Ventilatory Strategies ALI/ARDS

Dr S Manimala RaoSenior ConsultantEmeritus Professor

Dept. of Anaesthesiology & Critical CareNizam’s Institute Of Medical Sciences

Hyderabad, A.PINDIA

Pathophysiology

How is the lung in ARDS ?

It has 3 components• Diseased lung not

recruitable• Diseased lung

recruitable• Normal lung

20

-5

20

+5

Influence of chest wall compliance on lung distension and alveolar pressure

Normal Stiff Chest Wall

Problems in injured lungs• Over distension• High peak airway pressures • Low compliance• High FiO2 –absorption

atelectasis • Free radical injury

How & Why various ventilatory

strategies developed• Basis of supportive therapy• Mortality decreased• Quest for ideal strategy• Understanding the pathophysiology- baby lung

concept , CT scans ,Volutrauma, Atelectrauma, Biotrauma

• To decrease VILI• Survival with ventilatory strategy

Deleterious Effects of Mechanical Ventilation

• Has effect on surfactant

• Increases cytokines

• Migration of neutrophils

• Bacterial /Endotoxin translocation

History of Mechanical Ventilation

• 1774 Hunter Bellows for resuscitation• 1827 Leroy setback due to barotrauma• 1934 Freckner : Mech ventilators.• 1940 Crawford : Commercial Ventilators• 1945 Blease developed Prototype • 1952 Isben :Polio epidemic to and fro• 1967 Ashbaugh defined ARDS

PEEP improves oxygenation

Positive End Expiratory Pressure (PEEP) and

CPAP• Applied for controlled and spontaneous ventilation• Used to reduce or prevent atelectasis in ALI/ARDS• Reduce inspiratory load , work of breathing• Effects of PEEP and CPAP are similar for lung mechanics• Different effect on V/Q ratio ,CVS• CMV +PEEP – pressure gradient by mech ventilation, intrapleural

pressure • CPAP - pressure gradient by resp muscle ,intrapleural pressure

Controlled Mechanical Ventilation (CMV)Assist Control (AC)

• Use of large tidal volumes

• Square or sine wave flows

• Decreases ventilatory inequalities

• Better distribution of flow

• Keep plateau pressure < 45cmH20

• Assist control is a popular mode

• Retains spontaneous effort with back up

Volume Ventilation

• Constant flow rate

• Guaranteed tidal volume delivery

• Not affected by lung impedance

• Variable pressure

Pressure

Flow

Synchronized Mandatory Ventilation

(SIMV)• Preserves spontaneous

respiratory effort• Decreases WOB• Prevents patient and

ventilatory disharmony• Decreases the need for

sedatives and relaxants• SIMV volume preset with

decelerating flows

Pressure Support

•Patient spontaneous breaths supported by the preset Pressure Support.

• Elevates the inspiratory pressure above the baseline

•Decelerating, variableinspiratory flow rate

Flow

A B

Flow Cycled

Time cycled: (A)• Pressure ControlFlow cycled: (B)• Pressure Support

Continous Positive Airway Pressure

(CPAP)• Developed mode for ARDS

lung volumes • Popular for weaning & for

spontaneous breaths• Useful in ALI - shunting

& WOB• Non invasive ventilation to

improve oxygenation

PEEP

• Introduced to treat pulmonary odema

• Role in ARDS by Ashbaugh

• Increases FRC - shunt

• Reduces the shear stress associated with repetitive opening and closing of alveoli

• Prevents atelectrauma

• Collapsed lung units open – shear stress

Strategies to set Ideal PEEP levels

• Should recruit & prevent derecruitment• Least effect on Cardiac output• Should not contribute to VILI• Low levels may not open alveoli• Commonly used PEEP <20cm H2O• Observe pressure volume loops• Set upper and lower inflection points• Cumbersome ,may not be detectable• PEEP has become integral part of recruitment maneuvers

