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 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, 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.
Read more about this topic: Continuous Positive Airway Pressure
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