  Using a three-compartment model of blood pressure dynamics, we analyze
theoretically the short term cardiovascular variability: how the
respiratory-related blood pressure fluctuations are buffered by appropriate
heart rate changes: i.e. the respiratory sinus arrhythmia. The buffering is
shown to be crucially dependent on the time delay between the stimulus (such as
e.g. the inspiration onset) and the application of the control (the moment in
time when the efferent response is delivered to the heart). This theoretical
analysis shows that the buffering mechanism is effective only in the upright
position of the body. It explains a paradoxical effect of enhancement of the
blood pressure fluctuations by an ineffective control. Such a phenomenon was
observed experimentally. Using the basis of the model, we discuss the blood
pressure variability and heart rate variability under such clinical conditions
as the states of expressed adrenergic drive and the tilt-test during the
parasympathetic blockade or fixed rate atrial pacing. From the results of the
variability analysis we draw a conclusion that the control of blood pressure in
the HF band does not directly obtain the arterial baroreceptor input. We also
discuss methodological issues of baroreflex sensitivity and sympathovagal
balance assessment.