Plateau TidalPressure Volume

0 011 10014 20015 30017 40019 50021 60023 70024 80027 90037 1000

P-V Curve

0

200

400

600

800

1000

1200

0 10 20 30 40Plateau Pressure

TidalVolumes

Lower Inflection Point

Upper Inflection Point

Pressure Volume Curve

Development of Lung Protective Ventilatory

strategies• High VT v/s low VT

• volume v/s Pressure controlled

• Prone position• Recruitment Maneuvers

BEGINNING OF COLLAPSE OF UNSTABLE ALVEOLI

MOST UNITS COLLAPSED (INCLUDING SMALL AIRWAYS)

START OF RECRUITMENT (ESPECIALLY SMALL AIRWAYS)

END OF RECRUITMENT

Pflex

VOLUME

Goals of Ideal Lung Protective Strategy

• Keep P plat <35cmH20

• Bring down FiO2 to 0.5-0.6• Avoid over distension• Prevent barotrauma, volutrauma, atelectrauma, biotrauma• Maintain haemodynamics• Should maintain adequate end exp volume

While selecting ventilatory strategy

REMEMBER THE CONCEPT

Buy time –Doing least harm

Lower TV vs Conventional TV

• NIH ARDSnet Trial • VT 6ml/kg vs. 12ml/kg predicted body

weight• Plateau pressure limit 30cmH20

50cmH20• Higher PEEP requirement in low VT group• Reduced in hospital mortality• Level I evidence

NEJM 2000 , 342 , 1301-8

0.00.10.20.30.40.50.60.70.80.91.0

0 20 40 60 80 100 120 140 160 180Days after Randomization

Prop

ortio

n of

Pat

ient

s

Lower VT SurvivalLower VT DischargeTraditional VT SurvivalTraditional VT Discharge

Ventilation with Lower VT vs. Traditional VT for

ALI and ARDS

ARDS network NEJM 2000;342:1301ARDS network NEJM 2000;342:1301

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Traditional VT Lower VT

IL-6

(pg/

ml)

d1d3

Ventilation with Lower VT vs. Traditional VT for ALI and

ARDS

ARDS network NEJM 2000;342:1301ARDS network NEJM 2000;342:1301

Authors/yearAuthors/year N N BenefitBenefit Pplat (cmHPplat (cmH22O)O)

Amato/1998Amato/1998 53 53 yes yes 38 vs. 2438 vs. 24Stewart/1998Stewart/1998 120120 no no 28 vs. 2028 vs. 20Brochard/1998Brochard/1998 116116 no no 32 vs. 2632 vs. 26Brower/1999Brower/1999 52 52 no no 31 vs. 2531 vs. 25ARDSNET/2000ARDSNET/2000 861861 yes yes 37 vs. 2637 vs. 26

Comparison of Randomized Trials of Lower VT in ARDS

References: References: 1.1. NEJM 338:347 2. NEJM 338:355 NEJM 338:347 2. NEJM 338:355 3. AJRCCM 158:1831 4. CCM 27:1492 5. NEJM 342:13013. AJRCCM 158:1831 4. CCM 27:1492 5. NEJM 342:1301

Lessons from ARDS NET Trail

• Correct implementation of recruitment norms• How to set the appropriate PEEP• P plateau to be limited• Still taken as bench mark study

Slutsky & Ranieri : Resp Res 2001 (2) ;73-77

Revival of Pressure Controlled Ventilation

•Use in NRDS

•limit peak pressure

•PCV: constant square waves of pressure applied and released

•Pressure & time are constant, volume variable

•Decelerating flow

Pressure

Flow

A B

Time Cycled

Flow Cycled

Pressure Controlled Ventilation

• PCV-SIMV, PCV-CMV, PCV-IRV• Peak pressure , mean pressure - better

oxygenation• Pressure selected to deliver adequate VT

• Monitor , VT , compliance ,auto –PEEP, mean pressures

Pressure Controlled Ventilation

• 101 clinical investigations - 3264 patients reported

mortality in PCV v/s VCV

• Lack of power in the studies

• Intervention - PHC and IRV were used

Krafft et al ;An analysis of 101 clinical investigation.Intensive Care Med 22:519, 1996

