respiratory physiology biii - biv j lee 17.3.10

7/31/2019 Respiratory Physiology BIII - BIV J Lee 17.3.10

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Mechanics of Breathing &

Volumes and Ventilation

By Jackson LeeBy Jackson Lee

Primary CoursePrimary Course

17/03/201017/03/2010


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SyllabusVENTILATIONVENTILATION -- PERFUSION INEQUALITIESPERFUSION INEQUALITIESGeneral Instructional ObjectivesGeneral Instructional Objectives

1.1.An understanding of the normal matching of ventilation and perfusion, the mechanismsAn understanding of the normal matching of ventilation and perfusion, the mechanisms

causing ventilationcausing ventilation--perfusion inequality and an appreciation of its clinical significanceperfusion inequality and an appreciation of its clinical significance

Required AbilitiesRequired Abilities

1.1.To describeTo describe West's zonesWest's zones of the lung and explain the mechanisms responsible for them.of the lung and explain the mechanisms responsible for them.2.2.To explain theTo explain the shunt equationshunt equation..

3.3.To describe and explainTo describe and explain regional ventilationregional ventilation--perfusion inequalitiesperfusion inequalities, their, their clinicalclinical

importanceimportance, and changes with, and changes with postureposture..

4.4. To outline the methods used toTo outline the methods used to measure ventilationmeasure ventilation-- erfusion ine ualitieserfusion ine ualities..

5.5. To explainTo explain venous admixturevenous admixture and its relationship to shunt.and its relationship to shunt.6.6. To explain the clinical significance of changes in anatomical and physiologicalTo explain the clinical significance of changes in anatomical and physiological deaddead

spacespace..

7.7. To explain the effect ofTo explain the effect ofventilationventilation--perfusion inequalityperfusion inequality on oxygen transfer and carbonon oxygen transfer and carbon

dioxide elimination.dioxide elimination.


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Gross Factors

Lungs within thoraxLungs within thoraxseparated from thorax byseparated from thorax bypotential intrapotential intra--pleural spacepleural space

The lun s are elastic withThe lun s are elastic with

tendency to recoil in.tendency to recoil in.Chest wall has tendency toChest wall has tendency tospring outspring outTogether these produce aTogether these produce anegative intranegative intra--pleuralpleuralpressure.pressure.At FRC the two opposingAt FRC the two opposingforces are balancedforces are balanced


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Muscle of RespirationInspiration:Inspiration:

DiaphragmDiaphragm

Innervated by phrenic nerve C3,4,5.Innervated by phrenic nerve C3,4,5.

Normal tidal volume 1Normal tidal volume 1--2 cm2 cm

Forced: 10 cmForced: 10 cm

Moves abdominal content downwardMoves abdominal content downward

increased vertical dimensionincreased vertical dimension

Ribs are moved outwards increaseRibs are moved outwards increase

transverse dimensiontransverse dimension

External intercostalExternal intercostal

Slopes forwards and downwardsSlopes forwards and downwards

Pulls upwards and forwards increasePulls upwards and forwards increase

lateral and AP diameter. bucket handlelateral and AP diameter. bucket handle

Scalene and SternocleidomastoidScalene and Sternocleidomastoid

Elevation first two ribsElevation first two ribs

Important in exercise, resp failure orImportant in exercise, resp failure or

stressstress


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ExpirationExpiration Generally is passive due to the elastic property of lung and chestGenerally is passive due to the elastic property of lung and chest

wall to return to equilibrium.wall to return to equilibrium.

Abdominal muscles: Contracts to move the diaphragm upwardsAbdominal muscles: Contracts to move the diaphragm upwards

Internal costal muscles: pull the ribs down and inward. ImportantInternal costal muscles: pull the ribs down and inward. Importantto prevent bulging of intercostal space.to prevent bulging of intercostal space.


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Lung Volumes


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Tidal Volume (TV): amount of air entering and leaving with normal breath.

