Enthusiastic & innovative engineers with charitable intent are working to rapidly develop COVID-19 ventilators, sometimes without a deeper medical understanding of the mechanics of lung ventilation. This can be dangerous for the patients, if the medical implications of the operation modes are not well understood. This YouTube video “A Guide To Designing Low-Cost Ventilators for COVID-19” nicely explains the some risks.
Ventilators may be used in a variety of settings or operational “Modes”. The doctor evaluates the condition of the patient, and prescribes the mode in which the ventilator is to be operated. It is important for ventilator designers to have an understanding of these Modes, and design the operations and controls of the ventilator to meet at least some of the most essential Modes correctly.
However, getting a grasp of the jargon, and the details of the modes is not a trivial exercise. Our team studied the freely available standard: ISO 1923:2019 : Lung ventilators and related equipment — Vocabulary and semantics, and compiled a short summary of Ventilator Modes shared below. This is intended to serve as a basic reference. For a more in-depth understanding, the standard itself is a great resource, and, in addition we have some curated references in the Ventilator References section of this website.
The ventilator modes are best understood as shorthand to describe two independent parameters of ventilation. One parameter describes how each breath is initiated, and the other parameter describes how each full breath cycle is completed.
A breath might be initiated by the patient, or when the patient is unconscious or unable to breathe, the machine might initiate the breath. Modes that depend on the patient initiating the breath are called the Spontaneous Modes, while the modes that are started by the ventilator are called the Mandatory Modes.
There is also an “Assist Mode”, a sort of halfway house: The patient still initiates the breath, but as soon as he starts breathing, the machine kicks in with a predetermined inflation pattern that is set by the doctor.
Fine grained control on the breathing over the treatment duration is possible by combining and adding features to the basic modes: For example, in the Spontaneous mode the machine can maintain one positive pressure level (CPAP), or can alternate between two pressure settings, one for inhalation and the other for exhalation (BPAP). On the other side, if the Mandatory Mode allows some free time in between two cycles to allow spontaneous breathing, it is called Intermittent Mandatory Mode (IMV). Further, if in the IMV mode the machine waits for a small “window” of time to watch for a patient’s breath, and synchronises itself with it, the mode is called Synchronised Intermittent Mandatory Mode, or SIMV.
See figure 2 below for the spectrum of important Modes.
How about the other variable? Quite independent of the above discussion on how the breath is started, the doctor might also set how the breath inflation cycle is completed. Note that the machine only drives the inflation of the lungs – exhalation always occurs spontaneously, as the muscles stretched by the lung inflation return to the normal exhaled state.
There are broadly two methods: In the Pressure Control mode the machine increases the supply pressure to inflate the lungs, and the inflow stops when the internal pressure of the lungs matches the supply pressure. The volume of the air that went into the lungs is determined by the Gas Laws. The alternative is Volume Control where the Volume of air in each breath is set, and the pressure is slowly raised within the breath until the set volume has been inhaled (or if the pressure rise hits the safety limit).
The Mandatory and Assist Modes can be combined with either Pressure and Volume Control modes, but in the Spontaneous mode the machine cannot control the volume, so only Pressure Control modes are possible.
There are still other modes that are not seen in Figure 2. As per the ISO standard referenced above, over a period of time, alternative descriptions have emerged and there are close to 90 different mode names in use, including some proprietary names.