Operator-independent force-frequency relation monitoring during stress with a new transcutaneous cardiac force sensor

A system for non-invasive and automatic assessment of the cardiac force frequency relation (FFR) is presented. The method is based on a micro-electro-mechanical accelerometer which was used to build a cutaneous vibration sensor. The system was tested on 25 patients, 12 M and 13 F, age range 47 to 71 years, with normal ventricular function. Exercise echocardiography was scheduled in 17 patients and dipyridamole in 8. The cardiac force was measured as the cardiac tone amplitude in the isovolumic contraction period (the vibration generating the first cardiac sound on auscultation), and the FFR was obtained by plotting the force versus the cardiac frequency. Non myocardial noising vibrations (skeletal muscles, body movements, breathing) were filtered. Results were compared with another non-invasive measure which was obtained in the stress echo lab, where the force was computed as the systolic pressure (SP) / end-systolic volume index (ESVi) ratio. A consistent force signal was obtained in all patients: the cardiac tone amplitude increased from 0.012plusmn0.006g (g = 9.8 m/s²) to 0.032plusmn0.018g (+204plusmn164% vs. rest). A significant correlation with the SP/ESVi FFR slope was found: R = 0.68 p=0.0002. In conclusion, the cardiac FFR was measured both in pharmacological and exercise stress tests by a cutaneous sensor based on an accelerometer; a continuous assessment of the FFR was obtained which reflects the results given by measuring the force as SP/ESVi.