Inverse Ratio Ventilation

• First used in neonate• Prolongation of I time• Short expiratory time• Decelerating Insp Flow Pattern• Use of pressure controlled

ventilation• Use of sedation and muscle

relaxation • Permissive Hypercapnia

FLOW

Pressure

0

Benefits of IRV• Peak pressure , mean pressure • Prolonged inspiratory time - sustained inflation• Prevents collapse during short expiration• Uniform distribution of gases• V/Q mismatch is • FiO2 can be brought down faster

• PEEP is reduced , however intrinsic PEEP

Permissive Hypercapnia

• All protective strategies ++ PaCO2

• Low VT and short exp times

• Viewed as an unpleasant side effect • ? plays a major role in lung protection• Level ideally maintained at 60mmHg

Feihl EF: How permissive should we be? Am J Resp Crit Care Med 1994:150, 1722-37

Positive Effects of PHC

neutrophil function

platelet aggregation

cell adhesion

lipid peroxidation

injury with PaCO2

Prone position in ARDS

Proposed Explanations• Increased FRC

• Blood Flow Redistribution

• Changes in Diaphragmatic Motion

• Improved Secretion Removal

Ventral

Dorsal

Dorsal

Ventral

Mechanism of Prone Positioning

Prone Positioning: Procedure

• Appropriate staff to manage patient and “tubes”.

• Minimize abdominal pressure.• Maintain pt in Swimming position (one

arm extended over head, head turned to that side)

• Sedation generally required.

Prone Positioning: How Long?

Fridrich et al, Anesth Analg 1996;83:1206-1211

Prone Position• Prone-Supine Study Group• Multicenter randomized clinical trial• 304 adult patients prospectively

randomized to 10 days of supine vs. prone ventilation 6 hours/day

• Improved oxygenation in prone position• No improvement in survival

NEJM 2001;345:568-73

Open lung concept

• Law of Laplace• Critical opening pressure• Prevent destabilization• Open and keep it open• 30-40cmH2O PEEP with PCV

30-40sec

60cmH2O

40cmH2O

PEEP 5cm H2O

PIP 30 cm H2O

150 cm H2O1. Expiration 2. Inspiration

PIP 50 cm H2O

PEEP 12 cm H2O

PEEP 12 cm H2O

3. Opening procedure

4. Expiration

Recruitment Maneuvers

• Exact mech not understood• Airway opening move fluid to periphery

overcome shear stress• Open the collapsed alveoli• Requires high sustained pressures Barotrauma Haemodynamic instability

Criteria to apply RMS

• Perform early in disease• Maintain haemodynamic stability• Monitor HR, Art press, SpO2

FiO2 to 1 , 5 min before each RM• Use in line suction and aerosol therapy• Sedation mandatory• Multiple RMS may be required• Successful RMS PaO2/ FiO2 >300

Recruitment Manouvres

• Conceptual goal of RM maneuver- use single breath to provide max recruitment

• Bring lung down on deflation limb on PV curve

• PEEP required is less than opening pressure

• Airway Pressure required for ventilation is less

• Airspaces are open throughout ventilatory cycles

• Improves V/Q and reduces VILI

Approaches• Four different approaches• Single breath 1.5 – 2 times the set VT is applied every one or

two minutes• PEEP is temporarily ++,subsequent end inspiratory volume is • VT can be raised temporarily• High levels of CPAP applied for set point of time• RM can be applied with PCV 20cmH2O and PEEP 30-

40cmH20 for 1-2min

Karmarek RM strategies to optimize alveolar recruitment. Curr. Opin. Crit. Care 2001:7;15-20

