Physiology of positive pressure

ventilation

SAMIR EL ANSARY

Global Critical Carehttps://www.facebook.com/groups/1451610115129555/#!/groups/1451610115129555/

Wellcome in our new group ..... Dr.SAMIR EL ANSARY

Mechanical ventilation – Supports / replaces the normal ventilatory

pump moving air in & out of the lungs.

Primary indications – a. apneab.Ac. ventilation failurec. Impending ventilation failured.Severe oxygenation failure

Goals

Manipulate gas exchange↑ lung vol – FRC, end insp / exp lung

inflationManipulate work of breathing (WOB)Minimize CVS effects

ARTIFICIAL VENTILATION

- Creates a transairway P gradient by ↓ alveolar P to a level below airway opening P- Creates – P around thorax

e.g. iron lung chest cuirass / shell

- Achieved by applying + P at airway opening producing a transairway P gradient

Negative pressure ventilation Positive pressure

ventilation

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

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)

Understanding physiology of PPV

1) Different P gradients2) Time constant3) Airway P ( peak, plateau, mean )4) PEEP and Auto PEEP5) Types of waveforms

Pressure gradients

Distending pressure of lungs

Elastance load

Resistance load

Distending pressure

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).

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)

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

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.

Elastance(measure of stiffness) is the inverse of compliance(measure

of stretchiness)

An increase in elastance implies that the system is becoming stiffer.

Mean airway pressure Paw = Transrespiratory pressure

Mean alveolar pressure Palv = Transthoracic pressure

Transpulmonary pressure is the distending pressure in a spontaneously(negative)

breathing patient Transrespiratory pressure is the distending

pressure in positive pressure ventilation

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

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 H2O

R = 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

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

How -End inspiratory hold

• Pendelluft phenomenon• Visco-elastic properties of lung

End-inspiratory pause

Ppeak < 50 cm H2OPplat < 30 cm H2O

Ppeak = Pplat + Paw

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

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).

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.

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

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

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

Ppeak < 50 cm H2O; Pplat < 35 cm H2O – to avoid

barotrauma

• 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

Why

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 modes2. ↑insp time , ↓ exp time3. ↑ PEEP4. ↑ Resistance, ↓compliance5. Insp flow pattern

PEEP

BENEFITS 1. Restore FRC/

Alveolar recruitment2. ↓ shunt fraction3. ↑Lung compliance4. ↓WOB5. ↑PaO2 for given FiO2

DETRIMENTAL EFFECTS1. Barotrauma2. ↓ VR/ CO3. ↑ WOB (if overdistention)4. ↑ PVR5. ↑ MAP6. ↓ 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

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

Set PEEP above LIP – Prevent end expiratory airway collapseSet TV so that total P < UIP – prevent overdistention

Limitation – lung is inhomogenous - LIP / UIP differ for different lung units

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

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

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

Inadequate expiratory time - Air trapping

iPEEP

Flow curve FV loop

1. Allow more time for expiration2. Increase inspiratory flow rate3. Provide ePEEP

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 PEEP5. ↓airflow res – secretion management, bronchodilation,

large ETT6. ↓Insp time ( ↑insp flow, sq flow waveform, low TV)7. ↑ exp time (low resp rate )8. Apply PEEP to balance AutoPEEP

Cardiovascular effects of PPV

Spontaneous ventilation PPV

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

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

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

PPV

↓ preload, ventricular filling

↓ afterload , ↑ventricular

emptying

CO – 1. INCREASE2. DECREASE

1. Intravascular fluid status2. Compensation – HR, vasoconstriction3. Sepsis, 4. PEEP, MAP5. LV function

Effect on other body systems

Overview

1. Mode of ventilation – definition2. Breath – characteristics3. Breath types4. Waveforms – pressure- time, volume –time, flow-

time5. Modes - Volume & pressure limited 6. Conventional modes of ventilation7. Newer modes of ventilation

What is a ‘ mode of ventilation’ ?

A ventilator mode is delivery a sequence of

breath types & timing of breath

Breath characteristics

A= what initiates a breath - TRIGGER

B = what controls / limits it – LIMIT

C= What ends a breath - CYCLING

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

CONTROL/ LIMIT

Variable not allowed to rise above a preset valueDoes not terminate a breath Pressure Volume

Pressure Controlled • Pressure targeted,

pressure limited - Ppeak set

• Volume Variable Volume Controlled

• Volume targeted, volume limited - VT set

• Pressure Variable Dual Controlled

• volume targeted (guaranteed) and pressure limited

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

Breath types

SpontaneousBoth triggered and cycled by the patient

Control/Mandatory Machine triggered and machine cycled

AssistedPatient triggered but machine cycled

Waveforms

1. Volume -time2. Flow - time3. Pressure - time

a) Volume – time graphs

1. Air leaks2. Calibrate flow transducers

b) Flow waveforms

1. Inspiratory flow waveforms

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

Inspiratory and expiratory flow waveforms

2. Expiratory flow waveform

Expiratory flow is not driven by ventilator and is passiveIs negative by conventionSimilar in all modesDetermined by Airway resistance & exp time (Te)

