Cardiac autonomic balance versus cardiac regulatory capacity

Abstract

The concept of autonomic balance views autonomic states along a bipolar continuum from sympathetic (S) to parasympathetic (P) dominance, whereas regulatory capacity models emphasize overall autonomic flexibility as a marker of the capacity for regulation. These two concepts were evaluated for their utility in characterizing patterns of autonomic control. Measures of P (high frequency heart rate variability, HF) and S (preejection period, PEP) cardiac control were obtained. A measure of cardiac autonomic balance (CAB) was derived as the difference in the normalized P index minus the S index, and a measure of cardiac autonomic regulation (CAR) was derived as the normalized P index plus the S index. Results reveal that CAR, but not CAB, was a significant predictor of the prior occurrence of a myocardial infarction, net of demographic and other variables, whereas CAB, but not CAR, was a significant predictor of concurrent diabetes.

Figure 1

Bivariate representation of autonomic space, with a parasympathetic ordinate (estimated by normalized HF) and a sympathetic abscissa (estimated by normalized PEP). The 0,0 intersection illustrates the mean position along both dimensions, the diagonal of reciprocity represents a bipolar, model of reciprocally controlled autonomic divisions. The diagonal of coactivity illustrates another dimension of regulation, which more effectively indexes the aggregate or total autonomic activity. The axes and quadrants of this graphic represent the major modes of autonomic control as defined by Berntson et al., 1991. The locations along the parasympathetic and sympathetic axes represent patterns of independent parasympathetic and sympathetic control, respectively. The quadrants represent the remaining four modes of control (reciprocal parasympathetic, reciprocal sympathetic, coactivation and coinhibition.

Figure 2

Distribution of HFz and PEPz scores across the CHASRS population, and relation to the derived CAR and CAB metrics. The overall distribution deviates considerably from the reciprocal diagonal representing a bipolar model. Individuals in the Reciprocal Parasympathetic quadrant would have relatively high CAB scores, whereas those in the Reciprocal Sympathetic quadrant would have relatively low CAB scores. An additional dimension is reflected along the coactivity diagonal. Individuals in the Coactivation quadrant would have relatively high CAR scores, whereas those in the Coinhibition quadrant would have relatively low CAR scores.

Figure 3

CAR and CAB in disease states. Data points illustrate means and standard errors of CAR and CAB as a function of participant group. Compared to other participants, subjects with a prior myocardial infarction (MI) had lower CAR scores, indicating lower overall cardiac regulatory capacity, but were not highly deviate on CAB. In contrast, those with diabetes showed a lower CAB score, reflective of a predominant sympathetic balance, but were not highly deviant on CAB.