Non invasive measurement of blood pressure

0001The invention relates to a method and a device for non-invasively measuring the blood pressure of a patient.

0002Automatic electronic blood pressure monitors for non-invasively measuring the blood pressures are widely known. Such monitors determine the value of the systolic and the diastolic blood pressure e.g. on the basis of an oscillometric measurement. During such a measurement, the pressure within an inflated cuff wrapped around the arm of a patient is measured during deflation of the cuff. The systolic and the diastolic blood pressure and optionally the pulse rate are determined on the basis of the variation of the cuff pressure.

0003Automatic, electronic blood pressure measuring devices are usually applied to the upper arm of a patient, around the wrist or to a finger of the patient. There are always specific causes for errors during such measurements. Inaccurate measurement results may be due to normal physiological changes, insufficient rest condition of the patient, to pathologic blood pressure changes or to technical or application related artefacts. For example application artefacts may be caused by movements of the application site during the measurement cycle. The errors created by insufficient rest condition and by motion artefacts could be avoided if the patient correctly follows the user instructions of such a measuring device. It is, however, often the case, that measurements are carried out with the patient being not at a sufficient rest condition or patient application sites are moving during the measurement. This may lead to incorrect measurement results.

0004This problem has already been addressed in the copending European patent publication <patcit id="pcit0001" dnum="EP1101440A"><text>EP 1 101 440</text></patcit> of the same inventors.

0005The document <patcit id="pcit0002" dnum="EP409210A"><text>EP 409 210</text></patcit> describes a method of measuring blood pressure in which diastolic mean arterial and systolic pressures are measured; measurements are considered invalid if two of the three values are outside a range of the corresponding averages over several measurements.

0006It is an object of the present invention to overcome the drawbacks of the prior art, especially to provide an automatic, non-invasive blood pressure monitor and a method for measuring the blood pressure, which allow to reduce such incorrect measurement results. It is a further object of the invention to detect pathologic blood pressure changes during a blood pressure measurement.

0007According to the present invention, these objects are solved with a method and a device for non-invasively measuring the blood pressure of a patient according to the features of the independent patent claims.

0008There are mainly four reasons for sequential blood pressure values differing from each other. Differences may be due to normal physiologic changes, due to an insufficient rest condition of the patient or to pathologic blood pressure changes. Pathologic changes may derive from a broad variety of cardiovascular disorders. Finally, changes may be due to artefacts. Technical artefacts, i.e. disturbances caused by the device and application artefacts, e.g. artefacts caused by the motion of the patient are observed.

0009For each difference between sequential measurement results, there is a specific probability for every cause.

0010Low differences without a specific tendency occur mainly due to physiologic changes of the arterial blood pressure. Low to moderate differences, which follow specific dynamic changes known to those skilled in the art may be an indication of an insufficient patient rest condition. Normally, in such a case, the measurement results of the diastolic pressure rise while the values of the systolic pressure and the pulse frequency fall during a sequence.

0011Moderate changes occur due to a broad variety of pathologic blood pressure disorders. High changes are mostly due to artefacts.

0012The present invention allows to enhance the judgement of blood pressure measurement results. Especially, the invention allows to distinguish errors caused by artefacts from errors caused by pathologic and/or physiologic variations. In addition, the present invention allows to determine, whether or not a patient is properly settled to rest condition.

0013According to the method of the present invention, in a first step, the diastolic and the systolic blood pressure value and the pulse rate of a patient are non-invasively measured, preferably with the oscillometric method. The oscillometric method is known to those skilled in the art and does not need to be explained in detail.

0014After measuring the diastolic, the systolic blood pressure value and the pulse rate in a first step a.), step a.) is repeated at least once. Thereby, a series of consecutive results of the values of the systolic and the diastolic blood pressure and of the pulse rate are produced. Calculations, e.g. averages, of the values as defined below are performed subsequently.

