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Table of modes of mechanical ventilation
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In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing. Spontaneous breathing requires the rhythmic alteration of inhalation and exhalation to achieve its purpose: exchange of carbon dioxide with oxygen. In disease, the natural process of that rhythmic alteration may be replaced by an external device, a Ventilator, and the clinician has to control the following on the Ventilator to achieve gas exchange:
- Composition of gas mixture delivered (Setting: fraction of inspired oxygen, FiO2)
- Pressure at the end of exhalation (Setting: Positive End-Expiratory Pressure PEEP)
- Control of Breathing Support
 | This article needs additional citations for verification. (September 2011) |
FiO2 and PEEP settings are common to most Ventilators. Control of Breathing Support methods, however, can be done in different ways and are called "Modes of Mechanical Ventilation".[1] The settings to control Breathing Support follow the phases of breathing,[2] i.e., inhalation and exhalation:
- Inhalation:
- I-Trigger: Start of inhalation (Settings: time or patient Trigger)
- Inhalation mechanism: delivery of gas mixture (Settings: pressure or flow)
- Exhalation:
- Icycle: Cycling from inhalation to exhalation (Settings: time or pressure or volume or flow)
- Exhalation mechanism (Pcycle): Time and control of exhalation (Settings: time and end-expiratory pressure)
CAVEAT: Although manufacturers may offer identical Control of Breathing Support methods, the names of the ventilation mode may be different.
I-Trigger: The ventilator needs to know when to start delivering gas to the patient. If the patient does not breathe at all, a timing signal starts inhalation. If the patient has some breathing activity, the ventilator can sense this by measuring pressure or flow and start inhalation if pressure or flow drop below a certain threshold. That threshold is called Trigger Sensitivity.
Inhalation mechanism: Technically, two methods to deliver the gas mixture can be employed, flow controlled or pressure controlled. Flow control means that the ventilator outputs a pre-set flow and maintains that flow until the end of inhalation. Pressure control means that the ventilator outputs a pre-set pressure and maintains that pressure until the end of inhalation. Both methods have their advantages and disadvantages. Flow control will deliver the gas mixture independent of resistance to flow and guarantee a set delivery of gas. In the process, pressure might become very high and potentially dangerous to the patient. Pressure control will deliver the gas mixture at a pre-set level and never exceed that pressure. However, it may not succeed to deliver a set volume of gas mixture.
Icycle: Inhalation must eventually stop and give way to exhalation. If the patient does not breathe, the ventilator must switch to exhalation after a pre-set time or after a pre-set volume has been delivered. If the patient has some breathing activity left, the ventilator can sense this by measuring flow and start exhalation if flow drops below a certain threshold. That threshold may be termed "Expiratory Trigger Sensitivity".
Exhalation mechanism: Exhalation is only possible if the pressure inside the lungs is released. In normal breathing this pressure is ambient pressure. On a ventilator, the pressure is released to the level of PEEP (if PEEP is set to zero, pressure is released to ambient pressure, of course).
The table below lists the working principles of some of the common modes of ventilation. (V = controlled by ventilator; P = controlled by patient, based on flow or pressure measurent)[3]
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