Understanding Breath Types, Triggers, and Cycle

Date: November 29, 2023 | Posted by: Vikram Aditya Tirthani

Ventilation, a fundamental aspect of critical care, involves the mechanical support of breathing for patients who face respiratory challenges. In this exploration of the basics of ventilation, we’ll unravel the essential components, including breath types, triggers, and the distinction between volume and pressure breaths.

Breath Types

Breath Trigger Limit Cycle


Time Pressure Or Volume Time



Patient (P or F)


Pressure Or Volume






Patient (P or F)


Pressure Or Volume



  • Controlled – A Controlled (machine) breath delivers gas to the patient according to the control variable set by the operator.
  • Assisted – An assisted breath may either replace or be in addition to controlled breaths. An assisted breath is initiated when the patient’s inspiratory effort is equal to the sensitivity setting.
  • Spontaneous – Spontaneous breath is based on patient demand. Flow and volume are determined by the patient’s inspiratory effort. Flow is initiated when the patient’s inspiratory effort is equal to the sensitivity setting.

Trigger for ventilation

There are two types of triggers which control the breath cycle –

  • Time Trigger or Machine Trigger – Time trigger, often referred to as a machine trigger, is initiated by the ventilator at predetermined intervals, following the control values set by the doctor or user. For instance, if the user sets Ti (inspiratory time) at 2 seconds and Rate at 12 breaths per minute, each breath cycle will be 5 seconds long. During this cycle, the inspiratory phase lasts for 2 seconds, followed by a 3-second expiratory phase. These breaths are also known as mandatory or controlled breaths because the machine both starts and ends them.
  • Patient Trigger – Patient-triggered breaths occur when the patient actively initiates or terminates a breath cycle. These efforts are quantified, with a predefined threshold set by the doctor to validate the trigger and avoid false triggers. Patient-triggered breaths are significant in two main types:
  • Inspiratory Trigger – An inspiratory trigger occurs when the patient initiates inhalation during the expiratory phase. This trigger is identified by a negative pressure drop or an increase in flow. The inspiratory trigger has different time windows in various modes during the expiratory cycle, including the refractory window (a short period to prevent early triggering), the trigger window (a buffer for patient synchronization), and the synchronization window (used in assist modes to deliver assisted breaths).
  • Expiratory Trigger – In some modes, the ventilator switches to the expiratory phase based on the patient’s effort or lung’s compliance and resistance. The user sets an Exp% term to define these efforts, and when the inspiratory flow drops to peak flow * Exp%, the ventilator transitions to the expiratory phase during the inspiratory cycle.

These triggers play a crucial role in ensuring synchronized and effective mechanical ventilation, responding to both time-based intervals and patient-initiated breaths.

This can also be called as a synchronisation window which is considered at the end of inspiratory time. If the patient signal occurs during an inspiratory time synchronisation window, expiration starts and ends the breath mandatory breath.

It is set by the parameter named Exp% or ETS (End trigger sensitivity) and has a maximum value of 85% and a minimum value of 10%.

Relationship between Flow – Volume – Pressure

  • Purpose of MV is to put volume into the lung
  • Volume to get into lungs- should have a flow
  • Flow occurs only if there is a pressure difference
  • Flow for a certain time is volume delivered
  • From the machine’s side If the volume is set, pressure varies, If pressure is set, volume varies.
  • Compliance = ΔVolume/ ΔPressure

Breath Delivery types – Volume Vs Pressure

  • Volume-based breath cycles In these breaths a target volume (Tidal volume is set to be achieved regardless of the variation in pressure.

In the ventilation modes based upon volume-based breath cycles, the machine delivers a preset volume of gas into a patient’s lung and the breath stops once the volume is delivered. It has the advantage of the surety of achieving target minute volume, but the Patient is locked into whatever volume you set, this can lead to a significant rise in airway pressure in response to decreased respiratory compliance or increased airway resistance and if the patient inspiration demand is more than set peak flow the patient start fighting with a ventilator.

  • Pressure-based breath cycles – In these breath types, a target pressure (inspiratory pressure) is set to be achieved regardless of variation in pressure.

In the ventilation modes based upon pressure-based breath cycles, a preset pressure is delivered to the patient’s lung and breath is time cycled. These breath types are advantageous as the patient can take additional tidal volume, flow patterns are more comfortable and always meet the patient’s demand, and most importantly pressure to the lung is controlled. But it does have a few disadvantages as it gives Variable TV, MV, with compliance & resistance changes and also If the compliance improves then TV delivered for set pressure may be excessive causing volutrauma. Volutrauma refers to the local overdistention of normal alveoli.

Ventilation, with its intricate dance of breath types, triggers, and pressure or volume controls, serves as a lifeline in critical care settings. Healthcare professionals must navigate these fundamentals to provide optimal respiratory support tailored to each patient’s unique needs. As technology advances, understanding the basics of ventilation becomes increasingly essential, empowering healthcare teams to deliver precise and effective care to those in respiratory distress.

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