A novel approach to identify false positives due to body position changes during ST-segment monitoring

Abstract

The most common false alarms with ST-segment monitoring are ST fluctuations caused by body position changes. This study designs and implements a device to indicate the patient's body position as supine, right or left-lying or upright on the electrocardiogram (ECG) during ST-segment monitoring. Method: By using a strategy developed to sense body position for sleep research, a biosensor was designed by Inovise Medical (Newberg, OR) to communicate with the Mortara ELI 100 12-lead ST monitor (Milwaukee, WI). The biosensor is small (hockey puck-sized) and lightweight and is placed on the upper sternum. The biosensor interfaces between the electrode and the V2 lead wire and sends a specific signal every 2.1 s identifying in real time the patient's position. Visual indication of each patient orientation is achieved with dedicated light emitting diodes on the face of biosensor. Its 2 internal lithium batteries supply enough power to operate the lights continuously for 20 days and the pulse indicators for 208 days. It uses a low power PIC microprocessor to read a 2-axis accelerometer. The processor calculates the gravitational vector and is triggered to encode the information for a 45[deg ] turn. For patient isolation and to reduce artifact, the microprocessor is optically insulated from the lead wire and to save power, the microprocessor reverts to the sleep mode between each pulse stream. Results: In the first phase of the study, validation of the body sensor was performed by monitoring 35 cardiac telemetry unit patients during a sequence of the above four body positions typically observed in hospitalized patients. The biosensor correctly identified each of the positions. In the second phase of the study, a subset was monitored overnight and the 12-lead ECGs obtained during the sequential body positioning were used as fingerprints to identify false ST segment alarms. Fourteen percent or 40% of the sample showed a positional fingerprint in one or more of the positions. However, of the 5 patients in the positional group who were monitored overnight, all 5 produced at least 1 false positive ST-segment alarm as confirmed by comparisons with their earlier positional fingerprints. Conclusions: Clinically significant ST changes associated with body positional changes were frequently observed with overnight monitoring period and were accurately detected by biosensor monitoring. Concomitant body position and ST-segment monitoring aids in ECG interpretation of ST alarms by identifying periods in which the patient is not in a supine state. Future studies are needed to determine the sensitivity and specificity of ischemia monitoring with this approach.