Background: Anxiety symptoms are associated with a marked increase in sudden cardiac death, suggesting an abnormality in cardiac autonomic function. Our previous studies show a relationship between R amplitude variability and sympathetic function.Methods:We examined the effects of β-adrenergic stimulation on R and T amplitude variability in panic disorder patients by infusing the β-adrenergic agonist isoproterenol in 6 panic disorder patients and 11 normal subjects. The ECG signal was analyzed before the infusion and 5 min after the infusion was started. The outcome measures were the R and T detrended variance normalized for mean amplitudes (Rvm and Tvm) and the Rvi and Tvi, measures which are normalized for the inter-beat interval variability in addition. Results: Patients with panic disorder had significantly more variability in R and T amplitude than normal controls and the R amplitude variability was increased further by β-adrenergic stimulation with isoproterenol, which was more pronounced in the patients. Conclusions: The isoproterenol-associated increase in R amplitude variability occurred in controls in the absence of significant anxiety. However, the increase in R amplitude variability was greater in patients with panic disorder, suggesting a greater sensitivity to β-adrenergic effects of isoproterenol or to isoproterenol-induced anxiety.
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