Compliance (physiology)

Compliance (Eng. compliance) is used in physiology as a measure of the elasticity of body structures. It is used to describe and quantify the elasticity of the tissue under consideration. Compliance indicates how much gas or liquid can be filled in a walled structure until the pressure rises to a pressure unit.

General

For most observed in the medical context shows a non-linear relationship of the values. This means that compliance varies depending on the filling state of the system. Typically, it is for a certain range constant (same increase in volume creates equal pressure increase), but then drops when approaching the elastic limit of the tissue rapidly approaches 0 from ( even a small increase in volume produces large pressure increases ). For some of the investigated structures ( eg, lung ) can be found in addition in the range of small filling volumes, a low value, which reflects the development of the opposite action of adhesion forces and surface tension. Compliance ( ) is measured in the increase in volume ( ) by increasing the applied filling pressure ():

The unit is l / kPa. In medicine, yet the unit ml / cm H2O is used frequently.

Extra stretchy structures have a high compliance, particularly rigid structures show low values. The reciprocal of the elastance is the compliance (stiffness).

The increase in volume can be estimated by measuring the supplied volumes. The difference in pressure is given by the variation of the transmural pressure, which is the pressure difference between inside and outside.

For example, compliance of a balloon First high pressure must be applied to inject the first milliliter of air. When the balloon is again clamped, the pressure increases inside with a further volume of each supply by almost the same value? V is approximately proportional to Ap. The balloon volume approaches the maximum, the balloon skin stretched " to the breaking point ", and even the smallest volume supply leads to a massive increase in pressure. If the maximum is exceeded, bursting the balloon - the elastic limit has been reached.

An analysis of isolated values ​​is only of limited value, but rather interested in the change in the compliance depends on the filling state. Frequently, the connections are therefore shown in a pressure-volume diagram. Compliance corresponds to the " instantaneous slope " in one of the curve points, which is the first derivative of the curve.

In medical practice, the compliance of the following tissue plays a role:

  • Lungs and thorax
  • Blood vessels
  • Heart wall
  • Skull and meninges
  • Bladder wall

See also Young-Laplace equation

Compliance of the lung and thorax

The compliance of the lung is an important means of assessing the integrity of lung tissue and for controlling a ventilation therapy.

Volume with two elastic cases

Since the lung is located inside the thorax, a simple measurement of tidal volumes and the resulting transmural pressures will always reflect only the overall compliance of the chest and lungs. To determine the pulmonary compliance, therefore, the transpulmonary pressure must be used, resulting from the difference of the pressures in the air passages (Paw ) and pleural cavity ( pPleura ). The compliance of the thorax is determined using the Pleuradrucks pPleura. pPleura can be approximately determined by a pressure probe in the esophagus.

The overall compliance is related to the values ​​thus determined as follows:

There are thus actually the individual stiffness ( elastance ) are summed:

While the lung compliance is determined only by the tissue composition, the thorax compliance is also variable by the tone of the muscles. For issues of clinical medicine outside the lung function test to determine the total compliance is usually sufficiently accurate.

Differences within the lung

Due to gravity, the basal portions of the lung are better perfused. At the same time "hangs" the lung to the apical portions. It follows a top-down decreasing size of the alveoli (alveoli ). A particularly large alveolus is located in the upper flat portion of the compliance curve, a further extension is therefore hardly possible; in a particularly small alveolus the surface tension is so strong that the compliance markedly decreased. In a normal breath, the air is so unevenly distributed to the individual sections of the lungs, where it is preferred flow into the alveoli with medium pre-strain and thus maximum compliance.

Static and dynamic compliance

In clinical compliance investigations is measured after application of a defined tidal volume of either the mouth pressure or the pressure in the breathing tube. These pressures are then equated with the alveolar pressure when there is no gas flow takes place, ie when respiratory arrest. Otherwise, the additional necessary to overcome the airway resistance pressure leads to an underestimation of the actual compliance.

The measurement of static compliance requires respiratory arrest. In practical use the quasi-static determination of the compliance has been established, in which the patient breathes at a low respiratory rate of 4/min. In this way remains between breaths enough time for a complete pressure compensation.

In the dynamic compliance caused by the gas flow artifacts consciously go with one, so it is the highest measured during the breathing cycle airway pressure related. It provides an indication of the size of the viscous ( flow induced ) resistance, which can be derived from the difference between dynamic and static compliance.

