Probing the arterial baroreflex: is there a 'spontaneous' baroreflex?

The arterial baroreflex is important for beat-to-beat arterial pressure control and its sensitivity has predictive value for clinical outcomes in a myriad of cardiovascular conditions. Given this, researchers have sought approaches for baroreflex assessment that are not invasive and easily obtained. These techniques have exploited the beat-by-beat parallel changes in arterial pressure and heart period to produce estimates that have been termed 'spontaneous' baroreflex indices. The two most commonly used analyses--frequency domain or spectral analyses and sequence analysis have been evaluated in both animals and humans. The animal data suggests an important baroreflex role in linking spontaneous heart period and pressure variabilities, but do not resolve the extent to which these fluctuations reflect baroreflex gain. The human data suggest a high correlation between spontaneous indices and pharmacologically derived baroreflex gain, but also indicate a poor correspondence between them. This may be due to the fact that short-term fluctuations in RR interval are not intimately and always linked to those in pressure via the baroreflex and thus simple observation of arterial pressure and heart period alone may not reveal the extent of arterial baroreflex involvement. If baroreflex function is to be assessed with the fewest and safest assumptions, the input to the system should be driven externally to create large and apparent responses. Nonetheless, spontaneous baroreflex indices may have predictive power; although it remains unknown whether spontaneous indices provide predictive power beyond that provided by heart rate variability indices alone.