GB2575945A

GB2575945A - System for non-invasive blood pressure measurement

Info

Publication number: GB2575945A
Application number: GB1916351.8A
Authority: GB
Inventors: Harrison Tom
Original assignee: TTP Ventus Ltd
Current assignee: TTP Ventus Ltd
Priority date: 2019-11-11
Filing date: 2019-11-11
Publication date: 2020-01-29
Anticipated expiration: 2039-11-11
Also published as: GB2575945B; GB201916351D0

Abstract

A sphygomomanometer comprising an oscillating gas pump which inflates a cuff wherein the gas is substantially pulsation-free such that blood pressure measurements can be taken while the cuff is inflating. The cuff is in fluid communication with the resonant gas pump and a valve. The resonant gas pump comprises a substantially cylindrical cavity with walls, and inlet and outlet apertures e.g. valves. A drive circuit applies a voltage across the piezoelectric actuator at a frequency of at least 500 Hz to cause vibration of the gas pump walls. Preferably, the drive frequency is more than 20 kHz. Cuff inflation acts to compress e.g. an arm, so blood pressure pulses in the blood vessel can be measured while the gas pump is in operation. The rate of inflation is controlled in a feedback loop with the pressure sensor, where the amplitude of the voltage waveform across the piezoelectric actuator is adjustable.

Description

SYSTEM FOR NON-INVASIVE BLOOD PRESSURE MEASUREMENT

The present invention relates to non-invasive blood pressure measurement.

Whilst 24-hour Ambulatory Blood Pressure Measurement (ABPM) observation is routine, there are several shortcomings with the current generation of automated oscillometric measurement systems. The root of many of these shortcomings is in the use of traditional motor-driven diaphragm pumps, which are bulky, noisy, vibrate and introduce pulsation in the airflow.

During a 24-hour observation, blood pressure is measured at regular intervals throughout the day and night as the patient goes about their normal daily business. Such observation provides a dear picture of how blood pressure changes throughout the day; this may be, for example, to observe whether readings in a clinical setting are higher than in the home (known as white coat hypertension), or to see how the patient reacts to a course of drugs.

The standard oscillometric method requires careful measurement of the pulses in pressure that result from the expansion and contraction of the brachial artery. This measurement is carried out across a window just exceeding the systolic to diastolic arterial pressure. Most oscillometric systems use a traditional pump to inflate a brachial cuff to the top of the window, before disengaging the pump and bleeding the pressure out to the bottom of the window with a valve. The oscillometric measurements are made during deflation, from which the blood pressure is calculated.

In principle the oscillometric measurements can be taken during cuff inflation, which has several benefits.

First, the cuff need only be inflated to a pressure just exceeding the systolic; once the systolic pressure has been identified, the measurement can conclude. Therefore, the cuff is not inflated any higher than necessary, minimising the compression forces applied to the patient and therefore improving comfort. For 24-hour observation, some measurements will necessarily happen whilst the patient is sleeping, where maximising comfort and minimising disruption is therefore advantageous.

A second benefit of measurement on inflation is that the measurement cycle can be shortened, which also contributes to improved comfort. Whether measuring on inflation or deflation, the measurement step (controlled pressure change within the window) will take a similar length of time. Each approach has an additional step. For measurement on deflation, the cuff is first inflated with a pump. Typical product constraints mean that the pump selected is often of modest flow capacity and therefore it takes some time to inflate the cuff. For measurement on inflation, the cuff is vented with a valve once the systolic pressure has been measured. It is much easier to vent the cuff quickly with a valve with low flow restriction than to inflate it quickly with the size of pump used in conventional systems.

However, the applicant has recognised that the pulsation generated by traditional pumps can mask the underlying signal from the patient, and so in practice oscillometric measurements on inflation are difficult. Accordingly, there is a need for an improved pump system to enable measurement on inflation.

According to the invention there is provided a non-invasive blood pressure measurement system comprising: a resonant gas pump comprising a substantially cylindrical cavity defined by cavity walls, the cavity walls comprising a cavity inlet aperture and a cavity outlet aperture; a piezoelectric actuator arranged to generate oscillatory motion of the cavity walls to drive a gas between the inlet aperture and the outlet aperture; a drive circuit arranged to apply a voltage waveform across the piezoelectric actuator to generate said oscillatory motion of the cavity walls at a frequency of at least 500 Hz; a cuff for wearing on an ami or other body part having a blood vessel, the cuff comprising a pressure sensor for measuring blood pressure in the blood vessel, the cuff being in fluid communication with the resonant gas pump; and a valve in fluid communication with the cuff, wherein: the resonant gas pump is operable to inflate the cuff by generating a substantially pulsation-free flow of gas, such that, in use of the resonant gas pump to inflate the cuff so as to compress said arm or other body part, blood pressure pulses in the blood vessel can be measured whilst the resonant gas pump is in operation; and the resonant gas pump is precisely controllable such that the rate of inflation of the cuff is controllable in a feedback loop with the pressure sensor, the amplitude of the voltage waveform across the piezoelectric actuator being adjustable to control the rate of inflation.

Unlike traditional pumps, which typically cycle a few times per second, the resonant gas pump operates at high frequencies, optionally ultrasonic frequencies in excess of 20,000 cycles per second. With each 50-microsecond cycle of the pump moving less than a microlitre of air, the resulting pulsation in the cuff is entirely negligible. As a result, it is readily possible to measure the oscillometric signal whilst inflating the cuff. This provides the benefit of improved patient comfort.

