## Article
### 1 Introduction  
High-Frequency Ventilation (HFV) is a therapeutic technique, using a fast supply of very small breaths, comparable to the volume of the patient's anatomical dead space [2 ]. The typical respiratory rate is greater than four times the normal value of spontaneous breathing; tidal volumes are in the order of milliliters. This reduces the risk of damage to lung tissue. Therefore, HFV techniques are used in premature newborns, whose lung tissue is very fragile, tends to collapse and holds excessive air in the lungs--so-called air-trapping [3 ]. Two high-frequency ventilation techniques are currently used in neonatal clinical practice--HFOV (High-Frequency Oscillatory Ventilation) and HFJV (High-Frequency Jet Ventilation), with a unique ventilator--Life Pulse (Bunnell Inc., Salt Lake City, UT, USA) [3 ]. Both techniques are widely used by neonatology centers, especially in North America. In Europe, the use of HFOV is common, but the HFJV technique is used rarely. The reason is the need for the jet ventilator and standard ventilator combination. Then, there is the absence of flow monitoring or the absence of information on the supplied tidal volumes. This complicates the initial setting of the jet ventilator (currently performed according to sufficient chest vibrations). Simultaneously, it complicates the line of the high-frequency ventilation itself, and it is not easy to maintain the required PaCO\({}_{2}\). Given the importance of TV (Tidal Volume), it can be assumed that monitoring of this parameter must become an integral part of HFJV in the future [3 ]. In the HFJV Life Pulse ventilation circuit, it is theoretically possible to measure the flow in the Life Port fitting area, which contains the nozzle and allows the connection of a conventional ventilator, see Figure 1(a). In the nozzle, the gas is accelerated and enters the ETC. However, at the same time, the fast-flowing gas from the nozzle entrains the gas from the area above the nozzle (gas from the ventilation circuit of a conventional ventilator). In the case of exhaling, gas from the patient's respiratory system rises through the ETC and LifePort into a conventional ventilator circuit--Figure 1(b). There are theoretically two measurement options:  
* Measurement in the patient's ETC (AREA 1), where the flow sensor can register the entire amount of gas flowing into (from the inspiration) and from (the expiration) the patient;
* Measurement in the gas flow above the nozzle (upper part of the LifePort--AREA 2). A flow sensor located in this area can detect the total volume of expired gas. During inspiration, this area allows measurement of only the volume of gas that is entrained by the jet stream from the CV circuit. This means that the absolute value of the inspired gas cannot be measured. Placing the flow sensor in AREA 1 makes it possible to obtain absolute values of the amount of gas during inhalation and exhalation. Thus, it represents several advantages, such as monitoring the patient's minute ventilation and monitoring the size of gas leaks, etc. However, in terms of possible sensors usable in pipes with a diameter of 2.5-3.5 mm, it is necessary to consider the influence of the sensor used on the specific pulsatile flow of gases generated by the HFJV, with a duration of around 20-30 ms and respiratory rates used in clinical practice reaching values of up to 660 breaths/min. Orifices and, to a limited extent, hot-wire anemometers can be used for the above conditions. The choice of a specific type of flow sensor is always a compromise between flow resistance, sensor dead volume, resistance to vapor condensation and gas composition changes [11]. Laboratory experiments [10, 11] with flow measurement during HFJV using the orifice plate in the endotracheal tube confirmed the orifice plate's functionality. They pointed to the need for the compensation of pressure losses in the orifice.