njit-etd2003-081 - New Jersey Institute of Technology

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The baroreflex, an autonomic reflex which controls heart rate through efferent
vagal and sympathetic nerves by afferent inputs from pressure sensitive baroreceptors, is
still mostly unknown. The neural control of the total cardiovascular system with several
closed loops, of which the baroreflex is only a part, is very complex and that makes it
difficult to assess characteristics of this reflex. If one was able to create a large enough
broadband input signal and measure the input Blood Pressure (BP) as well as the output
Heart Rate (HR), one could estimate from spectral calculations of input, output and
cross- correlation, the amplitude and phase frequency characteristics of the baroreflex.
Respiration is known to modulate BP as well as HR, the latter phenomenon
called respiratory sinus arrhythmia (RSA). However, the precise relationships are still
under debate. Respiration affects primarily HR by central cardio-respiratory coupling
and changes BP through variations in cardiac output. Respiration also directly
modulates BP through mechanical effects of changing intrathoracic pressure on cardiac
filling, which through the baroreflex causes HR variations at the respiration frequency.
Recent studies [11[3] showed that respiration to a large extent primarily modulates BP
and through the baroreflex, HR. The baroreflex with the respiratory influence can
therefore schematically be visualized as in Figure 3.16. Under these assumptions one
does have the possibility to "control" baroreflex input (BP) by letting a person breathe
voluntarily according to a sinusoid on several fixed frequencies or a slowly changing
frequency (sweep). Here it is desirable to prove the possibility of assessing gain and
phase relations of the transfer function of BP and HR in a human, over a wide frequency
range by voluntarily breathing at predetermined frequencies.