The Map Between Lung Mechanics and Tissue Oxygenation

The Map Between Lung Mechanics and Tissue Oxygenation

Alveolar UnitsAlveolar Units

Gas exchangeCommunication with capillary

network

Gas exchangeCommunication with capillary

network

vv

O2

O2 O

2O

2

O2

O2

O2

O2

O2

O aaO2

O2 O

2O

2 O2

O22

Lung Compliance and ElasticityLung Compliance and Elasticity

Compliance is ability of lungs to stretch Low compliance in fibrotic lungs (and other restrictive

lung diseases) and when not enough surfactantElasticity (= Elastance) is ability to return to original

shape Low Elasticity in case of emphysema due to destruction

of elastic fibers.Normal lung is both compliant AND elastic

Compliance is ability of lungs to stretch Low compliance in fibrotic lungs (and other restrictive

lung diseases) and when not enough surfactantElasticity (= Elastance) is ability to return to original

shape Low Elasticity in case of emphysema due to destruction

of elastic fibers.Normal lung is both compliant AND elastic

Pulmonary circulationPulmonary circulation

• Thin walled, greater internal diameters and less vascular smooth muscle than the systemic vessels.

• Rapidly divides into 280 billion capillaries.• More distensible and compressible than the systemic

system and there is less resistance to blood flow.• Lower intravascular pressures• Are subject to alveolar and pleural pressures.• Form a sheet of blood

• Thin walled, greater internal diameters and less vascular smooth muscle than the systemic vessels.

• Rapidly divides into 280 billion capillaries.• More distensible and compressible than the systemic

system and there is less resistance to blood flow.• Lower intravascular pressures• Are subject to alveolar and pleural pressures.• Form a sheet of blood

Gas meets bloodGas meets blood

A >a >vA >a >v

a >A >va >A >v

a >v >Aa >v >A

Best V/Q !!!

ARDS…ARDS…

Ware & MatthayNEJM,2000

Lung Fluid Balance = Formation and Resolution of Lung Edema in ALI/ARDS

Lung Fluid Balance = Formation and Resolution of Lung Edema in ALI/ARDS

Gas meets bloodGas meets blood

A >a >vA >a >v

a >A >va >A >v

a >v >Aa >v >A

Best V/Q !!!

Gas meets bloodGas meets blood

A >a >vA >a >v

a >A >va >A >v

a >v >Aa >v >A

Best V/Q !!!

V/Q inequalityV/Q inequality• Increases A-a PO2

(norm. 5-20 mmHg)• Reduces PaO2

V/Q more blood than gas venous admixture V/Q more gas than blood venous admixture

• Q flows only through unaffected regions, thus it relatively there

• Increases A-a PO2

(norm. 5-20 mmHg)• Reduces PaO2

V/Q more blood than gas venous admixture V/Q more gas than blood venous admixture

• Q flows only through unaffected regions, thus it relatively there

Pulmonary MeasuresPulmonary Measures

• P/F ratio = Pa02/ Fi02 • 100/ 0.21= 470

• P/F ratio = Pa02/ Fi02 • 100/ 0.21= 470

Which is BESTWhich is BEST

• P/F ratio –1. 90 on 1.0

–2. 60 on 0.50

–3. 70 on 0.21

• P/F ratio –1. 90 on 1.0

–2. 60 on 0.50

–3. 70 on 0.21

What can I do about it?What can I do about it?

Blood flow increase towards lung basesBlood flow increase towards lung bases

• Zone 3: – resistance decrease due to distension

related to volume weight and gravity (dependence: water glass)

–Pa and Pv are higher than PA• What does this have to do with my

patient?

• Zone 3: – resistance decrease due to distension

related to volume weight and gravity (dependence: water glass)

–Pa and Pv are higher than PA• What does this have to do with my

patient?

Lung Fluid Balance = Formation and Resolution of Lung Edema in ALI/ARDS

Lung Fluid Balance = Formation and Resolution of Lung Edema in ALI/ARDS

When you ventilate only with…..When you ventilate only with…..

