This is Part 1 of a 5 part series on fundamental concepts in ventilator management.
What is a ventilator?
The answer to this question seems obvious…a ventilator ventilates. On a very simplistic level, a ventilator is just a mechanical BVM, however, there’s a lot more to ventilation than just moving air. A fundamental understanding of how a ventilator works builds a foundation for mastering the vent. This is divided into two sections: how ventilators move air, and how a ventilator decides when to move air.
How does a ventilator move air?
There are two main types of ventilators: compressor driven and turbine driven. A compressor driven vent operates by controlling the VOLUME of gas delivered directly. It can be thought of as a small piston that moves a certain amount of air with each stroke: the more strokes, the larger the volume. The faster the strokes, the higher the flow. A turbine driven vent operates by controlling the airway PRESSURE. It can be thought of a small fan that pushes air into the trachea: the faster the fan turns, the higher the pressure.
Why is this relevant?
There are multiple modes of ventilation, which can be broadly split into PRESSURE CONTROLLED and VOLUME CONTROLLED. Both types of vents can operate in both categories, which highlights that pressure and volume are directly related to each other. Controlling one is equivalent to controlling the other. A compressor driven vent delivering pressure controlled breaths delivers volume to maintain a certain set pressure. A turbine vent delivering volume controlled breaths increases the pressure until the set volume is reached.
Why have two types of control?
The simple answer, there are times when volume delivered is more important, and other time pressure in the airway is a priority. A patient can be adequately managed in either mode, but each one has some advantages and disadvantages. This type of control governs how gas is delivered to the patient. Mastering this aspect of ventilators revolves around understanding the relationships between the different parameters (discussed later).
How does the ventilator know when to give air?
Vents “listen” to the patient. When the patient has spontaneous respirations, the vent can detect this effort, and will then deliver a breath according to the parameters set. The vent can also deliver breaths on a schedule, even if the patient doesn’t try to inhale. Finally, in some ventilator modes, the vent may actually ignore when a patient tries to inhale. Setting a vent to NOT give breaths may seem counterintuitive. Mastering this area of vent management revolves around understanding how the vent interacts and responds to the patient’s respiratory drive.
How does a ventilator know when to stop giving air?
Only one thing will terminate a breath in any given mode. In a volume control, the breath ends once the full volume is delivered. In pressure control, the breath ends after a set amount of time at the inspiration pressure. There is one other type of mode, however, where the breath ends when the vent detects that the patient has stopped inhaling. Understanding the last one is a bit more challenging though, so focus mainly on volume control vs pressure control. Understanding when the vent stops giving air requires understanding how the previous two aspects of ventilator management interact with each other.
- Respiratory Cycle: One full inspiration and expiration
- Respiratory Rate (f): Breaths per minute
- Peak Inspiratory pressure (PIP): The maximum airway pressure reached during inspiration. In pressure control modes, this is the manipulated variable to control how much gas is delivered.
- Tidal Volume (Vt): The volume of each breath. In volume control modes, this is the manipulated variable to control how much gas is delivered.
- Positive End Expiratory Pressure (PEEP): Continuous vent pressure applied to the airway, in order to keep alveoli open.
- Airway pressure (Paw): Instantaneous pressure in the airway (varies during respiratory cycle).
- Alveolar pressure (Palv): Instantaneous pressure in the alveoli.
- Compliance (C): How much pressure is required to push a set volume of air into the lungs. Alternatively, how much lung volume changes when you move between two set pressures.
- Plateau pressure (Pplat): The maximum pressure seen by the alveoli during inspiration. Determined by Compliance and Tidal Volume.
- Airway Resistance (RAW): How much the airways resist flow.
- Inspiratory/Expiratory flow: Measured in liters per minute, determined by airway resistance and the difference between Paw and Palv.
- Inspiratory Time (I-time): The time the vent takes to deliver a breath
- Expiratory Time (E-time): The time it takes to go from the end of one inspiration to the start of another. In other words, how long the vent allows the patient to exhale.
- Inspiratory to expiratory ratio (I:E ratio): Ratio of the time spent during inspiration to the time spent during expiration.
- Fraction of Inspired Oxygen (FiO2): What percent of the delivered gas is oxygen. Room air has an FiO2 of 21%.
- Minute Volume (MV): Total amount of gas moved per minute
In the next section, we start working through the equations that describe the relationships between the various respiratory variables.