Chronic performance of a subcutaneous hemodynamic sensor.

BACKGROUND

A subcutaneous photoplethysmography (PPG) sensor uses light to detect changes in vascular volume from a location outside the bloodstream. Incorporation into a chronically implanted device, such as a pacemaker or an implantable cardioverter defibrillator, may facilitate therapy optimization and disease monitoring by providing continuous assessment of hemodynamic function and arterial oxygen saturation. However, performance of a chronically placed subcutaneous sensor has not been established.

METHODS

Six dogs were implanted with 2-4 PPG sensors subcutaneously in the neck or posterior thorax. Half of the sensors were directed toward deep tissue and half toward overlying cutaneous tissue. Each sensor contained a red and an infrared light emitting diode, a photodetector, and supporting electronics, which were encapsulated in epoxy and attached to a transcutaneous connector. Data were collected at implant and every 3 ± 1 days for 4-9 months starting 3 weeks postimplant. At explant, the fibrous encapsulation was histologically analyzed.

RESULTS

A minimally to moderately neovascularized encapsulation formed over all sensors, consisting of fibrous and granulation tissue. Higher cardiac pulse amplitudes and direct current (DC) components were recorded in sensors oriented toward deep tissue, but no significant difference between orientations was found in respiratory wave amplitude. Cardiac pulse amplitude, respiratory wave amplitude, and DC component amplitude, as recorded by the sensor, did not significantly change over time.

CONCLUSIONS

Despite fibrous encapsulation of PPG sensors, cardiac pulses and respiratory waves could easily be measured throughout the study and remained constant over time. These results suggest suitability of subcutaneous PPG technology for chronic applications.