The respiratory cycle and mechanical ventilation.

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!