There are several different concepts we have to discuss in order to understand mechanical ventilation. The following image represent the six key components of mechanical ventilation, each with their own sub-categories. We will discuss each of these in more depth below. Please note this is just an introduction to mechanical ventilation. There are textbooks devoted entirely to the subject! Other articles discuss when to intubate and start mechanically ventilating, as well as how to handle mechanical ventilation problems.
Ventilators follow a pattern of breathing. This is also known as the ventilator mode. The more common modes used during invasive mechanical ventilation include controlled mandatory ventilation (CMV), assist control (AC), intermittent mandatory ventilation (IMV), and pressure support (PS).
In controlled mandatory mode the machine does all the breathing for the patient. The ventilator will not allow spontaneous breathing efforts. This is the mode of ventilation used most commonly in the operating room when a patient is pharmacologically paralyzed and unable to breathe on their own.
In intermittent mandatory mode, ventilator controlled breaths are combined with spontaneous breaths initiated by the patient.
In assist control ventilation a patient may begin to spontaneously breathe, at which point the machine senses the effort, and then gives a full ventilator controlled breath. In this mode, both assisted and controlled (ie: ventilator given) breaths are given.
In pressure support mode the machine controls the amount of pressure each breath has behind it. However, the patient controls everything else, from the amount of air they breath in (aka: tidal volume) to the effort they put into breathing.
Cycling (How long does the air flow stay there?)
All ventilators deliver air flow to the lungs until a preset volume or pressure is met. Cycling refers to how long the air flow remains at that particular volume or pressure point. It is dictated by the mode discussed above.
The ventilator breathes via one of three different ways. The first way is mandatory breathing. In this setting the ventilator does all the breathing for the patient.
The second is assisted breathing where the machine assists the patients own spontaneous respirations. The amount of assistance given is dependent on what mode the ventilator is in.
The third type of breathing is spontaneous breathing, which is initiated by the patient. In this setting the patient is effectively "sprinting" or breathing on their own.
Breathing consists of two important components: tidal volume and airway pressure. This is exactly how the ventilator "controls" breathing; it does so either by providing a set pressure, or a set volume.
In pressure support ventilation the pressure of each breath remains constant. In other words, when the ventilator reaches a preset pressure limit inspiration is terminated. The amount of volume delivered is entirely dependent on the lung compliance of the patient. For example, if the patient has stiff non-compliant lungs (such as those seen in pulmonary fibrosis) the pressure from the machine will be used to overcome the non-compliant lung tissue, and relatively little air volume will actually be delivered to the patient.
The main benefit of pressure controlled ventilation is, you guessed it, that airway pressures are controlled. This makes being mechanically ventilated much more comfortable and better tolerated. It also decreases the risk of barotrauma to the alveoli and bronchi.In volume support, the ventilator delivers a preset volume of air per breath. The ventilator will continue to "force" that set volume into the lungs regardless of the pressures being generated. Therefore, high airway pressures can occur and lead to barotrauma (especially on older ventilators without peak limits).
The benefit of volume support is that a set volume is always delivered to the patient, and thus tidal volume is guaranteed with each breath.
We've seen that the ventilator can be set to mandatory ventilation (ie: CMV) or support modes (ie: AC, PS). However, how does the ventilator know when to breathe? This is easy to understand in controlled ventilation. The machine is set to give a breath every 8 seconds, or 10 seconds, or whatever rate you program into the machine. This is called "time triggering".
How does the ventilator know when the patient is attempting to take a breath, and then help that patient finish the breath? There are two different ways: pressure and flow triggering. In pressure triggering the ventilator circuitry detects the drop in intrathoracic pressure that occurs when a patient initiates a breath. When this occurs the ventilator delivers a breath to the patient. Other ventilators detect changes in flow that occur during patient induced inspiration.
Air flow is the whole point of mechanical ventilation. The flow of that air follows different patterns depending on what type of ventilator settings are being used. Sinusoidal air flow patterns indicate spontaneous breathing by the patient. Decelerating air flow patterns indicate that the ventilator is in some sort of pressure control setting. This makes sense because in pressure controlled modes the flow rate will start out rapidly, and as pressure builds towards the set point the flow rate will decelerate. Constant air flow patterns are seen in volume controlled settings. Under these conditions a set air flow is delivered until the pre-specified volume is achieved.
There are several important things to think about when handling mechanical ventilation. They include what mode the ventilator is in, how breathes are delivered, how triggering occurs, and what control is being used (ie: pressure or volume). Tinkering with these parameters can benefit the patient significantly from both a ventilatory and comfort point of view.
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