Identifying Patient-Ventilator Asynchrony Using
Ivan I Ramirez1, Daniel H Arellano2*
1PT, Division of Critical Care Medicine, Hospital Clinico Universidad de Chile, Santiago. Universidad San Sebastian, Santiago,
Chile and Universidad de los Andes, Santiago, Chile
2PT CRT MSc, Division of Critical Care Medicine, Hospital Clinico Universidad de Chile and Respiratory Care Laboratory,
Kinesiology Department, Santiago of Chile and Universidad San Sebastian.
Daniel H Arellano PT CRT Msc, Intensive Care Unit, Hospital Clinico Universidad de Chile, Santos Dumont 999,
Independencia, Santiago, Chile. Email:
Received: May 05, 2017; Accepted: June 07, 2017; Published: December 04, 2017
Daniel HA, Ivan IR (2017) Identifying Patient-Ventilator Asynchrony Using Waveform Analysis. Palliat Med Care 4(4): 1-6. DOI: 10.15226/2374-8362/4/4/00147
A significant percentage of mechanically ventilated patients in
Intensive Care Units (ICUs) show some type of patient-ventilator
asynchrony (PVA). The presence of PVA is associated with
complications that affect the clinical outcome and the goals for which
mechanical ventilation is used in critically ill patients. Currently,
mechanical ventilators are able to show different types of waveforms
that allow to identify the different types of PVA in a noninvasive and
reliable way. However, in order to perform an adequate interpretation
and management of the PVA the health care professionals must be
properly trained in the topic.
Mechanical ventilation is used in intensive care units (ICUs)
in order to decrease the work of breathing, maintain the adequate
gas exchange, and unloading the respiratory muscles [1-3].
However, these goals can be difficult to achieve if there is not an
adequate interaction between patient and ventilator, which is
known as patient-ventilator asynchrony (PVA)3. Epstein2 defines
PVA as “any condition where patient-ventilator interaction is not
Based on evidence, it is necessary to consider that a significant
percentage of patients will present some type of asynchrony
during mechanical ventilation (approximately 25%) . For that
reason, health care professionals should be aware, especially
when sedation levels are lowered, active humidification systems
are used and when parameters are selected, in order to ensure an
appropriate interaction and to prevent complications associated
with PVA [Table 1].
Currently, mechanical ventilators used in ICUs show different
types of waveforms, such as: pressure/time, flow/time and
volume/time. One of the main reason why current ventilators
show this visual information is to identify if there is an adequate
interaction between the patient and the ventilator5. Identifying
PVA using waveform analysis, is a non invasive and reliable
method, which has shown to have a good correlation with other
methods such as the identification of PVA by measurement of the
esophageal pressure .
Several authors point out that the interpretation of PVA using
waveform analysis, is a skill that every health care professional
who is in contact with ventilated patients, should develop in order
to prevent their appearance and their complications associated
[3,5-9] [Table 1]. However, the interpretation of PVA is a skill
that requires specific training. For example, in some cases health
care professionals may identify more than one type of PVA in the
ventilator graphics at the same time, which make it more difficult
to analyze. Therefore, the identification of PVA is not an easy
task. Ramírezet al  evaluated the ability of 366 professionals
that work in ICUs to identify PVA using waveform analysis. Their
results showed that only 21% of health care professionals were
able to recognize all types PVAs. In addition, they demonstrated
that neither experience nor profession proved to be a relevant
Complications associated to Patient-Ventilator Asynchrony (PVA)
- Increased work of breathing.
- Ineffective effort.
- Air trapping.
- Respiratory alkalosis and hyper inflation of the lungs (Auto- triggering).
- Dynamic Hyper inflation.
- Auto PEEP (also known as intrinsic PEEP (PEEPi)).
- Alters the outcome of Weaning.
- Increased levels of sedation.
- Confusion about the actual condition of the patient.
- Sleep disorders.
- Longer stay in Intensive Care Units and mechanical ventilation.
factor to identify asynchrony correctly using waveform analysis.
