MECHANCIAL VENTILATIONRESPIRATORY CYCLE (PART TWO)

RESPIRATORY CYCLE

Objectives:

• Examine the phases of the respiratory cycle:– Trigger– Inspiratory Phase– Cycling Mechanism– Expiratory Phase

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:

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.

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.

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.

EXPIRATORY PHASE• The expiratory phase begins when the inspiratory phase ends.

• The RT can manipulate various setting to increase or decrease the expiratory phase.

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

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.

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.

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!