Method and Apparatus for Controlling the Cuff Pressure in a Sphygmomanometer

The present invention relates to a method and apparatus for controlling the cuff pressure in a non-invasive sphygmomanometer. The cuff pressure control mechanism comprises one or a plurality of valves whose operation ion is controlled by means of one or a plurality of control elements in a manner that the valve or valves have an operating frequency which is above the heart beat rate of the heart.

The blood pressure is generally tested by pumping air into a cuff which is wrapped e.g. around the arm of a patient. When the cuff pressure is sufficiently high to stop arterial blood circulation, the reduction of pressure is initiated. As the cuff pressure drops so-called Korotkoff sounds are listened for. As the sound becomes audible, the reading of systolic pressure is obtained. As the pressure drop is continued, the Korotkoff sounds gradually cease altogether. The reading of diastolic pressure is obtained at the moment the sounds cease to be audible.

The Finnish Patent application No. 853781, corresponding to U.S. Pat. No. 4,768,518, described an apparatus used in automatic blood pressure testing, comprising a cuff, a pump and at least two magnetic valves provided with a throttle element, as well as a microprocessor. The throttle element consists of a thin tube whose length has an effect on the pressure reduction rate. It is customary to employ a plurality of throttle elements in a single apparatus, each throttle element being fitted with a tube of different length. Each throttle element is controlled by its own magnetic valve. The magnetic valves are controlled by means of a microprocessor which opens a given magnetic valve depending on which magnetic valve has behind it a throttle element that is best suitable for maintaining a linear cuff air pressure reduction rate at a given moment. If necessary, several magnetic valves can also be opened at the same time. A problem in this type of apparatus is that it requires a plurality of bulky components consisting of a magnetic valve and a throttle element and, nevertheless, it is not possible to obtain a linear rate of the cuff pressure reduction. In addition, opening an of valves leads to disturbances in testing. Also, a pressure drop profile cannot be freely adjusted.

The cuff pressure reduction can also be controlled by using a stepping motor driven needle valve. In order to maintain a uniform pressure drop, the valve must be adjusted during testing. This type of system requires expensive equipment as the manufacturing process is quite complicated. Another problem in this apparatus is malfunctions caused by dust.

An object of this invention is to eliminate the above problems. Thus, an object of the invention is to provide a simple, inexpensive and compact solution. Another object is to achieve a linear cuff pressure drop.

The characterizing features for an apparatus of the invention are set forth in the annexed claims.

The invention is based on a remarkable discovery that by using a pulse-controlled high-speed valve the cuff pressure drop can be effected in a completely linear fashion. The valve control is effected on the basis of a measured reading received from a cuff pressure measuring element.

The processing of a measured reading and the valve control effected on the basis thereof requires,,some special control means which can be e.g. an analog or digital circuit. A particularly noteworthy digital circuit is a microprocessor. Microprocessors compatible with this apparatus are commercially available. The microprocessor should be provided with a pulse-width modulated output.

In an analog controlled system, a signal coming from a pressure sensor is compared in a buffer amplifier with an analog uniformly falling control signal. The output of a buffer amplifier controls the pulse valve by means of pulse-width modulation.

The most important feature is, however, that, on the basis of a measured reading received from the pressure measuring element, the control means for effecting the valve control is capable of controlling the valve opening and closing to occur with a sufficient frequency and for periods of time of determined duration. One such control means is sufficient but of course more than one can be used.

By adjusting the width of a pulse transmitted by an analog or digital circuit it is possible to achieve a stepless regulation of the amount of a gas or liquid flowing through the valve per unit time. In practice, this is performed by having the valve open for a certain determined period of time, whereby a gas or liquid contained in the cuff tends to flow out of it, followed by having the valve closed for a determined period. The duration of these successive open-closed cycles is automatically controlled by these circuits according to measured readings provided by the pressure measuring means or element. Thus, the pressure drop profile is freely adjustable. The application of a sufficiently high pulse frequency serves to avoid the disturbing effect on blood pressure testing caused by pressure shocks. The optimum pulse frequency is 35-60 Hz. However, the pulse frequency must be higher than the heart so as not to disturb the actual blood pressure testing process In fact, there is no actual upper limit to the possible pulse frequency.