However, they found that health care professionals who have
specific training in mechanical ventilation increase their ability
to identify asynchrony using waveform analysis.
For this reason, the objective of this article is to review the
types of PVAs, their effects on the mechanically ventilated patient
and their analysis based on the mechanical ventilator waveforms.
The Asynchrony Index (AI) is defined as the number of
asynchrony events divided by the total respiratory rate (Number
of asynchronies / Number of total respiratory rate (Number of
asynchrony events + Number of cycles effectively delivered by the
mechanical ventilator) x 100 .
The AI has been used in different studies to evaluate the effect
and complications associated with PVAs. The value is expressed as
a percentage and values ≥ 10% are associated with complications
[4,12,13]. The complications associated to PVA are summarized
in Table 1.
Asynchronies Related To Trigger
Asynchronies related to trigger (the variable that start[s]
inspiration) are the most frequent and studied type of PVAs in
ventilated patients . Chao et al compared a group of patients
who presented PVA related to trigger vs. A group that did not
have this type of PVA . The results showed that only 16%
of the patients in the group with PVA related to trigger, had a
successful weaning process versus a 57% of success in the group
of patients who did not have PVA. It was also observed that in the
first group the average duration of the weaning process (in days)
was significantly higher compared to the second group, which
can be translated into a greater number of days in mechanical
ventilation and ICU.
Within the group of asynchronies related to trigger are:
ineffective efforts, auto-triggering, double triggering and reverse
Ineffective effort is defined as “patient efforts that are not
sensed by the ventilator” . In other words, the patient generates
an inspiratory effort, but the ventilator does not recognize it and
does not deliver a breath to the patient. The main characteristic
of ineffective efforts is to produce an airway pressure drop in
the pressure/time waveform, caused by the inspiratory effort of
the patient, which decreases the airway pressure and, a change
in the expiratory flow (which tends to return to zero due to the
inspiratory effort of the patient) without the delivery of a breath
from the ventilator [4,14][Figure 1]. This type of PVA is the most
frequent and occurs more frequently during the expiratory phase
(may also occur during the inspiratory phase), in all modes [4,15].
Thilleet al found that Ineffective triggering and doubletriggering
accounted for more than 98% of the total number
of asynchrony events (85% were ineffective triggering events,
and 13% were double-triggering events). Among ineffective
triggering events, 78% occurred during the expiratory period
Figure 1: White arrows show ineffective efforts in the pressure/time
waveform. Red arrows show ineffective efforts in the flow/time waveform
and 7% during the inspiratory phase. De Wit et al obtained
similar results, where ineffective efforts accounted for 88.3% of
total PVAs .
This type of PVA is common in patients with chronic
obstructive pulmonary disease (COPD), as they are patients
who must generate a great inspiratory effort to overcome the
Auto-PEEP, also known as intrinsic positive end-expiratory
pressure (PEEPi) [4,9]. On the other hand, these patients have
a short expiratory time which may cause air trapping, dynamic
hyperinflation and Auto-PEEP.
However, there are other causes of ineffective efforts; for
example, Inadequate programming of triggering sensitivity and
the level of pressure support [4,12,17]. Leung et al demonstrated
that as the level of pressure support increases the respiratory
drive decreases, resulting in breaths with a higher tidal volume,
longer inspiratory time and shorter expiratory time, which are
the characteristics of the breaths that precede ineffective efforts,
leading to air trapping, dynamic hyperinflation, Auto-PEEP and
ineffective efforts .
Auto-triggering is a type of PVA that can be caused by leaks
in the mechanical ventilator circuit, condensation in the circuit,
improper setting of sensitivity and cardiac oscillations [6,14,19].
Auto-triggering is defined as “a delivery of a breath that is neither
scheduled (based on the set respiratory frequency) nor initiated
by the patient” giving the impression of tachypnea or hyper
It is possible to identify auto-triggering by looking at the
pressure/time waveform. Auto-triggering shows in the pressure/
time waveform, a lack of airway pressure drop or variations in
the flow/time waveform at the begining of the inspiratory phase
[Figure 2]. Complications associated with this type of PVA
include dynamic hyper inflation and respiratory alkalosis, which
may lead to confusion in the clinical team. Due to this, when
auto-triggering is suspected, it is important to evaluate if there is
inspiratory effort in the pressure/time waveform, the breathing
pattern, the general condition, sedation levels, cuff pressure
levels, endotracheal tube position, presence of condensation
in the circuit, the programming of trigger sensitivity, cardiac
oscillations and the type of ventilator used [6,19-21].
