A self-calibrating telemetry system for measurement of ventricular pressure-volume relations in conscious, freely moving rats

Using Bluetooth wireless technology, we developed an implantable telemetry system for measurement of the left ventricular pressure-volume relation in conscious, freely moving rats. The telemetry system consisted of a pressure-conductance catheter (1.8-Fr) connected to a small (14-g) fully implantable signal transmitter. To make the system fully telemetric, calibrations such as blood resistivity and parallel conductance were also conducted telemetrically. To estimate blood resistivity, we used four electrodes arranged 0.2 mm apart on the pressure-conductance catheter. To estimate parallel conductance, we used a dual-frequency method. We examined the accuracy of calibrations, stroke volume (SV) measurements, and the reproducibility of the telemetry. The blood resistivity estimated telemetrically agreed with that measured using an ex vivo cuvette method (y = 1.09x - 11.9, r(2) = 0.88, n = 10). Parallel conductance estimated by the dual-frequency (2 and 20 kHz) method correlated well with that measured by a conventional saline injection method (y = 1.59x - 1.77, r(2) = 0.87, n = 13). The telemetric SV closely correlated with the flowmetric SV during inferior vena cava occlusions (y = 0.96x + 7.5, r(2) = 0.96, n = 4). In six conscious rats, differences between the repeated telemetries on different days ( 3 days apart on average) were reasonably small: 13% for end-diastolic volume, 20% for end-systolic volume, 28% for end-diastolic pressure, and 6% for end-systolic pressure. We conclude that the developed telemetry system enables us to estimate the pressure-volume relation with reasonable accuracy and reproducibility in conscious, untethered rats.