Patient ventilator interactions

Patient-ventilator Interactions
Mostafa Elshazly
Pulmonary Critical Care Unit (PCCU)
Kasr Alainy School Of Medicine
Cairo University

Objectives Of MV
• Safety
• Efficacy
• Oxygenation
• Ventilation
• Work of Breathing
• Comfort /Synchrony
• Surveillance of Flow and Pressure

• In a spontaneously breathing subject, the pressure
generated by the respiratory muscles (Pmus) during
inspiration is dissipated to overcome both the elastic
and resistive forces opposing respiratory system
inflation, as described by the equation of motion of
the respiratory system.

Ers is respiratory system elastance,
Rrs is respiratory system resistance,
V′ is inspiratory flow, and
V is volume of the respiratory system above FRC.

• During the controlled ventilation of a passive patient, Pmus
= 0, and the necessary pressure is applied by the ventilator
(Paw).
• Therefore, equation 1 becomes:

• During the controlled ventilation of a passive patient,
Pmus = 0, and the necessary pressure is applied by
the ventilator (Paw).
• Therefore, equation 1 becomes:

➢ In the case of assisted ventilatory modes, both the
ventilator and the patient provide the required
pressure, equation 1 becoming:

In the case of assisted ventilatory modes, both
the ventilator and the patient provide the
required pressure, equation 1 becoming:

where Pappl is total pressure applied to the respiratory
system.
➢From equation 3, one can easily see that, to maintain a
constant Pappl, a change in any one of its determinants
(Pmus or P aw) must be met by an opposite change in
the other.

• Mechanical ventilator support can be controlled
entirely by the ventilator, as in the CMV of a passive
patient, or can interact with patient’s respiratory
muscle efforts, as in assisted or supported ventilation
of an actively breathing patient

• CMV provides the benefit of a guaranteed minute
ventilation with a predetermined ventilatory pattern
but often at the cost of heavy sedation or even
neuromuscular blockade to silence dys-synchronous
ventilatory muscle activity.

• Assisted or supported ventilation, if synchronous with
the patient’s ventilatory muscle efforts, shares the
work of breathing, facilitates muscle recovery from
respiratory fatigue or failure, and avoids excessive
sedation.

For this ideal shared relationship to occur, synchrony must
exist between the flow and pressure delivery of the
ventilator and the patient’s effort during all 3 phases of
breath delivery: initiation, flow delivery, and termination.

• Synchronous support means that the ventilator’s
timing and pressure–flow delivery respond promptly
to patient effort, provide pressure and flow that avoid
excessive muscle loading, and terminate when patient
effort ends.

• Dyssynchronous interactions can overload ventilatory
muscles (“imposed” loads), compromise alveolar
ventilation, over distend alveolar units, disrupt sleep
patterns, and cause patient discomfort prompting
additional sedation.

• Importantly, Dyssynchronies can result from
either inappropriate patient ventilatory drive or
suboptimal ventilator settings (or both)

Breath Triggering
• Assisted or supported breaths are initiated or
triggered by patient effort.
• Patient effort is sensed by either a drop in circuit
pressure (pressure trigger) or circuit bias flow (flow
trigger) initiating breath delivery.

Breath Triggering
• Triggers must be sensitive enough to recognize patient
effort so as to not impose an additional load but not
too sensitive to predispose to auto triggering.
• Importantly, a triggering delay from onset of patient
effort to delivery of breath is often unavoidable due to
inherent valve system sensitivity or responsiveness

Breath Triggering
• Triggers must be sensitive enough to recognize patient
effort so as to not impose an additional load but not
too sensitive to predispose to auto triggering.
• Importantly, a triggering delay from onset of patient
effort to delivery of breath is often unavoidable due
to inherent valve system sensitivity or responsiveness

Breath Triggering
Point a represents onset of patient
effort.
Point b represents recognition of this
effort by the ventilator.
Point c marks the beginning of flow
delivery.
Point d marks the attainment of target
flow.

Breath Triggering
Point a represents onset of patient
effort.
Point b represents recognition of this
effort by the ventilator.
Point c marks the beginning of flow
delivery.
Point d marks the attainment of target
flow.
The pressure decline from a to b
represents trigger sensitivity, whereas
the duration from point b to point d is
considered the responsiveness of the
system.

Breath Triggering
• Trigger dyssynchrony is of two types
• Delayed/missed triggers
• Extra triggering

Breath Triggering
➢Delayed/missed triggers
➢One cause for this is an insensitive or poorly responsive
triggering system.
➢ There are also mechanical triggering delays due to the
inherent responsiveness characteristics of a ventilator’s
valving systems.

