Pressure sensors: 8 life-saving medical applications

Many medical devices now depend on accurate and stable pressure measurements in order to operate reliably. What’s more, patient care is expanding beyond the hospital and surgery and arriving in patients’ homes, in the form of home health monitoring.

As a result, developing with pressure sensors has become an integral part of designing medical applications. Below, we explore 8 different uses of pressure sensors in medical technology.

1. Getting the mix right in medical ventilators

A ventilator works by mixing air with pure oxygen to help the respiratory function of a patient. Differential or gauge pressure sensors are normally sited between valves and regulators to ensure the air and oxygen are mixed in the right quantities. In this kind of application, small surface-mount sensors are ideal; they will typically be specified for a pressure range of 2in or 5in H2O and are available with either analog or digital (I2C) outputs.

Despite being small and low power, these low pressure sensors can often include an integrated DSP (digital Signal processor) for compensating for non-linearity, offsets or the effects of temperature.

2. Monitoring oxygen therapy effectiveness

Oxygen therapy comes in a number of forms, as concentrated oxygen can be an effective initial treatment for asthma, bronchitis and oedemas, as well as heart failure.

Oxygen therapy systems use differential pressure sensors at several points in the system to monitor the pressure of the oxygen as it is mixed with atmospheric air. These sites are usually at the outlet of the oxygen tank, inline with the pressure regulator, and next to the flow control valve.The pressures sensors in this application are likely to be differential pressure sensors with a scale of up to 4 kPa.

3. Delivering hyperbaric therapy

Raising the air pressure in a sealed chamber containing a patient is known as hyperbaric therapy and can be effective for a number of conditions. It is used to treat decompression sickness experienced by divers, and can also help patients with skin grafts or burn injuries. It can also be effective in treating carbon monoxide poisoning and even some necrotising infections.

Pressure sensors are used to monitor the pressure inside the chamber and control the amount of pressure applied during treatment. This will typically take the form of an absolute pressure sensor capable of measuring pressures up to around 100 kPa.

Even this most industrial of treatments is making inroads into patient’s homes, as ‘soft’ chambers become increasingly available - although the pressures these soft chambers can achieve are lower than theprofessionalgrade ‘hard’ chambers. Typically, a soft chamber will require gauge pressure sensors capable of measuring around 0.3 to 0.5 bar, while a hard chamber would employ gauge pressure sensors able to measure as much as 6 bar.

4. Providing positive pressure masks to treat sleep apnoea

Sleep apnoea is a condition that causes the sufferer to stop breathing while asleep. Left untreated it can lead to a number of serious conditions, from chronic fatigue to potential heart failure.

The treatment involves using a device called a continuous positive air pressure machine, or CPAP, which delivers air at a positive pressure to a mask worn over the nose and mouth of the patient. An airflow pressure sensor is used to monitor the air pressure, detecting when the patient is breathing in and immediately turning on a fan to create positive pressure to open the airways (see diagram below). As the patient breathes out the fan is deactivated, allowing the patient to exhale without forcing them to fight against the positive pressure.

Sleep apnoea machines will likely employ a differential pressure sensor able to measure pressures up to 4 kPa.

5. Automating drug infusion

Drugs delivered in liquid form can be an effective form of treatment, as can other types of fluids e.g. for rehydration. These fluids can be administered either intravenously, subcutaneously or directly into a vein, and are typically delivered using infusion pumps. In order to ensure the correct volume of fluid is administered at the correct rate, the pumps use a number of sensors including gauge and differential pressure sensors, to closely monitor and control the flow of liquid.

Differential pressure sensors are used in drug delivery systems (see diagram to the right) to measure and control the flow of liquids into the patient. This ensures the right volume of drugs is delivered at the right time throughout the day and night, without the need for constant medical attention.

Differential pressure sensors are normally calibrated to measure flow rates in the range of 0.5 to 10.0 micro litres/min.

6,7,8. Measuring in vivo blood pressure, ex vivo blood pressure and intraocular pressure

In vivo blood pressure sensing involves implanting a sensor into the body. It can now be achieved using tiny absolute pressure sensors designed for this purpose.

Ex vivo blood pressure sensing, from outside the body, can be implemented using gauge pressure sensors to measure the blood pressure when the heart beats (systolic) and between the heart’s beats (diastolic).

Sensors for both in vivo and ex vivo blood pressure sensing need to be able to measure pressures up to 300 mm Hg (maximum). In vivo applications tend to use absolute pressure sensors, while ex vivo favour gauge pressure sensors.

MEMS-based gauge pressure sensors are now being used to measure the intraocular pressure of a patient’s eyes, which is particularly important after an operation to replace a cataract.

Manufacturers are now producing an ever-widening range of pressure sensors for medical applications, including disposable pressure sensors based on MEMS technology that can be used inside the body or in-line with fluids entering the body. These are produced in clean rooms and in accordance with industry-accepted guidelines including those generated by the Association for the Advancement of Medical Instrumentation (AAMI).

Pressure sensors have become an essential element of medical care, providing accurate and stable measurement of critical pressure levels in gas and liquids within the body and in treatments being applied to patients.

Future developments will enable more sophisticated, and ever smaller medical equipment to be developed, as well as lowering the price point for home-use devices.

One significant result will be an elevated quality of life for an ageing population.