Compliance, airway resitance, work of breathing

Chest wall

Lungs

Pleural space

Design of the ventilatory apparatus

Function : to move the air in and out of lungs

Made up two expansible chambers one inside the other

Mechanics of thorax and lungs (alone and combined)

Development of pleural pressure (negative)

Lungs and thoracic cage are both elastic structuresDisplay a constant relationship between distending pressureand change in volume

Compliance

Change in volume per unit change in pressure

Measure of distensibility

Determination of the combined compliance of thorax and lungs

Apply negative/positive pressure around the thorax and record the change in the volumes

C

Inhale an amount of air, relax the muscles and measure the pressure developed due to recoil

V is same for both lungs and chest wall

P for lungs is difference between alveolar pressure andpleural pressure

P for thoracic is difference between atmospheric pressure andpleural pressure

AP

PP

IP

Pressure-Volume curve of the Lungs

Ideally should be a straight line

Compliance of lungs --------------0.2L/cm water

What will happen if……..

•There is only one lung

•In children

Specific compliance…………..compliance/FRC

Compliance of thoracic cage

0.2L/cm water in adults

Compliance of total respiratory system………..0.1L/cm of water

Formula :

1/CT=1/CL+1/CW

Meaning of the Rahn Diagram

Surface Tension

• Force acting across an imaginary line 1 cm. long in a liquid surface

• Laplace’s Law:

For each surface of a bubble, pressure is equal to twice the tension divided by the radius

P1

P2

T

T

r1

r2

r

TP

2

2

rP

2

rPT 2211

1221 PP Then, rr If Result: Small Bubble Collapses

T/R = t/r P1 = P2

Role of surfactant

Small alveoli will not empty into large alveoli

Difference of pressure exists between the ends

Pressure difference depends on the rate and pattern of flow

3 patterns of flow

Laminar flow

Turbulent flow

Transitional flow

Airflow through tubes

Resistance during flow

P1 P2

Laminar flow

Turbulent flow

(for most of respiratory system)

What decides whether flow through a tube will be laminar or turbulent?

Reynold‘s number

Re = Vdρ/η

If reynold;s number > 2000, flow will be turbulent

Laminar airflow in tubes<2mm

Pressure flow characteristics

By poisseuill, for laminar flow

Critical importance of radius

For turbulent flow P= KV2

For airways

P= K1V +K2V2

Airway resistance

Large diameter

Parallel branching

Airway resistance

• Airway resistance is defined as the ratio of the pressure drop between the mouth and alveoli to the volume flow rate

• Chief site of airway resistance: medium sized bronchi• Smaller airways contribute very little to total resistance

due to their numerous parallel branching resulting in a large cross-sectional area

• Average normal airway resistance in healthy adults ……1.6cm water/L/s

R = P/V

Factors determining airway resistance

Radius of the tube

Nature of flow : turbulence and velocity of flow

Factors influencing airway resistance

State of contraction of bronchial musculature

Lung volume

Breathing….expiration much more difficult in asthmatic attack

Dust and smoke

Work of breathing

W.D = F X D

W.D = P X V

Plot the change in interpleural pressure against change in volume

Work done during inspiration

P

VWVR 35%

WE 65%

Work done to overcome elastic recoil of lungs and chest wall Work done to overcome viscous resistance

WTR 7%

WAR 28%

Work done during expiration

P

V

Dissipated as heat

Energy cost of breathing

The magnitude of elastic component of the work depends ondegree of expansion of the lungs

Patients with restrictive lung diseases…………….shallow breaths, increase frequency

Magnitude of viscous component of work of breathingdepends on the velocity of air flow

Obstructive lung disease patients……….slow and deep breaths

Thankyou