The new understanding of the term is also used as dynamic compliance, in addition to the flow -induced pressures, the dependence of the compliance is described by temporal processes ( history of pressure, flow and volume). The measurement of these dynamic compliance is usually carried out under the dynamic condition of continuous ventilation and requires mathematical method for calculation. Although the airway resistance is taken into account by this method in determining the dynamic compliance, to some extent marked differences between statically determinate compliance can be observed. These differences are partly caused by the influence of volume history and on the other hand (especially in the context of lung diseases ) through the systematic distortion of the static curve through recruitment.

Clinical Significance

A pathological decrease in the compliance leads to an increase in the work of breathing, as more (negative ) pressure must be applied in order to fill the same volume with stiff lungs. It is often found in restrictive lung disease, but also occurs in acute changes such as pulmonary edema, pneumonia or ARDS.

In emphysema however, it may even lead to an increase in compliance.

Compliance with ventilation

In mechanical ventilation, the analysis of the compliance of the lungs as gentle as possible adjustment of the ventilator used. Both pressure as well as volume-controlled ventilation with plateau occurs (assuming the pause between breaths is it long enough and the patient shows no own breathing efforts ) at the end of an administered breath for equalizing pressure between alveoli and respiratory system. Dividing the applied volume at this time by the prevailing pressure, one obtains the static compliance.

In volume-controlled ventilation can be used for the calculation of dynamic compliance in the classical sense of the respiratory peak pressure. He is inter alia dependent on the applied inspiratory flow and thus on the airway resistance. Is also a plateau phase adjusted, as above outlined can the static compliance are determined and thus an indication of the airway resistance can be made.

Newer methods use multiple linear regression analysis to solve the equation of motion: p = V / C V ' * R p0. By solving this equation, both the resistance and compliance of the respiratory system can be unambiguously determined in all forms of controlled ventilation.

With the following rule of thumb can be carried out a rough estimation of compliance, if no measurement is available apparatus:

With = tidal volume, peak airway pressure =, = positive end-expiratory pressure.

Upper and lower inflection point

To avoid unnecessary shear forces and pressure peaks, one aims at a ventilation regime whose tidal volume in the steep ascending portion of the compliance curve ( ie in the region of maximum compliance) moves the affected lung. If the lung volume extends not at full exhalation or at full inhalation in the flat portions of the curve, the time necessary breath volume to the lowest possible pressure to be administered.

From the S-shaped course of the compliance curve, two turning points arise (English inflection points, not to be confused with the mathematical definition of the inflection point ). The lower marks the transition from the flat " development " part of the curve in the near-linear high - compliance area, the top one shows the approach to the elastic limit at. A lung protective ventilation should therefore take place between these two points. By choosing an appropriate positive end-expiratory pressure ( PEEP) may be a waste to be avoided at the lower inflection point. The size of the tidal volume determines whether is exceeded by that base from the upper inflection point.

Compliance of the blood vessels

The compliance -represented in blood vessels the contribution of the elastic ( static ) resistance to the resulting blood pressure.

Under physiological circumstances, collapsing arteries even at low filling does not, so that one does not see a " development phase ", ie, the curve does not appear S-shaped. The compliance of the vessels can be controlled by the tone of the vascular smooth muscle, its rise leads to a decrease of compliance and thus to an increase in blood pressure.

The higher the compliance is particularly the large arteries, the more pronounced is the Windkessel function. Due to aging or pathological processes, the wall composition and thus the compliance of the vessel changed. As a result, it may lead to high blood pressure, or, in the case of selectively increased compliance, to the formation of aneurysms.

Compliance of the heart wall

It describes the intraventricular pressure in response to the ventricular performance and thus the stretchability of the heart wall. The variability of this size is the main reason that the measurement of pressures alone are not sufficient to assess the three-dimensional state of the heart.

The compliance of the heart wall varies continuously during the contraction of the heart muscle and reaches its maximum in the filling between heartbeats.

A reduced compliance of the heart wall leads to reduced filling of the ventricle with a corresponding drop in the pump power ( Frank -Starling mechanism ). In addition to the consequent Rückstauungsphänomenen in the pulmonary circulation, it can also lead to hypoperfusion of the right ventricle adjacent to the heart muscle layers by the increased pressure in the ventricle. One such event may be particularly acute volume overload, such as lying down, occur.

Compliance of the skull and the meninges

With an increase in volume of the brain, for example, as a result of trauma or edema, it is a function of the compliance of the enclosing covers at some point to a pressure increase, which may eventually lead to ischemia and necrosis.

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