Patient Monitor Electronics: AFEs, Isolation & Alarms

One box, many biosignals. Mission: capture clean data, alarm correctly, interoperate with the hospital network—and pass EMC without breaking a sweat.

Hospital monitoring network architecture: bedside monitors, central station, EMR by Ersa

1) System architecture & form factors

Bedside monitors: integrated screen + parameter boards; AC mains with BF/CF modules on isolation islands.
Transport monitors: battery-backed, ruggedized I/O; quick-release patient cables; low-power AFEs.
Modular platforms: plug-in parameter pods (ECG/SpO₂/NIBP/IBP/CO₂); hot-swappable; common backplane for power/data.
Central station: trending, alarm escalation, remote settings; gateway to EMR.

2) Signal chains by parameter

Parameter	Front end (AFE)	Sampling / bandwidth (typ.)	Notes
ECG	Inst. amp → DRL → anti-alias → ADC	250–1000 sps; ~0.05–150 Hz	Lead-off detect; defib protection; pace detect
RESP (impedance)	Low-amp AC injection → demod → filter	~25–100 sps	Inject μA-level AC via ECG leads; isolate path
SpO₂	LED driver (red/IR) → photodiode TIA → ADC	~200–1000 sps	Ambient rejection; motion resilience; LED calibration
NIBP	Pressure sensor + IA → ADC	~100–500 sps	Pump/valve control; oscillometric algorithm
Temp	RTD/thermistor bridge → precision ADC	Low rate	Linearization; patient isolation if applied
IBP (optional)	Bridge transducer → IA → anti-alias → ADC	250–1000 sps	Heated transducers; catheter kits vary
EtCO₂ (optional)	IR source → detector AFE → ADC	~100–200 sps	Mainstream or sidestream; sample line management

3) AFE/ADC design details

AFE layout best practices: Kelvin returns, short sensor traces, RC at entries by Ersa

ECG: high CMRR instrumentation amp; right-leg drive; high-pass for baseline wander; protect against defib pulses (gas tubes/TVS/resistors per design guide).
SpO₂: programmable LED current; ambient subtraction; TIA with low input bias and stable GBW; per-probe ID/coefficients.
NIBP: noise-aware routing near pumps/valves; differential sensing to resist EMI; oversampling + digital filtering.
ADC: 16–24-bit for low-level analog (ECG/IBP/temp); synchronize channels for derived parameters (RESP from ECG, etc.).

4) Isolation strategies (BF/CF)

Partition patient-applied modules behind isolation barriers with isolated DC-DC and digital isolators. Choose BF or CF based on contact type; respect leakage limits and creepage/clearance for your PCB stack-up.

Isolation barriers with separate patient and system domains by Ersa

Isolate ECG/SpO₂/IBP boards from mains; NIBP pump electronics may share system ground but isolate pressure transducer path if applied.
Use reinforced insulation where required; validate leakage across temperature/humidity and aging.

5) Power tree & display

AC-DC medical PSU + DC rails for logic, AFEs, backlight; optional battery for transport/ride-through.
Supervisors and brownout logs protect trend data; backlight dimming for ICU lighting policies.
Fans vs fanless heat spreaders—consider acoustic environment and cleaning workflow.

6) Alarms, trends & usability

Alarm priorities and indicators per IEC 60601-1-8 by Ersa

Conform to IEC 60601-1-8: distinct tones (high/med/low), visual indicators, acknowledge/mute behavior with time limits.
Smart alarm delays and plausibility checks (e.g., ECG loss before arrhythmia) reduce nuisance without masking true events.
Trending: minute-to-days retention; export to central station; bedside strips for events (arrhythmia, desat, hypertensive spikes).
UI for gloves, night mode, and quick lead/cuff status; color coding by parameter.

7) Networking & interoperability

Connectivity architecture: Ethernet/Wi-Fi, TLS, central station, EMR by Ersa

Ethernet (PoE optional) and/or Wi-Fi for mobility; VLAN segmentation recommended.
Protocols vary by deployment: gateway/central station often bridges to hospital HL7/11073 or vendor middleware—keep data models clean and time-synced (NTP).
USB service port for logs/updates; audit exported events with integrity checks.

