SAFETY ASPECTS OF AMBULANT ESOPHAGEAL MANOMETRY AND PH MONITORING

Nonperfused piezoresistive pressure transducers and ISFET (ion-sensitive field effect transistors) pH electrodes are increasingly used for research purposes and during ambulatory patient monitoring. The currently used multichannel dataloggers are usually powered by batteries with a voltage range of approximately 3-9 V. Piezoresistive pressure transducers need an excitation voltage between 0.5 and 2.0 V (constant voltage powered) or an excitation current of 0.5-2 mA (constant current powered). ISFET pH electrodes are characterized by the need of a wide range of supply voltages (-3 to +3 V). These transducers normally are employed in endoluminal multisensor catheters, possibly with an external reference electrode, which implies a galvanic coupling of the logger system with the patient. In the case of a loss of insulation of one or more of the sensors, net currents of up to several milliamps can occur, depending on the relative placement of the sensors and/or the reference electrode. It is conceivable that these currents can induce cardiac arrhythmias and even fibrillation. To prevent these potential problems, we have developed an eight-channel ambulant datalogger incorporating a safety circuitry to check for current leakage from each sensor. If leakage occurs, an automatic galvanic separation of the defective transducer is effected. This was achieved using a hardware design independent of the processor. The threshold of deactivation is chosen according to national regulations [10 mu A in the United States (UL, Underwriters Laboratories Inc.) and 100 mu A in the EEC (DIN IEC-601, International Electrotechnical Commission)].