Feature Article

How Healthy Are Your Vital Signs Monitors?


Posted in Testing Equipment by Camilla Andersson on November 15, 2012

Vital signs monitors need to be checked regularly to ensure they work accurately and safely. This has led to the development of high-performance simulators to undertake vital signs simulation and performance testing.


The main vital signs are blood pressure, temperature, electrocardiogram (ECG), respiration and blood oxygen saturation (SpO2). All vital signs are related to the operation and functioning of the respiratory system. The ECG shows the electrical activity of the human heart pumping oxygenated blood around the arteries, generating blood pressure. Respiration rates might show obstructions in the airways, affecting oxygen absorption in the lungs. The core body temperature—which is, with blood pressure, the most commonly measured vital sign—is maintained through good blood circulation.

The heart is the main engine of the respiratory system. It circulates blood through the body and lungs—the carburettor of the body attaching oxygen to the haemoglobin protein in the red blood cells—to ensure oxygen is able to reach the (brain) tissues and organs in order to sustain life.

 Figure 1: A simplified representation of the circulatory system.

To establish a single circulation cycle, blood flows through the heart twice, passing through the left and right side of the heart respectively (Figure 1). Acting as two pumps, the heart circulates oxygenated blood (the red circuit in the diagram, called systemic circulation) from the lungs through the left side of the heart, whilst deoxygenated blood from the tissues flows through the right side of the heart to the lungs to re-oxygenate the blood cells (the blue circuit, or pulmonary circulation).

During the cardiac cycle, the ventricles contract (systole) and blood pressure is at its highest (systolic); during complete cardiac diastole, blood pressure is at its lowest (diastolic), enabling the blood to circulate in the body through systemic and pulmonary circulation. The blood flow and pressure change at each stage of the cardiac cycle and are reported in millimetres of mercury (mmHg).

Vital signs monitor
To ensure the correct treatment, diagnoses or monitoring of a patient’s vital signs, it is critical that the vital signs monitor be able to provide accurate data across all available vital signs. Such accuracy is verified on a regular basis, based on risk assessment, manufacturer recommendations and stages of the monitor’s life cycle.

Performance tests (also referred to as quality or functional tests) are typically executed using calibrated simulators across a number of applications and are all part of an acceptance test, preventive maintenance cycle or repair.
A typical test cycle for a vital signs monitor might include:

  • visual inspection
  • self tests (where applicable)
  • electrical safety testing (earth bonding, leakage currents)
  • verification of integrity of the device under test (leak test, overpressure test)
  • parameter accuracy (temperature, pressure, SpO2, time)
  • alarms (pitch, frequency, volume)
  • physiological simulations (dynamic patient simulation).

Verification

 Figure 2: NIBP test setup.

Visual inspections form a critical part of the general safety and performance inspections during the functional life of medical equipment. Visual inspections are relatively easy procedures to ensure that the medical equipment in use is in the expected and intended condition as released by the manufacturer and has not suffered from any external damage or contamination. These inspections can include the following:

  • Check the integrity of the housing/enclosure (look for damage, cracks and so forth)
  • Check for possible contamination (spills, liquids)
  • Check the integrity of mechanical/moving parts for obstruction
  • Check the integrity of electrical wiring and connectors (cables and accessories)
  • Check the integrity of (safety) markings and labelling.

The correct function and operation of medical equipment is 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 maintenance must be technically competent, appropriately trained and aware of the various parameters being verified.

Testing and simulating NIBP/IBP

 Figure 3: Typical construction of a SpO2 finger probe.

Blood pressure can be measured noninvasively (NIBP) and invasively (IBP) and is associated with pressure in the arterial blood vessels. While the invasive method is more accurate, NIBP measurement is more common because it is relatively simple and can be done by skilled and unskilled people. NIBP monitors range from units designed for domestic use to comprehensive multiparameter monitors used in healthcare facilities, but all types require regular performance verifications to ensure they are operating correctly.

