What Are High-Flow and Low-Flow Oxygen Delivery Systems?
To the Editor:
We read with interest the article, “A Pilot Study of Normobaric Oxygen Therapy in Acute Ischemic Stroke.”1 However, we have certain reservations with the terminology and the method of oxygen delivery used by the authors.
First, regarding the terminology of “high-flow” oxygen used by the authors, the authors mention the flow rates but not the oxygen delivery system used. An oxygen delivery system is a device used to administer, regulate, and supplement oxygen to a subject to increase the arterial oxygenation. In general, the system entrains oxygen and air to prepare a fixed concentration required for administration. Oxygen delivery systems are generally classified as low-flow or variable-performance devices and high-flow or fixed-performance devices.2 Low-flow systems provide oxygen at flow rates that are lower than patients’ inspiratory demands; thus, when the total ventilation exceeds the capacity of the oxygen reservoir, room air is entrained. The final concentration of oxygen delivered depends on the ventilatory demands of the patient, the size of the oxygen reservoir, and the rate at which the reservoir is filled. At a constant flow, the larger the tidal volume, the lower the FiO2 and vice versa. In contrast, the high-flow systems provide a constant FiO2 by delivering the gas at flow rates that exceed the patient’s peak inspiratory flow rate and by using devices that entrain a fixed proportion of room air.3
Second, the authors have also not mentioned how they gave oxygen at flow rates of 45 L/min. The standard flow meters are calibrated to flow rates of oxygen at 15 L/min, although if one turns the thumbscrew of the valve wide open, a much greater flow is delivered.4
Also, there is a tendency to confuse flow systems with oxygen concentrations. However, both are mutually exclusive in that a high-flow system, viz. Venturi mask, can deliver FiO2 as low as 0.24, whereas a low-flow system like a nonrebreather mask can deliver FiO2 as high as 0.8. Thus, if the ventilatory demand of the patient is met completely by the system, then it is a high-flow system. In contrast, if the system fails to meet the ventilatory demand of the patient, then it is classified as a low-flow system.
Singhal AB, Benner T, Roccatagliata L, Koroshetz WJ, Schaefer PW, Lo EH, nanno FS, Gonzalez RG, Sorensen AG. A pilot study of normobaric oxygen therapy in acute ischemic stroke. Stroke. 2005; 36: 797–802.
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Sassoon CSH, McGovern JP. Oxygenation strategy. In: Schoemaker WC, Ayres SM, Grenvik A, Holbrook PR, eds. Textbook of Critical Care. Singapore: Harcourt Asia; 2001: 1308–1323.
We thank Drs Agarwal and Gupta for giving us the opportunity to elaborate on our method of oxygen delivery. As stated in our article,1 we delivered humidified oxygen at flow rates of 45 L/min through a simple facemask. “High-flow” refers to the high flow rates of oxygen used in the trial. Because the peak inspiratory flow rate of a resting individual is typically below 30 L/min,2 delivering oxygen at higher flow rates (eg, 45 L/min) precludes contamination of oxygen within the facemask with room air, so that the effective FiO2 is close to 1.0. The high flow rates were achieved by substituting the standard flowmeter (calibrated to 15 L/min) with a commercially available flowmeter that can deliver flow rates as high as 75 L/min (Timeter Classic Series Flowmeter model O-75; Allied Healthcare Products, Inc). This flow meter can only be attached to an oxygen wall outlet; therefore, during the short period of patient transport to the magnetic resonance imaging suite, we used the standard 0 to 15 L/min flow meter attached to oxygen cylinders and maximized oxygen delivery by fully opening the outflow valve and using a nonrebreather facemask (with this method, the effective FiO2 is 0.8 to 0.9). Although we were able to achieve encouraging results using this simple methodology, we emphasize that further studies, in unselected stroke populations, are needed to establish the safety and efficacy of high-flow oxygen before it can be used as an acute stroke therapy.
Singhal AB, Benner T, Roccatagliata L, Koroshetz WJ, Schaefer PW, Lo EH, Buonanno FS, Gonzalez RG, Sorensen AG. A pilot study of normobaric hyperoxia therapy in acute ischemic stroke. Stroke. 2005; 36: 797–802.
McNeill RMG, Brown R. A comparison of expiratory and inspiratory flow rates in health and in chronic pulmonary disease. Thorax. 1959; 14: 225–232.