One of the main mechanisms to improve a patient's work of breathing is to attempt to match their peak inspiratory flow demands with the use of a high-flow device. This will be discussed later in this review when we discuss the concept of oxygen dilution. If these patients with respiratory failure (with PIF rates of up to 60 - 120 L/min and high minute volumes (> 20 L/min in some adults)) are placed on a 15 L/min NRB mask, then this may not provide adequate support. This is important as patients in acute respiratory failure can become extremely tachypneic, and their peak inspiratory flows (PIF), which may normally be 30 L/min - 60 L/min at rest, can reach upwards of 120 L/min in acute respiratory failure . One obvious benefit is that the high-flow nasal cannula can deliver very high flow rates of gas in an attempt to match a patient's inspiratory flow demands. Thus, heated and humidified oxygen may improve secretion clearance, decrease airway inflammation, and also decrease energy expenditure, particularly in the setting of acute respiratory failure . This cold, dry gas can lead to airway inflammation, increase airway resistance, and impair mucociliary function, possibly impairing secretion clearance . Also, a significant amount of energy is expended by individuals to both warm and humidify gas during normal breathing .
Heated and humidified oxygen has a number of benefits compared to standard oxygen therapy. Standard oxygen therapy delivered through a nasal cannula or another device, such as a non-rebreather mask (NRBM), delivers cold (not warmed) and dry (not humidified) gas. The mechanisms of action below are not listed in the order of importance but rather to give the reader an easy to remember mnemonic (HIFLOW) for the mechanisms that have been attributed to the use of high-flow nasal oxygen. It is not clear which of the benefits are most important, and it may depend on the individual patient's etiology of respiratory failure. There are many beneficial mechanisms of action that have been attributed to the effectiveness of high-flow nasal cannula in adult and pediatric patients with respiratory failure. Each manufacturer has a maximum flow rate for each cannula size corresponding to the size and age of the patient.
The nasal prongs should fit snuggly in the patient’s nares in order to prevent entrainment of room air around the cannula, a problem which occurs in standard nasal cannulas. (Auckland, New Zealand), who offers both the Optiflow™, as well as the AIRVO™ 2 device, both of which can deliver flow rates of up to 60 L/min.Įach company offers smaller cannula sizes for premature neonates, as well as children of various ages, to adult-sized cannulas. There are two main companies who manufacture these devices: Vapotherm® (Exeter, NH), which has a device that can deliver flow rates of up to 50 L/min flow rates, and Fisher and Paykel Healthcare, Inc. The flow rate and FiO 2 can be independently titrated based on a patient’s flow and FiO 2 requirements. The heat and humidified high-flow nasal cannula or, as most call it, high-flow nasal cannula (HFNC), isn’t just a standard nasal cannula turned up to very high flow rates. It takes gas, is able to heat it to 37 oC with a 100% relative humidity, and can deliver 0.21 - 1.00% fraction of inspired oxygen (FiO 2) at flow rates of up to 60 liters (L)/min. The main pediatric indication is in infants with bronchiolitis, but other indications are being studied, such as its use in asthma, croup, pneumonia, transport of a critically ill child, and post-extubation. The data for the use of high-flow nasal cannula is growing, and currently, some of the main adult indications include hypoxemic respiratory failure due to pneumonia, post-extubation, pre-oxygenation prior to intubation, acute pulmonary edema, and use in patients who are "do not resuscitate or intubate". We will also examine some of the main indications for its use in both the adult and pediatric age groups. This article describes the mechanism of action in an easy to remember mnemonic (HIFLOW) Heated and humidified, meets Inspiratory demands, increases Functional residual capacity (FRC), Lighter, minimizes Oxygen dilution, and Washout of pharyngeal dead space. It is unclear which of the mechanisms of action is the most important, but it may depend on the cause of the patient’s respiratory failure. This article will examine the main mechanisms of actions attributed to the use of the high-flow nasal cannula and review the indications in adult and pediatric populations (outside of the neonatal period). The use of the heated and humidified high-flow nasal cannula has become increasingly popular in the treatment of patients with respiratory failure through all age groups.
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