Aggressive RMS

• CT images showed improvement in collapse lung

• Better oxygenation mortality

Amato MB et al : N Engl J Med 1998:338; 345-54

Unanswered Questions• Who is the ideal patient for RM?• Which is the best technique?• How to set PEEP?• How to monitor recruitment ?• Safety of the maneuver• Their effect on survival

Hess D R : The role of recruitment maneuvers, Respir Care 2002: 47;308-318

Million Dollar Question• At the end of RM’s are the lungs happy or

pretty?• Maximizing O2 tension by aggressive RMS• May be gratifying - short term effect• Whether it prevents lung injury and promotes

repair?

Hubmayor RD, A skeptical look at opening and collapse story :Am J Respir Crit Care Med 2002, 156; 1647-53

High frequency ventilation

• Introduced by Lunkenheimer 1972• Expiration and Inspiration active process

• VT 1-3ml/kg ,freq 100 - 2400/min

• No gas entrainment• Better humidification and weaning• Prevents air trapping,over distension and

CVS depression

High frequency ventilation

• Very low VT equivalent to dead space • high freq ventilation disappointing• HFO extensively used in neonates• Applied for severe ARDS – Rescue therapy• Mean pressures ++ better oxygenation• Set the PAW 5cmH2O above that used for conventional • Early institution may be beneficial

Mehta et al :Prospective trial Crit Care Med 2001: 29(7) 1360-69

High frequency ventilation

• MRCT 148 patients with ARDS

• Randomized to CMV and HFOV

• In hospital mortality no difference

• 30 days ( 87 vs. 52)

Derdak S etal Am J Respir Crit Care Med 2002 15 :166 (6) ;801-808

Airway Pressure Release Ventilation

• Can minimize lung volume expansion

• Inflation pressure is CPAP level – Best compliance , oxygenation

• APRV supports ventilation at optimal resting volumes

• Pulmonary volume is maximized at FRC

Airway Pressure Release Ventilation

• APRV used in patients with lung injury• Improved haemodynamics• Reduced peak and mean airway pressures• Decreased use of sedatives and relaxants• Improved cardiac index• Pressor agents usage is reduced• Shortened the length of mech ventilation

Kaplan et al , Crit Care 2001,5(4) ;221-226

Tracheal-Gas Insufflation

• ALI , rapid CO2

• Low tidal volume & cyclical pressures

• TGI tube at carina - as continuous flow - as phasic flow

anatomical dead space• Turbulence at tip CO2 limitation• adjunct to pressure ventilation

TGI CATHETER

Tracheal-Gas Insufflation

Disadvantages Disadvantages

• Mucosal damage• Barotrauma• secretion retention

Partial liquid ventilation

• Perfluorochemicals (PCF) liquids• Dissolve O2 and CO2 • Evaporates slowly• Dist . Homogenously• Low surface tension• Viscosity like water (Perflubron) • Both liquid & gas ventilation

Partial liquid ventilation

Mechanism of action

of interfacial surface tension of alveoli

• Physical distension of alveoli by fluid

• O2 exchange in alveoli opened by fluid

• Redirection of pul. arterial blood flow

Non invasive ventilation (NIPPV)

• Approach not new• Limited use in acute settings during 1970s

and 1980s• Most successful in COPD patients with

acute exacerbations• Today level I data supporting use of NIPPV

Five randomized controlled trials have been published

Pt type NPPV/Control

% intubatedNIV/Control

Mortality %NIV/Control

Bott et al COPD 26/30 0/0 3.8/27Kramer et al mixed 16/15 31/70 6.3/13Wysocki et al mixed 11/6 36/100 9/66Brochard et al COPD 43/42 26/74 9/29Barbe et al COPD 14/10 0/0 0/0Nava et al COPD 25/25 88/68 8/23Antonelli et al Hypoxaemic

Resp failure32/32 31/100 28/47

Dual modes

• Most modern ventilators offer useful new modes

• The benefit of pressure limited breaths• The security of assured VT

• Better synchrony and more comfort• Low work of breathing

Combined Pressure Volume Targetted Modes

• PCV- Permissive hypercapnia, ++sedation,

VT variability• VCV- Flow starvation

over distension• Future –look at combination of both

New modes of assisted ventilation

• Within breath adjustment - Volume assured pressure support- Automatic tube compensation- Proportional assist ventilation

• Between breath adjustment- Volume support- Pressure –regulated volume control- Adaptive support ventilation

What will last for future?

