The primary vital signs are clinical measurements that are displayed and recorded across the biomedical field, predominantly on patient monitors. Patient simulation provides accurate mimicking of physiological vital signs to calibrate and test these devices.
There are five vital signs that are regularly simulated: blood pressure, oxygen saturation, heart rhythm, respiratory rate and temperature.
Patient simulation is a fundamental part of service tests for patient monitors, which are required at regular service intervals. Service testing consists of a wide range of checks for each function or parameter. The first tests are usually visual and electrical safety tests. This is followed by performance testing and includes integrity testing (leak checks, over-pressure testing), performance accuracy (patient simulation), printer checks, alarm checks and physiological simulations.
Why perform patient simulations?
Vital signs communicate a patient’s condition to clinicians. Patient monitors observe these signs continuously and provide warnings in the case of a serious event. Patient simulation is a key part of performance test procedures, checking that monitoring devices measure in line with the manufacturer’s specifications and applicable standards.
Why each parameter is tested
NIBP (non-invasive blood pressure): to determine that transducer linearity and static pressure are within specification; to find leaks in the cuff or pressure system; and to confirm that the over-pressure (pop-off) valve safeguards against a build-up of pressure.
IBP (invasive blood pressure): to test pressure-transducer linearity and dynamic pressure behaviour for hypo- and hypertension.
SpO2: to find faulty probes (LED degradation, contaminated lenses or damaged wiring), to check the monitor’s SpO2 performance, and to verify audible alarms.
ECG: linearity for heart-rate amplitude and frequency, arrhythmia recognition, and gain-change sensitivity.
Temperature: linearity of temperature measurement and selection of the correct sensor type (for example YSI 400 / YSI 700).
Respiration: detection of apnoea, alarm verification, and linearity of respiration rates.
How simulations are performed
Patient simulation follows performance verification procedures from the medical-device manufacturer’s service manuals. A multi-parameter patient simulator is typically used so that a device can be tested in one sequence.
NIBP measurement commonly uses the oscillometric method. Testing involves static and dynamic pressure simulations, plus system leak and over-pressure tests.
IBP uses a liquid-filled catheter in an artery; the arterial pressure is converted by a transducer into an electrical signal (typically 5 µV/mmHg). Simulation is performed by outputting defined DC voltages.
SpO2 estimates oxygen in the blood by analysing absorption of red and infrared light. Simulation often uses optical “fingers” that attenuate those wavelengths.
ECG simulations are electrically generated arrhythmias or performance waveforms with pre-set amplitudes and frequencies. Respiration simulation uses baseline and delta impedances on the ECG leads to represent chest-wall movement.