0015In a further step, these series of consecutive results are automatically analysed for determining the validity of the results. During this analysis, it is verified, whether the results fulfil one or more of the following conditions: <ul id="ul0001" list-style="none"><li>Condition i.): it is verified, whether the results of the diastolic or the systolic blood pressure and of the pulse rate are steadily decreasing or increasing. If the value for the systolic blood pressure is continously decreasing over the consecutive measurements or if the value for the diastolic blood pressure is continously increasing or decreasing during the measurements and if at the same time the pulse rate is continously decreasing during the measurement, it is concluded that the patient is not at a sufficient rest condition. Continuously decreasing/increasing is defined as follows, referring to a three measurement analysis: <ul id="ul0002" list-style="none" compact="compact"><li>Si-S (i-1) < Si, i-1 (A)</li><li>Di-D(i-1) < Di, i-1 (B1)</li><li>Di-D(i-1) > Di, i-1 (B2)</li><li>Pi-P(i-1) < Pi, i-1 (C)</li><li>with typical constants: <ul id="ul0003" list-style="none" compact="compact"><li>S32= S21=-5 [mmHg]</li><li>D32= D21=-3 [mmHg] or +3 (mmHg)</li><li>P32=-3 [bpm]</li><li>P21=-5[bpm]</li></ul></li></ul></li><li>Condition i.) is met if either (A) AND (B1) AND (C) or (A) AND (B2) AND (C) holds valid.</li><li>Condition ii.): according to a preferred embodiment of the invention, a variation index of the series of the results of the values of the diastolic blood pressure, the systolic blood pressure and the pulse rate according to the following formula is formed respectively: <maths id="math0001"><math display="block"><mi mathvariant="italic">VI</mi><mo>=</mo><mfrac><mn>1</mn><mi>n</mi></mfrac><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>n</mi></munderover></mstyle><mfrac><mfenced open="|" close="|"><msub><mi>R</mi><mi>i</mi></msub><mo>-</mo><mi mathvariant="italic">mean</mi></mfenced><mi mathvariant="italic">mean</mi></mfrac><mo>=</mo><mfrac><mn>1</mn><mi>n</mi></mfrac><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>n</mi></munderover></mstyle><mfrac><mfenced open="|" close="|"><msub><mi>R</mi><mi>i</mi></msub><mo>-</mo><mi mathvariant="italic">Σ</mi><mo>⁢</mo><mfrac><msub><mi>R</mi><mi>i</mi></msub><mi>n</mi></mfrac></mfenced><mrow><mi mathvariant="normal">Σ</mi><mo>⁢</mo><mfrac><msub><mi>R</mi><mi>i</mi></msub><mi>n</mi></mfrac></mrow></mfrac><mo>=</mo><mfrac><mn>1</mn><mi>n</mi></mfrac><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>n</mi></munderover></mstyle><mfrac><mfenced open="|" close="|"><msub><mi>R</mi><mi>i</mi></msub><mo>⁢</mo><mi>n</mi><mo>-</mo><mi mathvariant="normal">Σ</mi><mo>⁢</mo><msub><mi>R</mi><mi>i</mi></msub></mfenced><mrow><mi mathvariant="normal">Σ</mi><mo>⁢</mo><msub><mi>R</mi><mi>i</mi></msub></mrow></mfrac></math><img file="EP1387635B1_D0001.tif" /></maths> whereby <maths id="math0002"><math display="block"><mi mathvariant="italic">mean</mi><mo>=</mo><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>n</mi></munderover></mstyle><mfrac><msub><mi>R</mi><mi>i</mi></msub><mi>n</mi></mfrac></math><img file="EP1387635B1_D0002.tif" /></maths> and whereby Ri are the results of the respective measurements of the diastolic, systolic blood pressure and of the pulse rate and whereby n is the number of subsequent measurements. The variation index is determined for all values of the systolic, the diastolic blood pressure and the pulse rate. The variation index is a figure which describes the measurement stability. If the variation index is high, this may be due to a cardiovascular disorder, affecting the blood pressure stability, e.g. deriving from arrythmia, heart insufficiency or a vascular control disorder. The variation index for each of the results is therefore compared with a predetermined or predeterminable variation index limit. If the variation index is above this limit, the patient/user is informed by displaying this particular result. The patient/user is advised to have a new measurement run..</li><li>Condition iii.): according to a further preferred embodiment of the invention, it is verified, whether the differences between the consecutive measurement results of the systolic blood pressure and of the diastolic blood pressure are below a predetermined or predeterminable similarity limit. The similarity limit may be e.g. set to +/- 12mmHg for the diastolic blood pressure and +/- 18mmHg for the systolic blood pressure. If the differences are below the similarity limit, it is assumed that the blood pressure results are sufficiently similar to each other. In this case, a normal arithmetic average is formed on the basis of the results of the measurements of the systolic and of the diastolic blood pressure</li><li>Condition iv.): according to a further preferred embodiment, it is verified where the differences between the consecutive measurement results of the systolic and diastolic blood pressure are above a predetermined alteration limit. The alteration limit may be typically set to +/- 25mmHg for the diastolic blood pressure and to +/-40mmHg for the systolic blood pressure. If one of the blood pressure measurement results is beyond this limit, it is assumed that this result is highly altered and is due to artefacts. Such results therefore are disregarded in the measuring sequence.</li></ul>