Typical problems affecting accuracy include leaks in the cuff or pressure system resulting in a lower blood pressure reading or an acoustic variance of the cuff caused by incorrect volume (due to positioning or wrongly applying the cuff to the patient) and varying cuff materials in relation to the original OEM versions. Changes in atmospheric pressure also can affect an NIBP monitor’s performance. Therefore, NIBP simulators are set up and used to perform pressure leak, overpressure value, static pressure and linearity tests, and to verify dynamic pressure to determine the correct operation of the monitors. Figure 2 shows a standard test setup using an NIBP or vital signs simulator.

 Figure 4: Test setup for SpO2 using optronic simulation.

The NIBP method has limitations, however: it only provides an indirect arterial pressure as it calculates pressure based on typical 30-second cycles and not in real time.

For greater accuracy or real-time testing, the most common approach is to use the invasive method. This involves placing a liquid-filled catheter in the artery; the arterial pressure is directly transferred to the liquid inside the catheter. An external pressure transducer converts the pressure to an electronic signal, which is then processed further using a monitor.

Testing IBP monitors can take two approaches:

  • a static pressure and linearity test, used to verify the performance of the pressure transducer
  • dynamic pressure simulation to check the accuracy of the pressure transducer.

The external pressure transducer produces a millivolt (mV) that the IBP simulator reacts to by producing a corresponding mV signal on the signal and excitation connections to the IBP monitor to simulate the external pressure transducer.

Testing and simulating SpO2
The quantity of oxygen absorbed (oxyhaemoglobin) is a sign of the respiratory system’s vitality (or performance), which is why it is another of the most commonly monitored vital signs. Displayed in percentage of oxyhaemoglobin (SaO2, a direct measurement) in relation to haemoglobin, pulse oximeters can provide a real-time indication of total oxygen saturation (SpO2) in the blood.

 Figure 5: Normal sinus rhythm.

A pulse oximeter in combination with a finger probe is used to determine SpO2. The pulse oximeter relies on the different light absorption characteristics of oxyhaemoglobin and haemoglobin at red (650–700 nm) and infrared (850–950 nm) spectra. A finger probe transmits and receives the red and infrared light through the tissue (Figure 3) and the pulse oximeter is able to measure the difference in light absorption between red and infrared, which is an indication of the SpO2 value.

The most common errors in SpO2 stem from the finger probe and cable that goes from the patient to the monitor. When testing for accuracy and performance, it is important to include the probe and cables in the test setup. When the LEDs start to degrade and shift in their light spectra, inaccuracies can occur. Testing the probe with the manufacturer’s own probes and light spectrum is the only way to test the true accuracy of the probe and monitor. Some simulators use an optical finger, which captures the red and infrared light but only emits predetermined red light generated by the optical finger. The reproduced light doesn’t match the exact light spectrum of the SpO2 probes and this can lead to errors or a false “pass” indication of a (near) faulty probe, especially in the infrared spectrum.

We recommend optronic simulations that do not alter or reproduce the finger probe light spectrum but, in fact, utilise the full light spectrum of the probe (Figure 4). This allows the user to identify near faulty probes in both the red and infrared light spectra.

Testing and simulating ECGs
An ECG machine is used to observe the difference between two amplified electrical signals at different points on the body and the electrical potentials displayed on the machine’s screen. Using an ECG machine can indicate such things as an abnormally fast heart rate (tachycardia), abnormally slow rate (bradycardia), a blockage in the heart or blood clot in the heart. A healthy heart produces an electrical signal that is referred to as a normal sinus rhythm, as shown in Figure 5.

 Figure 6: Transthoracic impedance measurement through ECG leads.

Testing an ECG machine, which is an important piece of analytical equipment, is crucial to ensure the input circuits are able to measure the small ECG signals accurately. A number of simulations and performance tests should be undertaken as part of a regular maintenance programme, including linearity of heart rate measurement, QRS beep, alarms, arrhythmia recognition, sensibility test and so forth. These can be done quickly and accurately using a patient or ECG simulator connected to the device under test (DUT) via an ECG interface.

