Method and device for measuring tissue pressure

Abstract

The method facilitates measuring of a tissue pressure non-invasively utilizing a negative pressure. The device has a pressure chamber, pressure sensor for measuring the pressure in the pressure chamber and a range sensor for measuring the skin tissue rising caused by the negative pressure.

Claims

1. A method for measuring a value for a tissue pressure of a tissue comprising: providing a Central Processing Unit (CPU) in operative engagement with a pressurizing unit that is in operative engagement with a pressure chamber having a pressure sensor and a range sensor, the range sensor being one of a transceiver operating on at least one of infrared, visible light or radio frequencies, a laser sensor, or a tonometer having a mechanically protruding part; the pressurizing unit creating a negative pressure in the pressure chamber; the pressure sensor measuring the negative pressure as a function of time and transferring measured information about the negative pressure to the CPU; the range sensor measuring a distance to the tissue to in turn measure a rising of the tissue caused by the negative pressure as a function of time, the range sensor transferring measured information about the rise caused by the negative pressure to the CPU; the CPU receiving the measured information about the negative pressure from the pressure sensor and the measured information about the rise from the range sensor; and the CPU using the measured information about the negative pressure and the rise to calculate a value for the tissue pressure; the calculation of the value for the tissue pressure being calculated iteratively as a function of the pressure chamber pressure, the ambient air pressure, an applied force, an elasticity coefficient and the rising of the tissue.

2. The method of claim 1 wherein the method further comprises the step of the CPU using an iterative calculation to determine an internal pressure.

3. The method of claim 1 wherein the method further comprises the step of measuring rising of the tissue by using an infrared sensor, laser sensor or tonometer.

4. The method of claim 1 wherein the method further comprises the step of the CPU measuring movement of a mechanically protruding part of the range sensor.

5. The method of claim 1 wherein the method further comprises the step of the CPU processing and saving data.

6. The method of claim 1 wherein the method further comprises the step of the pressurizing unit raising the skin to a first rise (hα) and permitting blood and lymph to be transferred from high pressure areas to a lower pressure area to fill up an expanded volume of the raised tissue.

7. The method of claim 1 wherein the method further comprises the step of a force sensor controlling a friction between the tissue and a measuring device facing the tissue.

8. The method of claim 1 wherein the method further comprises the CPU assuming that a total rise (h) is smaller than a radius (L) of a pressure chamber of a measuring device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following the invention is described in more detail with reference to the advantageous embodiments presented as examples and the attached figures, where

(2) FIG. 1 shows a measuring device according to a simple embodiment,

(3) FIG. 2 shows a measuring device according to a more versatile embodiment,

(4) FIG. 3 shows the geometry of a measuring device according to an embodiment, and

(5) FIG. 4 visualizes the relationship between skin rising and the negative pressure in use.

DETAILED DESCRIPTION

(6) In FIG. 1 there is presented a measuring device for measuring tissue pressure according to a simple embodiment. The measuring device according to the embodiment comprises a CPU 2, a pressurizing unit 6 and a pressure chamber 10. In connection with the pressure chamber 10 there is, in addition, a pressure sensor 12 and a range sensor 14. The range sensor 14 may for instance be a transceiver operating on infrared, visible light, laser or radio frequencies or a tonometer, having a mechanically protruding part, the movement of which is measured in order to measure the range 16. There may be several pressure sensors and/or range sensors.

(7) The CPU 2 comprises means to process and save data and run software. The CPU 2 is connected to the pressurizing unit 6, which in its turn is connected to the pressure chamber 10. The pressurizing unit 6 is arranged to create a negative pressure in the pressure chamber 10 by removing media, for instance air or water, from the pressure chamber in order to achieve a rising in the tissue 20.

(8) Because of the negative pressure formed in the pressurizing unit 10, the internal pressure in the inter-tissue/intercellular space, in the blood vessels and the lymphatic vessels expands tissue volume and stretches and/or raises the skin, which is elastic. At first, the tissue pressure in the expanded point is lower than in the surrounding tissue space. The inter-tissue fluid/interstitial fluid, the blood and the lymph are transferred from the higher pressure towards the lower pressure and fill up the expanded volume. Thus the skin rises until equilibrium finally is reached because of the elastic force of the tissue. In the state of equilibrium, the forces caused by the elasticity of the tissue and the air pressure in the pressure chamber are of the same magnitude as the force of the tissue pressure.

(9) In the above mentioned event, the pressure and the pressure change can, based upon the signal given by the pressure sensor 12, be measured as a function of time and likewise, based upon the measurements of the range sensor 14, the rising of the tissue and/or skin is known as a function of time. The results of the measurements are transferred to the CPU 2, where the data is saved and processed. Based upon these measurements, the CPU calculates the elastic force of the skin and/or the tissue and further the pressure or edema pressure of the skin and/or the tissue.

(10) In FIG. 2 there is shown a measuring device for measuring tissue pressure according to a more versatile embodiment. The measuring device of this embodiment comprises more sensors and/or measuring devices than the aforementioned measuring device of a simple embodiment. The range sensor 14 may for instance be a transceiver operating on infrared, visible light, laser or radio frequencies or a tonometer, having a mechanically protruding part, the movement of which is measured in order to measure the range 16. There may be several pressure sensors and/or range sensors. In addition, the pressure sensor or other sensor 13 may be used for measuring the force, wherewith the pressure chamber or other component of the measuring device is pressed against the skin. Thus the friction between the skin and the part of the measuring device facing the skin may be controlled.

