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 structures Display a constant relationship between distending pressure and 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 and pleural pressure P for thoracic is difference between atmospheric pressure and pleural pressure AP PP IP

Pressure-Volume curve of the Lungs Ideally should be a straight line

Compliance of lungs L/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/C T =1/C L +1/C W

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 P1P1 P2P2 T T r1r1 r2r2 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= KV 2 For airways P= K 1 V +K 2 V 2

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 V WVR 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 on degree of expansion of the lungs Patients with restrictive lung diseases……………. shallow breaths, increase frequency Magnitude of viscous component of work of breathing depends on the velocity of air flow Obstructive lung disease patients……….slow and deep breaths

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