FIGURE 1
Patient’s inspiration: pressure wave
35
30
25
20
15
10
5
Negative defl ection in pressure wave due to patient’s inspiratory eff ort (red arrow). The two
dotted lines indicate the beginning of the patient’s inspiratory eff ort (1) and the delayed
ventilator support (2) – inspiratory delay 1
FIGURE 2
Patient’s inspiration: fl ow wave
1500
1000
500
0
-500
-1000
-1500
The blue arrow shows the change in fl ow slope due to a patient’s inspiratory eff ort. The initial
part of the expiration phase is characterised by a certain slope, representing the passive
defl ation of the lungs. When any inspiratory muscle activity starts, it produces a change in fl ow
shape, making the slope steeper towards zero fl ow. Note that the ventilator is able to detect
patient’s activity only if it reaches the set inspiratory trigger level (always at positive values of
fl ow). Thus, delayed triggering will always be present if patient’s inspiration starts at negative
values of fl ow (for example in presence of hyperinfl ation). The two dotted lines indicate the
inspiratory delay 1 .
technologies able to optimise the issue (mainly
diaphragmatic electrical activity, other automatic
triggering systems available on a few modern ICU
ventilators). However, these tools are not always
available for every ICU patient. The second
approach is based on the observation of ventilator
waveforms, the direct recognition of asynchronies
and the optimisation of the ventilator settings.
Obviously this method is more applicable in the
ICU because it does not require special
technologies; however, a good knowledge of the
most frequent patterns, and of the underlying
pathophysiology, is essential to make it effi cient.
The waveform method is based on the
observation of the standard curves displayed on
the ventilator screen (fl ow and airway pressure),
because they are as sensitive and specifi c as
oesophageal pressure, which thus far is
considered the gold standard for detection of
asynchronies. The method is centred on the
identifi cation of the patient’s spontaneous
activity.
Patient’s inspiration
Typically, when a patient starts a breath, this
causes a negative defl ection on the pressure curve
(Fig.1); on the fl ow curve, a positive defl ection can
be detected, even if fl ow is still negative (Fig.2).
Flow and pressure changes correlate with
oesophageal pressure, thus they are suffi cient
to detect a patient’s respiratory activity in most
cases. 6,7,13
With these simple rules, a patient’s inspiratory
activity can be detected even when it is neither
detected nor assisted by the ventilator: in other
words, ventilator waveforms can reveal a patient’s
attempt to trigger the ventilator that does not
reach its aim, namely an ineffective effort (Fig.3).
The patient’s start of expiration can also be
detected on fl ow and pressure waves;
physiologically, it corresponds to a time point
between the nadir of muscular pressure curve and
its return to the baseline. This time point varies
from patient to patient depending on respiratory
mechanics and breathing pattern, but can be
conveniently approximated at half relaxation. 8
If a muscular pressure curve is not available,
indirect signs of relaxation can be detected on
fl ow wave and their appearance varies depending
on the assistance given from the ventilator.
FIGURE 3
1250
Delayed cycling to expiration
1000
750
35
30
500
25
250
0
20
-250
-500
15
-750
-1000
10
(A) Change in fl ow shape similar to the one previously observed in case of delayed triggering; here no ventilator assistance follows, that is in case of ineff ective eff ort.
(B) Wasted eff ort causes a typical depression in the expiratory phase on the pressure curve (red circle) 1
11
HHE 2019 | hospitalhealthcare.com