Download Dynamic Mechanics of Breathing

Dynamic Mechanics of Breathing
สรชัย ศรีสุมะ พบ., Ph.D.
ภาควิชาสรีรวิทยา
คณะแพทยศาสตรศิริราชพยาบาล
มหาวิทยาลัยมหิดล
Learning Objectives
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Define the flow, resistance, velocity and their
changes in their magnitude in airway during
airflow
Describe the factors that contribute to airway
resistance in the lungs
Define the concept of waterfall phenomeonon,
starling resistor, flow limitation and dynamic
compression of airway during expiration
including their principles leading to the
phenomenon
Integrate the principles that are necessary to
understand flow rate measurement, lung volume
change and flow-volume loop
Outline
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Dynamic compression of airway
Equal pressure point
Forced Expiratory Spirography
Work of breathing
Driving Pressure Provides Energy to Move Air
to the Lung
Atmosphere
pressure gradient
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Resistance, poiseuille’s law
Airflow: Laminar flow, Turbulent flow, Reynold
number
Velocity: Bernoulli’s principle
Factors determining airway cross sectional area
Waterfall phenomenon
Flow limitation
0 cmH2O
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ครอบคลุมสรีรวิทยา 2 หนา 497-509
Mouth/Nose
Upper Airway
Lower Airway
Alveoli
Airway Resistance in Respiratory System
Airway Resistance
(cmH2O/Litre/sec)
Percentage
(%)
Normal
Normal
Pharynx-Larynx
0.6
40
Airway > 2 mm
diameter
0.6
40
Airway < 2 mm
diameter
0.3
20
Total airway
resistance
1.5
100
Oral Breathing
Laminar Airflow
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Airflow is proportional to driving pressure or
pressure gradient (ΔP) but inversely proportional
to airway resistance (R) (Ohm’s law)
V=
„
„
ΔP
R
=
PB - Palv
R
V is airflow, measured in liters/second
The dot above indicates the time derivative of
volume
Resistance and Poiseuille’s Law
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When airflow is laminar, air molecules move
smoothly in the same direction
Poiseuille’s law states that airway resistance is
proportional to the viscosity of the gas (η) and
the length of the tube (ℓ), but inversely
proportional to the fourth power of radius (r)
R=
8ηℓ
πr4
Turbulent Flow
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High flow rates, particularly through branched
or irregular tubes
Turbulent flow is present when high
resistance to airflow exists
ΔP α ( V )
2
Turbulent Flow
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If the Reynolds number (Re) > 2000,
turbulent flow develops
2rυρ
Re =
η
r is the radius of tube
υ is gas velocity
ρ is gas density
η is viscosity
Airflow in the Lung
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Laminar: small airways distal to terminal
bronchioles
Transitional: mostly throughout tracheobronchial
tree
Turbulent: only in trachea, where radius is large
and linear air velocity is high during exercise
and coughing
Gas velocity and Airflow in the Lung
V = volume
time
=
=
Total area x length
time
Total area x velocity
Resistance in the Lung
The Bernoulli’s Principle
Total Energy
Velocity
140 cm/sec
Flow
1 liter/sec
Velocity
280 cm/sec
Velocity
140 cm/sec
1 liter/sec
Potential
(Pressure)
Energy
Kinetic
Energy
Distance along tube
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When a fluid moves through a tube at a constant flow
rate, the total energy of fluid (potential + kinetic energy)
decreases because frictional losses convert some of the
energy into heat
Increase in fluid velocity occurs where the tube narrows,
causing an increase in kinetic energy component at the
expense of potential energy, that is pressure decreases
Bernoulli’s Principle and Airplane
Higher velocity, lower pressure
Wing cross-section
Lower velocity, high pressure
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The flow of gas above and beneath the wing must be similar
Velocitytop > velocitybottom
Pressuretop < pressurebottom
The difference in pressure exerted on the wing lifts the
airplane off the runway
Bernoulli’s Principle and Flow Through Tubes
P4
P4
P1
υ2
υ1
Constant flow
(liter/min)
υ1
P4
P2
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P3
< υ2
P1 > P3 >
„
υ1
P2
The greater the increase in velocity, the
greater the decrease in pressure
The small tube will be compressed if P4 > P2
Remember transmural pressure
