Download - Conv. ventilation physi
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Physiology of positive pressure ventilation
SAMIR EL ANSARY
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Global Critical Carehttps://www.facebook.com/groups/1451610115129555/#!/groups/145161011512
9555/ Wellcome in our new group ..... Dr.SAMIR EL ANSARY
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Mechanical ventilation –
Supports / replaces the normal ventilatorypump moving air in & out of the lungs.
Primary indications –
a.apnea
b.Ac. ventilation failure
c. Impending ventilation failure
d.Severe oxygenation failure
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Goals
Manipulate gas exchange
↑ lung vol – FRC, end insp / exp lung inflation
Manipulate work of breathing (WOB)
Minimize CVS effects
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ARTIFICIAL VENTILATION
- Creates a transairway P gradient by ↓ alveolar P to a level below airway opening P- Creates – P around thorax
e.g. iron lungchest cuirass / shell
- Achieved by applying + P at airway opening producing a transairwayP gradient
Negative pressure ventilation Positive pressure
ventilation
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ventilation without artificial airway-Nasal , face mask
adv.1.Avoid intubation / c/c2.Preserve natural airway defences3.Comfort4.Speech/ swallowing + 5.Less sedation needed6.Intermittent use
Noninvasive
Disadv1.Cooperation2.Mask discomfort3.Air leaks4.Facial ulcers, eye irritation, dry nose5.Aerophagia6.Limited P supporte.g. BiPAP, CPAP
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Ventilatory support
FULL PARTIAL
All energy provided by ventilator
e.g. ACV / full support SIMV ( RR
= 12-26 & TV = 8-10 ml/kg)
Pt provides a portion of energy
needed for effective ventilation
e.g. SIMV (RR < 10)
Used for weaning
WOB total = WOB ventilator (forces gas into lungs)+ WOB patient (msls draw gas into lungs)
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Understanding physiology of PPV
1) Different P gradients
2) Time constant
3) Airway P ( peak, plateau, mean )
4) PEEP and Auto PEEP
5) Types of waveforms
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Pressure gradients
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Distending pressure of lungs
Elastance load
Resistance load
Distending
pressure
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Flow through the airways is generated by
Transairway pressure (pressure at the airway opening minus pressure in the lungs).
Expansion of the elastic chamber is generated by Transthoracic pressure (pressure in the lungs
minus pressure on the body surface).
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Transrespiratory pressure (pressure at the airway opening minus pressure on the body surface) is the sum of these two pressures and is the total pressure
required to generate inspiration.
Transrespiratory pressure can have two components, one secondary to the ventilator (pvent) and one
secondary to the respiratory muscles (Pmusc)
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Trans pulmonary pressure (pressure at airway opening minus pleural pressure) [= Transrespiratory pressure?]Transpulmonary pressure is the distending force of the
lungThe airway-pressure gauge on a positive-pressure
ventilator displays transrespiratory pressure
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Pressure, volume, and flow are functions of time and are called variables. They are all measured
relative to their values at end expiration.
Elastance and resistance are assumed to remain constant and are called parameters.
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Elastance(measure of stiffness) is the inverse of compliance(measure
of stretchiness)
An increase in elastance implies that the system is becoming stiffer.
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Mean airway pressure Paw = Transrespiratorypressure
Mean alveolar pressure Palv = Transthoracicpressure
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Transpulmonary pressure is the distending pressure in a spontaneously(negative)
breathing patient Transrespiratory pressure is the distending
pressure in positive pressure ventilation
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Airway pressures
Peak insp P (PIP)
• Highest P produced during insp.
• PRESISTANCE + P INFLATE ALVEOLI
• Dynamic compliance
• Barotrauma
Plateau P
• Observed during end insp
pause
•P INFLATE ALVEOLI
•Static compliance
•Effect of flow resistance
negated
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Time constant• Defined for variables that undergo exponential
decay• Time for passive inflation / deflation of lung / unit
t = compliance X resistance= VT .
peak exp flow
Normal lung C = 0.1 L/cm H2OR = 1cm H2O/L/s
COAD – resistance to exp increases → time constant increases → exp time to be increased lest incomplete exp ( auto PEEP generates).ARDS - inhomogenous time constants
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Why and how to separate dynamic & static components ?
