HFT, HFNC, and HHFNC: Terminology and Definitions

Definition of HFT
 Any delivered flow in excess of subject’s intrinsic inspiratory flow is called “high flow.”1
 Because inspiratory flow is dependent on several factors (see below) that, in turn, are dependent on the subject’s weight and age, no single flow rate can be given for the pediatric age group that could be called high flow. In general, flows in excess of 2–8 L/min in neonates are considered high flows.1 The terms HFT, HFNC, and HHFNC are often used interchangeably. HHFNC is a form of HFT. HFT if delivered via nasal cannula (as opposed to a mask interphase) is called HFNC.
 HFNC devices to deliver up to 8 L/min in neonates and 40 L/min have been approved.1
Calculation of inspiratory flow1
 Inspiratory flow = minute ventilation (minute vol)/inspiratory time fraction
 Minute ventilation = tidal volume (Vt) × respiratory rate
 Inspiratory time fraction = inspiratory time (Ti)/time for total respiratory cycle (Ttot). Example: If respiratory rate is 60 breaths per min, Ttot would be 1 s. If inspiratory time is 0.3 s, then inspiratory time fraction would be 0.3.
 There are normal predicted values for Vt and respiratory rate for children of different ages. Inspiratory time fraction varies according to disease process. In general, it is <0.5 in normal health.
  • HFT, high-flow therapy.