Basic Pulmonary Mechanics during Mechanical Ventilation

Points of Discussion

Basics , scalars and loops1. Equation of motion2. Airway pressures3. Compliance4. Resistance5. Pressure-Time6. Flow-Time7. Pressure-volume loop8. Flow-volume loop9. Work of breathing10.Hysterexis

Abnormalities1. Air Leak2. Auto PEEP and air trapping3. Active Exhalation4. Inadequate insp flow5. Obstruction6. Trigger sensitivity7. Increased airway resistance 8. Inadequate flow support9. Inadequate sensitivity10.Atelectasis11.Inadequate PEEP12.Over-distension

Inspiration

Spontaneous Breathing

Exhalation

Precondition of Inspiration

• Pa < Pb– Spontaneous

breath• Pb > Pa

– Mechanical ventilation

Pb

Pa

Gas Flow

Pa < Pb

Spontaneous Inspiration

Volume Change

Gas Flow

Pressure Difference

Mechanical Ventilation

Pressure Difference

Volume Change

Gas Flow

“The Feature of the Tube”

R = D PD F

Airway Resistance

Pressure Difference = Flow Rate x Resistance of the Tube

Volume Change = Pressure Difference x Compliance of the Balloon

Volume

Pressure

D V

D P

C = D VD P

Compliance

Tube + Spring Model

Elastic Forces

Resistive Forces

Basic Calculations

Cst = dV / (Pplat-PEEP)

R = (PIP-Pplat) / Flow

dP = R x Flow + dV / C st

time

Pressure

PEEP

Pplat

elastance = Dpressure / Dvolume

volume

transairwaypressure

transthoracicpressure

transrespiratorypressure

Lung Mechanics resistance = Dpressure / Dflowflow

Pressure vs TimeSpontaneous Breath

P aw (c

m H

20)

Time (sec)

Inspiration

Expiration

Pressure vs TimeMechanical Breath

InspirationExpirationP a

w (

cm H

2O)

Time (sec)

}TI

Peak Inspiratory PressurePIP

PEEPTE

Spontaneous vs. Mechanical

Mechanical

Time (sec)

SpontaneousPaw (cm H2O)

Inspiration

ExpirationExpiration

Inspiration

Assisted vs Controlled

Time (sec)

Assisted ControlledPressure (cmH20)

Components of Inflation Pressure

Begin Expiration

P aw (

cm H

2O)

Time (sec)

Begin Inspiration

PIP

Pplateau

(Palveolar)

Transairway Pressure (PTA)}

Expiration

Inspiratory Pause

Begin Inspiration Begin Expiration

P aw (

cm H

2O)

Time (sec)

Airway Resi

stance

Distending (Alveolar)

Pressure Expiration

Inflation Hold(seconds)

Begin Expiration

P aw (

cm H

2O)

Time (sec)Begin Inspiration

PIP

Pplateau

(Palveolar

Transairway Pressure (PTA)

}Exhalation Valve Opens

Expiration

PIP

PIP vs Pplat

NormalHigh Raw

High Flow

Low Compliance

Time (sec)

Paw

(cm

H2O

)PIP

PPlat

PIP

PIP PIP

PPlatPPlat

PPlat

Mean Airway Pressure

Increase FlowIncrease peak pressureLengthen Inspiratory TimeIncrease RateIncrease PEEP

P ressure

T im e

Increasing Mean Airway Pressure

1. Increase flow2. Increase peak pressure3. Lengthen inspiratory time4. Increase PEEP5. Increase Rate

Flow vs Time

Inspiration

Expiration

Time (sec)

Flow

(L/m

in)

Flow Patterns

ACCELERATING

DECELERATING

SINE

SQUARE

Flow Patterns and Effects of Volume

ACCELERATINGDECELERATING SINESQUARE

Inspiratory Flow Pattern

Inspiration

Expiration

Time (sec)

Flow

(L/m

in)

Beginning of inspirationexhalation valve closes

Peak inspiratory flow ratePIFR

Beginning of expirationexhalation valve opens

Total cycle timeTCT

Inspiratory Time

TI

Expiratory Time TE

Expiratory Flow Pattern

Inspiration

Expiration

Time (sec)

Flow

(L/m

in)

Beginning of expirationexhalation valve opens

Peak Expiratory Flow RatePEFR

Duration of expiratory flow

Expiratory time

TE

Spontaneous Breath

Time (sec)

