Continuous Noninvasive Assessment of the Autonomic Function

The synchronized measurement of continuous noninvasive blood pressure, beat-to-beat cardiac output, stroke volume and total peripheral resistance as well as a high-resolution ECG allows for an unprecedented insight into the autonomic function of the human body. Especially short term beat-to-beat variations of heart rate and blood pressure, but also the correlation between these signals are of interest. CNSystems has implemented three different software tools for the investigation of short term autonomic regulation mechanisms:

The first instrument is a mathematical prism for the online illustration of physiological rhythms. The software package of the Task Force^® Monitor evaluates the variability in real-time through implementation of "Adaptive Autoregressive Parameters" originally designed for the prediction of stock market changes [1]. The online tracking of transient events (e.g. during tilt table testing, autonomic function tests) delivers insight into the complex regulatory mechanism during the measurement. The following figure illustrates these mechanisms:

Physiological rhythms in heart rate and blood pressure are caused by breathing or by "Traube-Hering-Mayer" waves. The application of such modern digital filtering algorithms made it possible to display these oscillations. After splitting the frequencies through the mathematical prism, several characteristic frequency bands can be derived: High frequency (HF), low frequency (LF) and very low frequency (VLF) oscillations can be differentiated - this phenomenon is known as "Heart Rate Variability".

• The breath-dependent HF oscillations of HR give information about the cardiovagal activity and are usually about 0.2 – 0.3 Hz according to the usual breathing frequency (one breath every 3^rd to 5^th second).This rhythm is related to parasympathetic (vagal) activity.
  

• The LF oscillations of the blood pressure are caused by the sympathetic vasomotoric activity. This rhythm of the blood pressure has its summit at approximately 0.1 Hz ("10 sec rhythms") and is known as Traube-Hering-Mayer wave.  The LF oscillations of heart rate around 0.1 Hz depend on this slow blood pressure rhythm and result from the baroreceptor reflex as a response to Mayer waves. Thus, the LF oscillations of heart rate are, however, not a reliable index for the pure sympathetic activity, since different influences are involved in their origin. They become influenced by i.e. changes in the venous return, cardiac filling, the baroreceptor reflexes and the cardiac sympathetic activity. This is the reason why sympathetic drive is calculated by the Task Force^® Monitor software from the blood pressure variability.

• Further, very low frequency oscillations (VLF) can also be detected, which probably derive from thermo-regulation influences of the central nervous system on the heart activity or also the Renin-Angiotensin system.

In the first figure, the power spectra or heart rate variability (HRV) of a healthy patient is shown. Until head-up tilt (mark 2 at ~ 12min) a peak at breathing frequency (~ 0.25 Hz) can be observed. This corresponds with high vagal activity during rest. After head-up tilt, this 0.25 Hz peak immediately disappears and a new rhythmic activity around 0.1 Hz, which corresponds with sympathetic drive, comes up. Again, after tilting back (mark 3 at ~ 23min) the sympathetic peak at 0.1 Hz disappears and the vagal activity peak at 0.25 Hz slowly comes back.

The next figure displays the strong connection between hemodynamic signals using the sequence method [2]: It is used for the dynamic evaluation of the baroreceptor reflex sensitivity:

Baroreceptor reflex sensitivity measures the coupling between the length of the heart beat (RR-interval of ECG) and the systolic blood pressure. It is a marker for the ability of the autonomic nervous system to react on blood pressure changes by altering the length of the RR-interval. A change of blood pressure is detected by the human baroreceptor located in the aortic arch and in the carotid sinus. Afferent nerves transmit the information to the medulla oblongata, where either sympathetic or parasympathetic drives affect the cardiac sine node in order to change the RR-interval. If blood pressure is increased, the autonomic nervous system delays the RR-interval (heart rate decreases). In opposite, the RR-interval is shortened, if blood pressure drops. As the Task Force^® Monitor measures RR-interval and blood pressure on a beat-to-beat basis and with high resolution, baroreceptor reflex sensitivity can be detected.

A trend display for displaying autonomic function in real-time:

For a better time resolution, the software displays all relevant autonomic parameters within one trend plot. These parameters are:

• The HF-band of heart rate variability, which is the parasympathetic or vagal tone (blue line in first plot)
• The LF-band of blood pressure variability, which is the best guess of sympathetic tone (red line in first plot)
• SympatIco-Vagal Balance, which is the ratio between the above LF to HF bands
• Baroreceptor sequences found, whereas the height of the entire line (sequence) indicates the baroreceptor reflex sensitivity

References:

[1] Fortin J, Habenbacher W, Gruellenberger R, Wach P, Skrabal F: Real-time Monitor for Hemodynamic Beat-to-beat Parameters and Power Spectra Analysis of the Biosignals. Proceedings of the 20th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Vol 20, No 1, 360-3, 1998

[2] Parati G, Omboni St, Frattola A, Di Rienzo M, Zanchetti A, Mancia G. Dynamic evaluation of the baroreflex in ambulant subject. In: Blood pressure and heart rate variability, edited by di Rienzo et al. IOS Press, 1992, pp. 123-137.