Dynamic regulation of heart rate during acute hypotension: new insight into baroreflex function

To examine the dynamic properties of baroreflex function, we measured beat- to-beat changes in arterial blood pressure (ABP) and heart rate (HR) during acute hypotension induced by thigh cuff deflation in 10 healthy subjects u nder supine resting conditions and during progressive lower body negative p ressure (LBNP). The quantitative, temporal relationship between ABP and HR was fitted by a second-order autoregressive (AR) model. The frequency respo nse was evaluated by transfer function analysis. Results: HR changes during acute hypotension appear to be controlled by an ABP error signal between b aseline and induced hypotension. The quantitative relationship between chan ges in ABP and HR is characterized by a second-order AR model with a pure t ime delay of 0.75 s containing low-pass filter properties. During LBNP, the change in HR/change in ABP during induced hypotension significantly decrea sed, as did the numerator coefficients of the AR model and transfer functio n gain. Conclusions: 1) Beat-to-beat HR responses to dynamic changes in ABP may be controlled by an error signal rather than directional changes in pr essure, suggesting a "set point" mechanism in short-term ABP control. 2) Th e quantitative relationship between dynamic changes in ABP and HR can be de scribed by a second-order AR model with a pure time delay. 3) The ability o f the baroreflex to evoke a HR response to transient changes in pressure wa s reduced during LBNP, which was due primarily to a reduction of the static gain of the baroreflex.