Flow

(L/m

in) Inspiration

Expiration

Mechanical vs Spontaneous

Inspiration

Expiration

Spontaneous

Mechanical

Increased Expiratory Resistance

Time

Flow

Normal Resistance Increased Resistance

Response to Bronchodilator

Before

Time (sec)

Flow

(L/m

in)

PEFR

After

Long TE

Higher PEFR

Shorter TE

Insufficient Expiratory Time

TimeFlow

End-Expiratory Flow

Air Trapping

Inspiration

Expiration

NormalPatient

Time (sec)

Flow

(L/m

in)

Air TrappingAuto-PEEP

}

Excessive Secretions

Inspiration

Expiration

NormalPatient

Time (sec)

Flow

(L/m

in)

Air Leak (Flow Trigger, autotriggering)

Inspiration

Expiration

Time (sec)

Flow

(L/m

in)

Leak in LPM

Active Inspiration or Asynchrony

Flow (L/min)

Time (sec)

NormalAbnormal

Patient’s effort

Excessive Inspiratory Time

Inspiration

Expiration

NormalPatient

Time (sec)

Flow

(L/m

in)

Air TrappingAuto-PEEP

}

Increase WOB and “Fighting” of the ventilator

Obstruction vs Active Expiration

Obstruction Active Expiration

Time (sec)

NormalAbnormal

Flow (L/min)

Trigger Sensitivity

Pressure

Sensitivity levelTime

Flow

Time

Volume vs. Time

Inspiration

Expiration

Time (sec)

Volu

me

(ml)

Inspiratory Tidal Volume

TI

Active Exhalation

Volume (ml)

Time (sec)

Inadequate Inspiratory Flow

Adequate Flow

Time (sec)

Inadequate Flow

P aw

(cm

H2O

)

Air LeakVo

lum

e (m

l)

Time (sec)

Air Leak

Air Leak

Expiratory flow area less than inspiratory flow area

Expired volumeInspired volume

Leak

Pressure

Flow

Volume

FRC and PV Loop

FRC

VOLU

ME

TLC

Negative Positive0

DISTENDING PRESSURE

Normal Compliance

FRC

Components of Pressure-Volume Loop

Volume (mL)

Insp

iratio

n

Expi

ratio

n

PIP

VT

Paw (cm H2O)

Pressure-Volume Loop(Type of Breath)

Controlled Assisted Spontaneous

Vol (

ml)

Paw (cm H2O)

I: Inspiration E: Expiration

I

E

E

E

II

PEEP and P-V Loop

Volume (mL)

VT

PIPPaw (cm H2O)

PEEP

Inflection Points

Pressure (cm H2O)

Volume (mL)

Upper Inflection Point

Lower Inflection Point

Upper Inflection Point: Represents pressure resulting in regional overdistension

Lower Inflection Point: Represents minimal pressure for adequate alveolar recruitment

Decreased Compliance

Volu

me(

ml)

Pressure (cm H2O)

NormalPatient

Lung Compliance Changes and the P-V Loop

Volume (mL)

PIP levels

Preset VT

Paw (cm H2O)

Normal

Volume Targeted Ventilation

Decreased

Increased

Lung Compliance Changes and the P-V Loop

Volume (mL)

Preset PIP

VT

leve

ls

Paw (cm H2O)

Normal

Pressure Targeted Ventilation

Increased

Decreased

Hysteresis

Volume (ml)

Pressure (cm H2O)

Abnormal Hysteresis

Normal Hysteresis

Flow-Volume Loop

Volume (ml)

Inspiration

Expiration

Flow

(L /

min

)

PEFR

FRC

PIFR

VT

Work of Breathing

A: Resistive Work B: Elastic Work

Pressure (cm H2O)

Volume (ml)

B

A

Work of Breathing• WOB is a major source of caloric expenditure and oxygen

consumption• Appr. 70% to overcome elastic forces, 30% flow-resistive work• Patient work is a one of the most sensitive indicator of ventilator

dependency• Comparison of Ventilator and Patient work is a useful indicator

during weaning process• WOB may be altered by changes in compliance, resistance,

patient effort, level of support, PEEP, improper Ti, demand system sensitivity, mode setting

