Impulse oscillometry
By
Dr/ Hossam EL-din mohamed
Lecturer of chest diseases
Ain Shams university
At the end of this lecture you would be able to:
• List the different components of IOS.
• Interpret IOS results.
• Compare the variations in IOS results among different
disease states.
• Describe the limitations of IOS.
• Describe the assessment of bronchial hyperreactivity
using IOS.
Impulse oscilomety (IOS)
Ohm‘s Law:
Resistance = pressure
flow
AIRWAY RESISTANCE
• The relationship between pressure and
flow tells us about airway resistance.
Flow
Pressure
Low Resistance
High Resistance
IOS
Methods of measurement of airway
resistance
• Occlusion technique
• Impulse oscillometry
• Bodyplethysmography
OCCLUSION
occlusion 
pressure
PT 
flow
Forced Oscillation Technique
(FOT):
• Apply frequencies from outside
to the airways
• measure pressure changes
• Range: from 1 Hz to 30 Hz
One frequency
Two frequencies
Sum of two frequencies
Rectangular pulse: all frequencies
Simplified approach:
• pressure waves generate pressure in the lungs
• waves are reflected in the lungs
• reflected waves carry information about the pressure
at different places in the lungs
• flow measured via PT
• reflected waves are analysed in respect of pressure
and frequency
•  Resistance = pressure/flow
• can be calculated for different frequencies (= areas)
Different Frequencies
• High frequencies:
low penetration
upper (central) airway
• Low frequencies:
deep penetration
total airways (central and peripheral)
Impulse Penetration
Slow Impulses 5Hz
Fast Impulses 20 Hz
Impedance Z = R + jX
Impedance describes a complex (mathematical sense)
airway resistance which includes two components, the
real resistance R and the imaginary reactance X.
Reactance X (Capacitive-inertial storage of energy)
Reactance consists of two components. The positive
inertial portion increases with frequency and the negative
capacitive portion decreases frequency
X at 5 Hz: X5 Distal capacitive reactance
f at X=0: Fres Resonant frequency
The ability to store energy in the capacitance (elastic recoil) is
prerequisite for passive expiration.
Inertance has minor clinical relevance
Impedance Z
Respiratory impedance, i. e. the interaction between resistive
and reactive properties of the respiratory system, is primarily
measured by the oscillometric method.