Breath Triggering
➢Intrinsic PEEP (PEEPi)
➢This occurs because the patient’s ventilatory muscles must first
overcome the PEEPi in the alveoli before any circuit pressure or
flow change can occur to trigger a breath

Breath Triggering
➢At the bedside, triggering missed or delayed triggers
clinically appear as a patient effort with chest wall rise
and/or abdominal motion followed by absent or delayed
breath delivery.
➢This is best appreciated by placing a hand on the patient’s
chest and observing the ventilator’s response to effort.

Breath Triggering
➢On the airway P-T graphic, there may be marked
airway pressure deflections present before
breath triggering.
➢On the expiratory F-T graphic, there may be
evidence of transient flow reversal during missed
trigger efforts.

Breath Triggering
➢Patient effort and delayed/absent ventilator triggering
can be better appreciated if a diaphragmatic EMG or an
esophageal pressure (a surrogate for pleural pressure) is
available as these techniques directly assess the timing
of ventilatory muscle contraction.

How To Correct IIE:
• Decrease PEEPi: Lower tidal volume
or decrease the level of pressure
support, particularly in COPD
• Lengthen the expiratory time
• Decrease resistance of the airways
(i.e., bronchodilators)
• Decrease sedation levels
• Increase external PEEP
• Decrease trigger threshold (increase
trigger sensitivity). Te trigger should be as
sensitive as possible without causing
auto-triggering .
• Assess respiratory drive
• Use larger endotracheal tube (generating
greater pressure to overcome increased
airway resistance from smaller tubes
requires time and greater eﬀort)

Breath Triggering
• Trigger dyssynchrony is of two types
• Delayed/missed triggers
•Extra triggering

Breath Triggering
• Auto cycling
• Entrainment
• Double triggering

Breath Triggering
• Auto cycling
• These are mechanical breaths delivered by the
ventilator in the absence of the patient’s triggered
inspiratory eﬀort.
• Auto-triggering causes discomfort and can increase
PEEPi

Breath Triggering
• Auto cycling
• These extra breaths can result in significant
apparent “tachypnea” and hyperventilation.
• As a consequence, some insensitivity in the
triggering system often must be tolerated

Breath Triggering
• Auto cycling
• Auto triggering occurs when even small circuit leaks,
tube condensation, and/or cardiac oscillations may
trigger breaths and produce undesired hyperventilation
and/or breath stacking with PEEPi.

Auto Cycling
• Inspiration is not preceded by a pressure drop below
PEEP
• Variation in peak inspiratory flow in the absence of
dynamic hyperinflation
• Breath by breath variability in tidal volume
• Variation in shape of flow waveform (Flow– time)

Breath Triggering
• Auto cycling

Breath Triggering
• Double triggering is defined as two consecutive
inspiratory cycles not separated by an expiration,
occurring within a timeframe of less than half of the
mean inspiratory time.

Breath Triggering
• Causes of double triggering include high ventilatory
demand, short inspiratory time, insufficient level of
pressure support, inadequate inspiratory volume or
flow, sighs, and coughing.

Breath Triggering
• Another mechanism for extra triggering is in the setting
of persistent effort after the machine breath has
terminated (neural TI > machine TI) .
• Under these circumstances, the second breath is tightly
linked to the original breath and results in an increase in
the measured ventilator rate.

Breath Triggering
• In assist/control volume ventilation, double triggering
often results from a short preset inspiratory time, or
inadequate flow or volume settings.

Breath Triggering
• With pressure-control ventilation, double triggering
may be caused by a short inspiratory time or low
inspiratory pressure settings.

Breath Triggering
• In pressure-support ventilation, double triggering may
be due to an insufficient level of pressure support or a
high expiratory sensitivity.

Flow Delivery
• Once a breath is patient effort triggered,
diaphragmatic contraction continues to occur .
• If flow is synchronous with that contraction pattern,
the inspiratory muscle pressure–volume profile
conceptually should resemble a near normal pattern .

Flow Delivery
• Flow asynchrony may be due to ventilator flow being
either too fast or too slow for the patient, and may
occur with either flow-targeted breaths or with
pressure-targeted breaths.

Flow Delivery
• In flow-targeted breaths
the clinician typically
chooses the speed of
the flow and the pattern
of the flow.

Flow Delivery
• In a pressure-targeted breath
the speed at which the
targeted pressure is reached
is dependent on the rise time,
with faster rise times
resulting in higher flows and
shorter duration to achieve
the pressure set by the
clinician.

Flow Delivery
• Because volume-control breaths have preset peak
flow and flow pattern (ie, square, decelerating,
accelerating, or sinusoidal), the problem with flow
asynchrony may be particularly common in volume-
control breaths.
• In pressure targeted breaths, flow is variable and thus
more responsive to patient needs.

Flow Delivery
• When patient respiratory drive is elevated, the
pressure-time waveform may reveal a dip during
assisted inspiration.
• This dip occurs when the ventilator flow is below the
patient’s desired flow, and the patient “pulls down”
the pressure-time waveform during assisted
inspiration.

