IN A HOSPITAL ENVIRONMENT

A I Riemann and N E Evans
N Ireland Bioengineering Centre & School of Electrical and Electronic Engineering
University of Ulster, Shore Road, Newtownabbey, Co Antrim, N Ireland, UK, BT37 0QB

INTRODUCTION The potential of low-cost radio biotelemeters using basic modulation schemes such as PWM, PPM and FSK is limited by the effect of impulsive electrical noise mainly generated by electrical plant and radiated by equipment wiring and in-building cables. The resultant interference corrupts weaker signals input to the telemetry receiver’s detector stage, inducing output waveform distortion in analogue signals and bit errors in transmitted digital data streams. The telemedicine industry is already exploiting RF links in both hospital- and home-based patient monitors; but the nature of the electrical environment hosting these systems is largely unknown. This paper reports on initial measurements of 450 MHz radio noise made in the Coronary Care Unit at Belfast City Hospital, a 15 storey tower block with 576 inpatient beds.

METHOD The measurement setup consisted of an Icom IC-R7000 receiver modified for switchable AGC and with a pulse output derived immediately post AM-detection: this fed a PC equipped with custom counter hardware / software capable of recording pulses of 10 m s and higher duration. One-minute pulse totals were stored on the computer’s hard disk, together with the current time. Attenuators placed in the feeder of a folded dipole enabled input thresholds of -130, -120 and -110 dBm to be set at the receiver’s antenna port. The dipole could be oriented for both vertical and horizontal polarisation. The equipment was located on the 9th floor of the hospital, adjacent to the main receive-antenna bank for the CC Unit’s ambulatory ECG telemetry transmitters.

RESULTS The table below summarises the information derived for a series of 4-day observations, in terms of pulses counted per minute.

DISCUSSION Similar pulse counts were logged on a daily basis; there was no significant difference between weekdays and weekends, but the diurnal variations did follow working shift patterns within the hospital.