0016Results which are above the similarity limits as defined in condition iii but which are not above the alteration limit as defined in condition iv may be caused by pathologic cardiovascular events but may also be induced by artefacts: These results are probably not fully correct and therefore are not completely disregarded. The influence of these values is reduced by applying a weighted average of the results of the measurement. Thus, the suspicious results are not contributing as high as the normal results. By this measure, possible artefacts are reduced.

0017Results which are not exceeding the similarity limit as defined in condition iii are likely to be correctly measured results. They are used to calculate a classic average result.

0018Typically, a series of three measurements is made. If one of the results is disregarded, a fourth measurement may be made for generating three usable results. If an artefact is specifically detected by the measurement device, one repetition occurs. So the maximum of blood pressure takes is 5, comprising 3 measurements, one possible artefact-induced repetition and one possible variation-induced repetition. Larger number of measurements could also be made. Three measurements are, however, preferred in order to limit the complete measurement period, which should not exceed 5 to 7 minutes in practice.

0019The device according to the present invention is non-invasively measuring the blood pressure of a patient in the above described manner. For that purpose, the device is provided with means for determining the diastolic and the systolic blood pressure and the pulse rate of the patient with the oscillometric method.

0020The device is further provided with means for storing a series of consecutive measurement results of the systolic and the diastolic blood pressure and of the pulse rate. The device includes means for analysing the series of consecutive results of the diastolic and systolic blood pressure and of the pulse rate. The means for analysing the results are adapted to determine whether the results of the diastolic or the systolic blood pressure are steadily decreasing or increasing and whether at the same time the results of the pulse rate are decreasing.

0021According to a preferred embodiment, the means for analysing are further adapted to determine a variation index as defined above and for comparing the variation index with a predetermined or preterminable variation index limit.

0022According to another preferred embodiment, the means for analysing are adapted to determine whether the results of a series of measurements for the systolic and diastolic blood pressure, pulse rate or below/above predetermined or predeterminable similarity limits and/or alteration limits, respectively.

0023The device according to the present invention is further provided with means for calculating an average or a weighted average on the basis of a series of consecutive measurement results.

