2001 Respiratory Review

8/13/2019 2001 Respiratory Review

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Review of Respiratory

Physiology


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Steady State

Lung O2Uptake rate

= Cell O2Utilization rate

Cell CO2Production rate

= Lung CO2Release rate


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Gaseous Environment.

Atmosphere: Nitrogen and Oxygen,

negligible Carbon Dioxide.

Clinical Relevance of Environment Altitude: PO2depends on PB

Suffocation: PO2depends on fractional O2

Oxygen therapy: PO2depends on fractional O2

P F PI I BO O2 247 ( )


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Diffusion and Diffusion

Abnormality


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Ficks Law for Diffusion for Gases

O2

CO2

T

P1

P2

A

( )V A D

TP P

gas

1 2


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Single Breath DLCO

Single inspiration of a dilute CO

mixture

10 second breath-hold

Measure CO uptake using infrared

detector to compare inspiratory and

expiratory concentrations

Normal Value: 25 ml/min/mmHg


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Clinical Interpretation of DLCO.

Decreases with loss of surface area.

Decreases with increasing membrane

thickness

Decreases with ventilation/perfusionmismatching

Summary: DLCO better index of overalllung function than diffusion capabilitiesper se.


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Two Factors Affect Gas Transfer

Rate

Diffusion rate of a gas Perfusion Rate (Pulmonary blood

flow)

Gases must be carried away to maintainlocal diffusion gradients in the lung.


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O2Diffusion duringNormoxia


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Lung Mechanics in Obstructive

and Restrictive Disease


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Measuring vital capacity and its

subcomponents. Use a spirometer.

TLC

RV

VC

TV

FRC

ICIRV

ERV

RV

Can Use

Spiromenter

Cant Use a Spirometer


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Measuring Residual Volume

Cant use a Spirometer

Use instead:

Nitrogen Washout

Helium Dilution Method

Plethysmograph


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Obstructive Disease

Difficult to get air out of the lungs

Obstruct expiration

Examples:

emphysema

chronic bronchitis

asthma.


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Restrictive Disease

Difficult to get air in to the lungs

Restrict inspiration

Examples:

intersitial fibrosis

sarcoidosis

muscular diseases

chestwall deformities.


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Lung Capacity and Disease

Summary

Obstructive Disease:

Decreased VC

IncreasedTLC, RV, FRC.

Restrictive Disease:

Decreased VC

DecreasedTLC, RV, FRC.


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Fig 5: Lung Capacity and

Disease

Normal

RV

ERV

TV

IRV

FRC

VC

Restrictive

RV

ERV

TV

IRV

FRC

VC

Obstructive

RV

ERV

TV

IRV

FRC

VC

125

100

75

50

25

0

%NormalTLC


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Forced Vital Capacity

FEV1.0 / FVC Ratio


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Small Airways Disease

FEF25-75


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Flow -Volume Curves

Peak Flow


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Pulmonary Function Summary

Obstructive

Disease

Restrictive

Disease

FEV1.0

Decreased Decreased

FVC Decreased Decreased

FEV1.0/FVC Decreased Unchanged or

IncreasedPeak Flow Decreased Decreased or

Unchanged

RV/TLC Increased Unchanged


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CLINICAL USE OF IDEAL

ALVEOLAR GAS VALUES


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Ideal Alveolar Gas Equation.

P PP

RP F R

RA IA

A IO OCO

CO O2 2

2

2 21

Clinically Useful Form:

Complete Form:

P PP

RA I

A

O O

CO

2 2

2


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USE #1

Compare PAO

2to P

aO

2

Healthy people: PAO2= PaO2

Two Approaches to Comparison

(PAO2 - PaO2) difference

PaO2/ PAO2 ratio


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A-a Difference

PAO2- PaO2

Normally 5-20 mmHg

Because of normal anatomical shunt

Ventilation/Perfusion mismatching.

