Spectral measures of short-term blood pressure and heart rate variations:Methodological considerations

Since the non-invasive volume clamp technique (developed by Penaz) for continuous blood pressure (BP) recording became available, the assessment of short-term BP and heart rate (HR) variability and successive estimation of baroreflex sensitivity (BRS) by spectral analysis techniques has been inaugurated for clinical evaluation of autonomic neuropathy and various cardiovascular disorders. However, basic methodological requirements of short-term cardiovascular variability measures such as reliability and validity have to be investigated. In a first study we addressed reproducibility of short-term BP and HR variability, and in a second study we investigated the validity of BRS estimation based on spectral analysis of non-invasive BP and HR recordings.Non-invasive beat-to-beat BP (FINAPRES-system), HR and respiratory frequency were continuously recorded during rest and during paced respiration (0.3 Hz). Within a three weeks period 57 healthy subjects (age: 32 +/- 8 years, casual BP: 133 +/- 14 / 88 9 mmHg, HR: 76 +/- 9 bpm, mean respiratory frequency at rest: 0.24 +/- 0.08 Hz) were measured twice for 5 minutes respectively. power spectra and transfer functions between systolic BP and HR (coherence and modulus) were determined by a discrete Fourier analysis with frequency resolution of 0.01 Hz over an integral of 0.01 to 0.5 Hz (CARSPAN program). Log-transformed variability measures were calculated for the low (0.02-0.06 Hz), mid (0.07-0.14 Hz), high (0.15-0.40 Hz), respiratory related (mean respiratory frequency +/- 0.03 Hz) and total frequency band (0.02-0.5 Hz). BRS was mathematically defined by integration of the modulus function over the mid, high and respiratory related frequency bands respectively. For the purpose of validation BRS was determined by the spectral method, as well as by a standardized reference technique employing bolus angiotensine II (A2) injections in 18 observations. HR variability and systolic BP variability, as well as BRS estimated by spectral analysis showed significant test-retest stability (r > 0.6) in all frequency bands. Test-retest stability of BRS estimated by the spectral analysis further increased when periods of paced respiration were analysed. High correlations between BRS calculated by the standardized bolus A2 injections and spectral techniques could be found for the mid (r=0.62, p <0.006), high (r=0.86, p <0.0001), and respiratory related (r=0.79, p <0.0001) frequency bands.We conclude that short-term variability measures of non-invasive systolic BP and HR as well as measures of BRS estimated by spectral analysis fulfil basic requirements of clinically relevant diagnostic tools.