPAP prevents the airway from collapsing or becoming blocked.
Although delivery of CPAP through a nasal mask is the most common modality of treatment, other systems exist for interfacing with adults and children. Nasal CPAP is frequently used in infants, though its use is controversial. Studies have shown nasal CPAP reduces ventilator time but an increased occurrence of pneumothorax was also prevalent. Oral masks and naso-oral masks are often used when nasal congestion or obstruction is an issue. Devices that combine nasal pressure with maxillary advancement devices (MAD) also exist.
Humidified high flow nasal airway respiratory support is a method of delivering a high per-minute volume of respiratory gas via nasal cannula. The respiratory gas is heated to near body temperature and humidified, usually to saturation vapor pressure. This form of respiratory support is generally referred to as high flow therapy (HFT). HFT is also referred to as transnasal insufflation (TNI), especially when used for treatment of sleep apnea.
Nasal cannulae usually used for oxygen delivery typically deliver 1–6 liters of oxygen per minute. The FiO2 — the percentage of oxygen inhaled by the patient—usually ranges roughly from 24% to 35%, as 100% O2 delivered from the cannula is diluted with air at about 21% oxygen. Flow rates for delivery of oxygen using typical nasal cannulae are limited because medical oxygen is anhydrous, and when delivered from a pressurized source the gas cools as it expands with the drop to atmospheric pressure. Delivery of cold, dry gas is irritating to the respiratory mucosa, can cause drying and bleeding of the nasal mucosa, can trigger bronchospasm in asthmatics, and can increase metabolic demand by cooling the body. Thus oxygen delivery by nasal cannula is limited to less than 6 liters per minute.
Even with quiet breathing, the inspiratory flow rate at the nares of an adult usually exceeds 12 liters per minute, and can exceed 30 liters a minute for someone with mild respiratory distress. The typical upper limit for oxygen delivery via nasal cannula of six liters a minute does not meet the inspiratory flow rates of the average adult, and therefore the oxygen is diluted with room air during inspiration. Prior to the advent of HFT, when high FiO2 was required for respiratory support, special face masks or intubation were required. With HFT, respiratory gas flow volume is delivered which meets or exceeds the patient's inspiratory flow rate, and is heated and humidified, allowing for comfortable delivery of respiratory support.
For HFT, a source of oxygen is usually blended with compressed air. Hospitals usually have 350 kPa (50 psi) compressed O2 and air available for therapeutic use. This allows the delivery of air, blends of air and O2 from 22% to 99%, or delivery of 100% oxygen with the use of an oxygen blender. The gas is then heated, generally to about 37 °C (98.6 °F), and humidified to near 100% relative humidity using a humidifier. The gas is transported to the patient through a heated delivery tube to prevent cooling and condensation of the water vapor which has been added to the respiratory gas(es).
HFT requires the use of a special nasal cannula and tubing large enough to deliver flow rates of respiratory gas of up to 50 liters per minute in adults. At the same time, the nasal cannula must be small enough that it does not seal inside the nares, as this allows flow during exhalation and excess gas flow during inhalation to escape. If the cannula did seal, the high flow volume could produce excessive pressure in the airway and might provoke barotrauma.
Since the delivered flow rate of HFT can meet the inspiration flow rate, the delivered gases are not diluted by room air. The FiO2 is controlled by the clinician, and can be set from 21% to 100% oxygen. Nasal high flow therapy reduces respiratory dead space, and generates some positive airway pressure resulting from the expiratory resistance generated by continuous high-flow gas delivery. Flow rates exceeding inspiratory demand may also provide positive pressure during inspiration. Heated humidification of the respiratory gas facilitates secretion clearance and decreases the development of bronchial hyper-response symptoms. Some patients requiring respiratory support for bronchospasm benefit from using air delivered by HFT without additional oxygen. HFT is useful in the treatment of sleep apnea.
Nasal HFT has shown to be useful in neonatal intensive care settings for premature infants with infant respiratory distress syndrome (RDS), as it prevents many infants from being intubated, and allows safe respiratory management at lower FiO2 levels, thus reducing the risk of retinopathy of prematurity or other forms of oxygen toxicity.
HFT was originally developed for use in race horses.