Figure 1. A simplified representation of the circulatory system.

Medical devices and equipment used to check for and monitor people’s vital signs needs to be checked regularly to ensure they work accurately and safely, and do not pose a risk to operators and patients alike. This has led to the development of high performance simulators to undertake vital signs monitoring and performance testing.

The main vital signs described are blood pressure (invasive or noninvasive methods), temperature, electrocardiogram (ECG), respiration, and blood oxygen saturation (SpO2). All vital signs are related to the operation and functioning of the respiratory system. While the ECG shows the electrical activity of the human heart pumping the oxygenated blood around the arteries, blood pressure is generated. Respiration rates might show any obstruction in the airways, thus affecting the oxygen absorption in the lungs. The core body temperature, together with blood pressure—the most commonly measured vital signs—is maintained through good blood circulation.

The human heart is central to the respiratory system and can be seen as the main engine within. The heart circulates blood through the body and lungs (the carburetor of the body attaching oxygen to the hemoglobin protein in the red blood cells) in order to ensure oxygen is able to reach the (brain) tissues and organs in order to sustain life.

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 from the lungs through the left side of the heart, while deoxygenated blood from the tissues flows through the right side of the heart to the lungs in order to reoxygenate the blood cells.

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

Vital Signs Monitors

To ensure the correct treatment, diagnoses, or monitoring of a patient’s vital signs, it is of critical importance that the vital signs monitor is 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 a visual inspection, self-tests (where applicable), electrical safety testing (earth bonding, leakage currents), integrity of the device under test (leak test, over pressure test), parameter accuracy (temperature, pressure, SpO2, time, etc), check alarms (pitch, frequency, volume), and physiological simulations (dynamic patient simulation).

Verification Inspections

Figure 2. Typical construction of a SpO2 finger probe.

Visual inspections form a critical part of the general safety and performance inspections during the functional life of medical equipment. Visual inspections are a relatively easy procedure 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 and/or contamination.

These inspections can include the following: Housing (enclosure; look for damage, cracks, etc); contamination, checking for obstruction of moving parts, connector pins, etc; cabling (supply, applied parts and accessories, etc); looking for cuts, wrong connections, etc; fuse rating; checking correct values after replacement; markings and labeling; and checking the integrity of safety markings and the integrity of mechanical parts (checking for any obstructions).

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.

Testing and Simulating NIBP/IBP

Blood pressure can be measured both noninvasively (NIBP) and invasively (IBP), and is associated with the pressure in the arterial blood vessels. While the invasive method is more accurate, the NIBP is the most common because it is relatively simple and can be done by both skilled and unskilled people. NIBP monitors range from domestic use to comprehensive multiparameter monitors used in health care facilities, but all types require regular performance verifications to ensure their correct operation.

Typical problems affecting accuracy include a leak in the cuff or pressure system causing a lower blood pressure reading, or an acoustic variance of the cuff due to incorrect volume, or variety in materials used in positioning or applying the cuff to the patient. Changes in atmospheric pressure can also affect an NIBP monitor’s performance. Therefore, an NIBP simulator is set up and used to undertake a number of tests to determine the correct operation of the monitors, including the pressure leak test, over pressure valve test, static pressure and linearity test, and dynamic pressure test. Figure 2 (above right) provides a standard test setup using an NIBP or vital signs simulator.

However, the NIBP method has limitations since it only provides an indirect and non-real-time arterial pressure as it calculates pressures based on a typically 30-second cycle. So, for greater accuracy, or real-time testing, the most common approach is to use the invasive method (IBP). This involves a liquid-filled catheter being placed in the artery, where the arterial pressure is directly transferred to the liquid inside the catheter and tubing to an external pressure transducer, which converts the pressure to an electronic signal. This is then processed further using a monitor.

Testing IBP monitors to ensure they are working correctly can involve two approaches: a static pressure and linearity test (used for verifying the performance of the pressure transducer) and dynamic pressure simulation to check the pressure transducer’s accuracy. The external pressure transducer produces a milliVolt (mV), which 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

Figure 3. Normal sinus rhythm.

The quantity of oxygen absorbed (oxyhemoglobin) 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 oxyhemoglobin (SaO2, a direct measurement) in relation to hemoglobin, pulse oximeters can provide a real-time indication of the total oxygen saturation (SpO2) in the blood.

A pulse oximeter in combination with a finger probe is used to determine the SpO2. The pulse oximeter relies on the different light-absorption characteristics of oxyhemoglobin and hemoglobin at red (650 to 700 nm) and infrared (850 to 950 nm) spectrum. A finger probe transmits and receives the red and infrared light through the tissue, 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 are related to the finger probe and the patient cable to the monitor. It is important when testing the accuracy and performance to include the probe and cables in the test setup. When LEDs start to degrade and shift in their light spectrum, 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 does not 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.

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 a blood clot in the heart. A healthy heart produces an electrical signal—referred to as a normal sinus rhythm, as shown in figure 3.

Testing an ECG machine, which is an important piece of analytical equipment, is crucial for ensuring 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 program, including linearity of heart rate measurement, QRS beep, alarms, arrhythmias recognition, and a sensibility test. These can be done quickly and accurately using a patient or ECG simulator.

Testing and Simulating Respiration

Figure 4. Transthorasic impedance measurement through ECG leads.

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

The measurement of the transthoracic impedance between the ECG leads is the most common way to obtain respiration rates. Another approach to determine the respiration is by observing the change in the ECG amplitude (ECG-derived respiration, or EDR) as a result of changes in the position between electrodes and the heart as the chest cavity expands and the heart moves as the diaphragm position alters. Whatever approach is adopted, 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 apnea and testing apnea alarms (high and low). The latter includes ensuring alarms are at the set value(s) and that they are at the correct pitch and frequency.

Testing and Simulating Temperature

One of the most commonly monitored vital signs is the body temperature. A person’s core body temperature varies by gender and can fluctuate between 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 the placement, application, and method, different temperature readings are expected in healthy individuals. The most common temperature sensors used on bedside monitoring are electrical temperature sensors based on a temperature related varying resistor (thermistors). These thermistors are commonly known as NTCs (negative temperature coefficient, meaning that the resistance decreases when temperature increases) and PTCs (positive temperature coefficient, meaning that the resistance is increasing as 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 range of 0°C to 75°C, the YSI 700, which contains a dual element (Ra = 6kΩ @ 25°C and Rb = 30kΩ @ 25°C), is able to provide its accuracy over a wider range (-25°C to 100°C).

Body temperature is simulated by the different resistor values corresponding to the required temperature, and testing temperature function on multiparametric monitors is undertaken 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) temperature 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. (Always refer to the instruction manual.)

Planned preventive maintenance is an important aspect during the useful life of a medical electronic device. To ensure the safety of both the patient and operator, procedures are required to cover visual inspection, electrical safety testing (see 24×7‘s May 2012 article about IEC 62353), performance or functional testing, and record keeping.

Without fully understanding the function and/or operation, any visual inspections, electrical safety tests, and functional tests could be incorrect or incomplete. So ensure that the function and operation of the device under test 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 test equipment is properly trained on both the test equipment and the device under test to get valid measurements and prevent unnecessary danger during the safety test. Always ensure that the device under test does not pose any danger to the user and/or people within the vicinity of the safety test, for example from moving parts, open conductors, live components, heat, etc.

Make sure 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), and when determining the correct means of testing a specific medical device, make sure that the chosen test procedures are applicable to the device and are clearly documented for future use.

John Backes, MA, is the associate director of Rigel Medical, Peterlee, England, a manufacturer of portable biomedical test equipment. For more information, contact .