Physiology of Ventilation
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Transcript of Physiology of Ventilation
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Physiology of Ventilation
Principles of Ventilation
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Educational Objectives
• Define and differentiate between compliance,
elastance, and resistance
• List the normal values for the pressures
associated with the act of breathing
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Educational Objectives
• Describe the distribution of ventilation within
the lung, listing factors that affect how air is
distributed
• Describe the normal perfusion of the lung,
listing the factors that affect blood flow
• Define ventilation/perfusion ratio
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Definitions
• Ventilation – movement of air between the
atmosphere and the alveoli
• Respiration – movement of gas molecules
across a membrane
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Airflow Into And Out of The Lungs
• Intrapulmonary Pressure (Palv)
– Pressure at the alveolus; changes from positive
to negative during ventilatory cycle (-5 to +5 cm
H2O)
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Airflow Into And Out of The Lungs
• Intrapleural pressure (Ppl)
– Always negative during normal breathing –
(-5 to -10 cm H2O)
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Airflow Into And Out of The Lungs
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Normal Inspiration
• Diaphragm contracts
• Intrathoracic volume increases
• Intrapleural pressure increases in negativity
• Increase in volume causes decrease in
intrapulmonary pressure
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Normal Inspiration
• Decrease in intrapulmonary pressure creates
negative pressure gradient relative to the
atmospheric pressure
• Air flows into the lungs until pressures
equalize
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Normal Expiration
• Diaphragm relaxes, moving upward
• Intrathoracic volume decreases
• Intrapleural pressure becomes less negative
• Decrease in volume creates increase in
intrapulmonary pressure
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Normal Expiration
• Increase in intrapulmonary pressure creates
positive pressure gradient relative to the
atmospheric pressure
• Air flows out of the lungs until pressures
equalize
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Airflow Into And Out of The Lungs
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Factors Affecting Lung Volume
• Compliance
• Elastance (Elasticity)
• Resistance
• Muscle strength and endurance
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Compliance
• The ratio of the change in volume to a
given change in pressure
• Normal value – 100 ml/cm H2O
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Types of Compliance
• Dynamic compliance – measured
during normal breathing cycle
Cdyn = Volume _ Peak Inspiratory Pressure
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Types of Compliance
• Static compliance – measured during breath-
holding procedure
Cplat = Volume _
Plateau Pressure
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Elastance
• The physical tendency of an object to return
to its initial state after deformation
• Inverse of compliance
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Resistance
• Opposition to a force; ratio of pressure
change to flow change• Poiseuille’s Law – ΔP = 8nlV r4
• R = P1 – P2
Volume
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Factors Affecting Muscle Strength and Endurance
• Gender
• Age
• Training
• Position
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Factors Affecting Muscle Strength and Endurance
• Underlying cardiac, pulmonary, and muscular
disorders
• Electrolyte imbalances
• Acid-base disturbances
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Factors Affecting Muscle Strength and Endurance
• Endocrine abnormalities (e.g., thyroid
disorders)
• Prolonged use of steroids
• Neuromuscular blocking drugs
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Evaluation of Muscle Strength and Endurance
• Measurement of transdiaphragmatic
pressure
• Maximum voluntary ventilation (MVV)
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Distribution of Ventilation
• Dead Space
– Ventilation not involved in gas exchange
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Dead Space
• Anatomic dead space
– Volume of ventilation in conducting airways
• Alveolar dead space
– Volume of ventilation in alveoli which are under
perfused or not perfused
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Dead Space
• Physiologic dead space
– Sum of anatomic and alveolar dead space
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Normal Distribution of Ventilation (Upright Position)
• Pleural pressure lower (more negative) at
apex of lung
• Greater transpulmonary pressure at apex
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Normal Distribution of Ventilation (Upright Position)
• Alveoli at apex more distended at FRC than
those at base
• Alveoli at base receive greater ventilation
(are able to distend further) than the alveoli
at apex
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Factors Affecting Distribution of Ventilation
• Increased regional resistance (inflammation)
• Localized changes in compliance (blebs)
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Distribution of Perfusion
• Blood flow determined by difference between
pulmonary vascular pressure and alveolar pressure
– At apex, alveolar pressure greater than pulmonary
vascular pressure – no blood flow (Zone 1)
– At base, pulmonary vascular pressure greater than
alveolar pressure – minimal ventilation (Zone 3)
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Distribution of Perfusion
• Blood flow determined by difference between
pulmonary vascular pressure and alveolar pressure
– Between Zones 1 And 3 (Zone 2), blood flow determined
by the difference between pulmonary vascular pressure
and alveolar pressure
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Three Lung Zones
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Distribution of Perfusion
• Lowest resistance to blood flow is at FRC;
resistance increases at either residual volume or
total lung capacity
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Ventilation/Perfusion Ratio
• Ideally V/Q ratio is 1
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Shunt
• Perfusion Without Ventilation
• V/Q Ratio is 0
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Causes of Shunts
• Atelectasis
• Fluid in the alveolar space
• Airway obstruction
• Anatomic abnormalities
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Modified Shunt Equation
• Qs = (PAO2 – PaO2) x 0.003 _ QT (CaO2 – CvO2) + (PAO2) x 0.003
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Ventilation/Perfusion
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Oxygen Uptake and Diffusion Capacity
• Time of transit of RBC through the pulmonary
capillary
– At rest – 0.75 seconds
– During exercise – 0.25 seconds
• Number of RBCs available
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Oxygen Uptake and Diffusion Capacity
• Biochemical characteristics of hemoglobin
(e.g., sickle cell, carbon monoxide, presence
of fetal hemoglobin)
• Evaluation done by measuring single breath
carbon monoxide diffusion