Flow Delivery
• When patient
respiratory drive is
elevated, the
pressure-time
waveform may
reveal a dip during
assisted inspiration.

Flow Delivery
• Flow dyssynchrony from inadequate flow delivery can
be appreciated clinically by observing inspiratory
efforts that appear “flow starved” (vigorous
inspiratory efforts unrewarded by adequate flow) and
accompanied by marked patient discomfort.

Flow Delivery
• Examining the airway pressure–time profile can be
useful in assessing flow dyssynchrony .

Flow Delivery
The dotted airway pressure line
represents that observed during a
control breath with a similar tidal
volume as the assisted breath.
Left: The assisted breath airway
pressure profile remains smoothly
positive and tracks with the control
breath airway pressure profile,
suggesting that the inspiratory muscle
loading is likely not excessive.

Flow Delivery
Middle: The assisted breath airway
pressure profile is uneven and
appears to be markedly “sucked
down” by patient effort during much
of the breath.
This might suggest that the flow
delivery is inadequate for patient
demand to the point that inspiratory
muscle overload may be present.

Flow Delivery
Right: The assisted breath airway
pressure profile goes below the
baseline (expiratory) pressure.
Flow is thus inadequate to provide
any inspiratory muscle unloading.

Flow Delivery
• Flow dyssynchrony from inadequate flow delivery is
more common during acute respiratory failure when
inspiratory flow demands are high, vary from breath
to breath, and ventilator flow delivery is set
inappropriately low.

Flow Delivery
• When ventilator flow is faster than the patient’s
respiratory drive, the pressure-time waveform reveals
a peaking of airway pressure at the beginning of
inspiration, in cases where the flow pattern is
decelerating

Flow Delivery
• When ventilator flow is
faster than the patient’s
respiratory drive, the
pressure-time waveform
reveals a peaking of airway
pressure at the beginning of
inspiration, in cases where
the flow pattern is
decelerating

Flow Delivery
• This problem may be corrected by decreasing the peak flow
in the volume-control breaths.
• This problem may also occur in a pressure-support breath if
the rise time is set too high.
• An alarm signaling an elevated airway pressure may sound as
well.
• The patient may not want such rapid flow and it may result
in active exhalation or coughing

• Mechanical ventilators cycle or terminate delivered flow to
end inspiration based on multiple criteria, including
attainment of a set Ti (pressure assist breath), delivery of set
Vt (volume assist breath) or decline in inspiratory flow to a
set threshold (pressure support breath).
• The end of mechanical Ti must coincide with the end of the
patient’s neural Ti or a cycling dys-synchronies occur.
Breath Cycling

Breath Cycling
• Cycling dyssynchrony occurs when the neural TI
and the machine TI are mismatched .

• If the ventilator breath is longer than the patient’s
neural inspiratory time, the patient may actually fight
the ventilator, recruiting expiratory muscles in an
attempt to force expiration
Breath Cycling

Breath Cycling
When patients arrive at end-
inspiration, they expect the
airway to be free of resistance
and ready to allow expiration.
However, if the ventilator’s
inspiratory time (TIV) is longer
than the patient’s inspiratory
time (TIP), the airway is still
being pressurized.

Breath Cycling
If the delay in the ending of TIV is
long enough (or is systematic and
the patient can anticipate it), the
patient activates expiratory
muscles, increasing esophageal
pressure (PESO) (down) and the
pressure of the peripheral
compartment (up; gray).

Breath Cycling
• Machine TI is greater than
neural TI.
• As a consequence, the
lung inflation extends into
neural exhalation and the
patient may activate
expiratory muscles to
“turn the breath off.”
• This results in an elevation
in airway pressure at the
end of the inhalation.

• Mechanical TI less than neural TI can leave the patient
uncomfortable (air hungry) as inspiratory muscles continue
to contract into mechanical expiratory time (TE) against the
sudden elastic recoil of the chest wall
Breath Cycling

Breath Cycling
• Machine inspiratory
time (TI) is less than
neural TI.
• As a consequence the
persistent patient effort
“pulls” the airway
pressure profile
downward and reverses
expiratory flow after
breath termination.

Breath Cycling
• Machine inspiratory
time (TI) is less than
neural TI.
• As a consequence the
persistent patient effort
“pulls” the airway
pressure profile
downward and reverses
expiratory flow after
breath termination.
• This persistent effort
may trigger a second
breath.

• Cycling dyssynchrony occurs when the neural TI and
the machine TI are mismatched .
• Importantly, the mismatch may be because of an
abnormal ventilatory drive or because the cycle
criteria are set either too short or too long for an
appropriate ventilatory drive.
Breath Cycling

Strategies to Improve Patient–
ventilatory Interactions
• The challenge with ventilator management in actively
breathing patients is to match ventilatory support
with patient effort so as to ensure safe and effective
support without imposing inappropriate loads.

