Abstract—The function of heart rate variability (HRV) has been studied over the years, but less is known about the factors predicting recovery from work stress during sleep. The availability to register reliable data for short-time HRV has raised the interest to find the congestive HRV signal. The objective of this study was to identify individual-level factors related to work and leisure-time predicting the recovery of autonomic nervous system (ANS) during congestion measured by 24-h HRV. 15 subjects (11 men, aged 22 to 71, and 4 women, aged 54 to 63) with severe congestive heart failure (NYHA class 3-4). This group of subjects was part of a larger study group receiving conventional medical therapy prior to receiving the oral inotropic agent, milrinone. Questionnaire data was gathered with a self-administered questionnaire of individual characteristics, perceived work ability, stress and psychological resources. The individual recordings are each about 20 hours in duration, and contain two ECG signals each sampled at 250 samples per second with 12-bit resolution over a range of ±10 millivolts. In this study, we only calculate first 15 minus of each subject. Index Terms—Pointcaré plot, heart rate variability, chaos, congestive heart failure. I. INTRODUCTION Heart failure (HF), often called congestive heart failure (CHF) or congestive cardiac failure (CCF), occurs when the heart is unable to provide sufficient pump action to maintain blood flow to meet the needs of the body [1]-[3]. Heart failure can cause a number of symptoms including shortness of breath, leg swelling, and exercise intolerance. The condition is diagnosed by patient physical examination and confirmed with echocardiography. Blood tests help to the etiology diagnosis. Treatment depends from severity and etiology of heart failure. In a chronic patient already in a stable situation, treatment commonly consists of lifestyle measures such as smoking cessation, light exercise, dietary changes, and medications. Sometimes, depending from etiology, it is treated with implanted devices (pacemakers or ventricular assist devices) and occasionally a heart transplant is required. Common causes of heart failure include myocardial infarction and other forms of ischemic heart disease, hypertension, valvular heart disease, and cardiomyopathy [4]. The term heart failure is sometimes incorrectly used for other cardiac-related illnesses, such as myocardial infarction (heart attack) or cardiac arrest, which can cause heart failure but are not equivalent to heart failure. Heart failure is a common, costly, disabling, and potentially deadly condition. [4] In developed countries, around 2% of adults suffer from heart failure, but in those over the age of 65, this increases to 6–10% [4], [5]. It is widely recognized that exercise training induces acute and chronic adaptations in heart rate (HR), but the exact mechanisms that mediate these changes are not clear [1]-[4]. It is hypothesized that training can affect autonomic regulation causing reduction in the sympathetic nerve activity and increase in the parasympathetic outflow [5], [6]. Previous studies have shown that the autonomic modulation of HR can be studied by non-invasive methods utilizing heart rate variability (HRV) [7]–[12]. The HRV is associated with sympathovagal balance and it can be a practical and accurate method to assess the effects of acute exercise and training on the autonomic modulation of HR [6]-[13]. It is derived from analysis of consecutive beat-to-beat oscilations of sinus rhythm in time or frequency domains, which are mainly mediated by the autonomic nervous system branches’ activities. However, other neural, humoral, and metabolic factors might also induce changes on HR and on HRV parameters. Another study investigated the effects of aerobic training on HRV response during a progressive cycle ergometer test. The training involved cycling during 30 min at 50% of the difference between peak work rate during the progressive test and HRV threshold. The sessions were performed three times per week throughout three weeks. The results showed that moderate-intensity training caused an increase in work rate at HRV threshold while no significant changes were observed in the control group. However, during progressive exercise test, the effects of high-intensity interval training on HRV response have not been established yet. Therefore, the purpose of this study was to investigate the effects of high-intensity interval training on HRV threshold and on HRV-work rate curve during progressive exercise. We have hypothesized that significant changes would occur in the autonomic cardiac control in response to this form of training and consequently, HRV-work rate curve during progressive exercise would be shifted to the upward and to the right directions, with concomitant reduction in heart rate in submaximal stages. II. METHODS A. Data Acquisitions This database includes long-term ECG recordings from 15 Detecting Congestive Heart Rate Variability Failure Using PointcaréTrend Analysis Plot International Journal of Bioscience, Biochemistry and Bioinformatics, Vol. 3, No. 6, November 2013 566 DOI: 10.7763/IJBBB.2013.V3.277 Hoang Chu Duc, Hung Pham Manh, Thuan Nguyen Duc, and Dung Nguyen Viet Manuscript received April 9, 2013; revised July 11, 2013. Hoang Chu Duc, Thuan Nguyen Duc, and Hung Pham Manh are with the Department of Electronics Technology and Biomedical Engineering, School of Electronics and Telecommunications, Hanoi University of Scicence and Technology, Hanoi, Vietnam (e-mail: hoang.chuduc@hust.edu.vn, thuan.nguyenduc @hust.edu.vn) . Dung Nguyen Viet is with Center of Electronics and Biomedical, Hanoi University of Scicence and Technology, Hanoi, Vietnam (e-mail: trung.laihuuphuong@hust.edu.vn).