Kunihiro Yamada

Parametric system identification of arterial baroreflex with random perturbation of blood pressure in normal subjects

Summary: The purpose of the study was to evaluate the dynamic nature of heart rate baroreflex under closed‐loop conditions. We applied pressure perturbation with a random‐interval cuff inflation to parametric system identification of the baroreflex. In 10 healthy men (six young, 27 ± 3.9 years old, and four older, 50 ± 3.8 years old), blood pressure variations were produced by a random‐interval (0.06‐0.50 Hz) cuff inflation around bilateral thighs to identify characteristics of the baroreflex with an autoregressive moving average model with exogenous input. The model faithfully described the dynamic relationship between mean blood pressure and RR interval. A theoretical 1 mmHg step input of mean blood pressure caused a rapid increase in RR intervals toward the steady state within 10 s in younger subjects. In older subjects, however, the step response showed a delayed and gradual increase in RR interval over 20 s. The steady‐state gain obtained from the autoregressive moving average model with exogenous input was significantly greater than that determined from the slope of the pressure‐RR relationship. The conclusion was that the dynamic baroreflex gain was quantified using a parametric system identification technique and blood pressure perturbation by means of a random‐interval cuff inflation. This method would potentially be of great use in helping us gain insights into a closed‐loop system of complex and dynamic cardiovascular regulation in humans.

Circadian Dynamics of Heart Rate and Physical Activity in Patients with Heart Failure

The present study was designed to develop a method to continuously measure Holter electrocardiogram (ECG) and physical activity in terms of metabolic costs to examine circadian dynamics of RR intervals and physical activity in patients with heart failure. A total of 7 healthy subjects and 3 heart failure patients performed cardiopulmonary exercise test using four-stage graded treadmill walking at 0% grade to examine whether the acceleration signals in the vertical direction could reflect actual body energy expenditure during physical activity. Then, using this new method, 24-hr monitorings of ECG and physical activity were performed in 24 inpatients with heart failure while they were allowed to walk around freely. Our results showed the integral of rectified acceleration signals was closely correlated with actual metabolic cost in all subjects. Instantaneous changes in heart rate were quite concordant with physical activity. As compared with the asymptomatic patients (n = 12), the symptomatic patients (n = 12) had lower energy expenditure during 8-hr daytime periods but higher mean heart rate. Furthermore, a more prominent ultradian rhythm of circadian changes in heart rate and physical activity was found in 50% of all subjects studied. The simultaneous analysis of Holter ECG and physical activity as the same time series revealed that in patients with heart failure, sympathovagal balance shifted toward sympathotonic conditions and their physical activity could become subject to intrinsic ultradian dynamics of bodys homeostasis.