Biomedical Test & Calibration

Vital Signs and ECG Patient Simulator : Quality Requirements for Patient Monitoring

Durable medical equipment like patient monitors and ECGs depends on measuring the patient's clinical condition, so it must be safe, effective, and reliable. So scheduled preventative maintenance procedures (PMs) for safety checks or failure rates must be assessed for patient safety.

Current medical equipment management norms and practices should:

– ensure that each device is available,

– ensure that each device satisfies its functional specifications,

– ensure that each device is safe for both the patient and the user, and

– plan for the equipment's eventual replacement

Hospitals are subjected to a wide range of inspection or credentialing processes around the world. A relatively independent joint accreditation agency inspects most of them. There are numerous similar organizations, and there is a movement toward standardization. The World Health Organization (WHO) and the Joint Commission International (JCI) are two major global organizations.

Recommended protocol: Adult/infant electrocardiographs (ECGs)

For almost a century, clinicians have employed the electrical potentials generated by the heart for diagnostic purposes, and the electrocardiograph (ECG) is still one of the primary measures used by clinicians to examine the patient. Although there have been some variations, ECG electrode name, positioning, and color-coding are all standardized. A range of standard waveforms, with and without additional artifacts, should be used to test the ECG channel in a monitor. The unit under test (UUT) can also be tested with a variety of non-clinical waveforms to assess its frequency response

nse, gain, capacity to detect certain aspects of a typical ECG, and ECG printer performance.

The UUT should be linked to the simulator using the standard ECG cable to match the color-coding (the ECG connectors on the simulator should meet AAMI and IEC standards). Most simulators offer a choice of pre-sets that differ from the conventional ECG, allowing the technician to test the UUT at various heart rates, amplitude, S-T segment deviations, and artifact extremes. Verifying that the UUT responds to these variants according to requirements ensures that the UUT satisfies expectations.


Recommended protocol: Fetal/maternal monitors

Electronic fetal monitors detect, show, and record three characteristics for the patient in labor: fetal heart rate (FHR), intrauterine pressure (IUP), and uterine contractile force (UCF). The UUT's scalp electrode adapter acts as an ECG fetal scalp electrode, simulating the fetal heart rate. Between the simulator's ECG posts and the adaptor provided for the UUT, jumper wires are attached. Using simulators capable of testing fetal monitoring, the technician will vary the fetal heart rate from shallow levels like 60 b/m to as high as 240 b/m, with an average default value of 140.

During labor, a catheter is placed into the uterus to assess intra-uterine pressure. By attaching an adapter cable to the UUT and using the same pressure channel as the blood pressure simulation, this can be accomplished. The simulated contractions are represented by a bell-shaped pressure curve that peaks at 90 mmHg and can be configured to reoccur at clinically appropriate intervals like 2, 3, or 5 minutes. With each increase in contraction pressure, the simulated fetal heart rate decreases, imitating a typical fetal reaction to each contraction. Atypical fetal heart rate responses can be set up in the simulator for each contraction: early deceleration (FHR slows sooner than normal), late deceleration (FHR reaction is delayed), or uniform acceleration (FHR accelerates rather than slows).


Recommended protocol: Noninvasive blood pressure

The oscillometric method of blood pressure measurement is a variation of the original noninvasive (auscultatory) method. A cuff around a limb with a major artery is inflated to a pressure higher than the anticipated systolic blood pressure. A transducer in the monitor communicates with the cuff pressure. The moment where the cuff pressure equals the systolic blood pressure pulses will be achieved as the cuff pressure slowly declines (usually at 6 to 8 m Hg per second), causing the artery to "snap" open briefly.

This creates a pulse in the cuff's pneumatics, which is recognized by the transducer and recorded as the systolic pressure electronically. The amplitude of the arterial pulsations reaches its maximum as the pressure in the cuff is reduced further; this point is recorded electronically as the Mean Arterial Pressure. Finally, the electronics record the diastolic point when the pressure in the cuff drops to the point where no more arterial pulses are recorded. The systolic, mean, and diastolic pressures, as well as the heart rate derived from the pulsations, are displayed on the monitor.

