Disc Pump: a better solution for Ambulatory Blood Pressure Monitoring

Whilst 24-hour 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. Disc Pump from TTP Ventus is a novel, piezoelectric micropump whose unique combination of features addresses many of the disadvantages of diaphragm pumps, enabling:

  • Improved patient comfort through silent operation and ability to carry out BP measurement on inflation.
  • Fully integrated, arm-worn pump modules through compact and lightweight form factor.
  • Enhanced measurement accuracy through removal of hose and ability to drive adaptive inflation rates.

Introduction
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 clear 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.

Improving patient comfort: measurement on inflation
In principle the oscillometric measurements can be taken during cuff inflation, which would have the benefit of shortening the measurement cycle (the pressure can be immediately vented once the measurement is complete) thereby improving patient comfort. However, the pulsation generated by traditional pumps would mask the underlying signal from the patient, and so in practice oscillometric measurements on inflation are difficult. Unlike traditional pumps, which typically cycle a few times per second, Disc Pump operates at 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 (see Figure 1). This approach has the added benefit that the cuff pressure need not exceed the systolic pressure by much, minimising the compression forces applied to the patient.

Figure 1: blood pressure measurement taken during cuff inflation with Disc 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.

An unobtrusive approach
Disc 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, weighing just 5g (1/4 oz) and measuring 10 mm (<0.5 in) in height and 30 mm (1.25 in) in diameter. These two features allow the pump module to be tightly integrated with the brachial cuff, enabling the creation of wholly arm-worn systems.

Figure 2: Traditional 24-hour ambulatory blood pressure systems have a belt or strap-worn pump module connected to a brachial cuff via a long hose, which can be inconvenient to the user. Disc Pump (Figure 3, below), on the other hand, is compact, lightweight, and does not create sound or vibration during operation – enabling it to be mounted directly onto a brachial cuff.

Measurement accuracy
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 isn't just advantageous to the user experience. It also eliminates the need for the hose between the pump module and cuff. Given that any inadvertent pressure applied to this hose can interfere with the measurement, the revised architecture offers a more robust solution - not least during the night, when the hose can become kinked during sleep.

Oscillometric systems powered by Disc Pump also benefit from its 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).

Summary
Disc Pump offers improved patient comfort and through its support of measurement-on-inflation and silent operation. Its compact form factor enables the creation of fully-integrated, arm-worn pump modules, which further improves the user experience. Measurement robustness can be enhanced by eliminating the need to long lengths of tubing and by offering precise and adaptive control over inflation rates.

How can I test the pump?
Our series configuration models (DP-S2-007 and BL-S2-012) are best suited to blood pressure applications. For initial testing, we offer an evaluation kit which contains everything you need to get started, including two pumps, drive electronics, PC software (configuration, control and datalogging) and an accessories kit. The drive electronics can be controlled in a variety of ways, including providing a 0 to 2.5V signal that can be mapped to a pressure target; the evaluation kit can then control the output of the pump to reach the target pressure. This opens up the ability to carry out constant rate inflation testing, by supplying a triangle or saw-tooth waveform to the analog input.

If your application requires flow rates higher than those offered by our standard models, please let us know when you make your enquiry. We are developing a range of higher-flow pumps; pre-production prototypes of these models are now available to customers interested in testing them.

Head over to our products page to learn more about our product range, download datasheets and to request a quotation today.

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Figure 3: TTP Ventus' Disc Pump Evaluation Kit