Figure 2: Auto-triggering caused by leak in the circuit. Note that there is
no drop of the airway pressure in the pressure/time waveform (Upper
waveform) at the beginning of the inspiratory phase which means that
the breaths are not patient trigger
In a prospective randomized study Carteauxet al evaluated
the incidence of auto-triggering generated by invasive mechanical
ventilators, that included an algorithm to be used as non-invasive
ventilator, versus non-invasive ventilators . In a sample of
15 patients, they performed 3 consecutive measurements of
20 minutes each, where they ventilated patients first using an
invasive ventilator, with the option to be used as non-invasive
deactivated; second, with the option activated and then with a
non-invasive ventilator technique, using an oronasal interface
in all the cases. When a non-invasive ventilator was used, the
incidence of auto-triggering was significantly lower in the
invasive ventilator compared with the non-invasive option
activated, but this also showed a decrease in the incidence of
auto-triggering. Another interesting result of the study was that
27% of the 15 patients evaluated, presented an AI> 10% when
invasive ventilation with the option of the algorithm disabled
was used, 13% with the option activated and 0% when a noninvasive
ventilator was used. It is important to remember that
one of the main characteristics of non-invasive ventilation is that
they compensate leaks, so the appearance of auto-triggering will
not be a problem when this type of ventilator is used. Therefore,
in the case of invasive ventilation, the risk of auto-triggering will
always be present, even though the algorithm to be used as noninvasive
As it was mentioned, another cause of auto-triggering are
cardiac oscillations. Imanaka et al evaluated the effect of flow
triggering versus pressure triggering in 104 patients after
cardiac surgery, finding that when flow triggering was used, the
incidence of auto-triggering resulting from cardiac oscillations
was higher, which is associated with high ventricular filling
pressures, high cardiac output, and increased heart size . A
proper programming does not cause a great effort for the patient,
but neither auto-triggering.
Double triggering is a type of asynchrony that can occur in
modes controlled by pressure and volume . Thille et al found
that factors associated with a high incidence of double-triggering
were: a low pao2/FIO2 ratio, ACV (assist-control ventilation
mode), a shorter inspiratory time, a high maximal inspiratory
pressure, and a high level of PEEP [4,22]. Double trigger can
be defined as “two cycles separated by a very short expiratory
time, defined as less than one-half of the mean inspiratory time,
the first cycle being patient-triggered” [4,15,24]. This type of
PVA can occur when neural inspiratory time is longer than the
inspiratory time set on the ventilator2. This may trigger a second
breath by the ventilator if the patient inspiratory effort continues
after the ventilator cycle to expiratory phase and if these efforts
generated by the patient are able to overcome the trigger
threshold programmed by the clinician . Other studies agree
with the findings of Thilleet al., Takioka H et al also demonstrated
that there is a relationship between short-term inspiratory times
programmed in the ventilator and double triggering, which is also
related to a high ventilatory demand [4,23]. Double triggering can
cause great harm to the ventilated patient because if the patient
doesn’t have enough expiratory time, the tidal volume delivered
from the second breath will add to the tidal volume from the first
breath which may double the tidal volume delivered to the patient
causing over distension, volutrauma and barotrauma. [Figure 3]
Figure 3: Red arrow show double-triggering in the pressure/time waveform.
