Studying cardiac mechano-sensitivity in man

The human heart performs its function in a constantly changing mechanical environment, due to the action of both intrinsic and extrinsic mechanical stimuli. The cardiac cycle itself, alternating between fi lling and emptying of the cardiac chambers, involves conditions of muscle distension and shortening. Changes in intrathoracic pressure, venous return to the heart and peripheral resistance, associated with respiration, constitute additional sources of mechanical variability of the heart. Changes in the mechanical state of the heart are known to affect the behaviour of pacemaker cells as well as the conductive and refractory properties of the cardiac tissue via mechano-electrical coupling (MEC). These variations are mirrored by changes in the variability of heart beat interval series, determined from electrocardiogram (ECG) or endocardial signals, which allows in principle a non-invasive assessment of MEC effects in humans. The ability to determine mechanical modulation of cardiac performance and its resulting electrical variations depends on both the availability of suitable signal analysis methods (allowing to associate rhythm variations to their physiological correlates) and the choice of human models where mechanical effects are not overridden by other prevalent factors, such as autonomic modulation. This chapter reviews the evidence supporting the contribution of non-neural mechanical mechanisms in the modulation of sinus rhythm as well as the mechanical origin of atrial rate variability during re-entrant arrhythmias, such as atrial flutter.