Use1.Airtrapping & generation of AutoPEEP2.Exp flow resistance (↓PEFR + short Te) & response

bronchodilators (↑PEFR)

c) Pressure waveform

1. Spontaneous/ mandatory breaths2. Patient ventilator synchrony3. Calculation of compliance & resistance4. Work done against elastic and resistive forces5. AutoPEEP ( by adding end exp pause)

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

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

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 ↑

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)

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

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 – MANDATORY ASSISTED• Trigger – PATIENT TIME• 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

3. Intermittent mandatory ventilation (IMV)

Breath stackingSpontaneous breath immediately after a controlled breath without allowing time for 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 – MANDATORY SPONTANEOUS• Trigger – PATIENT VENTILATOR• Limit - VOLUME• Cycle - VOLUME

4.Synchronized Intermittent Mandatory Ventilation (SIMV)

• Breath – SPONTANEOUS

ASSISTED MANDATORY• Trigger – PATIENT TIME• Limit - VOLUME• Cycle – VOLUME/ TIME

• 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 inspiratory effort

• Weaning Adv Allows patients to exercise their respiratory muscles in

between – avoids atrophy Avoids breath stacking – ‘Synchronisation window’

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

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

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

Newer modes of ventilation

1. Volume assured pressure support (VAPS)2. Volume support (VS)3. Pressure regulated volume controlled (PRVC)4. Automode5. 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)

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 ?

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

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

1. Breath triggered (pt/ time) –2. P support level reached quickly – 3. ventilator compares delivered and desired/ set TV4. Delivered = set TV -------- Breath is FLOW cycled as in P controlled modes5. Delivered < set TV -------- Changeover from P to V limited ( flow kept constant + Ti ↑) P rises above set P support level

till set TV delivered

Dual control – breath to breath

P limited + FLOW cycled

Vol support / variable P support

P limited + TIME cycled

PRVC

Volume supportAllows automatic weaning of P support as compliance

alters.OPERATION –

C = V P

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

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

Pressure regulated volume controlled (PRVC)

• Autoflow / variable P control

• Similar to VS except that it is a modification of PCV rather than PSV

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

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)

Automode

Pitfalls :During the switch from time-

cycled to flow cycled ventilation Mean airway pressure hypoxemia may occur

Automode

Compensates for the resistance of ETT

Facilitates “ electronic weaning “ i.e pt during ATC mimic their breathing pattern as if extubated ( provided upper airway contorl provided)

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

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.

1. Feed resistive coef of ETT

2. Feed % compensation desired

3. Measures instantaneous flow

Calculates P support proportional to resistance throughout respiratory cycle

LimitationResistive coef changes in vivo ( kinks, temp,molding, secretions) Under/ overcompensation may result.

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

Higher plateau P – improves oxygenationRelease phase – alveolar ventilation & removal of CO2

Active patient – spontaneous breathing at both P levelsPassive patient – complete ventilation by P release

Settings 1.Phigh (15 – 30 cmH2O )

2.Plow (3-10 cmH2O ) == PEEP

3. F = 8-15 / min4. Thigh /Tlow = 8:1 to 10:1

If ↑ PaCO2 -↑ Phigh or ↓ Plow

- ↑ f

If ↓ PaO2 - ↑ Plow or FiO2

Advantages1. Preservation of spontaneous breathing and

comfort with most spontaneous breathing occurring at high CPAP

2. breathing occurring at high CPAP3. ↓WOB4. ↓Barotrauma5. ↓Circulatory compromise6. Better V/Q matching

Proportional Assist Ventilation

• Targets fixed portion of patient’s work during “spontaneous” breaths

• Automatically adjusts flow, volume and pressure needed each breath

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)

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.

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 levelsChangeover between 2 levels of CPAP synchronized with exp & insp

.

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

BiPAP

Bi- vent

Advantages1. Allows unrestricted spontaneous breathing2. Continuous weaning without need to change

ventilatory mode – universal ventilatory mode

3. Synchronization with pt’s breathing from exp. to insp. P level & vice versa

4. Less sedation needed

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

Global Critical Carehttps://www.facebook.com/groups/1451610115129555/#!/groups/1451610115129555/

Wellcome in our new group ..... Dr.SAMIR EL ANSARY

GOOD LUCK

SAMIR EL ANSARYICU PROFESSOR

AIN SHAMSCAIRO

elansarysamir@yahoo.com