physiology of ventilation
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Physiology of Ventilation. Principles of Ventilation. Educational Objectives. Define and differentiate between compliance, elastance, and resistance List the normal values for the pressures associated with the act of breathing. Educational Objectives. - PowerPoint PPT PresentationTRANSCRIPT
Physiology of Ventilation
Principles of Ventilation
Educational Objectives
• Define and differentiate between compliance,
elastance, and resistance
• List the normal values for the pressures
associated with the act of breathing
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
Definitions
• Ventilation – movement of air between the
atmosphere and the alveoli
• Respiration – movement of gas molecules
across a membrane
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)
Airflow Into And Out of The Lungs
• Intrapleural pressure (Ppl)
– Always negative during normal breathing –
(-5 to -10 cm H2O)
Airflow Into And Out of The Lungs
Normal Inspiration
• Diaphragm contracts
• Intrathoracic volume increases
• Intrapleural pressure increases in negativity
• Increase in volume causes decrease in
intrapulmonary pressure
Normal Inspiration
• Decrease in intrapulmonary pressure creates
negative pressure gradient relative to the
atmospheric pressure
• Air flows into the lungs until pressures
equalize
Normal Expiration
• Diaphragm relaxes, moving upward
• Intrathoracic volume decreases
• Intrapleural pressure becomes less negative
• Decrease in volume creates increase in
intrapulmonary pressure
Normal Expiration
• Increase in intrapulmonary pressure creates
positive pressure gradient relative to the
atmospheric pressure
• Air flows out of the lungs until pressures
equalize
Airflow Into And Out of The Lungs
Factors Affecting Lung Volume
• Compliance
• Elastance (Elasticity)
• Resistance
• Muscle strength and endurance
Compliance
• The ratio of the change in volume to a
given change in pressure
• Normal value – 100 ml/cm H2O
Types of Compliance
• Dynamic compliance – measured
during normal breathing cycle
Cdyn = Volume _ Peak Inspiratory Pressure
Types of Compliance
• Static compliance – measured during breath-
holding procedure
Cplat = Volume _
Plateau Pressure
Elastance
• The physical tendency of an object to return
to its initial state after deformation
• Inverse of compliance
Resistance
• Opposition to a force; ratio of pressure
change to flow change• Poiseuille’s Law – ΔP = 8nlV r4
• R = P1 – P2
Volume
Factors Affecting Muscle Strength and Endurance
• Gender
• Age
• Training
• Position
Factors Affecting Muscle Strength and Endurance
• Underlying cardiac, pulmonary, and muscular
disorders
• Electrolyte imbalances
• Acid-base disturbances
Factors Affecting Muscle Strength and Endurance
• Endocrine abnormalities (e.g., thyroid
disorders)
• Prolonged use of steroids
• Neuromuscular blocking drugs
Evaluation of Muscle Strength and Endurance
• Measurement of transdiaphragmatic
pressure
• Maximum voluntary ventilation (MVV)
Distribution of Ventilation
• Dead Space
– Ventilation not involved in gas exchange
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
Dead Space
• Physiologic dead space
– Sum of anatomic and alveolar dead space
Normal Distribution of Ventilation (Upright Position)
• Pleural pressure lower (more negative) at
apex of lung
• Greater transpulmonary pressure at apex
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
Factors Affecting Distribution of Ventilation
• Increased regional resistance (inflammation)
• Localized changes in compliance (blebs)
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)
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
Three Lung Zones
Distribution of Perfusion
• Lowest resistance to blood flow is at FRC;
resistance increases at either residual volume or
total lung capacity
Ventilation/Perfusion Ratio
• Ideally V/Q ratio is 1
Shunt
• Perfusion Without Ventilation
• V/Q Ratio is 0
Causes of Shunts
• Atelectasis
• Fluid in the alveolar space
• Airway obstruction
• Anatomic abnormalities
Modified Shunt Equation
• Qs = (PAO2 – PaO2) x 0.003 _ QT (CaO2 – CvO2) + (PAO2) x 0.003
Ventilation/Perfusion
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
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