ObjeCtiveS:To compare autonomic modulation in young patients with type 1 diabetes mellitus (DM1) (n=20) and healthy control subjects (n=23) using geometric indexes of heart rate variability (HRV) associated with linear domains of time and frequency; and to verify correlations between geometric indexes and those obtained by the time and the frequency domain of diabetic individuals. MethOdS: Heart rate was recorded as beat-to-beat with subjects supine for 30 min. The following indexes were calculated: RRtri, TINN, Poincaré plot, SD1, SD2, SD1/SD2, RMSSD, SDNN, LF and HF (ms²/un) (DM1) (1) because it affects the fibres of the autonomic nerves of the heart and blood vessels, causing heart rate abnormalities and vascular dynamic impairments (2,3), leading to increased mortality (4,5).Changes in the autonomic modulation of DM1 were previously described through heart rate variability (HRV) (6-9), a noninvasive assessment method that measures the oscillation of intervals between consecutive heart beats (RR), which are related to influences from the autonomic nervous system (ANS) on the sinus node (10,11).Most studies that assessed autonomic modulation by HRV included populations with type 2 diabetes mellitus (9,12-15), a population that normally have associated comorbidities that could interfere in autonomic modulation (12). Also, these studies used linear methods of analysis of frequency and time domains (14,15), which present limitations as stationary requirements, and show more sensitivity to interference and ectopic beats when compared with nonlinear methods (16). Nonlinear methods are based on chaos theory and may enable better discrimination between normal and abnormal ANS physiology (6). For example, the Poincaré plot is a method used for geometric and nonlinear analysis of the autonomic modulation (16-18). A previous study involving type 2 diabetic individuals assessed different methods of HRV and reported that geometric methods are more reliable for evaluating the presence of an autonomic diabetic disease (19), and studies that analyze this type of indexes in DM1 population were not deeply explored.Therefore, geometric methods could identify autonomic changes and help to plan a better treatment strategy in DM1 patients. Thus, the aim of the present study was: to compare HRV indexes obtained by geometric methods (RRtri, TINN and Poincaré plot -SD1, SD2 and SD1/SD2), associated to indexes of time domain (RMSSD and SDNN) and frequency domain (LF, HF (ms²/un) and LF/HF), of individuals with DM1 and healthy control subjects; and to verify correlations between geometric indexes and those obtained by the time and the frequency domain of diabetic individuals. MethOdS SubjectsThe present study included 43 young adults who were divided into two groups: the DM1 group (n=20 [nine men, 11 women]) with a mean (± SD) age 22.05±4.35 years, with 10.60±4.81 years of DM1 diagnosis, and a control group with 23 healthy subjects (15 men, eight women) mean age 21.69±2.42 years.The inclusion criteria were absence of cardiorespirato...