Compliance (physiology)

Compliance (dt. Resilience ) is used in the physiology as a measure for the elasticity of body structures. It is used to describe and quantify the elasticity of the tissue under consideration. The compliance indicates how much gas or liquid can be filled into a walled structure until the pressure increases by one pressure unit.

General

Representation of compliance as a gradient in a pressure-volume diagram: Depending on the pre-filling, different volumes ΔV can be added in order to cause the same pressure increase Δp (blue triangles). The vertical dashed line indicates the elastic limit (EG) above which the system will be damaged. A and B correspond to the lower and upper inflection point . p = pressure, V = volume.

For most of the relationships considered in medicine, there is a non-linear relationship between the values. This means that compliance changes depending on how full the system is. Typically it is constant for a certain range (the same increase in volume produces the same increase in pressure), but then drops sharply towards 0 when the elastic limit of the tissue is approached (even a small increase in volume produces large increases in pressure). In some of the structures examined (e.g. lungs) there is also a low value in the area of small filling volumes, which reflects the effect of adhesive forces and surface tension opposing development . Compliance ( ) is measured in volume increase ( ) per increase in the applied filling pressure ( ): ${\ displaystyle C}$ ${\ displaystyle {\ Delta V}}$ ${\ displaystyle {\ Delta p}}$

{\ displaystyle C = {\ frac {\ Delta V} {\ Delta p}}}

The unit of measurement is l / kPa . The unit ml / cm H ₂ O is still often used in medicine . Converted into SI units, the unit is: ${\ displaystyle {\ frac {m ^ {4} s ^ {2}} {kg}}}$

Particularly elastic structures have a high compliance , particularly stiff structures show low values. The reciprocal of compliance is elastance ( stiffness , also volume elasticity ).

The volume increase can be determined by measuring the volumes supplied. The pressure difference is given by the change in transmural pressure, that is the pressure difference between inside and outside.

An analysis of isolated values is, however, of little significance, rather the change in compliance depending on the filling status is of interest . The relationships are therefore often shown in a pressure-volume diagram. The compliance then corresponds to the "current slope" in one of the curve points, that is to say the 1st derivative of the curve.

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

Lungs and thorax
Blood vessels
Heart wall
Skull and meninges
Urinary bladder wall

Compliance of the lungs and thorax

The compliance of the lungs and the thorax is a measure of the elasticity of the breathing apparatus or its components and is defined as the ratio of volume change to the associated pressure change. Lung extensibility is an important means of assessing the integrity of the lung tissue and the compliance of the entire lung-thoracic system for controlling ventilation therapy .

Volume with two elastic covers

Since the lungs are located within the thorax, a simple measurement of the tidal volumes and the resulting transmural pressures will only ever show the overall compliance of the thorax and lungs. In order to determine the lung compliance, the transpulmonary pressure must therefore be used, which results from the difference in the pressures in the airways (p _aw ) and in the pleural space ( p _pleura ). The lung compliance is then calculated as the quotient of the change in volume and the pressure difference. The compliance of the thorax is determined using the pleural pressure p _pleura . p The _pleura can be approximately determined by a pressure probe in the esophagus.

The overall compliance (change in volume divided by change in airway pressure) is related to the values determined in this way as follows:

{\ displaystyle {\ frac {1} {C_ {Total}}} = {\ frac {1} {C_ {Lungs}}} + {\ frac {1} {C_ {Thorax}}}}

So actually the individual stiffnesses ( Elastance ) are summed up:

Because the lungs are enveloped by the thorax, the ratios must be considered for the same filling volume (if 3000 ml are in the lungs, then 3000 ml are also in the thorax.). However, different filling volumes are required for the thorax and lungs in order to achieve the same increase in pressure, which is reflected in the different compliances . By using the reciprocal value, however, the same filling volume is set for both structures and, if necessary, higher pressure rises are extrapolated accordingly.

While the lung compliance is only determined by the tissue composition, the thoracic compliance (change in volume divided by the pleural pressure difference) can also be changed by the tone of the muscles. For questions in clinical medicine outside of the lung function test , the determination of the overall compliance is usually sufficiently accurate.

Differences within the lungs

Due to the force of gravity, the basal parts of the lungs are better supplied with blood. At the same time, the lungs “hang” on the apical parts. This results in the size of the air sacs (alveoli) decreasing from top to bottom . A particularly large alveolus is located in the upper flat area of the compliance curve, which means that further expansion is difficult; In a particularly small alveolus, the surface tension becomes so strong that compliance is noticeably reduced. With a normal breath, the air will therefore be distributed unevenly over the individual lung sections, whereby it will preferably flow in alveoli with medium pre-expansion and thus maximum compliance.