The first thing to look for in an NIBP monitor is its pneumatic integrity—if there is a leak in the system, the NIBP data are worthless. The UUT and simulator are connected via a cuff or reservoir, and the UUT is then programmed to close its vents valve by setting it to "Service" or "Calibrate" mode. The simulator is now asked to pressurize the system to a user-selectable target pressure (we recommend at least 300 mmHg), with a real-time pressure visual displayed on the screen.

Most simulators can also be used to double-check the reading of a sphygmomanometer or the manometer built into any clinical monitor.



Recommended protocol: Invasive blood pressure

Direct, invasive blood pressure measurement provides a real-time, beat-to-beat graphic and a numeric indication of pressure, which is important for intense monitoring, especially in critically sick patients and those undergoing prolonged surgical procedures. The radial artery, directly proximal to the wrist, is the preferred location for a tiny in-dwelling catheter. A strain gauge type transducer, which is the variable element of a Wheatstone, is linked to the catheter.

It's recommended to examine the static pressure numbers after connecting the proper adapter cable from the UUT to the simulator. On the simulator, go to the appropriate screen and then zero the pressure channel on the UUT. Set a low target pressure on the simulator and check if the UUT displays it within the monitor's limits; then repeat the check with a clinically high number.

The anatomical position of the catheter in most simulators, such as the radial artery, left ventricle, pulmonary artery, and others, all have different characteristics.

Recommended protocol: Oxygen saturation

Noninvasive, beat-to-beat measurement of arterial blood oxygen saturation has been one of the most important innovations to clinical monitoring, with a substantial impact on the care of patients in emergency situations or undergoing surgery, as well as in ordinary outpatient care. Two (or more) wavelengths of infrared (IR) light are sent to well-perfused tissue (typically the distal end of a finger), where oxygen-carrying hemoglobin absorbs one and non-oxygenated hemoglobin absorbs the other. The two infrared signals are detected and compared, yielding a percentage of oxygenated hemoglobin (SpO2) as a result. The optimal saturation level is 100 percent; nevertheless, saturations as low as 90 percent are common, and saturations as low as 90 percent are clinically significant, necessitating care.

Not all pulse oximeter sensors react to different saturation levels in the same way. The ratio of IR signals for oxygenated and non-oxygenated hemoglobin at any given saturation level will vary from one manufacturer to the next. As a result, each sensor has its unique "R-curve" which is a graph that connects IR ratios to percent saturation. Knowing the manufacturer of the SpO2 sensor used by the UUT (many are licensed to the monitor maker) and selecting that make in the SpO2 Functional Tester is the first factor in setting up a SpO2 test.


Recommended protocol: Cardiac output (CO)

Despite the fact that there are other ways to determine the heart's output, the thermal dilution technique is the method of choice in clinical practice, especially during cardiac surgery. When a known volume of injectate is injected, the method monitors the temperature shift in the blood of a major artery. The temperature of the injectate might be room temperature (24 °C) or "iced" (zero °C). The cardiac output in L/m will be calculated from the area under the time-temperature curve using the blood temperature (Tb), the injectate temperature (Ti), the injectate volume (Vi), and a calibration constant particular to the catheter being utilized. Most monitors show the curve as well as the numeric CO value to visually check that the CO was calculated from an ideal, distortion-free waveform. (A precise measurement requires injecting in a smooth, fast motion.)


Recommended protocol: Temperature

Most monitors use one of Yellow Springs Instruments' two thermistor-based medical thermometry standards, which were developed in the 1950s: The 700-series utilizes two thermistors for higher linearity: 30 K and 6 K at 25 °C. The 400-series has a negative-coefficient thermistor with a resistance of 2252 at 25 °C.

Recommended protocol: Respiration

Many monitors use the ECG electrodes on the upper body to calculate a respiratory waveform and rate. The impedance across the electrodes changes slightly with each breathing cycle, which can be utilized to detect inhalation and exhale.

There is no need for an adapter if the simulator is already linked for ECG. The baseline impedance can be adjusted; common values range from 500 to 2000 ohms. The alarm limits on the UUT should be set to check their function, and the simulator should be set for at least two extremes of respiratory rates, such as 6 and 120 breaths per minute.

In addition, a full-featured simulator will allow the user to change the ratio of Inspiratory to Expiratory time (I: E). The simulator may also include the option of displaying a ventilated patient's respiratory waveform, which varies with each breathing cycle due to impedance variations.