Novel design of inspiratory flow generation and gas mixing for critical care ventilators suitable for rapid production and mass casualty incidents

Abstract

Scarcity of medical resources inspired many teams worldwide to design ventilators utilizing different approaches during the recent COVID-19 pandemic. Although it can be relatively easy to design a simple ventilator in a laboratory, a large scale production of reliable emergency ventilators which meet international standards for critical care ventilators is challenging and time consuming. The aim of this study is to propose a novel and easily manufacturable principle of gas mixing and inspiratory flow generation for mechanical lung ventilators. Two fast ON/OFF valves, one for air and one for oxygen, are used to control the inspiratory flow generation using pulse width modulation. Short gas flow pulses are smoothed by low-pass acoustic filters and do not propagate further into the patient circuit. At the same time, the appropriate pulse width modulation of both ON/OFF valves controls the oxygen fraction in the generated gas mixture. Tests focused on the accuracy of the delivered oxygen fractions and tidal volumes have proved compliance with the international standards for critical care ventilators. The concept of a simple construction using two fast ON/OFF valves may be used for designing mechanical lung ventilators and thus suitable for their rapid production during pandemics.

Figure 1

A scheme of inspiratory flow generation and gas mixing.

Figure 2

Examples of pulse sequences for generating inspiratory profiles with different flow patterns, magnitudes (i.e., tidal volumes) and fractions of oxygen in the gas mixture. Inspiratory profiles filtered by the low-pass acoustic filters are depicted on the left, the corresponding pulse sequences on the right.

Figure 3

Measuring setup for assessment of the technical properties of the experimental ventilatory set.

Figure 4

The recorded sequence of electric pulses and corresponding pressure pulses creating the inspiratory flow (a) and the resulting pressure and flow curves after the pneumatic filtering (b).

Figure 5

The dependance of measured FiO₂ on the set FiO₂ for RR = 20 breaths per minute (in black) and RR = 35 breaths per minute (in red).

Figure 6

The dependance of the measured tidal volume V_ti on the set inspiratory oxygen fraction FiO₂ and the set tidal volume.

Figure 7

The 3D plots of the dependance of the delivered volume and maximum pressure on the compliance and resistance of the model of the respiratory system.