Testing and simulating respiration
Monitoring the respiration rate of anaesthetised hospital patients, whose breathing is under the control of a mechanical ventilator, is also vital in providing immediate warning of changes to the respiration rates including obstruction of the air pipe (trachea). An obstruction prevents oxygen flow to the lungs and stops expiration of carbon dioxide from the blood, which can lead to cardiac arrest and death if left untreated.

Measuring transthoracic impedance between ECG leads is the most common way to obtain respiration rates (Figure 6). Another approach to determine respiration is by observing the change in the ECG amplitude—ECG-derived respiration (EDR)—as a result of changes in the position between electrodes and heart as the chest cavity expands and the heart moves as the diaphragm position alters. Regardless of the approach, the ECG leads are placed on the human chest at various points and respiration rates are then monitored through all limb and augmented leads.

Testing ECG monitors in this respect includes linearity of respiration measurement, a test to verify the capability of the monitor to measure and display respiration rate values. Other tests to perform to ensure the monitor is functioning correctly are sleep apnoea and testing apnoea alarms (high and low). The latter includes checking that alarms are at the set value(s) and at the correct pitch and frequency.

Testing and simulating temperature
One of the most commonly monitored vital signs is body temperature. A person’s core body temperature varies by gender and can fluctuate during different stages of the day. In women, the core body temperature also changes during the menstrual cycle, peaking at the time of ovulation. The average core body temperature is 37°C ±0.5°C.

Depending on placement, application and method, different temperature readings are expected in healthy individuals. The most common temperature sensors used for bedside monitoring are electrical temperature sensors based on a temperature-related varying resistor (thermistor). Thermistors are commonly known as NTCs (negative temperature coefficient, meaning that resistance decreases as the temperature increases) and PTCs (positive temperature coefficient, meaning that resistance increases as the temperature increases).

The YSI 400 and YSI 700 have become the standard NTCs used in the medical industry. While the YSI 400 is slightly more accurate over the 0–75ºC range, the YSI 700, which contains a dual element (Ra = 6 kΩ @ 25ºC and Rb = 30 kΩ @ 25ºC), is able to ensure accuracy over a wider -25º to 100ºC range.

Body temperature is simulated by different resistor values corresponding to the required temperature; temperature function on multiparametric monitors is tested by using linearity of temperature measurement and alarms (high and low) to ensure the correct temperature sensor (YSI 400 or 700) on the patient simulator is selected.

Using a patient simulator, normal (37°C), low (33°C), high (41°C) and room (25°C) temperatures may be simulated and you can record whether the alarm on the monitor occurs at the set value(s) and whether the alarm(s) is/are at the correct pitch and frequency. (Of course, you should always refer to the instruction manual.)

Planned preventive maintenance is an important aspect during the useful life of a medical electronic device. To ensure safety, procedures are required to cover visual inspection, electrical safety testing (IEC 62353), performance or functional testing and record keeping.

Without fully understanding the function and or operation of a device, any visual inspections, electrical safety tests and functional tests could be incorrect or incomplete. To ensure that the function and operation of the DUT is fully understood before commencing and also prior to any testing, confirm that the manufacturer’s recommendations are available, as they often supersede any general inspection guidelines.

Ensure also that the operator of the test system is properly trained on both the test equipment and the DUT to get valid measurements and prevent unnecessary danger during the safety test. Always ensure that the DUT does not pose any danger to the user or people within the vicinity of the safety test. Risk factors include moving parts, open conductors, live components and heat.

Make sure the manufacturer’s instructions are followed and that performance levels are checked against original documentation. Ensure high accuracy and repeatability of simulations and measurement readings (some manufacturers might specify full-scale accuracy, which will affect the accuracy of low-value readings or measurements); when determining the correct means of testing a specific medical device, make sure that the chosen test procedures are applicable to the DUT and are clearly documented for future use.

A free guidance booklet on measuring and simulating vital signs is available. It can be downloaded at www.rigelmedical.com/vitalsigns.

John Backes
is Associate Director at Rigel Medical, Bracken Hill, South West Industrial Estate, Peterlee, Co. Durham SR8 2SW, UK
tel. +44 1915 863 511
e-mail: johnb@rigelmedical.com
www.rigelmedical.com



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