0024The invention will be more clearly understood with reference to the following embodiments and to the following drawings, which show: <dl id="dl0001" compact="compact"><dt>Figure 1:</dt><dd>a flow chart of the method according to the present invention.</dd><dt>Figures 2a to 2c:</dt><dd>a graphic representation of the rest condition detection.</dd><dt>Figure 3:</dt><dd>a graphic representation of similarity /alteration detection.</dd></dl>

0025According to the present invention, for the measurement of the blood pressure, a number of consecutive measurements is made. Typically, three measurements, and under certain conditions, up to five measurements, are made to obtain the diastolic blood pressure D, the systolic blood pressure S and the pulse rate P. Each measurement lasts for about 60 seconds. Between two subsequent measurements, a pause of another 45 - 60 seconds is made. This is necessary because between two measurements, the venous reflow has to be completed. According to actual findings in medicine, this requires a minimum period of about 45 seconds.

0026The three measurements generate a series of measurement results D1, D2, D3 representing the diastolic blood pressure value, S1, S2, S3 representing the systolic blood pressure value and P1, P2, P3 representing the pulse rate. These values are stored within the blood pressure measuring device.

0027After three successful measurements an average is calculated (see below) and an analysis of the measurement results is made. In a first step, it is verified, whether or not the patient is in a proper rest condition. For that purpose, it is verified whether a first condition i.) as follows is fulfilled: <ul id="ul0004" list-style="none"><li>Systolic criteria for condition i.): S3<S2-dS<S1-2dS (A) whereby dS is approximately set to 5mmHg</li><li>Diastolic criteria for condition i.) D3>D2+dD>D1+2dD or (B1) D3<D2-dD<D1-2dD (B2) whereby dD is set to about 3[mmHg]</li><li>Pulse rate criteria for condition i.) P3<P2-dP2 AND P2<P1-dP1 (C) whereby dP1 is set to about 5[Bpm] (beats per minute), and dP2 is about 3 [Bpm]. The change of the pulse rate is decreasing in the course of time when a patient is not at a rest condition. For this reason, dP1 is set higher than dP2.</li></ul>

0028A patient is detected as being not properly settled, if the systolic criteria (A) AND one of the diastolic criteria (B1) OR (B2) AND the pulse rate criteria (C) are fulfilled.

0029The behaviour of the systolic blood pressure, the diastolic blood pressure and the pulse rate in case of an insufficient rest condition are schematically shown in <figref idref="f0003 f0004">figures 2a to 2c</figref>.

0030According to <figref idref="f0003">figure 2a</figref>, the systolic blood pressure is continously decreasing from the first to the third measurement result. The difference S between subsequent blood pressure values S1 and S2 and S2, S3 are above the preset limit of the parameter dS.

0031<figref idref="f0003">Figure 2b</figref> shows the behaviour of the diastolic blood pressure if a patient is not properly settled to rest condition. In such cases, the diastolic pressure is usually increasing steadily towards a steady state value in the course of the three measurements. According to <figref idref="f0003">figure 2b</figref>, the values D1, D2 and D3 are steadily increasing. The difference D between D2, D1 and between D3, D2 are both above the predetermined limit of change dD.

0032In some cases, the diastolic blood pressure values of a person, who is not properly settled, are decreasing towards a steady state value D3*. In this case D1* is larger than D2* which is larger than D3*.

0033Both conditions are observed in persons, being not properly settled .

0034<figref idref="f0004">Figure 2c</figref> shows the dynamics of the pulse rate of a patient who is still not properly settled.

0035The values of the pulse rate are steadily decreasing towards a steady state value. The difference P between the subsequent measurements P1, P2 and P2, P3 is above a predetermined limit dP.

0036If the conditions shown in <figref idref="f0003">figure 2a or 2b</figref> are fulfilled AND the condition in <figref idref="f0004">figure 2c</figref> holds valid as well, it is concluded that the patient is not properly settled.

0037The patient/user is informed about that fact and advised to have another measurement procedure after being intentionally in rest condition.