• Elevated WOB may contraindicate the weaning process

WOB Measurements

WOB = ∫0

ti P x Vdt• Elasic work: ABCA• Resistive work

– Inspiratory: ADCA– Expiratory: ACEA

PA

B C

D

E

V

Work of Breathing Measurements

WOB = ∫0

ti P x Vdt• Paw: Ventilator Work: The physical force required to

move gas into the lung, represents the total work of the resp. system (patient + ventilator)

• Peso: Patient Work: done by respiratory muscles, represents the pulmonary work of breathing

• Paw-Ptr: Imposed Work by the Endotracheal tube

P-V Loop and WOB

P

V

P

V

P

VNormal ComplianceNormal Resistance

Normal ComplianceIncreased Resistance

Decreased ComplianceNormal Resistance

Work of BreathingWork per breath is depicted as a pressure-volume areaWork per breath (Wbreath) = P x tidal volume (VT)Wmin = wbreath x respiratory rate

Pressure Pressure Pressure

Volu

me

Volu

me

Volu

me

VT

WR = resistive work

WEL = elastic work

The total work of breathing can be partitioned between an elastic and resistive work. By analogy, the pressure needed to inflate a balloon through a straw varies; one needs to overcome the resistance of the straw and the elasticity of the balloon.

Intrinsic PEEP and Work of Breathing

Volu

me

VT

VT

FRCPressure

PEEPi

Dynamic Hyperinflation

PEEPi = intrinsic or auto PEEP; green triangle = tidal elastic work; red loop = flow resistive work; blue rectangle = work expended in offsetting intrinsic PEEP (an expiratory driver) during inflation

When present, intrinsic PEEP contributes to the work of breaking and can be offset by applying external PEEP.

++

++

Ventilator

₊ ₊

The Pressure and Work of Breathing can be Entirely Provided by the Ventilator (Passive

Patient)

The Work of Breathing can be Shared Between the Ventilator and

the Patient

PAW

PES

patient machine

time

AC mode

The ventilator generates positive pressure within the airway and the patient’s inspiratory muscles generate negative pressure in the pleural space.

Paw = Airway pressure, Pes= esophageal pressure

Work of breath

Resistive Work

Elastic Work of Lung

Elastic Work of Chest

Paw

Pes

Volume

Pres

sure

Work to inflate the chest wall

Inflation Deflation

Relationship Between the Set Pressure Support Level and the Patient’s Breathing

Effort

Carrey et al. Chest. 1990;97:150.

The changes in Pes (esophageal pressure) and in the diaphragmatic activity (EMG) associated with the increase in the level of mask pressure (Pmask = pressure support) indicate transfer of the work of breathing from the patient to the ventilator.

Partitioning of the Workload Between the Ventilator and the Patient

How the work of breathing partitions between the patient and the ventilatordepends on:

• Mode of ventilation (e.g., in assist control most of the work is usually done by the ventilator)• Patient effort and synchrony with the mode of ventilation• Specific settings of a given mode (e.g., level of pressure in PS and set rate in SIMV)

Abnormalities• Air-leak• Air trapping• Increased airway resistance • Inadequate flow support• Inadequate sensitivity• Atelectasis• Inadequate PEEP• Airway obstruction• Over-distension

Air Leak

Volume (ml)

Pressure (cm H2O)

Air Leak

Air LeakInspiration

Expiration

Volume (ml)

Flow (L/min)

Air Leak in mL

NormalAbnormal

Air TrappingInspiration

Expiration

Volume (ml)

Flow (L/min)

Does not returnto baseline

NormalAbnormal

Increased Airway ResistanceInspiration

Expiration

Volume (ml)

Flow (L/min)

Decreased PEFR

NormalAbnormal“Scooped out”

pattern

Increased Raw

Pressure (cm H2O)

Higher PTA

Normal

Slope

Vol (mL)

Lower Slope

Airway Secretions/Water in the CircuitInspiration

Expiration

Volume (ml)

Flow (L/min)

NormalAbnormal

F

VV

F

After SuctionBefore Suction

Airway Obstruction

Optimising PEEP

V

P

PEEP: 3 cmH2O

V

P

PEEP: 8 cmH2O

Inadequate Sensitivity

Volume (mL)

Paw (cm H2O)Increased WOB

Replaced FRC

P

V

Lost FRC

V

P

Atelectasis

Overdistension

Volu

me

(ml)

Pressure (cm H2O)

With little or no change in VT

Paw rises

NormalAbnormal

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