White arrow show double-triggering in the flow/time waveform
It is important to realize that the first cycle delivered can be
triggered by patient effort or the ventilator. Liao et al classified
the double triggering in DT-P (double triggering where the
patient triggers these first cycle), DT-A (double trigger where
the first cycle is triggered byauto-triggering) and DT-V (when
the first cycle is triggered by the ventilator according to the
programmed criteria) . The results in the same study showed
that the DT-P is associated with short inspiratory time and that
a correct programing of the inspiratory time decreases or even
eliminate the presence of double triggering. On the other hand,
the DT-V is related to the programming of respiratory rate, and
could be solved by decreasing the inspiratory time or changing to
a spontaneous mode. These techniques were successfully applied
in 14 patients.
Reverse triggering is a poorly recognized type of PVA.
Muriaset al defined Reverse trigger as “a type of PVA in which the
patient’s respiratory center is activated in response to a passive
insufflations of the lungs” . During reverse triggering, there is
a delay between the start of the machine-triggered breath and the
start of the patient’s inspiratory effort. As a result, the patient’s
effort usually persists when the inspiratory phase is completed,
which could generate a double triggering if the inspiratory effort
of the patient is able to overcome the threshold of programmed
The pressure/time waveform shows a breath that is initiated
by the ventilator (there is no airway pressure drop at the beginning
of the breath), and/or also an airway pressure drop during the
inspiratory phase and part of the expiratory phase produced by
the activation of the respiratory of the patient with the consequent
contraction of the inspiratory muscles. In addition, an amputation
or deformation of the peak expiratory flow evidenced in the flow
/ time waveform product of the inspiratory effort of the patient
is observed [Figure 4]. Akoumianakiet al indicated that this
type of PVA can induce a continuous plyometric contraction of
the diaphragm . This contraction is associated with cytokine
release and damage of muscle fibers. In addition, it can produce
and increase of the respiratory work, oxygen consumption,
confusion in the monitoring of plateau pressure and increased in
the plateau pressure levels in modes controlled by volume.
Figure 4: White arrows show reverse triggering in the pressure/time
This type of PVA occurs when the programmed flow in the
ventilator does not meet the patient´s flow demands, causing
an increase in the work of breathing. It occurs frequently in that
ventilated with volume-controlled modes, with acute respiratory
failure and high ventilatory demand. Flow asynchrony is less
frequent in patients ventilated in pressure-controlled modes
because in the flow is variable [27-29]. Kalletet al compared
the effect of pressure-controlled ventilation and volumecontrolled
ventilation on the work of breathing of patients with
acute lung injury and respiratory distress syndrome . They
found that respiratory work was significantly lower in pressure
control modes. Yang et al compared respiratory effort produced
in volume-assist/control ventilation versus pressure-assist/
control ventilation in patients with acute respiratory failure
by measuring P0.1 (pressure generated in the first 100 m.sec)
. The results obtained showed that P0.1 decreased in a 25%
when pressure-assist/control ventilation was used, in addition
to decreasing patient discomfort. Macintyre et al induced a flow
asynchrony, by decreasing the programmed flow 50% in 16
stable patients ventilated with volume-assist/control ventilation
and subsequently attempted to correct it in two ways .
(1) Reestablishing the flow that was initially programmed by
adding 25% more of the original value and (2) using a pressurelimited
mode. Measurements of respiratory effort were made by
calculating the pressure-time product and the breathing pattern.
Both strategies demonstrated a significant decrease in flow
asynchrony: however, the strategy that involved switching to a
pressure controlled mode was more effective in those patients
who presented a severe degree of asynchrony. It is worth noting
that in most studies, work of breathing and discomfort caused
by inadequate flow decreases with pressure-controlled modes
in patients with a high ventilatory demand, because they modify
their effort and need for flow in each respiratory breath . For
this reason, programming a fixed flow level (such as occurs in
volume-controlled modes) would produce PVA not providing and
As Branson mentioned “the scalloped-out portion of the
pressure/time waveform during a patient-triggered volume
breath, is a well-recognized sign of flow asynchrony. Nilsestuen
and Hargett also mentioned how to identify flow asynchrony .
They explained that “as patient effort increases, the peak flow set
on the ventilator no longer meets the patient’s flow demand and
the airway pressure waveform becomes progressively dished out.
