Continuous positive airway pressure

Continuous positive airway pressure (CPAP) is the use of continuous positive pressure to maintain a continuous level of positive airway pressure. It is functionally similar to PEEP, except that PEEP is an applied pressure against exhalation and CPAP is a pressure applied by a constant flow. The ventilator does not cycle during CPAP, no additional pressure above the level of CPAP is provided, and patients must initiate all of their breaths. Nasal CPAP is frequently used in neonates though its use is controversial. Studies have shown nasal CPAP to reduce ventilator time but an increased occurrence of pneumothorax was also prevalent.

As a treatment or therapy, CPAP uses mild air pressure to keep an airway open. 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 to treat infants who have respiratory distress syndrome or 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.

CPAP at home utilizes machines specifically designed to deliver a constant flow or 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 CPAP prevents the airway from collapsing or becoming blocked.

Settings and measurements

 * Cpap — This is the pressure applied without pause or end to the airway. Generally utilizing flow to generate the pressure.
 * PEEP — PEEP is sometimes also called CPAP however the pressure generated in PEEP is by a backpressure valve forcing increased pressure on exhalation, ultimately creating a sustained (continuous) pressure.
 * FiO2 — The fractional O2 percentage that is being added to the delivered air.

High flow therapy


Humidified high flow nasal airway respiratory support is a method of delivering a high 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 cannula usually used for oxygen delivery usually deliver 1–6 liters of oxygen per minute. The FiO2; the percent 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 cannula 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, 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 a minute, and can exceed 30 liters a minute for someone with mild respiratory distress. The typical upper limit of 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 then 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 was 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 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, and humidified to near 100% RH 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 special nasal cannula and tubing large enough to deliver flow rates of respiratory gas, up to 50 liters per minute in adults. At the same time the nasal cannula must be small enough that they do 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.

Benefits
Since the delivered flow rate of HFT can meet the inspiration flow rate, the delivered gases is 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 using air delivered by HFT without additional oxygen. HFT is useful in the treatment of sleep apnea. During use of HFT the patient can drink, and speak. Most patients find HFT more comfortable than using oxygen masks. As this is a non-invasive therapy, it avoids the risk of ventilator-associated pneumonia in situations where it can supplant the use of a ventilator.

Use in neonates
Nasal HFT has shown to be useful in neonatal intensive care settings for premature infants with Infant respiratory distress syndrome, as it prevents many infants from intubation, and allows safe respiratory management at lower FiO2 levels, and thus reduces the risk of retinopathy of prematurity or other forms of oxygen toxicity.

Criticism
Critics of HFT point out that while there are many reports of the positive outcomes, little is known about the mechanisms of action of HFT and what levels may be unsafe. There has been concern that HFT delivery settings prescribed are largely empiric, with large variation of pressure produced by HFT between individuals.

Development
HFT was originally developed for use in race horses.