~500ml resting

Inspiratory Reserve Volume (IRV): volume that can be inspired above TV

(TLC (TV + FRC)) (~2000 ml)

Expiratory Reserve Volume (ERV): Additional volume that can be expiredfrom a normal end expiratory breath to RV (1500ml)

Residual Volume (RV): gas in lungs after maximal expiration (1500-1900ml)

Inspiratory Capacity: Volume able to be inspired from FRC

Vital Capacity (VC): Maximal forced expiration from TLC (4500-5000ml)

Total Lung Capacity ( TLC): total lung volume (around 6000ml)


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FRCThe amount of air remain in the lung after a quiet expiration.

The equilibrium point of the lung and chest wallResidual volume + Expiratory reserve volume

~ 2.2 L or 30 mL/kg

Factors influencing FRCFactors influencing FRC

HeightHeight ObesityObesity FRCFRC

Gender: female have FRC 10% less than maleGender: female have FRC 10% less than male

Age: increase by around 16 mL per yearAge: increase by around 16 mL per year

Diaphragm tone: FRC decrease in paralysed patient ~400 mLDiaphragm tone: FRC decrease in paralysed patient ~400 mL

Posture: The abdominal content can push against the diaphragmPosture: The abdominal content can push against the diaphragm

Lung disease:Lung disease:

Increase recoil decrease FRC fibrotic lung diseaseIncrease recoil decrease FRC fibrotic lung disease

Decrease recoil increase FRC asthma, emphysemaDecrease recoil increase FRC asthma, emphysema


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Consequence of FRC

FRC below Closing VolumeFRC below Closing Volume Atelectasis/Atelectasis/V/Q mismatch/shuntV/Q mismatch/shunt arterial hypoxiaarterial hypoxia

Increase Work of BreathingIncrease Work of Breathing

Decreased com liance o enin of colla sed alveoliDecreased com liance o enin of colla sed alveoli

Increased airway resistanceIncreased airway resistance

Decrease oxygen reserveDecrease oxygen reserve

Increase pulmonary vascular resistanceIncrease pulmonary vascular resistance


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Dead Space

This is the amount of air in the respiratory system that doesThis is the amount of air in the respiratory system that doesnot participate in gas exchange.not participate in gas exchange.

Anatomical dead space: This is the volume of air in theAnatomical dead space: This is the volume of air in the

conductin airwa .conductin airwa .

Alveolar dead space: This is the volume of air in unperfusedAlveolar dead space: This is the volume of air in unperfusedalveoli.alveoli.

Physiological dead space: This is sum of anatomical andPhysiological dead space: This is sum of anatomical and

alveolar dead space.alveolar dead space.


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Closing Capacity

Airways and alveoli in the dependent part of the lungs areAirways and alveoli in the dependent part of the lungs arealways smaller than those at the top. As the lung volume isalways smaller than those at the top. As the lung volume is

reduced closer to residue volume the dependent airway beginsreduced closer to residue volume the dependent airway begins

to colla se.to colla se.

The lung volume at which dependent airways begins toThe lung volume at which dependent airways begins tocollapse is known as the closing capacity (CC).collapse is known as the closing capacity (CC).

Closing volumeClosing volume == CCCC--RVRV

CC increases with age generally less than FRC in youngCC increases with age generally less than FRC in young

adultadult

FRC = CC supine age 44FRC = CC supine age 44

FRC = CC erect age 66FRC = CC erect age 66

Independent of body positionIndependent of body position


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Significance of CC

FRC < CC means pulmonary blood flow is delivered toFRC < CC means pulmonary blood flow is delivered toclosed airway in dependent airways.closed airway in dependent airways.

Creates a shuntCreates a shunt increased Aa gradientincreased Aa gradient

This means for a iven alveolar OThis means for a iven alveolar O there will be a decreasethere will be a decrease

of arterial POof arterial PO22Given that CC does not change with position but FRC doesGiven that CC does not change with position but FRC does

the effect is more marked in elderly supine patientsthe effect is more marked in elderly supine patients


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Determination of Dead Space & CC

Fowlers MethodAsk the subject to take a single breath of 100% O2 after maximal expiration.

a. Phase I- the beginning of expiration. This gas is coming from the anatomic dead space and ispure oxygen. The phase is barely seen on the tracing.

b. Phase II- seen as the first steep slope on the tracing. The gas is coming from both the bronchial

and alveolar areas and shows an abrupt increase in N2%. (50% dead space & 50% alveolar)

c. Phase III- the plateau phase in which the gas is coming from the alveoli. The percent of

nitrogen changes slowly and evenly producing a flatter line.

d. Phase IV- the percent of nitrogen once again increases sharply due to closure of the airway and

emptying of the apices (high in amount of nitrogen).