• Why – to find cause for altered airway pressures
• How – adding end insp pause
- no airflow, lung expanded, no expiration
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How -End inspiratory hold
• Pendelluft phenomenon• Visco-elastic properties of lung
End-inspiratory pause
Ppeak < 50 cm H2OPplat < 30 cm H2O
Ppeak = Pplat + Paw
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At the start of inflation, the airway pressure immediately rises because of the resistance to gas flow
(A), and at the end of inspiratory gas flow the airway pressure immediately falls by the same pressure (A) to
an inflexion point. Thereafter, the airway pressure more gradually declines
to the plateau pressure. The loss of airway pressure after the inflexion (B) is due to gas redistribution (Pendelluft) and the visco-
plasto-elastic lung and thorax behaviour
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P2(Pplat) is the static pressure of the respiratory system, which in the absence of flow equals the
alveolar pressure, which reflects the elastic retraction of the entire respiratory system.
The pressure drop from PIP to P1 represents the pressure required to move the inspiratory flow along
the airways without alveolar interference, thus representing the pressure dissipated by the flow-
dependent resistances(airway resistance).
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The slow post-occlusion decay from P1 to P2 depends on the viscoelastic properties of the system and on the
pendulum-like movement of the air (pendelluft).
During the post-inspiratory occlusion period there is a dynamic elastic rearrangement of lung volume, which
allows the different pressures in alveoli at different time constants to equalize, and depends on the
inhomogeneity of the lung parenchyma.
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The lung regions that have a low time constant (ie, rapid zones), where the alveolar pressure rises rapidly, are emptied in the lung regions that have higher time constants (ie, slow zones), where the pressure rises more slowly because of higher resistance or lower
compliance
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The static compliance of the respiratory
system mirrors the elastic features of the respiratory system, whereas
The dynamic compliance also includes the
resistive (flow-dependent) component of the airways and the endotracheal tube
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When the inspiratory pause is shorter than 2 seconds, P2 does not always reflect the alveolar pressure.
The compliance value thus measured is called quasi-static compliance.
In healthy subjects the difference between static compliance and quasi-static compliance is minimal,
whereas it is markedly higher in patients who have acute respiratory distress syndrome or chronic
obstructive pulmonary disease
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Ppeak < 50 cm H2O; Pplat < 35 cm H2O – to avoid
barotrauma
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• Pendulum like movement of air between lung units
• Reflects inhomogeneity of lung units
• More in ARDS and COPD
• Can lead to falsely measured high Pplat if the end-inspiratory occlusion duration is not long enough
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Why
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Mean airway P (MAP)
• average P across total cycle time (TCT)
• MAP = 0.5(PIP-PEEP)X Ti/TCT + PEEP
• Decreases as spontaneous breaths increase
• MAPSIMV < MAP ACV
• Hemodynamic consequences
Factors
1. Mandatory breath modes
2. ↑insp time , ↓ exp time
3. ↑ PEEP
4. ↑ Resistance, ↓compliance
5. Insp flow pattern
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PEEP
BENEFITS
1. Restore FRC/ Alveolar recruitment
2. ↓ shunt fraction
3. ↑Lung compliance
4. ↓WOB
5. ↑PaO2 for given FiO2
DETRIMENTAL EFFECTS
1. Barotrauma
2. ↓ VR/ CO
3. ↑ WOB (if overdistention)
4. ↑ PVR
5. ↑ MAP
6. ↓ Renal / portal bld flow
PEEP prevents complete collapse of the alveoli and keep them
partially inflated and thus provide protection against the development
of shear forces during mechanical inflation
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How much PEEP to apply?
Lower inflection point – transition from flat to steep part- ↑compliance
- recruitment begins (pt. above closing vol)Upper inflection point – transition from steep to flat part
- ↓compliance- over distension
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Set PEEP above LIP – Prevent end expiratory airway collapse
Set TV so that total P < UIP – prevent overdistention
Limitation – lung is inhomogenous
- LIP / UIP differ for different lung units
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Auto-PEEP or Intrinsic PEEP
• What is Auto-PEEP?
– Normally, at end expiration, the lung volume is equal to the FRC
– When PEEPi occurs, the lung volume at end expiration is greater then the FRC
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Auto-PEEP or Intrinsic PEEP
• Why does hyperinflation occur?