• Noninvasive positive pressure ventilation

• Lung protective ventilatory strategies

• Combined pressure –volume targeted modes

• Prone position ventilation

• Tracheal gas insufflation

Kacmarek RM :chestnet.org/edu/pccu/vol114

RICU Experience -NIMSBackground

• Established in 1990

• Started with minimum infrastructure

• Nurse patient ratio inadequate

• Developed over 13yrs

ARDS and ventilatory strategies

Our JourneyEarly half – 1990’s• CMV/SIMV• Used 10ml/kg + PEEP 10cmH2O• Mortality high-70%• Later use of 8ml/kg with higher PEEP 15cmH2O• Mortality –58%Year 1996 onwards• SIMV / A/C• Lower VT 5-6ml/kg while titrating PEEP• Aimed to maintain PaO2 >60 with SaO2 >90%• Haemodynamics monitored• Mortality 45%

ARDS and ventilatory strategies

Our Journey Year 2000• Lung protective ventilatory strategy using low VT effective

• Good survival rate in pt with mild to moderate lung injury scores-Mortality of 26.3%

• High mortality seen with LIS >3,66.6%• Implementation of Pressure control ventilation• Initial results disappointingYear 2003• Improved survival seen with use of pressure control

ventilation especially with early application of PCV in patients with LIS >2.5. Mortality 22.8%

Our Statistics

Use of Lung protective ventilatory strategies

NIMS RICU Census Jan 2002 – Dec 2002

ARDS cases – 94

• Age range – 13-68 • M/F ratio – 2.9:1

Causes Total Mortality

Pneumonia 14 9 (64%)

Systemic sepsis 37 21 (56%)

Postop sepsis 18 5 (27%)

Trauma 4 0

Fat embolism 7 0

Aspiration 6 3 (50%)

Drowning 1 0

Snake bite 2 0

Malaria 3 1 (33%)

Pancreatitis 2 1 (50%)

Total 94 40 (42%)

Statistics in Jan 2002 – Dec 2002

• VCV - 77• PCV - 12• NIPV - 5

No of patients

Mode of ventilation

Survival Mortality

74 LPVS- VCV 61.03% 38.9%

12 PCV 25% 75%

• Total Mortality 42%• 4-5 organ failure noted • Survival 58%

Our Experience with Pressure Control

VentilationYear 2003• 35 adult patients with ARDS ,LIS >2.5, • initially ventilated with LPVS VCV mode subsequently switched

over to PCV. • The reasons - high peak airway pressures and inability to maintain

oxygenation despite high PEEP. • Results:. • The mean PaO2/FiO2 ratio at VCV- 100±13, at 30min of institution of

PCV 136±17 • The mean time for achieving SpO2>90% was 37.28±5min, and for

attaining PaO2/FiO2>200 was 26.89± 14hrs.• The average number of ventilatory days was 7.05±2days. • The mortality was 22.85%.

Conclusions• Magic bullet ventilatory strategy –not yet• Mr GOOD MODE - still evading• Absence of definitive proof • ARDS NET trial bench mark• Happy lungs or pretty lungs- debate?• “Buy time – do least harm” - is the

prescription

Conclusions• Magic bullet ventilatory strategy to an

extent• Mr GOOD MODE - still evading• Absence of definitive proof • ARDS NET trial bench mark• Happy lungs or pretty lungs- debate?• “Buy time – do least harm” - is the

prescription