Controlled ventilation 1
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Convential
Mechanical ventilation
SAMIR EL ANSARY
ICU PROFESSOR
AIN SHAMS
CAIRO
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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 ventilatory pump moving air in &
out of the lungs.
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Primary indications apnea
Ac. ventilation failure
Impending ventilation failure
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
Disadv1.Cooperation2.Mask discomfort3.Air leaks4.Facial ulcers, eye irritation, dry nose5.Aerophagia6.Limited P supporte.g. BiPAP, CPAP
Noninvasive
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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
Different P gradients
Time constant
Airway P ( peak, plateau, mean )
PEEP and Auto PEEP
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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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 / unitt = 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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• 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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7.Continuous positive airway pressure (CPAP)
Breath –SPONTANEOUS
CPAP is actually PEEP applied to spontaneously breathing patients.
But CPAP is described a mode of ventilation without additional inspiratory support while PEEP is not regarded as a stand-alone mode
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Newer modes of ventilation
1. Volume assured pressure support (VAPS)
2. Volume support (VS)
3. Pressure regulated volume controlled (PRVC)
4. Automode
5. Automatic Tube Compensation (ATC)
6. Airway pressure release ventilation (APRV)
7. Proportional Assist Ventilation (PAV)
8. Biphasic positive airway pressure (BiPAP)
9. Neurally Adjusted Ventilatory Assist (NAVA)
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Newer modes of ventilation
• Recent modes allow ventilators to control one variable or the other based on a feedback loop
Volume controlled
Pressure controlled
Feedback loopIs the Airway Pexceeding set P limit ?
Has the desired/ set TV been delivered ?
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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 support
Allows 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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Limitations –
a) MV is fixed , pt may be stuck at that level of support even if pt demand exceeds MV chosen by clinician
b) If tachypnoea occurs – ventilator senses it as ↑ MV and ↓ses P support which is exactly OPPOSITE of what is required
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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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Had it been 1. Conventional V controlled mode – very high P would have resulted in an attempt
to deliver set TV -------- BAROTRAUMA2. Conventional P controlled mode – inadequate TV would have been delivered
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Automode
Shifts between P support (flow cycled)& P control (time cycled) mode with pt efforts
Combines VS & PRVCIf no efforts : PRVC (time cycled)As spontaneous breathing begins : VS (flow cycled)
Pitfalls : During the switch from time-cycled to flow cycled ventilation
↓Mean airway pressure ↓
↓hypoxemia may occur
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Automatic Tube Compensation
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
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Static condition – single P support level can eliminate ETT resistance
Dynamic condition – variable flow e.g. tachypnoea & in different phases of resp.
- P support needs to be continously altered to eliminate dynamically changing
WOB d/t ETT
1. Feed resistive coefof ETT
2. Feed % compensation desired
3. Measuresinstantaneous flow
Calculates P support proportional to resistancethroughout 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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Settings1.Phigh (15 – 30 cmH2O )2.Plow (3-10 cmH2O ) == PEEP3. F = 8-15 / min4. Thigh /Tlow = 8:1 to 10:1
If ↑ PaCO2 -↑ Phigh or ↓ Plow
- ↑ f
If ↓ PaO2 - ↑ Plow or FiO2
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Advantages
1. Preservation of spontaneous breathing and comfort with most spontaneous breathing occurring at high CPAP
2. breathing occurring at high CPAP
3. ↓WOB
4. ↓Barotrauma
5. ↓Circulatory compromise
6. Better V/Q matching
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Proportional Assist Ventilation
• Targets fixed portion of patient’s workduring “spontaneous” breaths
• Automatically adjusts flow, volume and pressure needed each breath
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WOB
Ventilator measures – elastance & resistanceClinician 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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Limitations
1. Elastance (E) & resistance (R) cannot be measured accurately.
2. E & R vary frequently esp in ICU patients.
3. Curves to measure E ( P-V curve) & R (P-F curve ) are not linear as assumed by ventilator.
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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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.
Can provide total / partial ventilatory support 1. BiPAP – PCV – if pt not breathing2. BiPAP – SIMV- spontaneous breathing at lower CPAP + mandatory
breaths by switching between 2 CPAP levels3. CPAP – both CPAP levels are identical in spontaneously breathing
patient4. BiPAP – P support – additional P support at lower CPAP5. Bi- vent – additional P support at both levels of CPAP
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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)
Electrical activity of respiratory muscles used as input Eadi (electrical activity of diaphragm)
Cycling on, cycling off: determined by Eadi
Synchrony between neural & mechanical inspiratory time is guaranteed
Patient comfort
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