Basic Pulmonary Mechanics during Mechanical Ventilation

Equation of Motion

dP = R x Flow + dV / C st

Points of Discussion

1.Equation of motion2.Airway pressures3.Compliance4.Resistance5.Pressure-Time6.Flow-Time7.Pressure-volume loop8.Flow-volume loop

Inspiration Inspiration

Spontaneous Breathing

Exhalation Exhalation

Precondition of Inspiration

• Pa < Pb– Spontaneous

breath• Pb > Pa

– Mechanical ventilation

Pb

Pa

Gas Flow

compliance = Dvolume / Dpressure

volume

transairwaypressure

transthoracicpressure

transrespiratorypressure

Lung Mechanics resistance = Dpressure / Dflow

flow

Tube + Spring Model

Elastic Forces

Resistive Forces

“The Feature of the Tube”

R = D

P D F

Airway Resistance

Pressure Difference = Flow Rate x Resistance

Pressure Difference = Volume Change / Compliance Pressure Difference = Volume Change / Compliance

Volume

Pressure

D V

D P

Cs = D VD P

Compliance

Cs= D

VD P

R = D P D

F

Compliance and Resistance

Equation of Motion

DYNAMIC CHARACTERISTICS:dP = dV / Cdyn

DYNAMIC CHARACTERISTICS:dP = dV / Cdyn

RESISTANCE:dPresistive = R x Flow

RESISTANCE:dPresistive = R x Flow

STATIC COMPLIANCE:dPdistensive = dV / Cst

STATIC COMPLIANCE:dPdistensive = dV / Cst

dP = dPresist. + dP dist.

dP = R x Flow + dV / C st

Equation of Motion

dP = R x Flow + dV / C st

Components of Inflation PressureComponents of Inflation Pressure

Begin Expiration

Pa

w

(cm

H2O

)

Time (sec)

Begin Inspiration

PIP

Pplateau

(Palveolar)

Transairway Pressure (PTA)}

Expiration

Inspiratory Pause

Begin Inspiration Begin Expiration

Paw

(cm

H2O

)

Time (sec)

Airway

Resi

stance

Distending (Alveolar)

Pressure Expiration

Inflation Hold(seconds)

Begin Expiration

Paw

(c

m H

2O)

Time (sec)

Begin Inspiration

PIP

Pplateau

(Palveolar

Transairway Pressure (PTA)

}Exhalation Valve Opens

Expiration

PIP

Spontaneous vs. Mechanical

Mechanical

Time (sec)

SpontaneousPaw (cm H2O)

Inspiration

ExpirationExpiration

Inspiration

PIP vs PplatPIP vs Pplat

NormalHigh Raw

High FlowLow Compliance

Time (sec)

Paw

(c

m H

2O

)

PIP

PPlat

PIP

PIP PIP

PPlatPPlat

PPlat

Mean Airway Pressure

Increase FlowIncrease peak pressureLengthen Inspiratory TimeIncrease RateIncrease PEEP

Inspiratory Flow Pattern Inspiratory Flow Pattern

Inspiration

Expiration

Time (sec)

Flo

w (

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

Peak Expiratory Flow Rate PEFR

Flow vs TimeFlow vs Time

Inspiration

Expiration

Time (sec)

Flo

w (

L/m

in)

Flow PatternsFlow Patterns

ACCELERATING

DECELERATING

SINE

SQUARE

Flow Patterns and Effects of Volume

Flow Patterns and Effects of Volume

ACCELERATINGDECELERATING SINESQUARE

Mechanical vs SpontaneousMechanical vs Spontaneous

Inspiration

Expiration

Spontaneous

Mechanical

Volume vs. TimeVolume vs. Time

Inspiration

Expiration

Time (sec)

Vo

lum

e (m

l)

Inspiratory Tidal Volume

TI

FRC and PV Loop

FRC

VO

LU

ME

TLC

Negative Positive0

DISTENDING PRESSURE

Normal Compliance

FRC

Components of Pressure-Volume LoopComponents of Pressure-Volume Loop

Volume (mL)

Insp

irat

ion

Expirat

ion

PIP

VT

Paw (cm H2O)

PEEP and P-V LoopPEEP and P-V Loop

Volume (mL)

VT

PIPPaw (cm H2O)

PEEP

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