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 Mechanical ventilation breaths can be considered as

• Controlled

• Assist- controlled

• Assisted-spontaneous

• (Fully) Spontaneous breaths

according to the settings selected for :

1. Breath initiation

2. Inspiration

3. Cycling to exhalation

1.Breath Initiation

• Breath may be initiated either by the machine or by the patient.

• Machine Initiation

  • Breath is initiated at a pre-set time, according to the setting for respiratory frequency.

  • Machine-initiation can take place only in the modes that include a control for frequency (Controlled and assist controlled breaths).

• Patient Initiation

• Patient initiation means that breath is initiated by the patient's inspiratory effort, by means of a pressure or flow based trigger.

• Patient initiation can take place during assist control, assisted spontaneous and spontaneous breathing modes.

2.Inspiration

• It can be either volume control or pressure controlled

• Volume controlled inspiration

• It is based on control of inspiratory flow, delivered for the set inspiratoy time and according to the set flow pattern, to achieve the set tidal volume.

• At any point of time during a volume controlled inspiration the pressure developed in airway depends on Inspiratory flow and volume change produced by ventilator and resistance, compliance and intrinsic PEEP of the lungs.

• Pressure controlled inspiration

• The ventilator applies a positive pressure above the baseline pressure. The ventilator generates a rapid increase in pressure to the user-set level, and then maintains that pressure throughout inspiration.

• At any point of time during a pressure controlled inspiration, the inspiratory flow and volume depends on the balance between inspiratory pressure and passive impedance of respiratory system.

3. Cycling to exhalation

• When the exhalation is controlled by the machine, it is time based.

• Time cycling means that the ventilator switches to exhalation as soon as the set inspiratory time has elapsed (Ti).

• The inspiratory time (Ti) may be set in a ventilator as

  • Duty cycle (Ti%) or the Inspiration : Expiration Ratio (I:E) combined with frequency.

  • Inspiratory peak flow combined with tidal volume and flow pattern.

• Time cycling is used in controlled and assist control breaths.

• Patient controlled cycling is used in assisted spontaneous and fully spontaneous breaths.

• It is based on expiratory trigger that allows the automatic adaptation of the ventilator Ti to patient's own Ti.

Updated on 20/9/2015

Reference

• Patient Centered Acute Care (PACT) . Mechanical Ventilation 2011.