FORCED OSCILLATION TECHNIQUE (FOT) FOR THE
ASSESSMENT OF BREATHING MECHANICS IN ANIMALS
H.-J. Smith, Erich Jaeger GmbH, Wuerzburg, Germany
1. Introduction
Lung function measurements in animals should be effort independent, able to
provide objective information and discriminate between the major components of
the respiratory tract, taking into account the particular lung physiology of the
measured species. Considering the huge differences in size and weight between
small animals like rodents [1, 6] and large animals, for example horses [2, 3], the
same measuring principle should be applicable with only minor modifications in
the mechanical components of the system, to fit it to the respiratory parameters
of the examined species and the needs of parameter determination.
Furthermore, it is required that the test is non-invasive and can be easily
performed in a short time.
The oscillometric methodology not only fulfils the above mentioned points, it also
offers additional properties which make the technique exceptionally well suited
for the measurement of the breathing mechanics in animals and the assessment
of their bronchial hyperreactivity.
The determination of respiratory impedance by the forced oscillation technique
was originally described by Du Bois et al. [4] in 1956.
In 1993, a workshop on mechanical respiratory impedance sponsored by the
Commission of the European Communities took place in Zeist (NL). As a result,
major recommendations concerning technology and clinical application of
oscillometry were established and published [5]. Although this workshop was
mainly focused on human medicine, these general recommendations can also
be transferred to respiratory measurements in animals.
The traditional technique to determine total pulmonary resistance RL and
dynamic compliance Cdyn by means of simultaneous recording of airflow and
oesophageal pressure correlates with oscillometry but is invasive.
The assessment of airflow limitation is most commonly achieved by the
spirometric technique, which is non-specific and less sensitive.
2. Forced oscillation technique
A characteristic feature of oscillometry is, that it does not derive pulmonary
impedance from the respiration signals, but from the pressure-flow-relationship
of artificial test signals which are produced by an external generator. These
artificial signals are superimposed on the respiratory tidal breathing waveform
within the measuring system. In this way they enter the respiratory tract and
allow determination of its properties. The advantage of artificial test signals is the
incomparable higher frequency contents with a relatively high consistency as far
as frequency range and amplitude are concerned. The distribution of all of these
frequencies is called spectrum. The knowledge of the physical behaviour of
every frequency is pre-condition for detailed differentiation of the respiratory
system.
2.1. Principle
The principle of input impedance measurements is shown in Fig. 1. The subject
whose respiratory impedance Zrs is to be determined, breathes ambient air via a
pneumotachograph, Y-adapter and terminating resistor. The terminating resistor
controls system pressure within the measuring head. Its low value guarantees an
insignificant effect on breathing. However, the resistance is high enough to make
sure, that a certain amount of the oscillometric pressure, which is produced by
the external generator, is forced into the respiratory tract.
Two separated transducers register flow V’ and the corresponding pressure P.
The respective signals of total pressure and total flow consisting of spontaneous
breathing and the superimposed test signal are recorded.
The principle of transfer impedance measurements [6, 7], which is mostly used in
small animals, differs from input impedance. In this technique the flow is still
recorded at the airway opening, whilst oscillometric pressure is applied to the
chest.
In order to determine the respiratory impedance from the test signal, it has to be
separated from the breathing by means of frequency discrimination. For the
oscillometric evaluation program, the range between 0.1 Hz and 50 Hz is of
particular importance. The frequency range of spontaneous breathing is
considerably less and therefore can be easily eliminated from the frequencies of
the test signal.
2.2. Technical aspects
To achieve high quality of all acquired data, the amplitude and phase of flowand pressure transducer must be matched. Both transducers should have linear
response and an extremely good common mode rejection ratio of more than 60
dB [5, 8]. This is especially important for the flow transducer.
Wave tube techniques [9] can improve the matching of the transducers, because
the flow sensor is replaced by a reference impedance and a second pressure
transducer of the same type. However, the dimensions of the wave tube and its
limited linearity properties have limited the usefulness of this technique.
2.3. Test signals
The various oscillometric techniques use different test signals. In the past
sinusoidal signals were more often applied, standard of today is the use of multifrequent signals like pseudo random noise or impulses [10, 11, 22].
The generation of these test signals ranges from electrically controlled
membrane deflections of loudspeakers to electrically or mechanically controlled
pumps, to sophisticated jet techniques which produce a pulse-shaped pressure
flow excitation.
3. Important oscillometric parameters
In multi-frequent applications such as Pseudo Random Noise Technique or
Impulse Oscillometry, the characteristics of the respiratory system are displayed
in form of an impedance spectrum Zrs.
Depending on the measured species, it is necessary to identify which of the low
frequencies can be effected by spontaneous breathing and which higher
frequencies have low sensitivity in the diagnostic evaluation.
The respiratory impedance Zrs, which mathematically can be described by its
two components, resistance Rrs and reactance Xrs, can be considered as an
objective and differentiated image of lung mechanics. Resistance and reactance
reflect quite specific and completely different properties of the lung.