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Bohrs Method MeasuringPhysiological Dead Space

VVTT = V= VAA + V+ VDD Total dead space is the sum of alveolar and anatomical dead spaceTotal dead space is the sum of alveolar and anatomical dead space

VVAA = V= VTT -- VVDDVVTT FFee = V= VAAFFAA Amount of COAmount of CO22 producedproduced

VVTT FFee = (V= (VTT -- VVDD )F)FAAVV FF = F= F VV -- FF VV

FFAA VVDD = F= FAAVVTT -- VVTT FFeeFFAA VVDD = V= VTT (F(FAA -- FFee ))

Enghoff modification: replace PEnghoff modification: replace PAACOCO22 withwith

measured Pmeasured PaaCOCO22


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Compliance

Compliance = Volume change per unitCompliance = Volume change per unit

pressure changepressure changeC =C = V/P ml/cmHV/P ml/cmH2200

Is the slope of the volumeIs the slope of the volume--pressurepressure

The steeper the curve the higher theThe steeper the curve the higher the

compliancecompliance easier to be stretched.easier to be stretched.

Slope flatter at high volumesSlope flatter at high volumes stifferstiffer

Normal value of LUNG ALONE =Normal value of LUNG ALONE =200ml/cmH200ml/cmH2200

Elastance = 1/compliance = stiffnessElastance = 1/compliance = stiffness

or resistance to deformationor resistance to deformation


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Compliance

Static compliance: The change in volume per unit pressure changeStatic compliance: The change in volume per unit pressure change

when the flow of gas has ceasedwhen the flow of gas has ceased Measured by inspiration to a certain volume and then relaxing against aMeasured by inspiration to a certain volume and then relaxing against a

closed airway for as long as possible, intrapleural pressure is measure byclosed airway for as long as possible, intrapleural pressure is measure by

oeso ha usoeso ha us

Dynamic compliance: Measure of volume V pressure changeDynamic compliance: Measure of volume V pressure changeduring normal breathing without breath holding.during normal breathing without breath holding.

Calculation is made when there is no gas flow at mouth at the end of inspCalculation is made when there is no gas flow at mouth at the end of insp

& exp parts.& exp parts.

It is frequency dependent and decrease with resp rate due to lung unitsIt is frequency dependent and decrease with resp rate due to lung units

have different time constant.have different time constant.

Specific Compliance: Compliance per unit volume of the lung i.e.Specific Compliance: Compliance per unit volume of the lung i.e.

static compliance/FRCstatic compliance/FRC


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Factors Affecting LungCompliance

Surface tensionSurface tension Most importantMost important

>50% of normal lung recoil>50% of normal lung recoil

Presence of surfactantPresence of surfactant

FRC: age, body size, postureFRC: age, body size, posture

Lung SizeLung Size

Large difference in volumeLarge difference in volume

comparing neonate and adult.comparing neonate and adult.

Tissue elastic fibresTissue elastic fibres Account for 20%Account for 20%

Emphysema, pulmonaryEmphysema, pulmonary

oedema, fibrosisoedema, fibrosis

Lung volume (Optimal at FRCLung volume (Optimal at FRC)) Lobar, lung resectionLobar, lung resection

Collapse or consolidationCollapse or consolidation

Pressure is less differentPressure is less different

GravityGravity

Pulmonary congestionPulmonary congestion


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Surface Tension (Dynes)

This is the imbalance ofThis is the imbalance ofintermolecular forces at the liquid andintermolecular forces at the liquid and

gas interface.gas interface.

molecules is stronger than with air.molecules is stronger than with air.Water would therefore reduce itsWater would therefore reduce its

surface area as small as possiblesurface area as small as possible

Given lung has a surface area of 50Given lung has a surface area of 50--100 m100 m22 it is a considerable force toit is a considerable force to

overcome.overcome.


