In a second possibility, the ventilator stops
aerogating air when the patient’s muscles are still
contracting, so expiratory fl ow is halted by the
patient’s inspiratory activity extended after the
opening of the expiratory valve, with a typical
effect on expiratory peak fl ow, which appears cut,
delayed or ‘doubled’ (Fig.5). Another possible
consequence of early cycling to expiration is
double triggering. Persistent patient activity
after expiratory valve opening can again
activate the trigger; thus the ventilator aerogates
another breath immediately after the previous
one, without a physiological exhalation in
between.
In a third case, the ventilator ends its support
exactly when the patient’s muscles relax: in this
case, inspiratory fl ow decay becomes faster and
faster, directly switching into expiratory fl ow,
with immediate peak and then slow exponential
decay. 1
FIGURE 4
Examination of fl ow wave
1500
1000
500
0
-500
-1000
-1500
Examination of the fl ow wave can reveal the end of the patient’s inspiratory activity: (green
arrow). The rest of the inspiratory phase is than passive, because the patient’s inspiratory
muscles are already relaxed. The ventilator, however, continues to infl ate the lungs until the
expiratory valve opens (red arrow). The dotted lines indicate the expiratory delay 1
FIGURE 5
Early cycling to expiration
700
500
300
100
-100
-300
-500
When the ventilator ends the inspiration before the patient (that is, the expiratory valve opens
while the patient’s inspiratory muscles are still contracting), the fi rst eff ect on the fl ow shape is
visible on the expiratory peak. It is not as deep as expected (red arrow) and it can appear
doubled. The two dotted lines indicate the expiratory mismatch between the patient’s (1) and
the ventilator’s (2) start of expiration, namely early cycling 1
Bedside optimisation
Once the clinician has identifi ed the patient’s
activity and asynchronies observing the ventilator
waveforms, there are a few interventions that
can effectively solve the issue. Firstly, any source
of external disturbance has to be eliminated
(circuit leaks, secretions, circuit occlusions,
deconnections), because they can lead to
changes in the waveforms and thereby lead to
misinterpretation. Second, clinicians have to be
aware of the effects of a ventilator’s settings on
asynchrony development and act on them
appropriately to promote synchronisation. 1
Inspiratory trigger
The appropriateness of the inspiratory trigger
facilitates the breath initiation and decreases
the patient’s work of breathing. Flow trigger is
considered better than pressure trigger because it
is more sensitive to a patient’s effort and does not
require a negative pressure to be produced in the
circuit to trigger the ventilator; a little fl ow
entering the inspiratory valve is enough. This
leads to more comfortable triggering; however,
pressure triggers on modern ventilators have
been improved, and the difference between fl ow
and pressure triggers is often very fi ne. 1 As a
general rule, trigger sensitivity should be set at
the highest value (lowest fl ow threshold) able to
avoid autotriggers, in order to optimise the
comfort of the patient.
Pressure support level
Overassistance facilitates asynchronies as well as
muscle atrophy very high pressure support levels
must be avoided. An excessive pressure support
can worsen hyperinfl ation, leading to diffi cult
triggering (trigger delay and ineffective efforts)
and late cycling to expiration. 21 When such
asynchronies are detected on ventilator
waveforms, physicians should consider a decrease
of pressure support level.
There are three possible cases 1 : late cycling, early
cycling and optimal cycling.
In the fi rst, the machine aerogates air for
longer than required; in this case, the patient’s
inspiratory muscles will relax during the
ventilator’s inspiratory phase, causing a sudden
change from fast to slow decrease of inspiratory
fl ow as shown in Fig.4. This often leads to
hyperinfl ation, causing other asynchronies such
as ineffective efforts and delayed triggering in the
following breaths. 20 This phenomenon (called late
cycling) is typical of COPD patients and is
promoted by a high level of pressure support.
Sometimes, patients react to late cycling with
active exhalation attemps while the ventilator’s
infl ation still ongoing, causing a positive
defl ection on the pressure wave.
Ramp
The ramp represents the fl ow speed to reach the
inspiratory peak. As a general rule, for the same
sensitivity of the expiratory trigger, a faster ramp
makes cycling earlier, whereas a slower ramp
makes cycling later. Therefore, a fast ramp can
facilitate expiratory synchronisation, especially in
12
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