Abstract P177: the Dissociation of Heart Rate and Blood Pressure Masks the Regulatory Role of Heart Rate in Systemic Hemodynamics

Blood pressure (BP) and heart rate (HR) are independent cardiovascular risk factors but there is little information about interactions among HR, BP, and systemic hemodynamics throughout the day. We performed 24-hour ambulatory pulse wave analysis (Mobil-O-Graph, IEM, Stolberg, DE) with measurements at 20 minute intervals. Variables measured were: systolic and diastolic BP (SBP and DBP), HR, oscillometric pulse wave velocity (oPWV), stroke volume index (SVI), and total vascular resistance index (TVRI). We also performed regression analyses within-subjects to determine the slopes of the SVI-HR and TVRI-HR relationships within individuals and for the group. We studied 89 individuals (mean age 61 years, weight 84 kg, BP 138/82 mm Hg (38 normal or controlled BP, 51 uncontrolled BP), resting HR 69 beats/min (bpm), 36% non-white, 64% female); 24-hour mean HR was not correlated with corresponding SBP or DBP values (r= -0.013 and r= 0.154, pNS, respectively) and this dissociation persisted in corresponding day-time vs. night-time comparisons and subgroup stratification by BP control status, age, and gender. In contrast, 24-hour mean HR was strongly correlated with 24-hour mean cardiac index (r=0.629, p<0.000) but strongly inversely related to mean 24-hour SVI and TVRI (r= -0.508 and r= -0.505, p<0.000 each); each 10 beat/min change in mean HR was associated with a 2.3 mL/m 2 (6%) change in SVI and a corresponding 6% change in TVRI. Analysis of individual 24-hour studies also revealed no correlations between HR and BP but the within-individual slopes and correlation coefficients between HR and SVI or TVRI were more variable; higher oPWV (which correlated with the SD of SBP, p=.01) was associated with a lesser sensitivity of SVI to HR change and a greater sensitivity of TVRI to HR change (p<0.001 each) independent of age, gender, or BP. We conclude that BP is normally independent of HR, which plays a hidden but important regulatory role in the underlying systemic hemodynamics between and within individuals. The clinical impact of differential sensitivity of systemic hemodynamics to HR change is not established but people with stiffer arteries display diminished SVI sensitivity and greater TVRI sensitivity to HR change, which may contribute to their greater SBP variability.