This phenomenon is informally recognized as “flow starved” 
Figure 5: White arrows show flow asynchrony in the pressure/time
The cycle variable can be defined as “the variable (usually
pressure, volume, flow, or time) that is measured and used to end
inspiration (and begins expiratory flow) . Premature cycling
as well as double triggering is a type of asynchrony that occurs
when the patient’s neural inspiratory time is greater than the
inspiratory time programmed in the ventilator . The difference,
with the double triggering, is that in premature cycling the
inspiratory effort of the patient is not enough to trigger a second
breath. Premature cycling produces a significant decrease in
airway pressure, which can be seen immediately after the end of
the inspiratory phase programmed in the ventilator, accompanied
by an increase of the inspiratory flow which can be seen in the
flow/time waveform [Figure 6]. This type of PVA could be
confused with an ineffective effort during the expiratory phase,
with the difference that premature cycling responds to changes
in programmed inspiratory time or cycling; where as ineffective
efforts responds to changes in the level of PEEP, sensitivity or
assistance levels . Another difference is that in premature
cycling the drop of the airway pressure occurs immediately after
the inspiratory phase has ended, indicating that the patient’s
inspiratory effort continues. Takioka et al evaluated the effect
of cycling (expiratory sensitivity) at 45%, 35%, 20%, 5% and
1% of the peak inspiratory flow in 8 ventilated patients under
supportive pressure mode . They found that there was an
increased respiratory work and asynchrony (premature cycling
and double triggering) when the ventilator cycled at 45 and 35%
of maximal inspiratory flow (0.31±0.12 J / L versus 0.51 ± 0.11 J /
L with cycling criterion of 1% and 45%, respectively). The greater
percentage of cycling criteria, the shorter the inspiratory time
.Therefore, when patient is ventilated with pressure support
ventilation, is very important consider this aspect in the setting
to ensure a proper ventilation
Figure 6: Example of premature cycling. White arrows show an inspiratory
effort that continues after the inspiratory phase ended in the pressure/
time waveform. Red arrows show a sudden change in the expiratory
flow caused by the inspiratory effort of the patient
TDelayed cycling occurs when the inspiratory time
programmed in the mechanical ventilator exceeds the patient
neural inspiratory time . This means that the system continues
in the inspiratory phase, once the patient’s inspiratory effort
has ended, which decreases the time available for the expiratory
phase. This may produce an activation of the patient’s expiratory
muscles before the established cycling criteria (active exhalation),
air trapping, dynamic hyperinflation and PEEPi, which may
increase the work of breathing and cause ineffective efforts [32-
34]. Parthasarathy et al mentioned that “a delay in relaxation
of the expiratory muscles could cause them to remain active
during the early phase of the next inspiration, and by opposing
the downward motion of the diaphragm could hinder the efficacy
of the subsequent inspiratory effort.”
This type of asynchrony is common in COPD patients
because of PEEPi and a short expiratory time. In these cases,
an effective solution would be decreasing the inspiratory time
in controlled modes such as pressure assist/control ventilation
and Synchronized Intermittent Mandatory Ventilation (SIMV)
[33,34]. The inspiratory time can be decrease, also, by modifying
the cycling criteria in pressure support ventilation . So,
by modifying the cycling criteria ineffective efforts and active
expiration caused by delayed cycling can be prevent [35,36].
Delayed cycling is evidenced in the pressure/time waveform
as an increase in airway pressure near the end of the inspiratory
phase and in the flow/time waveform by a sudden decrease in
inspiratory flow, resulting from activation of the expiratory
muscles [Figure 7
Figure 7: Example of Delayed cycling. White circles show, in the pressure/
time waveform, an increase in airway pressure near the end of the
inspiratory phase caused by the contraction expiratory muscles. White
arrows show, in the flow/time waveform, a rapid decrease in inspiratory
flow resulting from activation of the expiratory
Identifying patient-ventilator using waveform analysis is a very
useful and important skill that every health care professional that
work in the ICU should develop in order to prevent complications
that may affect the outcome of the mechanically ventilated
patient. It is also important to establish standard definitions for
all types of PVAs, considering that different definitions are used
to describe the same problem.
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