• Although there are many ventilatory adjustments that
can be made to accomplish this, as will b discussed,
attention must first be paid to the appropriateness of
the patient’s ventilatory drive

• If the ventilatory drive is depressed from disease or
drugs, simply supplying an appropriate backup control
breath rate and VT is all that is needed

• However, if the ventilatory drive is inappropriately excessive,
interactive support settings can become quite challenging .
• Under these circumstances, a search for reversible causes (e.g., pain,
anxiety, acidosis, hypoxemia, tube obstructions, mucus plugging, and
dyssynchronous settings) should be done initially and corrected if
possible, recognizing that achieving synchrony may ultimately require
sedation usage.

• However, if the ventilatory drive is inappropriately excessive, interactive
support settings can become quite challenging .
• Under these circumstances, a search for reversible causes
(e.g., pain, anxiety, acidosis, hypoxemia, tube obstructions,
mucus plugging, and dyssynchronous settings) should be
done initially and corrected if possible, recognizing that
achieving synchrony may ultimately require sedation usage.

• Achieving the most synchronous settings requires
careful assessments and often is a “trial and error”
exercise.
• Ultimately, the proper delivery of assisted/supported
breaths must focus on all three phases of interactive
breath delivery.

• Choose the trigger sensor (flow vs. pressure) that is most
sensitive and responsive to patient effort
• Adjust the sensitivity of the triggering system to be as
sensitive as possible without producing auto triggering.
Optimizing Breath Triggering

• In the setting of PEEPi trigger dyssynchrony, there are several
clinical strategies.

• minute ventilation ( RR , PI, VT, reduce ventilation
needs driving patient efforts), lengthening the TE, or
improving airway mechanics
PEEPi trigger dyssynchrony

• judicious amounts of applied circuit PEEP,

• judicious amounts of applied circuit PEEP,
• Ironically, the ventilator breathing frequency may actually increase (as
will minute ventilation) because more efforts that were previously
missed are now being triggered.
• This may require subsequent adjustments to avoid excessive
ventilation

• Ventilator autotriggering can be managed with a careful
search for reversible causes (e.g., water in the circuit, small
leaks) and/or adjustments to the trigger sensitivity settings
Autotrigger dyssynchrony

Optimizing Flow Delivery

• Ventilator setting adjustments for achieving flow synchrony
depend on whether flow-targeted volume-cycled breaths or
pressure targeted breaths are being used.

• Direct control over the flow magnitude, flow delivery pattern,
TI, and the ultimate volume delivered.
• Fixed flow delivery pattern cannot interact with the patient’s
ventilatory drive and thus achieving flow synchrony can be a
challenge.
Flow-targeted volume-cycled breaths

• It may provide synchrony advantage over flow-targeted breaths.
• This is because pressure targeting allows the ventilator to deliver
whatever flow is needed to attain the set pressure target. Flow thus
varies with patient effort, and this feature has been shown in many
clinical studies to thereby enhance flow synchrony
Pressure -targeted breaths

• Pressure rise time adjustment allows manipulation of the initial flow
delivery, thereby increasing or decreasing the rate of rise of PI .
• Vigorous efforts might synchronize better with a rapid pressurization
pattern; less vigorous efforts might synchronize with a slower
pressurization pattern.
Pressure -targeted breaths

• Achieving breath cycling synchrony involves delivery of an
appropriate VT in accordance with patient demands and
matching of neural and machine TI.
• With flow–volume targeting, adjusting the VT and machine TI
is relatively straightforward as these are set independent
variables that produce the machine TI
Optimizing Breath Cycling

• In conclusion, the patient-ventilator interaction should not
impose excessive WOB on the patient.
• The aim of patient-ventilator synchrony is to achieve
synchrony between patient and ventilator during all phases of
respiration, including breath initiation, delivery, termination,
and exhalation.

• Ineffective triggering is the most common asynchrony in
patients undergoing invasive mechanical ventilation.
• Flow asynchrony and cycling asynchrony can also be detected
by simultaneous examination of patient breathing (ie, facial
expressions, mouth breathing, accessory muscle use, and
active exhalation with contraction of abdominal muscles) and
waveforms displayed on the ventilator.

Patient ventilator interactions

Patient ventilator interactions

Recomendados

Recomendados

Mais conteúdo relacionado

Mais procurados

Mais procurados (20)

Semelhante a Patient ventilator interactions

Semelhante a Patient ventilator interactions (20)

Mais de Mostafa Elshazly

Mais de Mostafa Elshazly (6)

Último

Último (20)

Patient ventilator interactions