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Welcome back to the tasty morsels of critical care podcast.
Ventilators remain one of the dark arts and hidden mysteries of critical care. Intensive care is a fairly generalist specialty overall and we share our core knowledge with a lot of other specialties. But when it comes to ventilators you enter the niche world of intensive care where arcane knowledge of boutique ventilators from the late 70s reigns supreme.
My own fumblings around ventilation concepts are merely my attempt to grasp some of the basic concepts and apologies in advance for the inevitable errors.
Ventilator cycling refers to the variable used to end inspiration. Triggering (a whole different topic) refers to what is used to initiate the breath.
So when a breath is triggered, gas starts to flow into the patient. At some point the inspiratory flow has to come to an end and the ventilator has to cycle from inspiration to expiration. This can be time cycled eg after half a second of inspiration the breath converts to expiration. Or is can be flow cycled eg once the flow rate drops to a certain level the ventilator cycles to expiration.
It is worth noting that cycling is not the same as the control variable, for that we have pressure control and volume control ventilation. Again these are topics in themselves to be covered at a later date. For example in a pressure controlled, time cycled breath the ventilator will target a certain pressure during inspiration eg 20cmH20. Once it reaches that target it will maintain that pressure until the set time has passed for inspiration before cycling into expiration. The pressure control controls the pressure achieved by the ventilator but the time set is what determines the cycling from inspiration to expiration. For newbies that can be a subtle but important difference that it’s worth taking the time to understand.
As a gross generalisation you’ll find that most mandatory modes of ventilation (eg volume control) are time cycled while most spontaneous modes of ventilation (eg pressure support) are flow cycled.
What follows is a brief summary of the basics and advantages of each type of cycling.
- typically found in mandatory modes of ventilation
- the cycling is determined by the interaction of I:E and RR
- eg at a RR of 20 assuming a 3 second breath with an I:E of 1:2 there will be inspiration for 1 second before cycling to a 2 second expiration. Changing the resp rate and the I:E ratio will change when the cycle from inspiration to expiration happens
- Advantage of time cycling ventilation
- higher mean airway pressure overall – pressure/volume held in the lungs for a set time as opposed to patient breathing out whenever they want. The resultant higher mean airway pressure will improve oxygenation.
- “flow cycle threshold” is a reasonable descriptive term but it has a different name on every vent. In the almost ubiquitous Servo-I found in Irish ICUs this is controlled by the “insp cycle off” setting. If you want a prolonged breath then you set this low eg 2% – this means the flow in the circuit has to decrease to 2% of the peak inspiratory flow before it cycles to expiration
- there’s not really a good and bad to how you set it but you can cause over distension if you leave it for too long before cycling to expiration. Set the % too high and the breath will end earlier than the patient might wish. Ideally you want to cycle to expiration whenever the patient neurologically has completed inspiration. (which is not an easy thing to measure)
- this is probably more comfortable for the spontaneously breathing patient as the ventilator will likely be more in tune with when the patient wants to end the breath.
Some things that make flow cycling very tricky
- Cuffless trache
- mask leak in NIV
- all of these will make it difficult to set a very low insp cycle off % as the flow will never get that low.
Gentile, M. A. Cycling of the mechanical ventilator breath. Respiratory care 56, 52–60 (2011).
Hess, D. R. Ventilator waveforms and the physiology of pressure support ventilation. Respiratory care 50, 166-86-discussion 183-6 (2005).