Dynamic response of the Initial Systolic Time Interval to a breathing stimulus measured with impedance cardiography

The Initial Systolic Time Interval (ISTI) is a measure for the time delay between the electrical and mechanical activity of the heart. The present study reports about the dynamic response of ISTI to a Valsalva manoeuvre. This response was investigated in 22 young healthy volunteers, having different levels of training in sports. The time course of the ISTI during the Valsalva manoeuvre was found to follow a distinct pattern and to be analogous to the course of the Pre-Ejection Period (PEP), also obtained from ECG and ICG signals, reported earlier. The recordings show a definite influence of the Frank-Starling mechanism and are to some extent consistent with reports on the time course of sympathetic activation. The highly trained subjects showed an ISTI that was systematically longer at all moments of the manoeuvre.