Autonomic changes in patients with heart failure and in post-myocardial infarction patients

Abstract

A range of techniques for the assessment of autonomic tone are described. Impaired baroreflex control of heart period, measured in terms of heart rate variability or baroreflex sensitivity, is independently associated with adverse prognosis in patients with heart failure and following acute myocardial infarction. These techniques have not yet entered routine clinical practice.

Figure 1

Measurement of geometric time domain indices of heart rate variability (HRV). To perform geometric measures on the NN interval histogram, the sample density distribution D is constructed, which assigns the number of equally long NN intervals to each value of their lengths. The most frequent NN interval length X is established—that is, Y  =  D(X) is the maximum of the sample density distribution D. The HRV triangular index is the value obtained by dividing the area integral of D by the maximum Y. When the distribution D with a discrete scale is constructed on the horizontal axis, the value is obtained according to the formula HRV index  =  (total number of all NN intervals)/Y. For the computation of the triangular interpolation of NN intervals (TINN) measure, the values N and M are established on the time axis and a multi-linear function q constructed such that q(t)  =  0 for t N and t M and q(X)  =  Y, and such that the integral 0 + D(t) − q(t))2 dt is the minimum among all selections of all values N and M. The TINN measure is expressed in milliseconds and given by the formula TINN  =  M − N. Reproduced with permission from Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Circulation 1996;93:1043–65.

Figure 2

Lorenz (Poincare) plot. The current NN interval (x axis) is plotted against the previous NN interval (y axis). In healthy subjects there is a notable disparity between the two NN intervals; at longer NN intervals lie low heart rates. This is not so in patients with heart failure. Reproduced with permission from Woo MA, et al. J Am Coll Cardiol 1994;23:565–9.

Figure 3

Power spectral analysis of short term HRV in a healthy control (right hand side) and in a patient with heart failure (left hand side). Note that the y axis range has been reduced 10-fold for patient data in order to aid visual inspection of peaks. If the same scale had been used the plot would be almost indistinguishable from the zero line in the patient. Figure provided by Saqib Chowdhary and Dr John Townsend.

Figure 4

Phenylephrine bolus technique for measuring baroreflex sensitivity (BRS). After a bolus injection of phenylephrine there is a progressive increase in blood pressure and a reflex slowing of heart rate. BRS is calculated as the slope of the relation between systolic blood pressure and the subsequent RR interval during this ramp increase in blood pressure. Reproduced with permission from Persson PB, et al. J Hypertens 2001;19:1699–705.

Figure 5

Cross spectral analysis of BRS. Power spectral analysis of blood pressure and heart period. Cross spectral analysis is used to measure BRS as shown. Reproduced with permission from Parati G, et al. J Hypertens 2000;18:7–19.

Figure 6

Prognostic significance of low frequency power (LFP) in heart failure. Reduced LFP (⩽ 13 ms²) was associated with increased risk of sudden cardiac death in patients with heart failure. On multivariate modelling, reduced LFP and frequent ventricular premature beats (> 83/hour) were the two independent predictors of sudden cardiac death. Reproduced from La Rovere et al, with permission.