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We have previously described the use of non-parametric transfer function analysis in combination with broad-band respiration to compute the magnitude and phase relations between respiration, heart rate (HR), and arterial blood pressure (ABP). These experimental results have led to formulation of a model of HR and ABP control which includes the mechanical feedforward from HR to ABP, the baroreflex feedback from ABP to HR, and respiration as a noise source which directly affects HR via central modulation of autonomic activity and affects ABP via mechanical thoracic coupling. Combining results from the model with experimental observations, we have concluded that short-term ABP variability results from a combination of mechanical and autonomic influences, where the precise mixture depends on an individual's autonomic state.

This paper presents a time domain technique to estimate transfer characteristics from fluctuations of instantaneous lung volume (ILV) to heart rate (HR). Pure moving average (MA) and autoregressive moving average (ARMA) system identification techniques are introduced to characterize the system. Pre-processing (pre-filtering of the HR fluctuations and pre-enhancement of the high frequency contents of ILV fluctuations) are shown to be effective in obtaining stable estimates of MA coefficients. Model order selection of the ARMA system based on a priori knowledge of the system characteristics is proposed. Data analysis using the time domain techniques revealed some temporal transfer characteristics which are poorly resolved using frequency domain analysis.

Quantitative assessment of the coupling between arterial blood pressure (ABP) and heart rate (HR) is discussed. Traditionally, the HR baroreflex has been studied directly by controlling sensed ABP independent of HR changes. A method is presented that, by combining broadband stimulation and parametric system identification, makes it possible to assess baroreceptor function without disturbing the normal closed-loop function of the cardiovascular system. Preliminary results suggest that the methodology can detect changes in baroreceptor sensitivity in response to the administration of atropine and propanolol. >

The authors previously determined that the transfer function between broadband respiratory activity and the heart-rate (HR) response can be used to elucidate the frequency-dependent phase and magnitude characteristics of the respiratory sinus arrhythmia (RSA). To understand the complex links between respiratory activity and arterial blood pressure (ABP), the authors determined the transfer function between instantaneous lung volume (ILV) and the systolic, diastolic, and pulse pressure (SBP, DBP, and PP) in healthy human volunteers during 6-minute periods in which the respiratory rate was voluntarily controlled in predetermined but erratic fashion. A pharmacologic autonomic blockage was used to separate the direct links between respiration and ABP from the effects of RSA and ABP. Results suggest that with HR fluctuations eliminated, respiration affects ABP in proportion to the rate of change in ILV; however, when neural control of HR remains intact, the RSA plays an important role in the genesis of ABP fluctuations. >