0038In a further step, it is verified by condition ii.), whether or not the measurement results disclose an indication of a disorder of the blood pressure stability. For that purpose, a variation index according to the following formula is applied to the diastolic, the systolic blood pressure and to the pulse rate: <maths id="math0003"><math display="block"><mi mathvariant="italic">VI</mi><mo>=</mo><mfrac><mn>1</mn><mi>n</mi></mfrac><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>n</mi></munderover></mstyle><mfrac><mfenced open="|" close="|"><msub><mi>R</mi><mi>i</mi></msub><mo>-</mo><mi mathvariant="italic">mean</mi></mfenced><mi mathvariant="italic">mean</mi></mfrac></math><img file="EP1387635B1_D0003.tif" /></maths> whereby <maths id="math0004"><math display="block"><mi mathvariant="italic">mean</mi><mo>=</mo><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>n</mi></munderover></mstyle><mfrac><msub><mi>R</mi><mi>i</mi></msub><mi>n</mi></mfrac></math><img file="EP1387635B1_D0004.tif" /></maths> and whereby Ri are the respective measurement results D1, D2, D3 and S1, S2, S3 and P1, P2, P3. The variation index VI describes the typical deriation of results in the course of the measurement series and is independent on the number of measurements.

0039For example if it is presumed, that the values of the systolic blood pressure are S1=112mmHg, S2=124mmHg and S3=124mmHg, the mean value is calculated as follows: <ul id="ul0005" list-style="none" compact="compact"><li>Mean= (112 +124+124)/3=120mmHg In this example the variation index VI_s for the systolic blood pressure consequently is defined as follows: <maths id="math0005"><math display="block"><msub><mi>VI</mi><mi mathvariant="normal">s</mi></msub><mo>=</mo><mfrac><mn>1</mn><mi>n</mi></mfrac><mrow><mo>(</mo><mn>120</mn><mo>-</mo><mn>112</mn><mo>+</mo><mn>124</mn><mo>-</mo><mn>120</mn><mo>+</mo><mn>124</mn><mo>-</mo><mn>20</mn><mo>)</mo><mo>/</mo><mn>120</mn><mo>=</mo><mn>16</mn><mo>/</mo><mn>120</mn><mo>=</mo><mn>4.4</mn><mo>%</mo><mn>.</mn></mrow></math><img file="EP1387635B1_D0005.tif" /></maths></li></ul>

0040If the variation index is above a variation index limit value, the user/patient is additionally informed by displaying this matter of fact. If the variation index frequently exceeds the predefined limits, the user is advised (operating manual) to undergo further diagnosis. The variation index limits are typically as follows:.10 [%] for the systolic blood pressure, 10 [%] for the diastolic blood pressure and 8 [%] for the pulse rate.

0041Prior to the calculation of averages of the results, it is verified, whether consecutive blood pressure values are sufficiently similar to each other, i.e. it is verified, whether the following condition iii.) is fulfilled: <ul id="ul0006" list-style="none"><li>Condition iii for the systolic blood pressure: S1-18mmHg&lt;S2&lt;S1+18mmHg S2-18mmHg&lt;S3&lt;S2+18mmHg</li><li>Condition iii for the diastolic blood pressure: D1-12mmHg&lt;D2&lt;D1+12mmHg D2-12mmHg&lt;D3&lt;D2+12mmHg</li></ul>