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Laplace Law

For a bubble (2 airFor a bubble (2 air--fluid interfaces)fluid interfaces)

P = 4T/RP = 4T/R

For a fluid lined alveolus (one airFor a fluid lined alveolus (one air--fluid interface)fluid interface)P = 2T/RP = 2T/R

P = pressure (cmH20)P = pressure (cmH20)T = surface tension, CONSTANTT = surface tension, CONSTANT(without surfactant)(without surfactant)R = radiusR = radius

As PAs P 1/R then R P1/R then R PAnd there would be a tendency forAnd there would be a tendency forsmall alveoli to collapse and emptysmall alveoli to collapse and emptyinto larger alveoliinto larger alveoliThe lung would be unstableThe lung would be unstable


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Surfactant

Composition:Composition: PhospholipidPhospholipid 90% (90% (DipalmitoylDipalmitoyl PhophatidylcholinePhophatidylcholine DPPC)DPPC)

10% protein10% protein

Produced:Produced:

Type 2 Alveolar epithelial cellsType 2 Alveolar epithelial cellsFunctionFunction

Reduce TReduce Tss in alveoliin alveoli

lung recoil and work of breathing lung recoil and work of breathing

Stabilise alveoli of different sizesStabilise alveoli of different sizes

Prevent emptying of small alveoli into large alveoliPrevent emptying of small alveoli into large alveoli

Prevent pulmonary oedema via decrease transudationPrevent pulmonary oedema via decrease transudation


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Compliance of Lung and Chest Wall

1/C1/CTT = 1/C= 1/CLL + 1/C+ 1/CCWCWCompliance of lung and chest wall are both 200 mL/cmHCompliance of lung and chest wall are both 200 mL/cmH22OO

1/200 + 1/200 = 1/1001/200 + 1/200 = 1/100

The total lun and chest wall com liance is 100 mL/cmHThe total lun and chest wall com liance is 100 mL/cmH OO


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Determinants of Compliance

Lung FactorsLung FactorsDecrease Compliance (Stiff lungs)Decrease Compliance (Stiff lungs)Pulmonary fibrosisPulmonary fibrosis

Pulmonary/alveolar oedemaPulmonary/alveolar oedema

Increased blood in lungsIncreased blood in lungs 22ndnd increaseincrease

Chest Wall FactorsChest Wall FactorsDecrease Compliance (stiff chest wall)Decrease Compliance (stiff chest wall)Bone disordersBone disorders RARA

DeformityDeformity kyphosiskyphosis, scoliosis, scoliosis

BurnsBurnspulm venous pressure (LVF, MR etc)pulm venous pressure (LVF, MR etc)

Increased Surface tensionIncreased Surface tension loss ofloss of

surfactant (premature,drowning)surfactant (premature,drowning)

ARDSARDS

Increased ComplianceIncreased Compliance

EmphysemaEmphysema destruction of parenchymadestruction of parenchyma decreased surface area and decreased decreased surface area and decreased

elastic tissueelastic tissue

Diaphragmatic paralysisDiaphragmatic paralysisIncreasedIncreased intraabdominalintraabdominal pressurepressure

(obesity, pregnancy,(obesity, pregnancy,

pneumoperitoneumpneumoperitoneum,, ascitesascites, peritonitis), peritonitis)

PositionPosition supine, prone,supine, prone, trendelenburgtrendelenburg


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Regional Variation in LungVentilation

In an erect subject the ventilation at the bases is greater thanIn an erect subject the ventilation at the bases is greater thanthe apices.the apices.

The base has a lower resting volume so a larger change inThe base has a lower resting volume so a larger change in vvolumeolume

It has a better com lianceIt has a better com liance

The weight of the lung requires that to support it the pressureThe weight of the lung requires that to support it the pressurebelow needs to be higher than that above.below needs to be higher than that above.

