Biomedical Engineering Reference
In-Depth Information
Chapter 3
The Respiratory Impedance
3.1 Forced Oscillation Technique Lung Function Test
Although standardized and currently used in clinical environment, spirometry has
several limitations. It requires maximal, reproducible efforts, which in turn requires
several measurement sessions from the patient, resulting in fatigue and time con-
sumption. The maximal expiratory flow is dependent on the lung recoil pressure,
the dynamic airway resistance and the airway properties at the flow limiting segment
(i.e. flow plateau) [
112
,
125
]. Spirometry has also difficulties to clearly evaluate ob-
structive lung diseases. Even paired with bronchoprovocation, spirometry cannot
reliably differentiate patients with both asthma and COPD (chronic obstructive pul-
monary disease) features from either asthma or either COPD, and it is rather insen-
sitive to early airway changes [
38
,
88
]. Additionally, neither spirometry nor body
plethysmography can provide information upon the resonance and anti-resonance
frequencies in the lungs.
The forced oscillation technique (FOT) is defined as superimposing external
pressure signals on spontaneous breathing (tidal breathing) [
32
,
116
,
141
,
158
]. The
effect of these oscillations on the airways and lungs provides an effort-independent
assessment of respiratory mechanics. There is a significant amount of literature in
pediatric applications [
15
,
25
,
27
,
123
,
140
] and there is an increasing interest in
adult lung function testing [
19
,
28
]. Typically, the forced oscillations can employ
a mono- or a multi-frequency excitation signal, typically in the range 4 Hz to 30-
50 Hz. It can be continuous (e.g. pseudo random noise, optimized multisine), or
time discrete (e.g. impulse oscillations) [
78
,
79
,
141
]. FOT has been broadly used
for screening purposes: upper airway obstruction, small airways disease in COPD
[
64
], for bronchoprovocation testing [
36
], vocal cord dysfunction evaluation [
106
],
with bronchodilator response, and respiratory mechanics in obstructive sleep ap-
nea [
110
].
The measurements of the signals analyzed in this topic have been performed
using the device depicted in Fig.
3.1
: the FOT standard setup, modified from a com-
mercially available device, able to assess the respiratory mechanics in the range
