Trigger, Limit, Cycle: The Waterfall We Should Chase?

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Max Hockstein

Academic emergency medicine and surgical intensive care in Washington, DC. Small k_m for echocardiography, cars, and diet coke.

The Pre-brief

Every good story has a beginning, a middle, and an end.  Such is the case with breaths delivered by a ventilator.  Every machine-delivered breath has three parts: trigger, target (“limit”), cycle.  Every good story also has a hero and a villain.  Our hero – ventilator synchrony; the villain – dyssynchrony.  Understanding how the ventilator delivers breaths allows us to deliver mechanical ventilation safely and comfortably.


The trigger variable is a monitored parameter that “tells” the ventilator to start the inspiratory phase of a breath.  Assist-control modes of mechanical ventilation rely on the ventilator’s ability to deliver assisted breaths (triggered by the patient) and control breaths (triggered by the ventilator).  The three common trigger variables are time, pressure, and flow.  

  • Time.  Time-triggered breaths are, by definition, control breaths.  Let’s say you set the respiratory rate to 12 breaths/minute.  That’s one breath every five seconds.  If at five seconds, the patient hasn’t initiated a breath, the ventilator delivers a control breath.  If the patient has a respiratory rate of over 12, the preset time-trigger variable is not exceeded and the ventilator does not deliver a control breath.  The idea of a time trigger is a similar phenomenon to VVI on a pacemaker.  
  • Pressure or flow.  Pressure or flow triggered breaths are assisted breaths.  For both pressure and flow triggers, clinicians set a sensitivity – or an amount of effort that the patient needs to contribute for the ventilator to recognize the effort as an intended breath.  The subject of whether a breath should be flow or pressure-triggered has been a subject of debate for many years.
  • Pressure.  When the machine senses a drop in airway pressure (because of a patient-initiated effort), an assisted breath is delivered.  For a pressure trigger to be “met” it requires work done by the patient and can result in discomfort.
  • Flow.  As work VTinspired×∆Ptrigger, a flow trigger requires less work by the patient.  Flow triggers are subject to a phenomenon known as “autotriggers” – a phenomenon where non-patient signals trigger a breath (ex: cardiogenic oscillations, leaks or water in the circuit). 

Target (“limit”)  

The target is a variable which you want to sustain (and not exceed) during the breath.  Pressure, flow, or volume are common target variables.  Prior to 2010, the target variable was known as a limit variable (changed in 2010 by the international standards organization, ISO).  Think of the target variable as either the pressure, flow, or volume that will not get exceeded while achieving the cycle variable.  Note: when the target variable is achieved, the breath is not terminated – the value is just held constant  

Cycle (“control”) 

The cycle (aka the “control”) variable (pressure, flow, volume, time), when met, switches the breath from inhalation to exhalation.  Put simply, it is what turns the breath off.  

  • Volume cycled: the cycle variable necessary for volume control ventilation.
  • Flow cycled: commonly used in pressure support and other spontaneous modes.  The breath cycles when the inspiratory flow decreases to a set threshold (usually designated as the percentage of peak inspiratory flow). 
  • Time cycled: the inspiratory valve is open for a pre-set time and delivers a constant pressure.  This is technically how “pressure control” is cycled.
  • Pressure cycled: the major use of this is as a safety feature. When the peak airway pressure exceeds the value of the high pressure alarm, breath is cycled off to prevent further increase in airway pressure . 

Figure 1: Example of changing flow cycle threshold in pressure support ventilation

The Debrief

  • There are three phases to all breaths delivered by a ventilator: a trigger, a target/limit, and a cycle.  
  • Triggers start the breath, the target is the variable which is not exceeded during the breath, and the cycle variable is what turns the breath from inhalation to exhalation.
  • Understanding each phase of a mechanically-delivered breath lends itself to understanding the enemy of mechanical ventilation, dyssynchrony. 
  • For more on fundamentals of synchrony, see this related post! (and part 2 here)
  • More to come on these topics in future posts!


  1. Tobin, M. J. (2012). Principles and practice of mechanical ventilation, third edition (3rd ed.). New York, NY: McGraw-Hill Medical.
  2. Respir Care. 2011 Jan;56(1):52-60.  doi: 10.4187/respcare.01088.


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