= pressure inside – pressure outside/
surrounding tissue
Bernoulli Effect and Airflow
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During expiration, gas velocity must increase
dramatically as flow travels toward the
trachea, because the total airway area
decreases
This causes the fall in the airway pressure
(Paw)
Determinants of the Cross-Sectional Area of
the Airway
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Airway Structure
Bronchial Smooth Muscle Tone
Lung Volume
Elastic Recoil of the Lung
Determinants of the Cross-Sectional Area of
the Airway: Airway Structure
Determinants of the Cross-Sectional Area of
the Airway: Bronchial Smooth Muscle Tone
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Autonomic Nervous System
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Parasympathetic (vagus nerve) stimulation via
Muscarinic 3 receptor Æ bronchoconstriction
Sympathetic stimulation via β2 Adrenergic
receptorÆ bronchodilation
Determinants of the Cross-Sectional Area of
the Airway: Bronchial Smooth Muscle Tone
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Autonomic Nervous System
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Non-adrenergic non-cholinergic system
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Bronchodilator nervous pathway
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Nitric oxide, vasoactive intestinal peptide
Bronchoconstrictor nervous pathway
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Substance P and neurokinin A from non-myelinated
C fibers
Determinants of the Cross-Sectional Area of
the Airway: Bronchial Smooth Muscle Tone
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Pharmacological substances
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Parasympathomimetics
β2 agonist, epinephrine
Anticholinergics
Reflex bronchoconstriction from irritants in
airway
Chemicals: histamine, ↓ PAco2 Æ
bronchoconstriction
Determinants of the Cross-Sectional Area of
the Airway: Lung Volume
pleural space
resting
chest wall
airway
alveoli
inspiration
chest wall
pleural space
alveoli
airway
Determinants of the Cross-Sectional Area of
the Airway: Lung Volume
Determinants of the Cross-Sectional Area of
the Airway: Lung Elastic Recoil
End of inspiration
chest wall
pleural cavity
alveoli
Expiration
airway
pleural cavity
alveoli
airway
Waterfall Phenomenon and Respiratory System
Porigin
PTM
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Pdestination
If PTM > Porigin, no flow occurs
If PTM < Pdestination,
driving pressure = Porigin – Pdestination
If PTM > Pdestination,
driving pressure = Porigin – PTM
Waterfall Phenomenon and Respiratory System
Porigin
PTM
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Pdestination
Pressure in the airways decreases during air
flows from alveolus to the mouth
Transmural pressure and force on the airway
determine the airflow
Equal Pressure Point andStarling Resistor
Equal Pressure Point andStarling Resistor
Equal Pressure Point andStarling Resistor
Equal Pressure Point andStarling Resistor
Dynamic Compression of Airway and Equal
Pressure Point
Dynamic Compression of Airway and Equal
Pressure Point
Dynamic Compression of Airway and Equal
Pressure Point
Dynamic Compression of Airway and Equal
Pressure Point
Dynamic Compression of Airway and Equal
Pressure Point
Dynamic Compression of Airway and Equal
Pressure Point
Expiratory Flow Limitation
Expiratory Flow Limitation
Transmural Pressure and Flow Limitation
during Forced Expiration
alveoli
airway
PTM’ = Paw – Ppl
Ppl
PTM = Pel= Palv – Ppl
Ppl
PTM’
PTM = Pel
Palv
Paw
Transmural Pressure and Flow Limitation
during Forced Expiration
Ppl
Ppl
PTM = Pel
PTM = Pel
= Palv – Ppl
Vmax =
Palv
ΔP
R
Paw
=
=
Vmax
airway
PTM’ = Paw – Ppl
PTM’
=
Palv - Paw
R
(Ppl + Pel) – (Ppl + PTM’)
R
Pel – PTM’
R
Factors Determining Flow Limitation during
Forced Expiration
Pel – PTM’
R
=
Vmax
Reduced Pel Æ ↓ Vmax
Increased PTM’ Æ ↓ Vmax
Increased R Æ ↓ Vmax
Measurement of Airway Resistance
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Body plethysmograph: to determine Palv -- ไมเรียน
R=
ΔP
=
V
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PB - Palv
V
Closing volume -- ไมเรียน
Determine the decrease in expiratory airflow
among population
V=
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Δvolume
Δtime
Forced expiratory spirography ------- Lab Nov9,10
Flow-volume loop/curve
------- KSA Nov 8