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 (P alv ) –Pressure at the alveolus; changes from positive to negative during ventilatory cycle (-5 to +5 cm H 2 O)

Airflow Into And Out of The Lungs Intrapleural pressure (P pl ) –Always negative during normal breathing – (-5 to -10 cm H 2 O)

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 H 2 O

Types of Compliance Dynamic compliance – measured during normal breathing cycle C dyn = Volume _ Peak Inspiratory Pressure

Types of Compliance Static compliance – measured during breath- holding procedure C plat = 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 r 4 R = P 1 – P 2 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 = (PAO 2 – PaO 2 ) x _ QT (CaO 2 – CvO 2 ) + (PAO 2 ) 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