Multiparameter patient monitor
A set of sensor front ends (ECG, SpO2, NIBP, IBP, temperature, CO2) feeding one processor that displays trends and raises alarms, often over a hospital network.
Principle
Each parameter has its own measurement chain. ECG uses skin electrodes and a high-impedance differential amplifier with strong common-mode rejection to pick up millivolt-level cardiac potentials. Pulse oximetry (SpO2) shines red and infrared light through tissue and computes oxygen saturation from the ratio of the pulsatile absorptions. Non-invasive blood pressure (NIBP) inflates a cuff and uses the oscillometric method, analysing pressure oscillations during deflation to estimate systolic, mean and diastolic pressure. Invasive pressure (IBP) uses a disposable strain-gauge transducer in a fluid-filled line. Temperature is measured with thermistors; capnography measures CO2 by infrared absorption either mainstream at the airway or sidestream by aspirating a sample. The patient-connected front ends are electrically isolated from mains and from each other to keep leakage currents within limits and to survive defibrillation. A main processor runs algorithms, trends and alarms, and the monitor usually connects to a central station over a wired or wireless network. Most field complaints come from accessories (cables, sensors, cuffs, hoses) rather than the monitor itself.
The chain, block by block
| Block | What it does | How it fails |
|---|---|---|
| ECG leads, trunk cable and front end | Electrodes, lead wires and trunk cable feed an isolated amplifier with filtering, pace detection and lead-off detection. | Noise and 'lead off' alarms from broken lead wires, worn connectors or poor electrodes; baseline wander; loss of defibrillation protection after a damaged cable. |
| SpO2 sensor, cable and module | LEDs and photodetector in the sensor; module computes saturation and pulse rate. | Most common failure is the sensor or extension cable (broken wires, damaged LEDs); also motion and ambient-light artefacts; using non-compatible sensors. |
| NIBP cuff, hose and pneumatic module | Pump, valves and pressure transducer inflate and deflate the cuff and measure oscillations. | Leaks in cuff, hose or connectors cause failed measurements; pump wear, valve faults and transducer drift give inaccurate readings; over-pressure safety must work. |
| IBP transducer and channel | Disposable transducer and fluid line connected to an isolated bridge amplifier; requires zeroing to atmosphere. | Drift from not zeroing, damped waveforms from air bubbles or kinks, damaged interface cables. |
| Temperature probes | Thermistor probes (skin, oesophageal, rectal) on isolated input. | Wrong probe type, broken probes, calibration offset. |
| CO2 (capnography) module | Infrared measurement of CO2 mainstream or sidestream with water trap and sampling line. | Occlusion alarms from water or blocked sample lines, pump wear, calibration drift. |
| Main processor, display and alarms | Algorithms, display, trends, alarm management with audible and visual alarms. | Touch screen and display failures, alarm speaker failure, software errors, settings lost after updates. |
| Power supply and battery | Mains supply with internal battery for transport and outages. | Short battery autonomy, failure to charge, unexpected shutdown during transport. |
| Network and central station | Wired or wireless connection to central station, EMR or alarm management system. | Monitor drops off central station, Wi-Fi coverage gaps, IP or configuration errors, time not synchronised. |
Symptom → where to look first
| Symptom | Suspect first |
|---|---|
| Noisy ECG or frequent lead-off alarms | Lead wires and trunk cable first, then electrodes and skin preparation, then ECG module; check for nearby interference sources. |
| SpO2 shows no value or dashes | Sensor and extension cable first (swap with a known-good sensor), then module. |
| NIBP measurement fails or takes very long | Cuff and hose leak, wrong cuff size, then pump and valves; run the leak test. |
| NIBP values inconsistent | Static pressure calibration of the transducer, then cuff size and placement. |
| IBP waveform damped or drifting | Transducer line (air, kinks), zeroing procedure, then interface cable. |
| CO2 occlusion alarm | Water trap and sample line, then sampling pump. |
| Monitor not visible on central station | Network cable or Wi-Fi coverage, IP configuration, then central station licence/configuration. |
| Shuts off when unplugged | Internal battery capacity and charging circuit. |
Safety
Site and infrastructure
- Grounded mains outlets; equipotential bonding where required in critical care and theatres.
- Wired network ports or Wi-Fi coverage for all monitored bedspaces where a central station is used.
- Secure mounting (wall, rail, ceiling pendant) rated for the monitor and modules.
- Room temperature and humidity within operating range; adequate ventilation around the monitor.
- Stock of compatible accessories (ECG cables, SpO2 sensors, cuffs, CO2 lines); incompatible accessories cause both errors and safety problems.
Preventive maintenance basics
- Inspect all accessories: ECG cables and leads, SpO2 sensors, NIBP cuffs and hoses, temperature probes, CO2 lines; replace damaged items.
- Verify ECG, SpO2, IBP and temperature with a patient simulator; check alarm activation and limits.
- Perform NIBP checks: static pressure accuracy against a calibrated manometer, leak test, over-pressure protection test.
- Verify capnography calibration and check the water trap and sampling pump.
- Test battery capacity and mains/battery switch-over.
- Perform electrical safety testing including patient leakage on each applied part (IEC 62353 or local standard).
- Check network connection to the central station and time synchronisation; review error logs.
Safety
- Patient isolation must never be compromised: do not use damaged cables or non-compatible accessories; an insulation failure on an applied part can create a leakage path to the patient.
- Defibrillation-proof inputs depend on correct cables and modules; after repair, confirm the module and cables are the specified type.
- Disconnect from mains before opening; some power supplies retain charge.
- Never disable or lower alarm volume and limits as part of servicing without restoring the hospital's configured defaults.
- Treat accessories and the monitor surface as contaminated; clean and disinfect before work.
Manufacturers
Philips · GE HealthCare · Mindray · Dräger · Nihon Kohden · Spacelabs Healthcare · Masimo · Edan · Comen · Schiller · Fukuda Denshi