0042If all of the above conditions are fulfilled, it is concluded that the measurement results are correct. Consequently, a normal arithmetic average of the results is calculated as follows: <maths id="math0006"><math display="block"><mi mathvariant="normal">S</mi><mo mathvariant="normal">=</mo><mrow><mo mathvariant="normal">(</mo><mi mathvariant="normal">S</mi><mo>⁢</mo><mn mathvariant="normal">1</mn><mo mathvariant="normal">+</mo><mi mathvariant="normal">S</mi><mo>⁢</mo><mn mathvariant="normal">2</mn><mo mathvariant="normal">+</mo><mi mathvariant="normal">S</mi><mo>⁢</mo><mn mathvariant="normal">3</mn><mo mathvariant="normal">)</mo><mo mathvariant="normal">/</mo><mn mathvariant="normal">3</mn></mrow></math><img file="EP1387635B1_D0006.tif" /></maths><maths id="math0007"><math display="block"><mi mathvariant="normal">D</mi><mo mathvariant="normal">=</mo><mrow><mo mathvariant="normal">(</mo><mi mathvariant="normal">D</mi><mo>⁢</mo><mn mathvariant="normal">1</mn><mo mathvariant="normal">+</mo><mi mathvariant="normal">D</mi><mo>⁢</mo><mn mathvariant="normal">2</mn><mo mathvariant="normal">+</mo><mi mathvariant="normal">D</mi><mo>⁢</mo><mn mathvariant="normal">3</mn><mo mathvariant="normal">)</mo><mo mathvariant="normal">/</mo><mn mathvariant="normal">3</mn></mrow></math><img file="EP1387635B1_D0007.tif" /></maths>

0043The values S and D are subsequently displayed in the blood pressure measuring device.

0044The above-mentioned ranges (+/- 18mmHg and +/- 12mmHg are referred to as the similarity limits.

0045It has been found that similarity limits of +/- 18mmHg for the systolic blood pressure and +/- 12mmHg for the diastolic blood pressure are leading to sufficiently precise results. Other similarity limits could be used as well.

0046In addition, it is verified, whether or not condition iv.) holds valid. It is checked whether results are highly altered compared to each other. The condition iv.) is defined as follows: <ul id="ul0007" list-style="none"><li>Systolic blood pressure: S1+40mmHg&lt;S2&lt;S1-40mmHg S2+40mmHg&lt;S3&lt;S2-40mmHg</li><li>Diastolic blood pressure: D1+25mmHg&lt;D2&lt;D1-25mmHg D2+25mmHg&lt;D3&lt;D2-25mmHg</li></ul>

0047If at least one of the above four conditions is fulfilled, it is concluded that at least one of the measurement results is due to an incorrect measurement. The range +- 40mmHg for the systolic blood pressure and +- 25mmHg for the diastolic blood pressure are also referred to as the alteration limits. One can savely assume that values beyond the alteration limits represent artefacts. These results are rejected.

0048If the above condition iv.) is not met, i.e. if all results are not highly altered and if the above condition iii.) is not met, i.e. the results are not sufficiently similar to each other, a weighted average is formed.

0049It is assumed, that blood pressure results which are neither similar nor highly altered, either represent an artefact or a severe pathologic blood pressure variation. Thus, the influence of these values is reduced by calculating a weighted average as follows: <ul id="ul0008" list-style="none" compact="compact"><li>S=2S1+2S2+S3/5, whereby S3 is the non-similar value and D=2D1+2D2+D3/5, whereby D3 is the non-similar value.</li></ul>

0050In a more general form: <maths id="math0008"><math display="block"><mi>S</mi><mo>=</mo><mfrac><mrow><mi>A</mi><mo>⁢</mo><mn>1</mn><mo>⁢</mo><mi>S</mi><mo>⁢</mo><mn>1</mn><mo>+</mo><mi>A</mi><mo>⁢</mo><mn>2</mn><mo>⁢</mo><mi>S</mi><mo>⁢</mo><mn>2</mn><mo>+</mo><mi>A</mi><mo>⁢</mo><mn>3</mn><mo>⁢</mo><mi>S</mi><mo>⁢</mo><mn>3</mn></mrow><mrow><mi>A</mi><mo>⁢</mo><mn>1</mn><mo>+</mo><mi>A</mi><mo>⁢</mo><mn>2</mn><mo>+</mo><mi>A</mi><mo>⁢</mo><mn>3</mn></mrow></mfrac></math><img file="EP1387635B1_D0008.tif" /></maths><maths id="math0009"><math display="block"><mi>D</mi><mo>=</mo><mfrac><mrow><mi>B</mi><mo>⁢</mo><mn>1</mn><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mn>1</mn><mo>+</mo><mi>B</mi><mo>⁢</mo><mn>2</mn><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mn>2</mn><mo>+</mo><mi>B</mi><mo>⁢</mo><mn>3</mn><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mn>3</mn></mrow><mrow><mi>B</mi><mo>⁢</mo><mn>1</mn><mo>+</mo><mi>B</mi><mo>⁢</mo><mn>2</mn><mo>+</mo><mi>B</mi><mo>⁢</mo><mn>3</mn></mrow></mfrac></math><img file="EP1387635B1_D0009.tif" /></maths> whereby A1>A3; A2>A3 if S3 is not similar to S1 and S2 and B1>B3; B2>B3 if D3 is not similar to D1 and D2.