– Airflow limitation because of dynamic collapse
– No time to expire all the lung volume (high RR or Vt)
– Lesions that increase expiratory resistance
Function of-Ventilator settings – TV, Exp time
Lung func – resistance, compliance
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Auto-PEEP or Intrinsic PEEP
• Auto-PEEP is measured in a relaxed pt with an end-expiratory hold maneuver on a mechanical ventilator immediately before the onset of the next breath
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Inadequate expiratory time - Air trapping
iPEEP
Flow curve FV loop
1. Allow more time for expiration2. Increase inspiratory flow rate3. Provide ePEEP
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Disadv1. Barotrauma / volutrauma2. ↑WOB a) lung overstretching ↓contractility of diaphragm
b) alters effective trigger sensitivity as autoPEEP must be overcome before P falls enough to trigger breath
3. ↑ MAP – CVS side effects4. May ↑ PVR
Minimising Auto PEEP1. ↓airflow res – secretion management, bronchodilation,
large ETT2. ↓Insp time ( ↑insp flow, sq flow waveform, low TV)3. ↑ exp time (low resp rate )4. Apply PEEP to balance AutoPEEP
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Cardiovascular effects of PPV
Spontaneous ventilation PPV
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Determinants of hemodynamic effects
due to – change in ITP, lung volumes, pericardial P
severity – lung compliance, chest wall compliance, rate & type of ventilation, airway resistance
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Low lung compliance – more P spent in lung expansion & less change in ITP
less hemodynamic effects (DAMPNING EFFECT OF LUNG)
Low chest wall compliance – higher change in ITP needed for effective ventilation
more hemodynamic effects
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Effect on CO ( preload , afterload )
Decreased PRELOAD 1. compression of intrathoracic veins (↓ CVP, RA
filling P)2. Increased PVR due to compression by alveolar
vol (decreased RV preload)3. Interventricular dependence - ↑ RV vol
pushes septum to left & ↓ LV vol & LV output
Decreased afterload1. emptying of thoracic aorta during insp2. Compression of heart by + P during systole 3. ↓ transmural P across LV during systole
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PPV
↓ preload, ventricular filling
↓ afterload , ↑ventricular
emptying
CO –1. INCREASE2. DECREASE
1. Intravascular fluid status
2. Compensation – HR, vasoconstriction
3. Sepsis,
4. PEEP, MAP
5. LV function
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Effect on other body systems
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Overview
1. Mode of ventilation – definition
2. Breath – characteristics
3. Breath types
4. Waveforms – pressure- time, volume –time, flow-time
5. Modes - Volume & pressure limited
6. Conventional modes of ventilation
7. Newer modes of ventilation
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What is a ‘ mode of ventilation’ ?
A ventilator mode is delivery a sequence of
breath types & timing of breath
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Breath characteristics
A= what initiates a breath -
TRIGGER
B = what controls / limits it –
LIMIT
C= What ends a breath -
CYCLING
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TRIGGER
What the ventilator
senses to initiate a
breath
Patient
• Pressure
• Flow
Machine
• Time based
Recently – EMG monitoring of phrenic Nerve via esophageal transducer
Pressure triggering
-1 to -3 cm H2O
Flow triggering
-1 to -3 L/min
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CONTROL/ LIMIT
Variable not allowed to rise above a preset value
Does not terminate a breath
Pressure
Volume
Pressure Controlled
• Pressure targeted, pressure limited - Ppeakset
• Volume Variable
Volume Controlled
• Volume targeted, volume limited - VT set
• Pressure Variable
Dual Controlled
• volume targeted (guaranteed) and pressure limited
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CYCLING VARIABLE
Determines the end of
inspiration and the
switch to expiration
Machine cycling
• Time
• Pressure
• Volume
Patient cycling
• Flow
May be multiple but
activated in hierarchy as
per preset algorithm
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Breath types
SpontaneousBoth triggered and cycled by the patient
Control/Mandatory Machine triggered and machine cycled
AssistedPatient triggered but machine cycled
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Waveforms
1. Volume -time
2. Flow - time
3. Pressure - time
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a) Volume – time graphs
1. Air leaks
2. Calibrate flow transducers
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b) Flow waveforms
1. Inspiratory flow waveforms
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Sine
Square
Decelerating
• Resembles normal inspiration
• More physiological
• Maintains constant flow• high flow with ↓ Ti &
improved I:E
• Flow slows down as alveolar pressure increases
• meets high initial flow demand in spont breathing patient - ↓WOB
Accelerating• Produces highest PIP as
airflow is highest towards end of inflation when alveoli are less compliant
Square- volume limited modes
Decelerating –pressure limited modes
Not used
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Inspiratory and expiratory flow waveforms
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2. Expiratory flow waveform