A-a difference increases with

pulmonary disease. Problem: Normal range changes on

100% O2.


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a/A ratio

Normally averages just over 0.8 (Am.Rev. Resp. Dis. 109: 142-145, 1974).

a/A ratio falls with pulmonarydisease.

Lower limit normal:

young (room air) : 0.74 older(room air) : 0.78

Both groups (100% O2): 0.82


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(A-a) Difference vs. a/A Ratio

Normal

Normal

(A-a)PO2Difference(mmHg)

a/A

PO2

ratio

Sick

Sick


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Use #2

PAO

2Estimates P

cO

2

Useful for calculation of VenousAdmixture or Shunt


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Venous Admixture

'

'

Q

Q

C C

C C

S

T

c a

c v

O O

O O

2 2

2 2

QT CaO2

(QT - Qs ) CcO2

Qs CvO2

PAO2


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Diagnosis of True Shunt

Breathing 100% oxygen -

will notabolish hypoxemia due toshunt

REASON: shunted blood never

exposed to the high alveolar PO2.


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Blood Gases


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Oxygen Blood Gas Quantities

Partial Pressure

Saturation

Content

Carrying Capacity: O2content at

100% saturation.


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Significance of Sigmoid Curve

4 Point Curve

Critical PO2

V


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Defining Content and Capacity

C Hb SOO

2

2136100%

. [ ] %


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Blood

Hemoglobin --> Allows Blood to hold

more oxygen. P50: Reciprical to Hb-O2Affinity

(H+, CO2, Temp, & 2,3-DPG)

Capacity: Anemia, Polycythemia

Hematocrit

Hemoglobin

Hemoglobin variants exist (e.g. Hbf)


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Four (+one) Things Change

Oxyhemoglobin Affinity Hydrogen Ion Concentration, [H+]

Carbon Dioxide Partial Pressure, PCO2

Temperature [2,3-DPG]

Special Case: Carbon Monoxide


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Three Things That Change O2

Carrying Capacity

Changes in Hb Concentration

Presence of Carbon Monoxide

Formation of Methemoglobin


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Minute Ventilation

Flow (vol/time) moved into or out of

the lungs.

Measured by collecting expiredvolume for a fixed time.

Normal value is 7.5 L/min (BTPS).

V V fE T


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Partitioning Minute Ventilation.

Alveolar Ventilation: the volume per

min entering gas exchange surfaces.

Dead space Ventilation: the volumeper min that is wasted

( )V V V V V f V f V f A E d T d T d


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Alveolar Ventilation Equation.

(Rearranged)

PV (STPD)

863 mmHgACO

CO2

2

( )V BTPSA


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Defining Adequate Ventilation

Normal--> PaCO2= 40 mmHg

Hypoventilation --> High PaCO2

Hyperventilation --> Low PaCO2


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Respiratory Acid-Base

Henderson-Hasselbach Equation:

Changes in PCO2cause changes in [H+] bymass action. Increased PCO2 resp. acidosis

Decreased PCO2 resp. alkalosis.

CO H O H CO H H CO 2 2 2 3 3 Carbonic Anhydrase

pHH CO

PCO

6 1

0 03

3

2

. log[ ]

( . )


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Hypoxemia Analysis

Step 1

Is PACO2> 40 mmHg

ANDa/A > 0.74 or (A-a) < 20 mmHg

PureHypoventilation

yes

Choose between:

ShuntDiffusion AbnormalityV/Q Mismatching

No

Continue


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Hypoxemia Analysis

Step 2

CanHypoxemia be eliminated

by 100% O2

TrueShunt

No

Choose between:

Diffusion AbnormalityV/Q Mismatching

Yes

Continue


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Hypoxemia Analysis

Step 3

Is the DLCO

Normal?

Diffusion Normalmust be

V/Q Mismatching

yes

No

Cant choose between:Diffusion AbnormalityV/Q Mismatching

or Combination


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Questions?

Documents

2001 Respiratory Review