Method for detecting the disconnection of an extracorporeal device using a patient's endogenous electrical voltages.
Tubing (especially venous) disconnections using pumped devices cause significant hemorrhage, and current monitoring techniques are imperfect because they rely on intraluminal pressure changes. We devised a passive detection method based on a patient's electrical voltages being transmitted via blood tubing to our alarm circuit. As the arterial and venous access sites are in close proximity, the signals are nearly identical during connection, and markedly different with disconnection. We built a prototype and tested it in vitro with saline and during hemodialysis treatments (n=7). The connection status is determined by examining the difference between endogenous voltages in the blood tubing from and to the patient, and when it exceeds a threshold an alarm condition is triggered. We tested for possible confounding by an electrical shunt through the dialyzer and determined that pathway had an impedance approximately three times (>350 kOmega) that of the tubing to the patient. As the roller blood-pump periodically occluded the tubing, the resultant intermittent very high impedance prevented that potential shunting problem and improved the sensitivity of our device. Disconnections were detectable at various bloodline sites (needles, sampling ports, drip chambers). Thus, the circuit's sensors can be placed remotely at the dialysis machine, with electrical continuity to blood made by inexpensive conductive elements at the tubing wall or drip chambers. Appropriate threshold and noise-eliminating circuitry, as well as alarm states that alert the staff and stop the blood pump, make our prototype a promising low-cost safety enhancement.