Expiratory flow is not driven by ventilator and is passive
Is negative by convention
Similar in all modes
Determined by Airway resistance & exp time (Te)
Use
1.Airtrapping & generation of AutoPEEP
2.Exp flow resistance (↓PEFR + short Te) & response bronchodilators (↑PEFR)
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c) Pressure waveform
1. Spontaneous/ mandatory breaths
2. Patient ventilator synchrony
3. Calculation of compliance & resistance
4. Work done against elastic and resistive forces
5. AutoPEEP ( by adding end exp pause)
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Classification of modes of ventilation
Volume controlled Pressure controlled
TV & inspiratory flow are preset
Airway P is preset
Airway P depends on above & lung elastance & compliance TV
& insp flow depend on above & lung elastance & compliance
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Volume controlled Pressure controlled
Trigger - patient / machine
Patient / machine
Limit Flow Pressure
Cycle Volume / time time / flow
TV Constant variable
Peak P Variable constant
Modes ACV, SIMV PCV, PSV
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Volume controlled Pressure controlled
Advantages1. Guaranteed TV2. Less atelectasis3. TV increases linearly with MV
Advantages1. Limits excessive airway P2. ↑ MAP by constant insp P – better
oxygenation3. Better gas distribution – high insp flow
↓Ti & ↑Te ,thereby, preventing airtrapping
4. Lower WOB – high initial flow rates meet high initial flow demands
5. Lower PIP – as flow rates higher when lung compliance high i.e early insp. phase
Disadvantages1. Limited flow may not meet
patients desired insp flow rate-flow hunger
2. May cause high Paw ( barotrauma)
Disadvantages1. Variable TV
↑TV as compliance ↑↓TV as resistance ↑
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Conventional modes of ventilation
1. Control mandatory ventilation (CMV / VCV)
2. Assist Control Mandatory Ventilation (ACMV)
3. Intermittent mandatory ventilation (IMV)
4. Synchronized Intermittent Mandatory Ventilation (SIMV)
5. Pressure controlled ventilation (PCV)
6. Pressure support ventilation (PSV)
7. Continuous positive airway pressure (CPAP)
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1. Control mandatory ventilation (CMV / VCV)
• Breath - MANDATORY• Trigger – TIME• Limit - VOLUME• Cycle – VOL / TIME
• Patient has no control over respiration
• Requires sedation and paralysis of patient
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2. Assist Control Mandatory Ventilation (ACMV)
• Patient has partial control over his respiration – Better Pt ventilator synchrony• Ventilator rate determined by patient or backup rate (whichever is higher) – risk of
respiratory alkalosis if tachypnoea• PASSIVE Pt – acts like CMV• ACTIVE pt – ALL spontaneous breaths assisted to preset volume
• Breath – MANDATORYASSISTED
• Trigger – PATIENTTIME
• Limit - VOLUME• Cycle – VOLUME / TIME
Once patient initiates the breath the ventilator takes over the WOBIf he fails to initiate, then the ventilator does the entire WOB
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3. Intermittent mandatory ventilation (IMV)
Breath stackingSpontaneous breath immediately after acontrolled breath without allowing timefor expiration ( SUPERIMPOSED BREATHS)
Basically CMV which allows spontaneous breaths in between
Disadvantage
In tachypnea can lead to breath stacking - leading to dynamic hyperinflation
Not used now – has been replaced by SIMV
• Breath – MANDATORYSPONTANEOUS
• Trigger – PATIENTVENTILATOR
• Limit - VOLUME• Cycle - VOLUME
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4.Synchronized Intermittent Mandatory Ventilation (SIMV)
• Breath –SPONTANEOUS
ASSISTEDMANDATORY
• Trigger – PATIENTTIME
• Limit - VOLUME• Cycle – VOLUME/ TIME
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• Basically, ACMV with spontaneous breaths (which may be pressure supported) allowed in between
• Synchronisation window – Time interval from the previous mandatory breath to just prior to the next time triggering, during which ventilator is responsive to patients spontaneous inspiratoryeffort
• Weaning
Adv Allows patients to exercise their respiratory muscles in
between – avoids atrophy
Avoids breath stacking – ‘Synchronisation window’
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5.Pressure controlled ventilation (PCV)
• Breath – MANDATORY• Trigger – TIME• Limit - PRESSURE• Cycle – TIME/ FLOW
Rise timeTime taken for airway pressure to rise from baseline to maximum
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6.Pressure support ventilation (PSV)
• Breath – SPONTANEOUS• Trigger – PATIENT• Limit - PRESSURE• Cycle – FLOW
( 5-25% OF PIFR)
After the trigger, ventilator generates a flow sufficient to raise and then maintain airway pressure at a preset level for the duration of the patient’s spontaneous respiratory effort
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Newer modes of ventilation
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Dual modes of ventilation
Devised to overcome the limitations of both V & P controlled modes
Dual control within a
breath
Switches from P to V
control during the same
breath
e.g. VAPS
PA
Dual control from breath
to breath
P limit ↑ or ↓ to maintain a
clinician set TV
ANALOGOUS to a resp
therapist who ↑ or ↓ P limit
of each breath based on
TV delivered in last breath
