Effect of coronary arterial occlusion on vagal control of heart rate

Heart rate variability in ischemic heart disease

Major untoward events, such as life-threatening arrhythmias and acute coronary events, have been suggested to be triggered by the activation of the autonomic nervous system in patients with coronary artery disease. Analysis of heart rate variability by conventional time and frequency domain methods, as well as by newer methods derived from nonlinear system theory, has offered a novel approach for studying the abnormalities in cardiovascular neural regulation in ischemic heart disease. Heart rate variability has been shown to be altered among patients with ischemic heart disease as compared to their age-matched controls without the evidence of ischemic heart disease. There are also obvious differences in various measures of heart rate variability between patients with uncomplicated coronary artery disease and those with coronary artery disease with complicated myocardial infarction. Impaired high-frequency oscillations of heart rate is the most prominent feature in patients with uncomplicated coronary artery disease, suggesting mainly an impairment in vagal autonomic regulation. Patients with prior myocardial infarction have a reduced overall heart rate variability, and a specific spectral pattern with a reduced low-frequency spectral component has been observed in patients with prior myocardial infarction and impaired left ventricular function. Recent studies have shown that the new nonlinear measures, particularly fractal analysis methods of heart rate dynamics, can detect subtle changes in heart rate behavior that are not easily detected by traditional analysis methods from ambulatory recordings. Patients with prior myocardial infarction have steeper power-law slope analyzed from the ultra and very low-frequency spectral bands, and they also have more random short-term heart rate dynamics analyzed by the detrended fluctuation method. A large body of data indicate that reduced overall heart rate variability is associated with an increased risk of mortality and nonfatal cardiac events in patient with ischemic heart disease. Of particular note, recent studies indicate that fractal analysis methods perform even better than the traditional analysis methods of heart rate variability as predictors of death and the onset of life-threatening arrhythmic events in post-infarction populations. These findings support the notion that heart rate variability analysis methods, such as fractal and complexity measures as well as conventional techniques, give valuable clinical information among patients with ischemic heart disease.

Left-sided stellate ganglion ablation or "rate-controlled" vagal nerve stimulation decreases regional myocardial metabolic impairment during acute ischemia in dogs

This study was designed to see whether during ischemia a metabolic advantage results with left-sided ablation of the stellate ganglion (SGA), an available clinical technique. Its effects on hemodynamics and regional metabolism during myocardial ischemia were compared with those of electrical stimulation of the left vagus nerve (VS), a nonclinical technique, and those of a control condition (ischemia without intervention). The left anterior descending coronary artery (LADa) of 30 dogs was constricted to reduce blood flow by 50% and then 75% from that before constriction and after autonomic intervention (baseline). Electrocardiogram, left-ventricular (LV) first-time derivative (dP/dt), and systemic, LADa, and LV end-diastolic pressures were continuously recorded. Before and during each constriction, cardiac output and regional myocardial blood flow (by microspheres), blood gas tensions, pH, hemoglobin O2 saturation, lactate, glucose, sodium, and potassium concentrations were measured. During ischemia, SGA and VS each decreased heart rate, myocardial contractility (dP/dt), and filling pressures, the decrease in each variable being greater with VS. Also during ischemia, myocardial O2 delivery and consumption decreased to the same extent in the ischemic zone with VS, but the O2 delivery/consumption ratio was higher only with SGA. In addition, ischemic lactate production was lower with SGA and VS than with no autonomic intervention. It is concluded that left-sided SGA or VS to a heart rate of 80 to 90 beats per minute similarly mitigated metabolic impairment during myocardial ischemia. Although the study was only designed to compare modification of ischemia by two different techniques, the results suggest that ischemic zone O2 delivery/consumption ratio and hemodynamic stability were better with SGA.

Frequency domain measures of heart rate variability before the onset of nonsustained and sustained ventricular tachycardia in patients with coronary artery disease

BACKGROUND

Low heart rate variability (HRV) is associated with an increased risk of arrhythmic death and ventricular tachycardia (VT). The purpose of this study was to examine whether there is a temporal relation between changes in HRV and the onset of spontaneous episodes of VT in patients at high risk of life-threatening arrhythmias.

METHODS AND RESULTS

Components of HRV in the frequency domain were analyzed before the onset of 28 episodes of nonsustained VT (more than four impulses; duration < 30 seconds) and 12 episodes of sustained VT (> 30 seconds or requiring defibrillation) in 18 patients with coronary artery disease. Seven patients had survived cardiac arrest not associated with acute myocardial infarction, and 11 had a history of sustained VT. All frequency domain measures of HRV, i.e., total power (p < 0.001), high-frequency power (p < 0.05), low-frequency power (p < 0.01), very-low-frequency power (p < 0.01), and ultralow-frequency power (p < 0.05), were significantly lower before the onset of sustained VT than before nonsustained VT. Total power of HRV was also lower during the 1-hour period before the onset of sustained VT than the average 24-hour HRV (p < 0.05). An indirect correlation existed between the length of VT and the total power of HRV analyzed during the 15 minutes before the onset of VT (r = 0.54, p < 0.01). HRV had a trend toward increasing values before the onset of nonsustained VT (p < 0.01) but not before the sustained VT episodes. The ratio between low-frequency and high-frequency powers increased substantially before both nonsustained and sustained VT episodes (p = 0.06 and p = 0.05, respectively). The rate of VT or the coupling interval initiating the VT did not differ significantly between the nonsustained and sustained VT.

CONCLUSIONS

Spontaneous episodes of VT are preceded by changes in HRV in the frequency domain. Divergent dynamics of HRV before the onset of nonsustained and sustained VT episodes may reflect differences in factors that can facilitate the perpetuation of these arrhythmias.