Lewis Lennard, application engineer, Rigel Medical, explains why and how some medical devices measuring vital signs need to be maintained.
Vital signs are crucial in communicating a patient’s condition and severity of a disease to healthcare clinicians. Patient monitors observe these vital signs continuously and provide warnings in the case of a serious event. It’s important any anomalous vital sign values alert clinicians, and to some extent, provide direct physiological input data to control connected life support devices.
COVID and other factors have led to a surge in patient admissions increasing the demand for patient monitors worldwide. Ageing populations, increasing complexity of patient conditions and healthcare cutbacks have taken their toll on healthcare organisations. Furthermore, a shortage of clinicians and technological advances has led to remote patient monitoring becoming commonplace.
Recalls of medical equipment up to the present day remain as relevant as ever. Vital sign monitoring equipment has been reported to the FDA with a multitude of different problems and failure to conform - sometimes resulting in patient injuries. The FDA database includes many reports of NIBP failures, incorrect oxygen values, ECG signal errors, and no alarms sounding for anomalous vital sign values. This compounds the importance of ensuring patient monitoring systems are accurate and safe to use.
Moreover, like all healthcare technology, these monitors must also be thoroughly checked and tested on a regular basis to ensure that they work accurately and safely so they do not pose a risk to operators and patients alike. To ensure that monitors are performing as intended regular performance tests at regular intervals are essential.
The main function of a patient monitor is to provide accurate data across all available vital signs. Such accuracy is verified on a periodic basis, based on risk assessment, manufacturer recommendations and stages of the monitor’s life cycle.
Performance tests are typically executed using calibrated simulators across several vital sign parameters and are all part of an acceptance test, preventative maintenance cycle or repair. A typical test cycle for a vital signs monitor might include a visual inspection, self-tests, electrical safety testing, integrity of the device under test, parameter accuracy of temperature, pressure, SpO2, ECG and respiration. Alarms checks for pitch, frequency, volume are required as well as dynamic physiological simulations.
Patient simulation is implemented by the following performance verification procedures from medical device manufacturer service manuals. Ideally, a multi-parameter patient simulator is used to test a device in one test sequence, which provides a practical approach to the biomedical technicians. Each parameter has a different method for performance testing.
- Non-invasive Blood Pressure
NIBP measurement principles primarily rely on the oscillometric method. It determines systolic, diastolic and mean arterial values by detecting the vibrations in the arterial wall at various pressure points by means of an inflated cuff. Testing the accuracy of the monitor involves both static and dynamic pressure simulations at specific values while system leak and over pressure tests are also part of the procedure, executed using a manometer and a built-in pump, to ensure patient safety.
- Invasive Blood Pressure
IBP is an invasive form of blood pressure measurement and uses a liquid filled catheter, which is placed in an artery. The arterial pressure is converted by a pressure transducer into an electrical signal. This is typically 5µV/V/mmHg. Testing the monitor for its linear sensitivity is essential in determining its accuracy. Patient simulation is performed by outputting defined DCV values.
- Pulse oximetry
SpO2 estimates the amount of oxygen in the blood by analysing the absorption of light by haemoglobin across two different wavelength LEDs (RED/IR). If more red than infrared light is being absorbed there are less oxygenated blood cells. SpO2 simulation is often implemented using optical simulation “fingers”. These devices provide variable attenuation to light in the red and IR wavelengths.
- Electrocardiograph
ECG measures tiny electrical signals from the heart using ECG leads placed on various parts of the body. These signals are amplified, measured, and displayed on a patient monitor. ECG simulations are electrically generated cardiac arrhythmias or performance waveforms with pre-set amplitudes and frequencies.
- Respiration
Respiration utilises the ECG leads to measure transthoracic impedance. As the thoracic cavity expands during inspiration, the impedance of the chest increases. During expiration the impedance of the chest decreases. Simulating respiration involves set baseline impedances with delta impedances providing respiration rates.
- Temperature
Temperature measurements in patient monitors are primarily carried out using NTC thermistors. This means that when temperature increases the resistance of the thermistor decreases. Temperature simulation is provided through a set number of resistances depending on the type of sensor (YSI400/YSI700).
Why is it important to test? The correct function and operation of medical equipment is equally as important as the function it performs. An incorrect reading or missed condition might have considerable consequences for the patient, therefore; the person carrying out the maintenance must be technically competent, appropriately trained, and aware of the various parameters being verified. Therefore, like all technology, equipment needs to be thoroughly checked and tested on a regular basis to ensure that they work accurately and safely so they do not pose a risk to operators and patients alike.
It is the responsibility of the medical equipment manufacturer to provide verification procedures to ensure optimum performance is being achieved and maintained. The person or organisation carrying out the maintenance must make themselves aware of the required procedures and operation of the medical equipment. If there is any doubt, contact the manufacturer.
Planned preventative maintenance is also an important aspect during the product life of medical electrical equipment. To ensure safety of the patient and operator, procedures are required to cover visual inspection, electrical safety testing (typically NFPA 99), performance or functional testing and record keeping.
Always ensure that the function and operation of the DUT is understood before commencing on the planned preventative maintenance. Without fully understanding the function and/or operation, visual inspections, electrical safety tests and functional tests might be incorrect or incomplete. Prior to any testing, ensure that the manufacturer’s recommendations are available as they often supersede any general inspection guidelines.
Regular performance checks of patient monitoring devices maintain accuracy and performance. This is paramount for modern healthcare providers and hospitals. Take one of Spain’s leading biomedical device service companies, for example. Hermed Spain’s team of biomedical service engineers and technicians has grown from 35 to 50 and all are equipped with the latest Rigel Medical test technology, including the Uni-Sim vital signs simulator and the Rigel 288 Plus electrical safety analyser.
Dr Marta Villarejo, director general of leading biomedical device service company Hermed Spain, said: “The Uni-Sim Vital Signs Simulator has proven itself to be a very practical tool. By bringing together NIBP, SpO2, ECG, temperature, IBP, and respiratory functions in one piece of equipment, it greatly facilitates the field service activities by Hermed engineers, saving time and avoiding the need to carry several pieces of equipment.”
Biomedical engineers, technologists and technicians have important responsibilities, but the use of the latest vital signs and patient monitoring test equipment will contribute effectively to them meeting compliance and performance requirements and ensure safety measures can be effectively maintained in all hospitals and healthcare facilities without the imposition of an overly excessive test regime.