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Dual control within a breath
Combined adv –
1. High & variable initial flow rate of P controlled breath ( thereby - ↑ pt – vent synchrony, ↓WOB, ↓sense of breathlessness)
2. Assured TV & MV as in V controlled breaths
Starts as P limited breaths but change over to V limited breath by converting decelerating flow to constant flow if minimum preset TV not delivered
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1. Breath triggered (pt/ time) –
2. P support level reached quickly –
3. ventilator compares delivered and desired/ set TV
4. Delivered = set TV -------- Breath is FLOW cycled as in P controlled modes
5. Delivered < set TV -------- Changeover from P to V limited ( flow kept constant + Ti ↑)
P rises above set P support level
till set TV delivered
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Dual control – breath to breath
P limited + FLOW cycled
Vol support /
variable P
support
P limited + TIME cycled
PRVC
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Volume supportAllows automatic weaning of P support as
compliance alters.OPERATION –
C = VP
changes during weaning & guides P support level
Preset & constant
P support dependent on C
compliance↑ - P support ↓ ↓ - P support ↑
By 3 cm H2O /
breath
Deliver desired
TV
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Pressure regulated volume controlled (PRVC)
• Autoflow / variable P control
• Similar to VS except that it is a modification of PCV rather than PSV
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1. Conventional V controlled mode – very high P would have resulted in an attempt to deliver set TV -------- BAROTRAUMA
2. Conventional P controlled mode – inadequate TV would have been delivered
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Shifts between P support (flow cycled)& P control (time cycled) mode with pt efforts
Combines VS & PRVC
If no efforts : PRVC (time cycled)
As spontaneous breathing begins : VS (flow cycled)
Automode
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Pitfalls :
During the switch from time-cycled to flow cycled ventilation
Mean airway pressure
hypoxemia may occur
Automode
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Compensates for the resistance of ETT
Facilitates “ electronic weaning “ i.e pt during ATC mimic their breathing pattern as if extubated ( provided upper airway contorlprovided)
Operation
As the flow ↑ / ETT dia ↓, the P support needs to be ↑to ↓WOB
∆P (P support) α (L / r4 ) α flow α WOB
Automatic Tube Compensation
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Static conditionSingle P support level can eliminate ETT
resistance
Dynamic conditionVariable flow e.g. tachypnoea & in different
phases of resp.
P.support needs to be continously altered to eliminate dynamically changing WOB.
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1. Feed resistive coefof ETT
2. Feed % compensation desired
3. Measuresinstantaneous flow
Calculates P support proportional to resistance throughout respiratory cycle
Limitation
Resistive coef changes in vivo ( kinks, temp,molding,
secretions) Under/ overcompensation may result.
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Airway pressure release ventilation (APRV)
• High level of CPAP with brief intermittent releases to a lower level
Conventional modes – begin at low P & elevate P to accomplish TV
APRV – commences at elevated P & releases P to accomplish TV
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Higher plateau P – improves oxygenation
Release phase – alveolar ventilation & removal of CO2
Active patient – spontaneous breathing at both P levels
Passive patient – complete ventilation by P release
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Settings
1.Phigh (15 – 30 cmH2O )
2.Plow (3-10 cmH2O ) == PEEP
3. F = 8-15 / min
4. Thigh /Tlow = 8:1 to 10:1
If ↑ PaCO2 -↑ Phigh or ↓ Plow
- ↑ f
If ↓ PaO2 - ↑ Plow or FiO2
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Proportional Assist Ventilation
• Targets fixed portion of patient’s work during “spontaneous” breaths
• Automatically adjusts flow, volume and pressure needed each breath
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WOB
Ventilator measures – elastance & resistance
Clinician sets -“Vol. assist %” reduces work of elastance
“Flow assist%” reduces work of resistance's
Increased patient effort (WOB) causes increased applied pressure (and flow & volume)
ELASTANCE (TV)
RESISTANCE (Flow)
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Biphasic positive airway pressure (BiPAP)
PCV & a variant of APRVTime cycled alteration between 2 levels of CPAP
BiPAP – P support for spontaneous level only at low CPAP level
Bi-vent - P support for spontaneous level at both low & high CPAP
Spontaneous breathing at both levels
Changeover between 2 levels of CPAP synchronized with exp & insp
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BiPAP
Bi- vent
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Advantages
1. Allows unrestricted spontaneous breathing
2. Continuous weaning without need to change ventilatory mode – universal ventilatorymode
3. Synchronization with pt’s breathing from exp. to insp. P level & vice versa
4. Less sedation needed
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Neurally Adjusted Ventilatory Assist (NAVA)
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