Usefulness of hemodynamic sensors for physiologic cardiac pacing in heart failure patients

Abstract

The rate adaptive sensors applied to cardiac pacing should respond as promptly as the normal sinus node with an highly specific and sensitive detection of the need of increasing heart rate. Sensors operating alone may not provide optimal heart responsiveness: central venous pH sensing, variations in the oxygen content of mixed venous blood, QT interval, breathing rate and pulmonary minute ventilation monitored by thoracic impedance variations, activity sensors. Using sensors that have different attributes but that work in a complementary manners offers distinct advantages. However, complicated sensors interactions may occur. Hemodynamic sensors detect changes in the hemodynamic performances of the heart, which partially depends on the autonomic nervous system-induced inotropic regulation of myocardial fibers. Specific hemodynamic sensors have been designed to measure different expression of the cardiac contraction strength: Peak Endocardial Acceleration (PEA), Closed Loop Stimulation (CLS) and TransValvular Impedance (TVI), guided by intraventricular impedance variations. Rate-responsive pacing is just one of the potential applications of hemodynamic sensors in implantable pacemakers. Other issues discussed in the paper include: hemodynamic monitoring for the optimal programmation and follow up of patients with cardiac resynchronization therapy; hemodynamic deterioration impact of tachyarrhythmias; hemodynamic upper rate limit control; monitoring and prevention of vasovagal malignant syncopes.

Figure 1

Peak Endocardial Acceleration (PEA) sensor (Sorin Group, Italy). An accelerometer mounted on the tip of a pacing lead placed in the right ventricle detects PEA-I, related to the isovolumic contraction (and the first cardiac tone) and PEA-II, related to the isovolumic relaxation (and the second cardiac tone).

Figure 2

Closed Loop Stimulation (CLS) system (Biotronik, Germany). (a) Changes in intracardiac impedance are closely related to myocardium-blood ratio in the volume around the ventricular tip. (b) An increase in blood volume (and the consequent solid angle) produces a decrease in intracardiac impedance (Z). (c) An enhanced contractility induces an increase of impedance around the electrode tip (morphological modification of impedance curve moves it to the left).

Figure 3

Trans Valvular Impedance (TVI) system (Medico, Italy). The impedance signal is derived between right atrium and the ventricle, by the tip (a) or the ring (b) electrodes of standard pacing leads.

Figure 4

Trans Valvular Impedance (TVI) system (Medico, Italy). TVI waveform mirrors the time-course of ventricular volume along the cardiac cycle, detecting by M-mode Echo. TVI increases during ventricular systole and decreases during passive and active filling period.