Increased Nonlinearity and Decreased Chaoticity of Heart Rate Time Series In Patients With Panic Disorder

This study investigated measures of nonlinearity (NL) and a measure of predictability and chaoticity of heart rate time series by quantifying the Largest Lyapunov Exponent (LLE) in 23 normal control subjects (31.3 6 7.6 years) and 32 patients with a diagnosis of panic disorder (31.6 6 5.1\ud years). The mean supine and standing heart rates were significantly higher in patients with panic disorder (Controls: 61.7 6 8.2 and 71.3 6 8.7; Patients: 71.3 6 8.7 and 85.9 6 10.0 beats per minute; p , 0.05). NL score was significantly higher in patients with panic disorder, especially in supine posture (6.1 6 2.9 vs 10.6 6 2.7; t 5 5.94; p 5 0.00001), possibly due to an increased sympathetic activity. Embedding dimension was significantly higher in patients in supine posture compared to controls (12.1 6 1.8 vs 14.2 6 1.8; t 5 4.4; p 5 0.0001) possibly due to an increase in the number of parameters that were affecting the system. LLE was significantly lower in patients in supine (0.138 6 0.024 vs 0.110 6 0.017; t 5 5.13; p 5 0.00001) as well as in standing (0.183 6 0.036 vs 0.153 6 0.014; t 5 4.13; p 5 0.0001) postures. Decreased LLE signifies increased predictability and decreased chaoticity in patients, which may be due to a decrease in some aspects of cardiac vagal function. Sex did not have any effect on these measures. Analysis of covariance using heart rate as a covariate did not change the significance of the results.\u