Referring to Figure 1, blood pressure measurement is taken during cuff inflation with a resonant gas pump, with the pump controlled to achieve a constant inflation rate of 3 mmHg/s. The lack of pulsation from the pump enables clear measurement of the arterial pulse pressure, from which the oscillation envelope and blood pressure can be extracted.

Using a resonant gas pump brings other benefits too. The resonant gas pump’s ultrasonic operation is inaudible and vibration-free. This means that the pump module can function without disturbing sleep. Further, the pump is compact, an exemplary pump weighing just 5g (1/4 oz) and measuring 10 mm (<0.5 in) in height and 30 mm (1.25 in) in diameter. These features allow the pump module to be (optionally) tightly integrated with the brachial cuff, enabling the creation of wholly arm-worn systems.

The relocation of the pump module from its traditional ‘home’ whilst in use - the belt (or bedside table, if at night) - to be directly mounted on the cuff is not only advantageous to the user experience. It also eliminates the need for a hose between the pump module and cuff. Given that any inadvertent pressure applied to such a hose can interfere with the measurement, the novel architecture offers a more robust solution - not least during the night, when the hose could become kinked during sleep, or where the user may roll onto it.

Oscillometric systems powered by resonant gas pump also benefit from the pump’s high-precision controllability, allowing the output of the pump to be adjusted to achieve a constant inflation rate. This could be time-based (mmHg per second) and mirror the rule-of-thumb 2-3 mmHg/s deflation rate used by many systems (see Figure 1 for a demonstration), or a pulse-rate based adaptive approach (mmHg per heartbeat).

Claims

1. A non-invasive blood pressure measurement system comprising:

a resonant gas pump comprising a substantially cylindrical cavity defined by cavity walls, the cavity walls comprising a cavity inlet aperture and a cavity outlet aperture;

a piezoelectric actuator arranged to generate oscillatory motion of the cavity walls to drive a gas between the inlet aperture and the outlet aperture;

a drive circuit arranged to apply a voltage waveform across the piezoelectric actuator to generate said oscillatory motion of the cavity walls at a frequency of at least 500 Hz;

a cuff for wearing on an arm or other body part having a blood vessel, the cuff comprising a pressure sensor for measuring blood pressure in the blood vessel, the cuff being in fluid communication with the resonant gas pump; and a valve in fluid communication with the cuff, wherein:

the resonant gas pump is operable to inflate the cuff by generating a substantially pulsation-free flow of gas, such that, in use of the resonant gas pump to inflate the cuff so as to compress said arm or other body part, blood pressure pulses in the blood vessel can be measured whilst the resonant gas pump is in operation; and the resonant gas pump is precisely controllable such that the rate of inflation of the cuff is controllable in a feedback loop with the pressure sensor, the amplitude of the voltage waveform across the piezoelectric actuator being adjustable to control the rate of inflation.

2. A system according to claim 1, wherein the rate of inflation is controllable in terms of pressure change per second.

3. A system according to claim 1, wherein the rate of inflation is controllable in terms of pressure change per measured heartbeat.

4. A system according to claim 2, wherein the rate of inflation is controllable to be set at a value representing an optimum combination of accuracy (slower inflation) and comfort (faster inflation).

5. A system according to any preceding claim, wherein the resonant gas pump and the pressure sensor are connected to the cuff via a single fluid path.

6. A system according to any preceding claim, wherein the drive circuit is arranged to apply the voltage waveform across the piezoelectric actuator to generate said oscillatory motion of the cavity walls at a frequency of more than 20,000 Hz.

7. A system according to any preceding claim, wherein the drive circuit is configured to compare the measured blood pressure pulses to a predetermined threshold value to determine a fault condition.

8. A system according to claim 7, comprising a user interface for indicating the fault condition.

9. A system according to claim 8, wherein the user interface comprises one or more of: a light-emitting element; a sound-emitting element; and a vibrating element.

10. A system according to any preceding claim, wherein the cavity radius, a, and height, h, of the resonant gas pump satisfy the following inequalities:

— is greater than 1.2; and h

h‘‘a is greater than 4 x 10’wm.

11. A system according to any preceding claim, wherein at least one of the inlet aperture and the outlet aperture is a valved aperture.

12. A system according to any preceding claim, wherein the volume of the cavity is less than 10mL, preferably less than 10pL.

13. A system according to any preceding claim, wherein the drive circuit comprises an electronic output stage arranged to drive the resonant gas pump.

14. A system according to claim 13, wherein the electronic output stage comprises an H-bridge.

15. A system according to any preceding claim, wherein cavity of the resonant gas pump comprises two pumping chambers, each chamber comprising a said inlet aperture and a said outlet aperture.

16. A system according to any preceding claim, comprising a second resonant gas pump arranged in series or in parallel with the first resonant gas pump.

17. A system according to claim 15 or 16, comprising a manifold arranged to connect the inlet apertures and outlet apertures of the resonant gas pumps or pumping chambers.

18. A system according to any preceding claim, wherein each of the resonant gas pump, the piezoelectric actuator, the drive circuit, the pressure sensor, and the valve, is integrated into the cuff such as to be substantially contained by the cuff.