Time-Courses of Central Frequencies of Low-Frequency Components of Systolic and Diastolic Pressures and RR Interval Variabilities in Response to Incremental Isometric Exercise

Alejandra Guillén-Mandujano and Salvador Carrasco-Sosa
Universidad Autónoma Metropolitana-I


While there are few reports describing the effects of sympathetic maneuvers on the central frequency of the low-frequency component (CFLF) of RR intervals (CFLFRR), there are none examining those of systolic (CFLFSP) and diastolic pressures (CFLFDP), nor the relation between them and their respective low-frequency powers (PLFRR, PLFSP and PLFDP). Thus, we assessed, in 29 healthy subjects, the effects of continuously increasing muscular force (CIMF) by isometrically extending both legs at a rate of 0.2 kg.s-1 until fatigue on the instantaneous 2-min time-courses of PLFSP, PLFDP, PLFRR, CFLFSP, CFLFDP and CFLFRR, estimated by a time-frequency distribution. Comparisons and correlations between CFLF and low-frequency power (PLF) of the three variables were obtained. PLF increment onset (threshold) was detected by V-slope method for response measures characterization into before (BTP) and after threshold (ATP) phases. For statistical analysis, 20-s epoch means (EM) of the dynamics were computed. Instantaneous time-courses of CFLFSP, CFLFDP and CFLFRR showed: 1. similar patterned responses of gradual increment in BTP and abrupt decrease in ATP, roughly inverse to the pattern presented by PLF dynamics; 2. differences (p<0.001) in EM, with respect to control and among CFLF according to the inequality: CFLFRR>CFLFSP>CFLFDP. CFLF-PLF correlations (p<0.01) of the three variables, shown in the table, were negative and greater (p<0.01) in ATP than BTP. The significant and characteristic changes elicited by CIMF in CFLFSP, CFLFDP and CFLFRR, correctly indicate, but inversely, the known responses of reduction and abrupt increment of sympathetic activity before and after metaboreflex triggering. CFLF indicating capability improves in ATP, by its substantially greater negative correlations. These findings imply that CFLFSP, CFLFDP and CFLFRR can be trustable sympathetic indicators that complement and strengthen the performance of autonomic activity spectral measures. The different CFLF levels found suggest that the cardiac modulatory sympathetic effect presents greater frequency than the vasomotor one.