(11) The device according to the embodiment may in addition comprise devices 15 for measuring and/or imaging properties of the skin or the subcutaneous tissue. The imaging may take place for instance by ultrasonic, infrared, X-ray and/or any other medically used imaging method. The properties to be measured may for instance be blood pressure, skin temperature, the temperature of the subcutaneous tissues. By subcutaneous tissues is meant one or several of all the tissues under the skin, such as fatty tissue, muscles, bones, tendons etc. The mentioned devices 15 may be situated in- or outside of the pressure chamber 10 and, in addition, in the case of several devices 15 both in- and outside. The obtained measuring results may used as aids in calculating edema pressure.

(12) In an exemplary embodiment for calculating edema pressure, there are used equations, which are based upon a simplified 2D model under the standard pressure acting in the normal direction of the skin. This is a classic “velaria” problem, where elastic material is used. In FIG. 3 there is shown a visualizing image of the situation and the variables used in the equation. In the equation it is assumed that the total rising h of the raised skin is smaller than the radius of the pressure chamber (h<L). It is assumed that the edge of the pressure chamber that is placed towards the skin is circular, in which case the skin rising determined by the edge can be assumed to be shaped as the calotte of a ball with a radius R, as can be seen from the cross section shown in FIG. 3.

(13) The following values are determined:
ΔP.sub.0=P.sub.int−P.sub.atm,ΔP.sub.t=P.sub.atm−P.sub.t
ΔP=P.sub.int−P.sub.t=ΔP.sub.0+ΔP.sub.t  (1), where P.sub.t is the pressure chamber pressure, P.sub.int is the internal tissue pressure, edema pressure P.sub.atm is the ambient air pressure

(14) In this embodiment it is assumed that the length of half of the skin under the pressure chamber is L, which is the radius of the pressure chamber aperture towards the skin. In FIG. 4 there is presented the qualitative behaviour of the skin under negative pressure. There are two areas visible in FIG. 4. In the first area there occurs a fast growth of the skin rising, as negative pressure is applied. This goes on until the turning point (30), where the change in question retards and the response starts to saturate. The first area also seems to correspond to a situation, where the “loose” skin around the pressure chamber slides under the negative pressure without the occurrence of any significant elastic elongation. If the friction between the surface of the pressure chamber towards the skin and the skin is big, the skin sliding in question does not necessarily occur at all.

(15) The second area corresponds to skin behaviour, when all possible peripheral loose skin already has been brought under the pressure chamber and the growth of the rising is due to elastic deformation of the skin.

(16) In order to explain the looseness in question, we determine the parameter a, which is the relation between the half-part s of the length of the skin that has been deformed under the pressure chamber and the half-part L of the length of the skin under normal conditions. The value α in question is dependent upon the original subcutaneous pressure and is determined:

(17) $\begin{array}{cc}\alpha \left(\Delta P_0\right)=\{\begin{array}{cc}1& \Delta P_0\ge \Delta P_{\alpha }\\ {\alpha }_{\max }\left(1-\frac{\Delta P_0}{\Delta P_{\alpha }}\right)+\frac{\Delta P_0}{\Delta P_{\alpha }}& \Delta P_0\le \Delta P_{\alpha }\end{array},\mathrm{where}\text{:}& \left(2\right)\\ {\alpha }_{\max }=\frac{1}{2}\left(\frac{h_{\alpha }}{L}+\frac{L}{h_{\alpha }}\right)a\cos \left(\frac{L^2}{h_{\alpha }^2+L^2}\right),& \left(3\right)\end{array}$
and the parameters h.sub.αja P.sub.α are determined in FIG. 4.

(18) Thus, the half-part of the length of the skin under the pressure chamber as the measuring pressure is applied is:

(19) $\begin{array}{cc}s=L.Math.\alpha .Math.\left(1+\frac{\left(\Delta P_0+\Delta P_t\right).Math.R}{k}\right),& \left(4\right)\end{array}$ where k is an elasticity coefficient, obtained by presuming that:
F=kΔs  (5)

(20) Because the solution obtained for the skin deformation is based upon the length of the circle circumference, the following equations may be obtained based upon the trigonometry of FIG. 2:

(21) $\begin{array}{cc}R=\frac{h^2+L^2}{2h}& \left(6\right)\\ s=\mathrm{Ra}\cos \left(\frac{R-h}{R}\right)& \left(7\right)\end{array}$

(22) By combining the equations (6) and (7) one obtains:

(23) $\begin{array}{cc}s=\frac{h^2+L^2}{2h}a\cos \left(\frac{L^2}{h^2+L^2}\right)& \left(8\right)\end{array}$

(24) By using the equations (4) and (8), we may thus compile the following equation:

(25) $\begin{array}{cc}\Delta P_0=k.Math.\left(\frac{1}{L.Math.\alpha }a\cos \left(\frac{L^2}{h^2+L^2}\right)+\frac{2h}{h^2+L^2}\right)-\Delta P_t& \left(9\right)\end{array}$

(26) The afore presented solution is valid only when ΔP.sub.0≦ΔP.sub.α. The equation (9) is dependent upon α, which also is a function of ΔP.sub.0. Because of this, one may have to solve the equation iteratively by using initial guesses as ΔP.sub.0 values in the equation (2).

(27) In order to calculate edema pressure one may also use other methods than those presented in the aforementioned example.

(28) For professionals in the field it stands clear that the previously presented example embodiments of demonstrational reasons are relatively simple in terms of construction and function. By following the model presented in this patent application it is possible to construct different and even very complex solutions utilizing the patent idea presented in this patent application.