The intrapleural pressure at the apices (The intrapleural pressure at the apices (--10 cmH10 cmH22O) and atO) and at

the base (the base (--2 cmH2 cmH22O)O)


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Regional Variation in Lung Ventilation

Basal AlveoliBasal AlveoliIntrapleural Pressure =Intrapleural Pressure = --2cmH2cmH2200Resting volume is lowResting volume is lowOn steep part of VOn steep part of V--P curve and hence is veryP curve and hence is verycompliant and expands more per unit pressurecompliant and expands more per unit pressurechange than the apexchange than the apex

Alveoli at ApexAlveoli at ApexMore negative resting expanding pressure =More negative resting expanding pressure = --10cmH10cmH2200Resting Volume highResting Volume high

-- ,,

hence increase in volume per unit pressure ishence increase in volume per unit pressure isless than base and hence its proportion ofless than base and hence its proportion ofalveolar ventilation is less.alveolar ventilation is less.

Note: with position change the weight andNote: with position change the weight andventilation redistributes to the dependent sideventilation redistributes to the dependent side


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Time Constants of the Lung

A lung unit can be considered as an airway with the alveoli it suppliesA lung unit can be considered as an airway with the alveoli it supplies

The resistance and compliance of a lung unit affect the rate of air flow into andThe resistance and compliance of a lung unit affect the rate of air flow into andout of the unit and the time to filling/emptying of that unit.out of the unit and the time to filling/emptying of that unit.A higher resistance airway will fill/empty slowerA higher resistance airway will fill/empty slowerA lower compliance (stiffer) unit will fill quickerA lower compliance (stiffer) unit will fill quickerAnd ViceAnd Vice--VersaVersa

The timeThe time--dependent filling/emptying of a lung unit can be expressed by its TIMEdependent filling/emptying of a lung unit can be expressed by its TIMECONSTANTCONSTANT

= RC= RCR: Resistance C: ComplianceR: Resistance C: ComplianceAnd is used to describe the rate of change of an Exponential ProcessAnd is used to describe the rate of change of an Exponential Process

TC = the time it would take to complete the process if it continued at its originalTC = the time it would take to complete the process if it continued at its originalraterateAn exponential process (eg flow of gas into lung unit) is 95% complete after 3An exponential process (eg flow of gas into lung unit) is 95% complete after 3Time ConstantsTime Constants


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For normal lung tissueFor normal lung tissueResistance = 2cmHResistance = 2cmH220/L/sec0/L/secTotal Compliance = 100ml/cmHTotal Compliance = 100ml/cmH2200Which gives a Time Constant of 0.2 secondsWhich gives a Time Constant of 0.2 secondsTherefore 95% of filling/emptying is complete in 0.6 secondsTherefore 95% of filling/emptying is complete in 0.6 seconds

SLOW ALVEOLISLOW ALVEOLI (slower filling/emptying)(slower filling/emptying)Resistance length of TCResistance length of TCCompliance TCCompliance TC

Time Constants of Lung Units

R TCR TCC TCC TC

The presence of lung units with different time constants means that gas flow can stillThe presence of lung units with different time constants means that gas flow can stilloccur between units at the end of insp/exp when there is not gas flow at the mouthoccur between units at the end of insp/exp when there is not gas flow at the mouthIt also means that regional ventilation depends on RR with RR leading to It also means that regional ventilation depends on RR with RR leading to ventilation of the long TC lung units as a percentage of Tidal Volume.ventilation of the long TC lung units as a percentage of Tidal Volume.


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Gas Flows

Physics of Gas FlowPhysics of Gas Flow

For gas to flow a pressure difference must existFor gas to flow a pressure difference must existThe pressure difference required to achieve a certain flow is dependentThe pressure difference required to achieve a certain flow is dependenton the type of flowon the type of flow

LaminarLaminar

TurbulentTurbulent TransitionalTransitional

For Laminar flow driving pressure is proportional to flowFor Laminar flow driving pressure is proportional to flowP = kV (where k is a constant)P = kV (where k is a constant)

For Turbulent flow driving pressure is proportional to flow squaredFor Turbulent flow driving pressure is proportional to flow squaredP = kVP = kV22


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Poiseuilles Law

Describes Laminar gas flow in a straight circular tubeDescribes Laminar gas flow in a straight circular tube

PP rr44 P = driving pressureP = driving pressureVolume flow rate =Volume flow rate = V =V = ------------------------ r = radiusr = radius

88ll = viscosity= viscosityl = lengthl = length

Most significant factor is the radius!Most significant factor is the radius!

and from Ohms Law as:and from Ohms Law as:Resistance (R) = Driving pressure / Flow = P / VResistance (R) = Driving pressure / Flow = P / VThenThen

88 llResistance = R =Resistance = R = ------------------------

rr44


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Turbulent or Laminar?