Thus, an apparatus of the invention can operate in a manner that either the valve has a constant pulse frequency but a varying pulse width, as mentioned above, or the valve has a varying pulse frequency but a constant pulse width. The apparatus can also be adapted to operate in a manner that both pulse frequency and pulse width of the valve can be varied during a blood pressure testing period. Naturally, both pulse frequency and pulse width of the valve can be maintained constant, but a problem then is that the cuff pressure drop does not occur in a linear fashion. In this latter case, an attempt to correct the linearity can be made by varying pulse frequency, pulse width or both stepwise at properly selected intervals during the same testing cycle. The first-mentioned alternative must be considered preferable.

In an apparatus of this invention, a single valve is sufficient. Of course, more than one valve can be used in parallel. One advantage gained by the use of several valves can be considered to be a prolonged service life for the valves. On the other hand, the space required by several valves can be considered a drawback.

Suitable valves include so-called on-off valves which, accordingly, are either in an open or closed position. This type of valves includes e.g. a solenoid valve, a piezoelectric valve and a magnetostrictive valve. However, a solenoid valve is perhaps the best choice for this type of blood pressure tester. A solenoid valve is highly durable. The durability of a piezoelectric valve would be even better but its drawback is a rather high operating voltage.

The valve can be placed at many different locations in a blood pressure testing apparatus. A preferred position is in connection with a tube running from pump to cuff. Other possible locations are e.g. the cuff itself, a separate tube issuing from the cuff, a tube running from cuff to pressure sensor, or perhaps the pump or a separate tube issuing from the pump. However, location of the valve is not of critical importance in terms of this invention.

An apparatus of this invention does not require a throttle element in connection with the valve, since the valve alone is capable of controlling a linear drop for the cuff pressure. Hence, this is simply effected by opening and closing the valve as often as necessary and by adjusting the durations of on- and off-periods for the valve.

Due to the pneumatic attenuation of tubes, connectors and cuff, no major shock-like changes are found in the cuff pressure even though the valve operates on the on-off technique. The cuff pressure drop is indeed linear.

A cuff pressure drop system is by no means dependent on the size of a cuff. Thus, cuffs of varying sizes can be used without problems.

The invention will now be described in more detail with reference made to the accompanying drawing, in which the drawing shows a general plan for an apparatus of the invention.

The apparatus shown in the drawing includes any conventional pump 1 with a tube 2 leading to a cuff 3. The tube 2 is fitted with a valve 4 for the reduction of pressure. The valve is controlled by a control element 5 which in turn operates on the basis of a measured reading received from a pressure sensing member 6 in connection with the cuff. The same control element 5 is also capable of controlling the operation of pump 1.

Pump 1 is used to pump a sufficiently high pressure in cuff 3. As pressure is reduced, a pulse-controlled high-speed valve 4 opens and closes repeatedly several times during a single blood pressure testing cycle. By using the, valve-regulating control element 5, such as an analog or digital circuit, it is possible to regulate the pressure drop of cuff 3 in a completely linear manner. The pressure drop rate can also be freely controlled by means of said control element 5 which regulates valve 4. The operation of valve 4 can be controlled by means of control element 5 primarily in four different ways:

(A) The pulse frequency of valve 4 is maintained constant but the pulse width is varied. When the pressure in cuff 3 is high, said valve 4 is open for a short period per one on-off cycle. When the pressure is lower than the initial pressure, for example, said valve 4 is open for a longer period per one on-off cycle, since the amount of outflowing air per unit time is smaller than in the case when the pressure in cuff 3 is higher. The frequency of the valve opening times remains unchanged.

(B) The pulse width of valve 4 is maintained constant but the pulse frequency of valve 4 is varied. In this case, the opening period of valve 4 in successive on-off cycles remains the same throughout a blood pressure testing process. Instead valve 4 is opened in a certain unit time more frequently the lower the pressure drops cuff 3.

(C) Both pulse width and pulse frequency of valve 4 are varied. In this case, both the opening period of valve 4 in successive on-off cycles and frequency of the opening times of valve 4 can be varied.

(D) Both pulse width and pulse frequency of valve 4 remain constant either for the entire period required by a blood pressure testing process or just for part of the period required by a testing process. Thus, in the former case, the opening period of valve 4 in successive on-off cycles remains constant and the same applies to the frequency of the opening times of valve 4. In the latter case, the opening period of valve 4 in successive on-off cycles remains constant for a certain period of time as also does the frequency of the opening times of valve 4 but the opening period and the frequency of the opening times of a valve or both of these can be varied at certain intervals during one and the same testing cycle.