This document discusses the respiratory cycle during mechanical ventilation. It describes the trigger that initiates the inspiratory phase, during which positive pressure is delivered to the lungs for 0.8-2 seconds. The inspiratory phase ends based on a preset volume, pressure, or flow/pressure limit. The expiratory phase then begins. Different modes, like assist-control volume cycled (AC-VC), assist-control pressure cycled (AC-PC), and pressure support ventilation (PSV) are reviewed in terms of how they deliver controlled, assisted, or spontaneous breaths using these phases and limits.

1. MECHANCIAL VENTILATION
RESPIRATORY CYCLE (PART TWO)

2. RESPIRATORY CYCLE
Objectives:
• Examine the phases of the respiratory cycle:
– Trigger
– Inspiratory Phase
– Cycling Mechanism
– Expiratory Phase

3. TRIGGER
Time Trigger: no patient
effort (no dip: no change
in flow or pressure)
followed by a controlled
breath
Flow Trigger: patient effort
(dip: a change in flow or
pressure) followed by an
assisted or spontaneous
breath (depending on mode
– see below)
The trigger allows the inspiratory phase to begin:

4. INSPIRATORY PHASE
• During the inspiratory phase positive
pressure is delivered to the lungs.
• The inspiratory phase lasts between
.8 seconds to 2 seconds.
• The inspiratory phase is adjusted by
the RT depending upon the clinical
situation.

5. CYCLING MECHANISM
• Volume cycled (or limited) will end the inspiratory
phase at a set volume (i.e. 500cc).
• Pressure cycled (or limited) will end the
inspiratory phase at a set pressure (i.e. 30
cmH20).
• Flow/Pressure cycled (or limited) will end the
inspiratory phase at a preset set pressure (i.e. 30
cmH20) or flow (i.e. <15 lmp). Which ever one
come first.

6. CYCLING MECHANISM
500cc
30cm
H20
30cm
H20 OR <15
lpm
The inspiratory phase ends, or cycles, at a preset
volume, pressure, or flow/pressure.
The RT will adjust these parameters depending
upon the clinical situation.

7. EXPIRATORY PHASE
• The expiratory phase begins when the inspiratory phase ends.
• The RT can manipulate various setting to increase or decrease
the expiratory phase.

8. RESPIRATORY CYCLE
Let us review:
• The trigger marks the start of the inspiratory phase.
• Positive pressure is applied to the lungs during the inspiratory phase.
• The inspiratory phase lasts between .8 and 2 seconds.
• The inspiratory phase ends (or cycled) at a preset volume (vT), pressure (cmH2O) or
flow (lpm).
• At the end of the expiratory phase the ventilator is ready to start another respiratory
cycle.
Trigger
Inspiratory
Phase
Cycling
Expiratory
Phase

9. MECHANCIAL VENTILATION
• How dose all this fit?
• AC-VC indicates that the ventilator will deliver a combination of assisted and
controlled breaths (AC).
• The inspiratory phase will end or be cycled at pre-set volume (VC)
• Assist Control – Volume Cycled (AC-VC) is also know as CMV and VC.
• AC-PC indicates that the ventilator will deliver a combination of assisted and
controlled breaths (AC).
• The inspiratory phase will end or be cycled at a pre-set pressure (PC).
• Assist Control – Pressure Cycled (AC-PC) is also know as PCV or PC.

10. MECHANICAL VENTILATION
• PSV indicates the ventilator will deliver only
patient triggered spontaneous breaths.
• The inspiratory phase will end when the flow
decreases or the maximum pressure is
achieved.

11. MECHANICAL VENTILATION
• You are doing great!
• We understand how the trigger, inspiratory
phase and the three breath types are used to
build AC-VC, AC-PC and PSV.
• Next, we will cover how mechanical
ventilation can improve oxygenation!