Reynolds NumberReynolds Number

: density: densityd: diameterd: diameter

: velocit: velocit

: viscosity: viscosityRe < 1000 consider laminarRe < 1000 consider laminar

Re > 1500 (>2000) almost entirely turbulentRe > 1500 (>2000) almost entirely turbulent


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Laminar Turbulent

Smooth, cylindrical rigid tubes Not Smooth, cylindrical rigid tubes

Occurs at low flows Usually occur at fast flow rate

Fast moving central spike with

decreasing velocity from axis to edge

No high axial flow velocity

Density is less important Density is important

Pressure is proportion to flow Pressure is proportional to flow squared


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Where in Bronchial Tree doesLaminar and turbulent flow occur

Bronchial tree is a rapidly branching system and turbulence occurs at branch pointsBronchial tree is a rapidly branching system and turbulence occurs at branch points

True Laminar FlowTrue Laminar FlowOnly in very small airwaysOnly in very small airwaysHere Re is very low ( approx 1 in terminal airways)Here Re is very low ( approx 1 in terminal airways)Low velocity, low radius (but high combined calibre)Low velocity, low radius (but high combined calibre)

True Turbulent FlowTrue Turbulent FlowAt tracheaAt tracheaEspecially with exercise when velocity is highEspecially with exercise when velocity is high

Transitional FlowTransitional FlowMixture of laminar and turbulentMixture of laminar and turbulentMost airwaysMost airways

Evidence of this by decreased resistance with HeliumEvidence of this by decreased resistance with HeliumHence driving pressure (P) is determined by flow rate and its squareHence driving pressure (P) is determined by flow rate and its square

P = kP = k11V + kV + k22VV22


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Respiratory Resistance

Airway resistanceAirway resistance

Tissue resistanceTissue resistance

Inertia (negligible)Inertia (negligible)


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Sites of Airway Resistance (AWR)

FromFrom PoiseuillesPoiseuilles law one wouldlaw one would

think most resistance would be atthink most resistance would be atsmall airways because of tinysmall airways because of tiny radiusradius

Most resistance/pressure dropMost resistance/pressure drop

generationsgenerationsAirways < 2mm diameter (gen 8+)Airways < 2mm diameter (gen 8+)

contribute


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Lung volume and AWR

Like extraLike extra--alveolar blood vesselsalveolar blood vessels

lung volume radial traction lung volume radial traction

increased calibre resistanceincreased calibre resistanceConductance = reciprocal of resistanceConductance = reciprocal of resistance

Airway Conductance V Lung volume aAirway Conductance V Lung volume a

linear linear

Closing Capacity:Closing Capacity:

At very low lung volumes small dependentAt very low lung volumes small dependent

airways can close completely.airways can close completely.

Pts with high airway resistance often breathPts with high airway resistance often breath

at high lung volumes to help reduceat high lung volumes to help reduce

AWRAWR


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Factors Influencing AirwayResistanceRadius of airwayRadius of airway

Intramural: secretions, foreign bodies, oedema or intraIntramural: secretions, foreign bodies, oedema or intra--luminal lesionsluminal lesions

Mural: Smooth muscle toneMural: Smooth muscle tone

Cholinergic: AChCholinergic: ACh

NonNon--adrenergic Non cholinergic VIP, SP and Neurokinin Aadrenergic Non cholinergic VIP, SP and Neurokinin A Extramural: Compression by tumours, tension pneumothorax, dynamic airwayExtramural: Compression by tumours, tension pneumothorax, dynamic airway

compression during forced expiration.compression during forced expiration.

