Oscillometry

The Oscillometry referred to as the " impulse oscillometry " a group of the pulmonary function test (in the Pulmonology also referred to as oscilloresistometry) for determining the respiratory impedance and a form of indirect blood pressure measurement.

In the oscillometric examination of the airways, their reaction to an oscillating pressure signal is determined. A frequency mixture is generated in the form of a pressure signal and applied to the patient. Oscillometry for measuring blood pressure measures the oscillations of the blood flow in the arteries .

Use as a lung function test

history

The basic measurement method was developed in 1956 by Dubois et al. introduced. This applies above all to the theory of lung diagnosis using electrical equivalent circuit diagrams. In the beginning, this idea did not enjoy much recognition. In 1971, Smidt and Muysers presented a measurement method that was state-of-the-art. The pulmonary tract is stimulated with a single frequency and its reaction is measured in the form of pressure (monofrequency oscillometry). In 1976 Landser et al. a device which already worked with multi-frequency excitation, a synthesized noise (pseudo random noise). Five years later, Müller and Vogel presented a technically simpler method that used a pulse instead of the noise signal ( pulse oscillometry ).

Theoretical foundations

Dubois' theoretical approach includes the characterization of the complex pneumatic system of the pulmonary tract with the help of physical laws of one-dimensional wave propagation . The latter was mainly used in electrical engineering and describes the influence of electrical components on the amplitude and phase of an electrical signal.

Consider the airways as a mechanical system with a certain response behavior to a pneumatic stimulus. A sinusoidal pressure signal at the entrance, i.e. the mouth opening of a test person, creates a flow course that depends on the properties of this system. The elasticity of the alveoli , the opening width and length of the trachea and the lung volume play a role.

We denote the pressure curve applied to the patient

${\displaystyle p(t)={\hat {P}}\cdot \sin(\omega t+\varphi _{p})}$

and the associated air flow with

${\displaystyle {\dot {V}}={\hat {\dot {V}}}\cdot \sin(\omega t+\varphi _{\dot {V}})}$.

If the imaginary part of a sinusoidal oscillation is included, this signal can be described as a vector in the complex numerical level and is thus reduced to the representation of magnitude and phase.

${\displaystyle p={\hat {P}}e^{j\varphi _{p}}}$

${\displaystyle {\dot {V}}={\hat {V}}e^{j\varphi _{\dot {V}}}}$

If the airways of a patient have a higher flow resistance, less gas volume flows into the lungs and also out of the lungs at the same pressure. The volume flow into the airways decreases over time, due to the capacitive effect of the sum of the pulmonary alveoli, the alveoli.

In the case of a sinusoidal pressure profile, this would mean that the time profile of the volume flow is not in phase with that of the pressure.

We take this into account by dividing the complex quantities of pressure and flow in the amount-phase representation. The resulting size of the impedance consists of the quotient of the absolute values ​​and the difference between the phase values.

${\displaystyle Z={\frac {p}{\dot {V}}}={\frac {\hat {P}}{\hat {\dot {V}}}}\cdot e^{j(\varphi _{p}-\varphi _{\dot {V}})}}$

The associated real and imaginary parts of the impedance are often evaluated for clinical diagnoses. Analogous to electrical engineering, the real part is called resistance and the imaginary part is called reactance .

${\displaystyle Z=R+jX}$

Meaning of the measurement parameters

The fundamentally measured parameters are resistance and reactance of the airways as summands of the airway impedance. The terms are based on electrical engineering and must be further explained in the clinical context.

Physically, the impedance Z is an alternating flow resistance. Analogous to an electrical resonant circuit with a coil, capacitor and ohmic resistance, the physiological impedance of the pulmonary system can be identified as follows.

• Impedance: As a composite quantity, it reflects the total resistance of the pulmonary system.
• Resistanz / Resistance: Here energy in the form of v. a. Lost friction. In the electrical circuit it corresponds to the ohmic resistance, so as a physiological variable it represents the flow resistance in the airways and the viscous resistance in the lungs and thorax .
• Reactance : As an imaginary component of the impedance, the reactance only arises through the action "breathing" and is dependent on the extent of the action. It can also be characterized by Intertence and Compliance.
• Inertance : It is a measure of the inertia of the lungs and thorax and has hardly any clinical relevance.
• Compliance: The volume expandability corresponds to the capacitive resistance in the electrical circuit. On the lung model, it stands for resistance that is created by stretching v. a. The bronchial system , lungs and thorax arise.

Use to measure arterial blood pressure

In this case, the oscillations triggered by the blood flow in the arteries are recorded using an oscillometric method . The method is not suitable for use in patients with cardiac arrhythmias.

(→ main article blood pressure measurement )

swell

1. ↑ Pulse oscillometry by Johannes Vogel and Udo Smidt
2. ↑ Diagnostic relevance of pulse oscillometry in comparison to body plethysmography in childhood by Alexander Kraus, JMU Würzburg

This article is about a health issue. It is not used for self-diagnosis and does not replace a diagnosis by a doctor. Please note the information on health issues !