VENTILATOR BREATH DELIVERY ALGORITHMS AND NOMENCLATURE

2. VENTILATOR BREATH DELIVERY ALGORITHMS AND NOMENCLATURE
During mechanical ventilatory support, there are four phases during each ventilatory cycle:
a) the trigger phase (breath initiation), b) the flow delivery phase, c) the cycle phase
(breath termination), and e) the expiratory phase. Mechanically delivered breaths can be
described by what determines the trigger, flow delivery, and cycle parameters for that breath.
A. Triggers are of three types: machine timer,
pressure change, and flow change. With a
machine timed trigger, the clinician sets a rate
and mechanical breaths are initiated by a
machine timer. With a pressure trigger, a
patient effort pulls airway/circuit pressure
negative and mechanical breaths are initiated
when pressure exceeds the set negative
pressure threshold (pressure sensitivity). With
a flow trigger, a patient effort draws flow from
the circuit (often from a continuous bias flow)
and mechanical breaths are initiated when flow
into the patient exceeds the set flow threshold
(flow sensitivity). In Figure 3 are illustrated three
different triggering episodes. The pressure
triggering threshold is indicated by a negative
pressure level below baseline; the flow triggering
threshold is indicated by an inspiratory flow above
baseline bias flow.
FIGURE 3 EXERCISES
Identify which breath is:
Machine timer triggered? __________________________
Pressure triggered?______________________________
Flow triggered? _________________________________
B. Flow delivery is governed by a clinician set target
or limit for the ventilator during inspiration. There
are two commonly used targets/limits: Flow target
(Clinician sets flow magnitude and pattern; pressure
therefore varies with mechanics/effort in order
to deliver that flow); and Pressure target (Clinician
sets inspiratory pressure (Pi); flow/volume therefore
varies with mechanics /effort to maintain that
pressure) (Figure 4)
FIGURE 4 EXERCISES
Identify which breath is:
Flow targeted? _______________________________________________________
Pressure targeted? ___________________________________________________
Figure 3
Figure 4
L1441 Rev. B September 2001 3
C. Breath (Inspiratory) termination (cycling) can be determined by several factors
(Figure 5). Commonly used cycling criteria are: Volume (breath terminates when target
volume achieved); Time (breath terminates when set inspiratory time achieved); Flow
(breath terminates when flow has fallen to set level); and Pressure (breath terminates
when set pressure is achieved). Note that the pressure cycle can be primary cycle variable
(e.g. older “IPPB” devices) or can be a “backup” cycle variable with those above to prevent
over-pressurization.
An expiratory effort during a mechanically delivered breath can be handled in one of three
ways by the ventilator. First, if flow is the primary cycling mechanism, expiratory effort may
terminate inspiratory flow and thus cycle the breath. Second, if inspiratory flow is
maintained, airway pressures will rise and the ventilator will pressure cycle. Third, flow and
volume can adjust (including some exhalation if necessary) to maintain a target inspiratory
pressure such that the primary cycling variable (usually time) remains intact. This latter
strategy is termed Pressure Relief/Release.
Finally, some ventilators are designed to have both volume and flow cycling features
(Pressure Augmentation or Volume Assured Pressure Support). With this approach, the
breath begins like a pressure targeted breath (i.e. inspiratory pressure is clinician set).
However, a minimum volume is also clinician set. The breath is flow cycled if the minimum
volume has been achieved or exceeded at that flow cycle value. If the volume is below the
set level at that flow point, however, a constant backup flow is supplied to achieve the set
volume (volume cycled).
FIGURE 5 EXERCISES
Identify:
Time cycle? ________________________________________________________
Flow cycle? ________________________________________________________
Volume cycle? ______________________________________________________
Pressure cycle? ____________________________________________________
Time and pressure cycle? _____________________________________________
Time and pressure relief/release? _______________________________________
Flow with backup volume cycle? ________________________________________
Figure 5
4 L1441 Rev. B September 2001
D. Breath types. Trigger, target/limit, and cycle criteria can be used to characterize
breath types during mechanical ventilation.
FIGURE 6 EXERCISES - Indentify the breath type from Figure 6.
TRIGGER TARGET/LIMIT CYCLE
Volume control (VC) time flow vol*
Volume assist (VA) effort flow vol*
Pressure control (PC) time pressure** time*
Pressure assist (PA) effort pressure** time*
Pressure support (PS) effort pressure** flow*
Pres. release/relief (PR) time/effort pressure time
Spontaneous effort pressure*** effort
*Expiratory effort elevating airway pressure will also cycle breath
**On some machines, the ventilator can adjust the clinician set inspiratory pressure to achieve
a target volume
***Set Pi is equal to set expiratory pressure (CPAP)
Figure 6
L1441 Rev. B September 2001 5
E. Modes. Ventilator modes are defined by the breath types available and the
algorithm that supplies them according to set the following table:
BREATHS TYPES AVAILABLE
MODES VC VA PC PA PS PR SP
Volume Assist Control X1,2 X
Pressure Assist Control X1,2,3 X3
Volume SIMV X X X1 X
Pressure SIMV X X X1 X
APRV (BIPAP) X2 X
PSV X4
1. Rate of control breaths can be automatically adjusted according to minute ventilaion criteria and called such things as “Apnea Ventilatiov” ,
“Minimum Minute Ventilation” ect. Rate can also turned to
10 to pure assist mode of support.
2. When inspiratory times of these breaths are extended beyond expiratory time, the term “Inverse
Ratio Ventilation” is often used.
3. Inspiratory pressure can be automatically adjusted according to tidal volume or minute ventilation criteria on some machines (Pressure
regulated volume control.)
4. Inspiratory pressure can be automatically adjusted according to tidal volume or minute ventilation criteria on some machines
(Volume support, pressure augmentation, volume assured pressure support.)