Simple Ventilation termsSimple Ventilation terms• Fraction of Inspired Oxygen (FIO2)

– Amount or percent of oxygen mixed into the atmospheric pressurized gas

• Optimal < 40%• Any patient on > 50% for more than two days is very sick and at

risk for long vent stays

• Pressure– A means of measuring and/or controlling the effects of

the volume delivery to the lung• Peak pressures > 50 cm H20 indicate stiff sick lungs and risk

for long vent stays• Mean pressures or Plateau pressures > 35 cm H20 may

indicate respiratory distress syndrome and very long vent stays• Pressure control ventilation may also indicate respiratory

distress syndrome and very long vent stays

• Fraction of Inspired Oxygen (FIO2)– Amount or percent of oxygen mixed into the

atmospheric pressurized gas• Optimal < 40%• Any patient on > 50% for more than two days is very sick and at

risk for long vent stays

• Pressure– A means of measuring and/or controlling the effects of

the volume delivery to the lung• Peak pressures > 50 cm H20 indicate stiff sick lungs and risk

for long vent stays• Mean pressures or Plateau pressures > 35 cm H20 may

indicate respiratory distress syndrome and very long vent stays• Pressure control ventilation may also indicate respiratory

distress syndrome and very long vent stays

Simple Ventilation TermsSimple Ventilation Terms

• Minute ventilation (VE or MV): – the amount of volume exhaled per minute (VE) or

measured as RR × VT (MV) – Normal 8-10 liters/minute

• Peak Inspiratory Pressure (PIP)– peak pressure measured when the

tidal volume is pushed into the airways– value used to set high and low

pressure alarm limits

• Minute ventilation (VE or MV): – the amount of volume exhaled per minute (VE) or

measured as RR × VT (MV) – Normal 8-10 liters/minute

• Peak Inspiratory Pressure (PIP)– peak pressure measured when the

tidal volume is pushed into the airways– value used to set high and low

pressure alarm limits

Simple Ventilation TermsSimple Ventilation Terms

• Flow rate: The rate of speed of volume delivery– impacts I:E ratio–slow flow: ↑ I time, ↓ E time– fast flow: ↓ I time, ↑ E time

• Flow rate: The rate of speed of volume delivery– impacts I:E ratio–slow flow: ↑ I time, ↓ E time– fast flow: ↓ I time, ↑ E time

SIMPLE METHODSSIMPLE METHODS

• Ventilatory Modes–CMV, IMV, SIMV, A/C, PCV

• Adjuncts to Mechanical Ventilation–PEEP, CPAP, PSV

• Ventilatory Modes–CMV, IMV, SIMV, A/C, PCV

• Adjuncts to Mechanical Ventilation–PEEP, CPAP, PSV

Simple Ventilation Modes and AdjunctsSimple Ventilation Modes and AdjunctsWhen anticipation is that the patient will not require lengthy

support > 3-5 days, simple modes are generally applied:

• Modes– Controlled Mandatory or assist control ventilation:

CMV or ACMV– Intermittent Mandatory Ventilation: IMV– Synchronized Intermittent Mandatory Ventilation:

SIMV

When anticipation is that the patient will not require lengthy support > 3-5 days, simple modes are generally applied:

• Modes– Controlled Mandatory or assist control ventilation:

CMV or ACMV– Intermittent Mandatory Ventilation: IMV– Synchronized Intermittent Mandatory Ventilation:

SIMV

Volume vs… Pressure Control VentilationVolume vs… Pressure Control Ventilation

Volume Ventilation

• Volume delivery constant• Inspiratory pressure varies• Inspiratory flow constant• Inspiratory time determined by

set flow and VT

Volume Ventilation

• Volume delivery constant• Inspiratory pressure varies• Inspiratory flow constant• Inspiratory time determined by

set flow and VT

Pressure Ventilation

• Volume delivery varies • Inspiratory pressure constant• Inspiratory flow varies• Inspiratory time set by

clinician

Pressure Ventilation

• Volume delivery varies • Inspiratory pressure constant• Inspiratory flow varies• Inspiratory time set by

clinician

Lung Compliance and ElasticityLung Compliance and Elasticity

Compliance is ability of lungs to stretch Low compliance in fibrotic lungs (and other restrictive

lung diseases) and when not enough surfactantElasticity (= Elastance) is ability to return to original

shape Low Elasticity in case of emphysema due to destruction

of elastic fibers.Normal lung is both compliant AND elastic

Compliance is ability of lungs to stretch Low compliance in fibrotic lungs (and other restrictive

lung diseases) and when not enough surfactantElasticity (= Elastance) is ability to return to original

shape Low Elasticity in case of emphysema due to destruction

of elastic fibers.Normal lung is both compliant AND elastic

Pre

ssur

e

time

PIP:complianceresistancevolumeflowPEEP

PEEP

PEEP

PIP

Pplat

resistanceflow

compliancetidal volume

No active breathingTreats lung as single unit

end-inspiratoryalveolar pressure

What else can I look at??What else can I look at??