0051The values of A1, A2, A3 and B1, B2, B3, respectively are empirically set to 2 for similar values (e.g. S1 and S2) and to 1 for non similar values. Further findings in research may lead to an amendment of these factors without leaving the scope of the present invention.

0052In <figref idref="f0004">figure 3</figref>, the most likely causes generating specific blood pressure differences during a series of measurements are graphically shown.

0053Curve 1 represents the probability of measuring physiologic blood pressure differences. Due to physiologic reasons, small blood pressure differences occur relatively frequently. As long as the differences are within the definitions of similarity, i.e. meet the condition iii.) a normal arithmetic average is made. Generally, in healthy persons physiologic blood pressure changes are small in rest condition.

0054Relatively large consecutive blood pressure differences which are above the alteration limit, i.e. which meet the condition iv.) are most probably due to artefacts. The probability of artefacts is shown by curve 3 in <figref idref="f0004">figure 3</figref>. When the results meet condition iv.), by referring to the criteria of high alteration no average is calculated and the values are subsequently rejected.

0055If none of the conditions iii.) and iv.) are met, i.e. if a result is above the similarity limit but below the alteration limit, the difference may be due to pathologic reasons or to artefacts. The probability of blood pressure differences due to pathologic reasons is shown by curve 2 in <figref idref="f0004">figure 3</figref>. In such a case, a weighted average is calculated.

0056The similarity limits are set to 18mmHg for the systolic blood pressure and 12mmHg for the diastolic blood pressure. The alteration limit is set to 40mmHg for the systolic blood pressure and to 25mmHg for the diastolic blood pressure.

0057In case one of the values is rejected, a fourth measurement is initiated. The same analysis, especially verification of the conditions i.), ii.), iii.) and iv.) is applied to the new set of results. If again one value has to be rejected, an error message is displayed and no measurement result is given. If the fourth measurement is not highly altered, i.e. if condition iv.) is not fulfilled, a measurement result is calculated and displayed as decribed above.

0058Averages are processed as follows:

0059After obtaining three blood pressure measurement results, the following cases have to be distinguished. <ul id="ul0009" list-style="none"><li>1) V1 (S) V2 (S) V3 ⇒ A_N= <maths id="math0010"><math display="inline"><mfrac><mn>1</mn><mn>3</mn></mfrac></math><img file="EP1387635B1_D0010.tif" /></maths> (V1+V2+V3)</li><li>2) V1 (NS) V2 (S) V3 ⇒ A_W= <maths id="math0011"><math display="inline"><mfrac><mn>1</mn><mn>5</mn></mfrac></math><img file="EP1387635B1_D0011.tif" /></maths> (V1+2V2+2V3)</li><li>3) V1 (HA) V2 (S) V3 ⇒ Initiate a fourth measurement and check V2, V3; V4</li><li>4) V1 (S) V2 (NS) V3 ⇒ A_W= <maths id="math0012"><math display="inline"><mfrac><mn>1</mn><mn>5</mn></mfrac></math><img file="EP1387635B1_D0012.tif" /></maths> (2V1+2V2+V3)</li><li>5) V1 (S) V2 (HA) V3 ⇒ Initiate a fourth measurement and check V1, V2, V4</li><li>6) V1(NS)V2(NS)V3 and <ul id="ul0010" list-style="none" compact="compact"><li>a) V3 (S) V1 ⇒ A_W= <maths id="math0013"><math display="inline"><mfrac><mn>1</mn><mn>5</mn></mfrac></math><img file="EP1387635B1_D0013.tif" /></maths> (2V1+V2+2V3) and</li><li>b) V3 (NS) V1 or V3 (HA) V1 ⇒ Error display</li></ul></li><li>7) V1 (HA) V2 (NS) V3 and <ul id="ul0011" list-style="none"><li>a) V3 (S) V1 ⇒ Initiate a fourth measurement and check V1, V3, V4 and</li><li>b) V3(NS)V1 or V3(HA)V1 => Error display</li></ul></li><li>8) V1(NS)V2(HA)V3 and <ul id="ul0012" list-style="none" compact="compact"><li>a) V3 (S) V1 ⇒ Initiate a fourth measurement and check V1, V3, V4 and</li><li>b) V3(NS)V1 or V3 (HA) V1 ⇒ Error display</li></ul></li><li>9) V1(HA)V2(HA)V3 and <ul id="ul0013" list-style="none" compact="compact"><li>a) V3(S)V1 ⇒ Initiate a fourth measurement and Check V1, V3, V4 and</li><li>b) V3 (NS) V1 or V3(HA)V1 ⇒ Error display</li></ul></li></ul> whereby <ul id="ul0014" list-style="dash"><li>V1,V2,V3,V4 designate the values of the systolic or the diastolic blood pressure</li><li>(S) designates that subsequent values are similar</li><li>(NS) designates that subsequent values are non-similar</li><li>(HA) designates that subsequent values are highly altered</li></ul>

0060The method of the present invention is shown in more detail in the flow chart in <figref idref="f0001">figure 1</figref>.

0061<figref idref="f0001">Figure 1</figref> shows a flow chart of an embodiment of the invention. A blood pressure measuring device may be operated either in a single mode or in an average mode. In the case of single mode operation, the variable meancountend is set to 1. In the case of average mode operation, the variable meancountend is set to the pre-defined number of measurements, e.g. to 3. If the device operates in average mode, measurements are repeated e.g. three times for generating three consecutive results of values for the systolic and for the diastolic blood pressure and pulse frequency. If e.g. three consecutive results of the systolic and the diastolic blood pressure have been obtained, the similarity and the degree of alteration of these values is checked. If similarity of all three values is detected, a conventional average is formed (see left branch of <figref idref="f0001">Figure 1</figref>).

0062If the second result is not similar to both the first and third result, it is checked whether the first and the third results are similar to each other. If the first and the third results are similar, an additional measurment is made. If not, an error is indicated. If an additional measurement is made, a variable, "meanflag" is set to 1. If the variable meanflag has previously been set to 1, an error message is displayed. This means, that in case of non-similar values, only a limited number of, e.g. one repetition may occur.

0063If the second result is not similar to the first or the third result, it is verified whether the first or the third value is highly altered to the second value. If there is no high alteration, a weighted average is formed. If there is a high alteration, a repetition is initiated in order to acquire an additional measurement result. If an additional measurement is scheduled, the variable meanflag is checked. If the variable meanflag has been previously set to 1, no additional measurement is taken and an error message is displayed.

0064After calculation of the conventional or of the weighted average and before displaying of the results, the following procedure is performed:

0065On the basis of the measurement values, it is verified whether the patient is properly settled. If the patient is found to be not properly settled, a no-settling indication is displayed and the patient is asked to properly settle before taking a further measurement.

0066In addition, the variation index is formed on the basis of the measurement values. If the variation index VI is above the variation index limit VI _Limit, an error message is displayed.

0067In other words: after calculation of the average, it is verified whether one the above defined condition i) or condition ii) is fulfilled. If yes an appropriate indication is displayed to the user.