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