Pulmonary resistance Rrs covers all frictional components which consume
breathing energy within the respiratory tract. The breathing mechanical
equivalent of the resistance is the flow resistance in the airways, dependent on
airway calibre and structure of the surface of the airway walls.
In comparison, reactance Xrs describes the ability of the lung to store energy,
which is needed for passive expiration. The reactance itself consists of two
components which also differ in their properties. The negative part of reactance
Xrs reflects capacitive behaviour (C) and corresponds not only to the thoracopulmonary elasticity but also to changes in the available lung volumes in the
distal compartments of the lung, including the visco-elastic properties of the lung
tissue. The positive part of reactance Xrs has inertial characteristics (I)
describing the moving air column in the bronchial tree.
Additionally, the point of intersection of the reactance/frequency plot with the
zero axis, the resonant frequency Fres, is useful for clinical interpretation of the
results of the measurement.
The Fast Fourier Transformation (FFT) is used to separate complex multifrequent test signals into their different sinusoidal components (spectrum) and
assumes linearity of the measured system.
The linearity of the respiratory system can be documented by the calculation of
the coherence function for pressure and flow. Criterion for linearity is a
coherence which is the square of the modulus of the cross power spectrum,
divided by the product of the pressure and flow auto spectrum at any particular
frequency higher than 0.95 [12].
4. The clinical contents of measured impedance parameters and their
relationship to abnormal lung function
4.1. Interpretation of pulmonary resistance Rrs and reactance Xrs
For clinical interpretation of lung function measurements, it is necessary to
establish a link between measured impedance parameters and their assessment
of lung physiology.
Weibel [13] described the physiology of the lung with the help of a trumpet
model. To get an impression of the dimensions of this trumpet one should
remember that the surface of all alveoli covers an area which is 10,000 times
larger than the cross sectional area of the trachea.
Although the calibre of the single airway is decreasing in the respective airway
generation, the cross sectional area of all airways in this generation increases
because of the quadratic increase of the number of airways. Simultaneously,
with the increase of cross sectional area, the airflow decreases.
Whereas the central airways limit the flow because of their narrow calibre and
therefore generate airway resistance, in distal lung compartments, which
comprise approximately 60% of the total lung volume, there is no flow limitation
and therefore no resistance at all.
Respiratory impedance is well suited to establish a relationship to this special
lung physiology. The resistive part is located more or less in the central and
upper airways. In contrast, the peripheral chamber is nearly independent of
resistance, expressed by the low frequent capacitive reactance.
Increasing resistance Rrs, but also decrease of reactance Xrs (loss of
capacitance), describe abnormalities of the respiratory system. The absolute
value of these parameters is equivalent to the degree of disability.
4.2. The influence of a face mask
Conscious animals are adapted to the measuring apparatus via a face mask. As
the face mask influences the outcome of the measurement, it has to be
considered in the interpretation of the results.
The face mask itself generates a shunt compliance which appears in parallel to
the respiratory impedance Zrs. Furthermore, the nasal cavity of the animal adds
a resistance which is in series to the lung. With increasing size of total resistance
(nose + lung), the effect of mask compliance becomes more important,
especially in higher frequencies and nasal resistance can dominate across
pulmonary resistance. As a consequence, low frequent reactance becomes more
sensitive in the determination of pulmonary properties compared to resistance.
5. Mathematical modeling
Conventional lung function diagnostics is mainly based on the time trend of
different signals like flow (V'), volume (V) or pressure (P), which are derived from
the respiratory tract while the subject is breathing. The behaviour of these
signals is easy to follow and to interpret.
Quite different is the outcome of oscillometry with mainly spectral parameters.
The clinical interpretation therefore needs a completely different approach.
To overcome this problem and to be able to use the advantages of oscillometry,
mathematical models were developed in order to transfer spectral data into
structural data describing different structures of the lung. This is a modern form
of interpretation of the measurement and a method of significant data reduction.
Although different models are available [14, 15, 16, 17, 18, 19], none of them are
able to fulfil the most important clinical requirement: to provide an extensive
interpretation of any measured respiratory system.
The definition of the model structure as well as the algorithms for numerical
parameter calculation are problematic. A high number of elements in the
mathematical model simplifies interpretation and fits to the complex structure of
the respiratory tract, but makes parameter calculation unstable. Simple models
allow reliable calculations but are still difficult to interpret.
However, some authors use models for further partitioning of pulmonary
impedance into tissue and airway components [19, 20]. Other authors imply
models to recalculate the influence of the face mask on the acquired impedance
[21].
6. Conclusions
Abnormal lung function means limited fitness of animals. Oscillometry offers
sufficient sensitivity to detect the changes in the respiratory system noninvasively.
The knowledge of the specific characteristics of resistance and reactance, their
relationship to lung physiology and the physical properties of various
frequencies, which are used in the multifrequent versions of the oscillometric
technique, offer the possibility to differentiate proximal and distal airway
resistance and to determine the capacitance of lung and thorax. This is possible
even if we take into account the distinguishing characteristics of measurements
in animals, as lung measurement across the nose and nose breathing.
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