Lung VolumeLung Volume

Anatomical siteAnatomical site

Highest resistance bronchi due to the small cross sectional area.Highest resistance bronchi due to the small cross sectional area.

Types of FlowTypes of Flow

TurbulentTurbulent

LaminarLaminar

TransitionalTransitional


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Measurement of Airway Resistance

AWR =AWR = pressure difference Mouthpressure difference Mouth Alveoli Alveoliflow rateflow rate

Alveolar pressure can be measure byAlveolar pressure can be measure by1.1. Deduction from measurement by a whole body plethysmographDeduction from measurement by a whole body plethysmograph

using Boyles Law Pusing Boyles Law P11VV11 = P= P22VV22 = K= K

2.2. Estimated from oesophageal pressure during normal breathingEstimated from oesophageal pressure during normal breathing


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Work of Breathing

Work = Pressure * Volume (joules)Work = Pressure * Volume (joules)

During normal tidal breathing the inspiratory muscles do all work andDuring normal tidal breathing the inspiratory muscles do all work andexpiration is passive.expiration is passive.

Inspiratory muscles do work againstInspiratory muscles do work against1.1. Frictional Forces (airway and tissue resistance) 35% of work, lost asFrictional Forces (airway and tissue resistance) 35% of work, lost as

heatheat2.2. Elastic Forces 65%, stored as potential energyElastic Forces 65%, stored as potential energy

During Expiration the inspiratory muscles relax, elastic materials return toDuring Expiration the inspiratory muscles relax, elastic materials return totheir original length and release stored potential energy which istheir original length and release stored potential energy which isused to overcome frictional airway and tissue forces and lost as heat.used to overcome frictional airway and tissue forces and lost as heat.

Normal Value of WOB = 3 watts= 3joules/min, 3ml of 0Normal Value of WOB = 3 watts= 3joules/min, 3ml of 022

Di f i l l h


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Area (1) = work against ElasticArea (1) = work against ElasticForces, stored as potential EForces, stored as potential E

( increased work with( increased work with TV or if lowTV or if low

lung compliance (highlung compliance (highelastance))elastance))

Area 2 = work a ainst nonArea 2 = work a ainst non--elasticelastic

Diagram of intrapleural pressure changeduring inspiration and expiration ofnormal tidal breath from FRC.

forces (airway and tissueforces (airway and tissueresistance) lost as heat ( withresistance) lost as heat ( withfaster flow rates/RR or AWR)faster flow rates/RR or AWR)

Total Work of Breathing (WOB) =Total Work of Breathing (WOB) =sum of elastic and nonsum of elastic and non--elasticelastic

workwork

Area (3) = Expiration, work againstArea (3) = Expiration, work againstnonnon--elastic forces, lost as heatelastic forces, lost as heat

H Mi i i WOB


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How to Minimise WOB

Work = Pressure * VolumeWork = Pressure * VolumeTo minimise work aim to reduce pressure to achieve aTo minimise work aim to reduce pressure to achieve acertain volumecertain volume

Increase compliance = volume change per unitIncrease compliance = volume change per unitpressure changepressure changeMinimise resistance = pressure change per unit flowMinimise resistance = pressure change per unit flow

compliance curvecompliance curveControl RRControl RRRR flow/velocity resistance WOB (nonRR flow/velocity resistance WOB (non--elastic)elastic)RR and TV advantageous inRR and TV advantageous in obstructive lungobstructive lungdiseasedisease as minimises frictional work and increasesas minimises frictional work and increases

stored potential, also maximises alveolar ventilation asstored potential, also maximises alveolar ventilation as% of MV and decreases dead space.% of MV and decreases dead space.InIn Restrictive Lung diseaseRestrictive Lung disease (low complaince/stiff(low complaince/stifflung) RR (small rapid breaths) minimises WOB thelung) RR (small rapid breaths) minimises WOB themajority of which is from elastic work.majority of which is from elastic work.

From Poiseuilles law and Reynolds no. multipleFrom Poiseuilles law and Reynolds no. multipleways of minimising AWR and hence WOBways of minimising AWR and hence WOB


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The End

respiratory physiology biii - biv j lee 17.3.10

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