Static and dynamic compliance

In clinical compliance examinations, either the mouth pressure or the pressure in the ventilation hose is measured after a defined tidal volume has been applied. These pressures can only be equated with the alveolar pressure when there is no more gas flow, i.e. when breathing has stopped. Otherwise the additional pressure required to overcome the breathing resistance leads to an underestimation of the actual compliance .

The measurement of static compliance (under ventilation: tidal volume divided by the difference between end-inspiratory and end-expiratory pressure), which corresponds to the overall compliance, requires respiratory arrest. In practical use, the determination of the quasi-static compliance has become established, in which the patient breathes with a low breathing rate of 4 / min. This way there is enough time between breaths for a complete pressure equalization.

The artefacts caused by the gas flow are deliberately included in dynamic compliance (under ventilation: tidal volume divided by the difference between the peak inspiratory pressure and the end-expiratory pressure), i.e. the highest airway pressure measured during the breathing cycle is used. It enables a statement to be made about the size of the viscous (flow-related) resistance, which can be derived from the difference between dynamic and static compliance.

According to more recent understanding, the term dynamic compliance is also used in such a way that, in addition to the flow-related pressures, the dependence of the compliance on time processes (previous history of pressure, flow and volume) is described. This dynamic compliance is usually measured under the dynamic conditions of uninterrupted ventilation and requires mathematical calculation methods. Although the airway resistance is taken into account by this method in the determination of the dynamic compliance, z. Sometimes there are clear differences to the statically determined compliance. These differences are caused on the one hand by the influence of the volume history and on the other hand (especially in the context of lung diseases) by the systematic falsification of the static curve through recruitment.

Clinical significance

A pathological decrease in compliance due to changes in the parenchyma of the lungs, surfactant dysfunction and volume reduction leads to an increase in the work of breathing , as more (negative) pressure has to be applied to fill the stiff lung with the same volume (work = pressure times volume). It is often found in restrictive lung diseases , but also occurs in acute changes such as pulmonary edema , pneumonia or ARDS .

In contrast, pulmonary emphysema can even lead to an increase in compliance .

Compliance under ventilation

With mechanical ventilation , the compliance analysis is used to set the ventilator as gently as possible . With both pressure- and volume-controlled ventilation with a plateau, at the end of a given breath, the pressure between the alveoli and the ventilation system is equalized (provided that the pause between two breaths is long enough and the patient does not make any effort to breathe). If you divide the volume applied at this point in time by the prevailing pressure, you get the static compliance .

Under volume-controlled ventilation, the peak airway pressure can be used to calculate dynamic compliance according to the classical understanding . He is u. a. depending on the applied inspiratory flow and thus on the airway resistance. If a plateau phase is set at the same time, the static compliance can be determined as outlined above and thus a statement can be made about the airway resistance.

Newer methods use multiple linear regression analysis to solve the equation of motion: p = V / C + V '* R + p ₀ . By solving this equation, both the resistance and the compliance of the respiratory system can be clearly determined for all forms of controlled ventilation.

The following rule of thumb can be used to make a rough estimate of compliance if no instrument-based measurement is available:

{\ displaystyle C = {\ frac {TV} {PPeak-EEP}}}

with = tidal volume = airway peak pressure = End Expiratory Pressure. ${\ displaystyle TV}$ ${\ displaystyle PPeak}$ ${\ displaystyle EEP}$

The effective compliance is calculated as the ratio of tidal volume and the difference between plateau pressure and PEEP.

Upper and lower inflection point

In order to avoid unnecessary shear forces and pressure peaks, a ventilation regime is sought whose tidal volume is in the steep, rising area of the compliance curve (i.e. in the area of maximum compliance ) of the affected lungs. If the lung volume does not extend into the flat parts of the curve either with full exhalation or with maximum inhalation, the necessary respiratory time volume can be administered with the lowest possible pressure.

The S-shaped course of the compliance curve results in two inflection points (not to be confused with the mathematical definition of the inflection point ). The lower one marks the transition from the flat “unfolding part” of the curve to the almost linear high compliance area, the upper one shows the approach to the elastic limit. Lung protective ventilation should therefore take place between these two points. By choosing an appropriate positive end-expiratory pressure (PEEP) , a drop below the lower inflection point can be avoided. The size of the tidal volume determines whether the upper inflection point is exceeded from this base.

Blood vessel compliance

In the case of blood vessels, compliance represents the contribution of the elastic (static) resistances to the resulting blood pressure.

Pressure-volume diagram in blood vessels.

Under physiological circumstances, arteries do not collapse, even if they are not completely filled, so that no “unfolding phase” is seen, i.e. the curve does not appear to be S-shaped. The compliance of the vessels can be controlled by the tone of the vascular muscles, the increase of which leads to a decrease in compliance and thus to an increase in blood pressure.