8) Cybersecurity & update strategy

Secure boot; signed updates (offline and OTA via gateway); SBOM tracking; role-based access for clinical vs service menus.
Encrypt PHI at rest and in transit; wipe/transfer modes for decommissioning.
Event/audit logs with tamper-evident storage; time-stamped with secure RTC.

9) PCB layout & EMC practices

EMC layout zones: analog AFEs, digital, power conditioning, drive by Ersa

Zone placement: quiet AFEs away from switching supplies and backlight drivers; short guarded traces to ADCs.
Filter/TVS at cable entries; common-mode chokes on USB/Ethernet; chassis bonding with defined return paths.
Validate with worst-case harness lengths and patient cables; simulate incoming ESD/EFT paths.

10) Compliance mapping (IEC/ISO)

Topic	Standard	Engineering artifact
Basic safety	IEC 60601-1	Schematics, creepage/clearance, leakage test plan/results
EMC	IEC 60601-1-2	Filter/shield plans, test matrix, immunity recovery behavior
Alarms	IEC 60601-1-8	Priority tables, sound patterns, usability validation
Multiparameter monitor	IEC 60601-2-49	Performance tests, labeling, essential performance
ECG	IEC 60601-2-27	Defib protection, pace detect, bandwidth verification
SpO₂	ISO 80601-2-61	LED/PD performance, motion/ambient tests
NIBP	ISO 80601-2-30	Oscillometric accuracy, cuffs/hoses verification
Respiratory gas (EtCO₂)	ISO 80601-2-55	CO₂ accuracy, response time, alarm checks
Thermometry	ISO 80601-2-56	Accuracy, drift vs temp range
Software	IEC 62304	SRS, risk class, verification/traceability
Usability	IEC 62366	Use-related risk files, formative/summative tests
Risk management	ISO 14971	Hazard analysis, FMEA/FMEDA, residual risk eval
QMS	ISO 13485	DHF/DMR, change control, CAPA linkage

Which “particulars” apply depends on your feature set; align your essential performance and tests accordingly.

11) Risk analysis (hazards → mitigations)

Hazard	Cause	Mitigation
Incorrect ECG reading	CMRR loss, lead reversal	High-CMRR IA, DRL integrity, lead-reversal detect, calibration checks
SpO₂ false desat	Motion/ambient light	Red/IR ratio plausibility, ambient subtraction, averaging with limits
Missed NIBP alarms	Pump/valve failure	Self-test valves/pumps, pressure rate-of-change checks, backup cycle
Leakage exceedance	Isolation fault	Robust barriers, hipot/leakage at production & service intervals
Data loss	Power fail mid-write	Transactional trend writes to FRAM/Flash; supervisors; hold-up energy
Unauthorized change	Unsecured service menu	Role-based access, signed configs, audit logs

12) Sample BOM highlights

Function	Component class	Selection cues
ECG/RESP	Inst. amp, DRL amp, protection, 24-bit ADC	CMRR, input noise, defib withstand, lead-off
SpO₂	LED driver, photodiode TIA, ADC	Current range, noise density, ambient rejection
NIBP	Pressure sensor, IA, pump, valves	Drift, hysteresis, lifetime cycles, acoustic noise
Temp	RTD/thermistor AFE, precision ADC	Linearity, excitation stability
Isolation	Digital isolators, isolated DC-DC	CMTI, creepage, leakage current limits
Processing	MCU/SoC + display	RTOS resources, graphics accel, power
Networking	Ethernet/Wi-Fi modules	Security features, EMC, driver support
Storage	FRAM/Flash/eMMC	Trend/event endurance, power-fail safety
Audio	Beeper/amp	Loudness profile per alarm priority
Security	Secure element/TPM	Key storage, X.509, lifecycle states

13) Manufacturing QA, calibration & service

ICT/FCT: ECG gain/bandwidth/pace detect; SpO₂ LED/PD loop; NIBP pump/valve timing; temp accuracy checks.
Calibration: ECG lead-off thresholds; SpO₂ LED current/PD baseline; NIBP zero/span; temp coefficients stored to NVM.
Acceptance: Alarm tests (60601-1-8 patterns/priorities), leakage/hipot, EMC spot checks on golden units.
Service: Cable/connector life, battery capacity (if transport), isolation re-tests at intervals.