2 types of breath-
Mandatory- these will be delivered regardless of what the patient does. We could set, for example, 12 breaths per minute of 500 mls.
Spontaneous- these are initiated by the patient. If there is no assistance from the ventilator, then it is the patients effort that determines the size of the breath.
Because it is narrower than the patients own airway. Laws of flow say that if you halve the diameter of a tube you increase resistance by 16 times! So the patient is going to have to work very hard to breath for themselves through the ventilator. They also have to overcome the resistance from the ventilator tubing.
So when the patient triggers the ventilator in SIMV we can add Pressure Support to that breath.
Typically this will be somewhere between 10-20 cmH2O. So the patient takes a breath, and the ventilator then adds pressure to that breath to make it easier for the patient to achieve a good tidal volume.
Breathing with pressure support, rather than the ventilators mandatory breath, means that the patient will be utilising their diaphragm and the normal muscles of inspiration.
This helps them maintain more muscle tone. They are also using negative pressure ventilation rather than positive pressure which creates a more normal distribution of gases and perfusion within the lungs.
Pressure support can be used with the spontaneously breathing patient. This patient is initiating all the breaths they get and they do not receive any mandatory breaths at all. This is commonly known as pressure support ventilation (PSV). The patient therefore sets their own rate and as their lungs become stronger the level of pressure support can be reduced, allowing a weaning process.
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