compliance, airway resitance, work of breathing
DESCRIPTION
Compliance, airway resitance, work of breathing. Design of the ventilatory apparatus. Function : to move the air in and out of lungs. Made up two expansible chambers one inside the other. Lungs. Pleural space. Chest wall. Mechanics of thorax and lungs (alone and combined). - PowerPoint PPT PresentationTRANSCRIPT
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