0 20 40 60-20-40-60

0.2

0.4

0.6

LITERS

Paw

cmH2O

VT

Pressure-Volume Loop ChangesPressure-Volume Loop Changes

Indicates a drop in compliance (higher pressure for the same volume)

0 20 40 602040-60

0.2

0.4

0.6

LITERS

Paw

cmH2O

VT

Changes in CompliancesChanges in Compliances

B

A

0 20 40 60-20-40-60

0.2

0.4

0.6

LITERS

Paw

cmH2O

C

A = inspiratory pressure

B = upper inflection point

C = lower inflection point

VT

OverdistensionOverdistension

• Respiratory rate

• Max RR at 35 breaths/min

• Efficiency of ventilation decreases with increasing RR

• Decreased time for alveolar emptying

• TV

• Goal of 10 ml/kg

• Risk of volutrauma

• Other means to decrease PaCO2

• Reduce muscular activity/seizures

• Minimizing exogenous carb load

• Controlling hypermetabolic states

• Permissive hypercapnea

• Preferable to dangerously high RR and TV, as long as pH > 7.15

RR and TV are adjusted to maintain VE and PaCO2

• I:E ratio (IRV)

• Increasing inspiration time will increase TV, but may lead to auto-PEEP

• PIP

• Elevated PIP suggests need for switch from volume-cycled to pressure-cycled mode

• Maintained at <45cm H2O to minimize barotrauma

• Plateau pressures

• Pressure measured at the end of inspiratory phase

• Maintained at <30-35cm H2O to minimize barotrauma

Vent settings to improve ventilationVent settings to improve ventilation

•PEEP • Increases FRC

• Prevents progressive atelectasis and intrapulmonary shunting

• Prevents repetitive opening/closing (injury)

•Recruits collapsed alveoli and improves V/Q matching• Resolves intrapulmonary shunting• Improves compliance

•Enables maintenance of adequate PaO2 at a safe FiO2 level

•Disadvantages• Increases intrathoracic pressure (may

require pulmonary a. catheter)• May lead to ARDS• Rupture: PTX, pulmonary edema

PEEP and FiO2 are adjusted in tandem

Oxygen delivery (DO2), not PaO2, should be used to assess optimal PEEP.

Vent settings to improve oxygenationVent settings to improve oxygenation

• Pressure Limited– FiO2

– Rate– I-time– PEEP– PIP

• Pressure Limited– FiO2

– Rate– I-time– PEEP– PIP

• Volume Limited– FiO2– Rate– Tidal Volume– PEEP– I time

• Volume Limited– FiO2– Rate– Tidal Volume– PEEP– I time

MV

MAP

Dealers ChoiceDealers Choice

ARDSnetARDSnet

NIH NHLBI ARDS Clinical Trials Network NIH NHLBI ARDS Clinical Trials Network

Downloaded from www.ardsnet.org

APRVAPRV

HFOVHFOV

Effects of PPV on AfterloadEffects of PPV on Afterload

• RV afterload (overall: increased)– increased: Starling resistor phenomenon– decreased: RV compression, pulmonary

vasodilation due to increased lung volume• LV afterload: decreased due to LV and

thoracic aorta compression

• RV afterload (overall: increased)– increased: Starling resistor phenomenon– decreased: RV compression, pulmonary

vasodilation due to increased lung volume• LV afterload: decreased due to LV and

thoracic aorta compression

Pulsus ParadoxusPulsus Paradoxus

SummarySummary

• What we can do?–hold to a standard of care–define and report appropriately–communicate scientifically–evaluate in the presence and the

absence of invasive monitoring

• What we can do?–hold to a standard of care–define and report appropriately–communicate scientifically–evaluate in the presence and the

absence of invasive monitoring

SummarySummary

• Discuss, defend, discuss• turn turn turn• oygenate?• ventilate?• flow that blood

• Discuss, defend, discuss• turn turn turn• oygenate?• ventilate?• flow that blood