Figure 2
The isovolumic contraction force sensor for FFR building. The transcutaneous force sensor is based on a linear accelerometer. The device includes in one single package a MEMS sensor that measures a capacitance variation in response to movement or inclination and a factory trimmed interface chip that converts the capacitance variations into analog signal proportional to the motion. The device has a full scale of ±2·g (g = 9.8 m/s2) with a resolution of 0.0005·g. The acceleration signal is converted to digital and recorded by a laptop PC, together with an ECG signal. The system is also provided with a user interface that shows both the acceleration and the ECG signals while the acquisition is in progress. A QRS detection algorithm is used to automatically locate the beginning of the isovolumic ventricular contractions and to record isovolumic contraction force vibrations (which audible components give rise to the first heart sound). All the parameters are acquired as instantaneous values at baseline and during stress; mobile mean is utilized to assess baseline value (1 minute recording), at each incremental stress test, at peak test, and during recovery.