Effect of DDD Pacing Titrated by Stepwise Changing of AVD on Aortic dP/dt

The maximum value of rate of left ventricular pressure increase, denoted by left ventricular peak dP/dt value (dP/dt max), corresponds to the highest point on the curve of first derivative of ventricular pressure curve with respect to time. It is one of the important systolic performance parameters, which is primarily determined by ventricular contractility, and emerges in isovolumetric contraction phase when aortic valves are closed. Thus it is not influenced by parameters beyond ventricle such as aortic valve area, aortic elastance and peripheral resistance. Despite these unique advantageous, the main reason for its limited usage in assessing ventricular systolic performance instead of commonly used ejection fraction is the difficulties in its calculation, which requires incessant intraventricular pressure recordings.

The maximum value of acceleration rate of aortic pressure increase can be named as aortic peak dP/dt. It, likewise, corresponds to the maximum value of first derivative of pressure curve with respect to time. Unlike left ventricular peak dP/dt, it emerges after aortic valve opens in early systolic ejection phase and thus is influenced by parameters beyond ventricle in addition to left ventricular contractility such as aortic elastance and compliance, vascular resistance, interference of forward and backward propagating pressure waves, stroke volume and its ejection rate (dV/dt). Notably, aortic peak dP/dt can be expected to be one of principal determinants in pressure-related damages in the vascular system. In structures exposed to variable pressures, the extent of damage depends on the magnitude and rate of acceleration of applied pressure in addition to susceptibility of the structures. Whether this mechanism, which has numerous implications from the real world, operates in the organism as well can be exemplified by the pulse pressure, which is an independent risk factor for adverse cardiovascular events apart from systolic and diastolic blood pressure levels. A wide pulse pressure suggests an accelerated rate of pressure increase. Therefore applications lowering left ventricular and aortic peak dP/dt values may open a new therapeutic avenue in management of pressure-related vascular damages such as aortic aneurisms. However, aortic peak dP/dt concept is seldom found a place even in research articles.

Since it is one of the principal determining factors of aortic peak dP/dt, changing left ventricular contractility thereby left ventricular peak dP/dt value is expected to change aortic peak dP/dt in the same direction since all other variables being unchanged. Yet to conclude the extent of this association and magnitude of change, measuring these parameters for various contractility levels is necessary. In this context, it has been demonstrated that narrowing of the QRS complex by cardiac resynchronization therapy (CRT) was shown to increase left ventricular peak dP/dt value. This finding strongly suggests that widening of the QRS complex, on the contrary, will decrease left ventricular peak dP/dt value. However, impact of decreasing left ventricular peak dP/dt by gradually lowering ventricular contractility upon aortic peak dP/dt has not been investigated in previous studies.

In the present study, changes in left ventricular and aortic peak dP/dt values in response to gradual reduction in left ventricular (LV) contractility with stepwise changing (decremental shortening) atrio-ventricular delay (AVD) by dual chamber pacing will be investigated in patients without LV systolic dysfunction.