ABSTRACT
The B point on the impedance cardiograph waveform corresponds to the opening of the aortic valve and is an important parameter for calculating systolic time intervals, stroke volume, and cardiac output. Identifying the location of the B point is sometimes problematic because the characteristic upstroke that serves as a marker of this point is not always apparent. Here is presented a reliable method for B point identification, based on the consistent relationship between the R to B interval (RB) and the interval between the R-wave and the peak of the dZ/dt function (RZ). The polynomial function relating RB to RZ (RB = 1.233RZ - 0.0032RZ(2) - 31.59) accounts for 90%-95% of the variance in the B point location across ages and gender and across baseline and stress conditions. This relation affords a rapid approximation to B point measurement that, in noisy or degraded signals, is superior to visual B point identification and to a derivative-based estimate.
Subject(s)
Cardiography, Impedance/statistics & numerical data , Heart Rate/physiology , Adolescent , Adult , Aging/physiology , Cardiac Output/physiology , Data Interpretation, Statistical , Electrocardiography , Female , Humans , Male , Reproducibility of Results , Stroke Volume/physiology , Ventricular Function, Left/physiologyABSTRACT
The root mean square successive difference (RMSSD) in heart period series is a time domain measure of heart period variability. The RMSSD is sensitive to high-frequency heart period fluctuations in the respiratory frequency range and has been used as an index of vagal cardiac control. By transfer function simulations, the RMSSD statistic is shown to represent a high-pass filter that effectively captures respiratory sinus arrhythmia but also passes lower frequency fluctuations that can include sympathetic influences. These simulations, together with analysis of actual heart period series, reveal that the RMSSD is biased by basal heart period. Although between-subjects levels of RMSSD covary highly with spectral estimates of high-frequency variability, within-subject RMSSD change scores account for only 50-60% of the variance in spectral estimates. The present findings raise caveats in the applications and interpretation of the RMSSD statistic.
Subject(s)
Heart Rate/physiology , Adult , Algorithms , Data Interpretation, Statistical , Female , Humans , Male , Models, Statistical , Vagus Nerve/physiologyABSTRACT
The onset of ventricular depolarization defines the start of the preejection period (PEP), which is commonly used as an index of myocardial contractility and sympathetic control of the heart. Although the fiducial point for this onset has traditionally been the onset of the Q wave of the electrocardiogram, other measurement points have also been used in the literature, including the peak of the Q wave (i.e., the onset of the R wave). Conceptual, physiological, and empirical considerations addressing the reliability and validity of these alternative metrics support the application of the Q-wave peak/R-wave onset as the fiducial point for PEP measures.
