Cross-bridge activation rate constant determined from systolic time intervals in patients with systemic lupus erythematosus.

Cardiac adrenergic activity was investigated in 38 patients with systemic lupus erythematosus (SLE) who were not receiving cardiovascular agents using a new index of the adrenergic tone of the working left ventricular (LV) myocardium and using the heart rate to represent sinus node adrenergic tone. According to the active cross-bridge model employed in this study, the cross-bridge activation rate constant (Ka) of the LV myocardium, corresponding to the rate constant for the binding of Ca2+ to troponin C, can be approximately expressed as Ka = 3/electromechanical systole (sec-1). The Ka value corrected for heart rate (Kac) remains nearly constant in normal individuals, but is increased without change of the heart rate by dobutamine infusion. Fifty patients who fulfilled the American Rheumatism Association criteria for SLE underwent echocardiography, as well as simultaneous recording of the electrocardiogram, phonocardiogram, and carotid pulse wave tracing. Kac was calculated from the interval between the onset of the QRS complex to the second heart sound (QS2 interval) and the heart rate (HR) as follows: Kac = 3/QS2 + 0.0249 (66-HR). The 38 SLE patients were divided into a normal Kac group (n = 26, 6.9 < Kac < 8.3 sec-1) and a high Kac group (n = 12, Kac > or = 8.3 sec-1).(ABSTRACT TRUNCATED AT 250 WORDS)