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Competency

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No. PY6.X (Respiratory Physiology)

CompetencyDescribe the dead space and alveolar ventilation & their clinical significance.

D/L/Core K/KH/Y

AM Written/Viva voce

Integration No

Imp. ConceptRapid shallow breathing Vs Slow deep breathing

Phy. Trivia Radford’s Formula (DS)

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Definition

Normal Value

Types

Measurement

Factors affecting:

Physiological, Pathological

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Definition

The part of the respiratory tract where there is no

gaseous exchange is called dead space.

The air present in the dead space is called dead space

air.

Value: 150 ml (approximately equal to the body weight

in pounds). It is 30 % of Tidal volume.

Only the first 350 mL of the 500 mL inspired with each

breath at rest mixes with the air in the alveoli.6

Types of Dead space

1. Anatomical dead space:

It is the space from nose up to Terminal

bronchioles (up to generation 16 conducting zone).

2. Physiological (total) dead space:

It is the anatomical dead space + Alveoli with

wasted ventilation, wasted ventilation occur due to

Underperfusion / overinflation of alveoli

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Total Dead space (Physiological Dead space) =

Anatomical Dead space + Alveolar Dead space

In normal person, Alveolar Dead space is zero

∴ Physiological Dead space = Anatomical Dead space

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Measurement of Dead space

1. Bohr’s equation:

for physiological dead space

2. Fowler’s method:

by single breath N2 wash out method for

anatomical dead space (pure O2 is inhaled then

expired into N2 meter)

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Measurement of Dead space

3. The Radford’s Formula:

By this one can predict anatomical dead space

By this formula……

Weight in pounds = Dead space in ml.

However, this prediction is fairly reliable for

healthy individuals, but not in patient with

respiratory problems.

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Factors affecting

Physiological factors:

• More in males ,

• ↑es with age (as lung inflate)

• Body height

Pathological factors:

• ↑Es in emphysema (as lung inflate),

• Bronchiectasis ( as bronchi dilates),

• Pulmonary embolism (as pulmonary blood flow ↓es) 12

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Alveolar Ventilation

The amount of air in lungs that participate

in gaseous exchange every minute is called

Alveolar Ventilation.

Alveolar Ventilation = (TV – DS) x RR

= (500 – 150) x 12 = 4.2 L/min

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Effect of variations in respiratory rate and depth on alveolar ventilation.

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Alveolar Ventilation…

The alveolar ventilation - the amount of air

reaching the alveoli per minute, is less than the

RMV.

Note that because of the dead space, rapid

shallow breathing produces much less

alveolar ventilation than slow deep breathing

at the same RMV

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Effect of variations in respiratory rate and depth on alveolar ventilation.

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Rapid shallow breathing Slow deep breathing

Measurement of Rs parameters

R parameter Method or Technique

1) TV, IRV, VC, IC simple / computerized spirometer,

respirometer,

2) RV, FRC & TLC He – dilution (close circuit) & N2

wash-out method

3) PEFR Wright’s peak flow meter

4) Sample of air (for

partial pressure)

alveolar (by Haldane-Priestly tube)

expired (by Douglas bag) & blood

(by Haldane’s appara.)

5) Diffusion capacity By DC of CO (x 1.23 for O2)

6) Dead space Indirect - by single breath oxygen / CO2 /

nitrogen,

direct- Bohr equation

7) Work done Pressure-volume (relaxation) curve by P

X V

8) Static

compliance

Slope of P-V curve by P / V

9) Intra pleural

pressure

Indirectly by intra-esophageal balloon

10) Partial

pressure of air

in alveoli

By alveolar gas equation

20Flow-volume curve

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