The higher the compliance , especially of the large arteries, the more pronounced their air chamber function . Aging or pathological processes change the composition of the wall and thus the compliance of the vessel. This can lead to high blood pressure or, in the case of increased compliance at certain points , to the formation of aneurysms .

Compliance of the heart wall

It describes the intraventricular pressure as a function of the ventricle filling and thus the ductility of the heart wall. The variability of this quantity is mainly responsible for the fact that the measurement of pressures alone is not sufficient to assess the three-dimensional condition of the heart.

The compliance of the heart wall changes continuously during the contraction of the heart muscle and reaches its maximum in the filling phase between two beats.

A reduced compliance of the heart wall leads to a reduced filling of the heart chamber with a corresponding drop in the pumping capacity ( Frank-Starling mechanism ). In addition to the resulting backflow phenomena in the pulmonary circulation, the increased pressure in the ventricle can also lead to a reduced blood flow in the myocardial layers directly adjacent to the ventricle. Such an event can occur especially with acute volume exertion, such as when lying down.

Classification of diastolic compliance on the heart based on echocardiographic criteria
(gender-unspecific for people over 60 years of age)
parameter	normal diastolic function	I ° disturbed relaxation	II ° pseudo normalization	III ° reversible restriction	IV ° fixed restriction
Mitral Flow (I / O)	0.75 - 1.5	<0.75	0.75 - 1.5	> 1.5	> 1.5
Deceleration time (DT in ms)	> 140	-	> 140	<140	<140
Myocardial velocity (E 'in cm / s)	> 7	> 7	<7	<< 7	<< 7
Filling index E / E '	<10	<10	≥ 10	≥ 10	≥ 10
Pulmonary venous flow	s ≥ d	s> d	s <d	s <d	s <d
LV compliance	normal	normal to slightly decreased	reduced	greatly diminished	very much reduced
LA print	normal	normal	slightly increased	elevated	greatly increased

Compliance of the skull and meninges

With an increase in volume of the brain, e.g. As a result of trauma or edema, for example, depending on the compliance of the enclosing sheaths, a pressure increase occurs from a certain point on, which can ultimately lead to ischemia and necrosis .

literature

Robert F. Schmidt, Florian Lang, Gerhard Thews : Human physiology with pathophysiology. Springer, Berlin 2004, ISBN 3-540-21882-3
Stefan Silbernagl, Agamemnon Despopoulos: Pocket Atlas of Physiology. Thieme, Stuttgart 2003, ISBN 3-13-567706-0
Jonathan L. Benumof: Anesthesia in Thoracic Surgery. Urban & Fischer bei Elsevier, 1991, ISBN 3-437-00609-6
JR Levick: Physiology of the Cardiovascular System. UTB, Stuttgart 1998 ISBN 3-8252-8129-9 .
Hilmar Burchardi: Etiology and pathophysiology of acute respiratory failure (ARI). In: J. Kilian, H. Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 , 2nd, unchanged edition, ibid. 1994, ISBN 3-540-57904-4 , pp. 47-91; here: pp. 53–58.
Thomas Pasch, S. Krayer, HR Brunner: Definition and measurands of acute respiratory insufficiency: ventilation, gas exchange, breathing mechanics. In: J. Kilian, H. Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 , 2nd, unchanged edition, ibid 1994, ISBN 3-540-57904-4 , pp. 93-108; here: pp. 102-104.

Individual evidence

↑ Peter Lotz: Anatomy and Physiology of the Respiratory Tract. In: J. Kilian, H. Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 , 2nd, unchanged edition, ibid 1994, ISBN 3-540-57904-4 , pp. 3–45; here: p. 21 f.
↑ J. Kilian, H. Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 , 2nd, unchanged edition, ibid 1994, ISBN 3-540-57904-4 , p. 178.
↑ Wilkenshoff / Kruck: Manual of echocardiography . 5th edition. Thieme, Stuttgart, New York 2011, ISBN 978-3-13-138015-9 .

[1] Peter Lotz: Anatomy and Physiology of the Respiratory Tract. In: J. Kilian, H. Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 , 2nd, unchanged edition, ibid 1994, ISBN 3-540-57904-4 , pp. 3–45; here: p. 21 f.

[2] J. Kilian, H. Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 , 2nd, unchanged edition, ibid 1994, ISBN 3-540-57904-4 , p. 178.

[Wilkenshoff-3] Wilkenshoff / Kruck: Manual of echocardiography . 5th edition. Thieme, Stuttgart, New York 2011, ISBN 978-3-13-138015-9 .