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Nonetheless, in recent years, several studies have reported obtaining information similar to HRV from other signals that also contain information related to the cardiac cycle, such as pulse waves. HRV is measured using electrocardiographic signals (ECG), which represent the electrical activity generated by the heart conduction system 3, 7, and standards of measurement have been established to align the methodologies used in HRV studies, in order to allow for comparisons among results 8. As explained in 6, HRV has also been found to aid in the diagnosis and increase the prognostic value of predisposing conditions for critical illness, such as hypertension, and some HRV parameters have been found to be abnormal even in the early stages of hypertension. Several authors have found that HRV is associated with cardiovascular conditions, such as myocardial infarction and heart failure 4, coronary artery disease, and sudden death 5. Heart rate variability (HRV), which is defined as changes in heart rate over time 1, is an indicator that is used to evaluate the activity of the cardiac autonomic nervous system (ANS) because of its relationship with the parasympathetic and sympathetic activity directed into the sinus node in the heart, which controls the heart rate 2, 3. The findings indicated that there were differences between PRV and HRV, especially in short-term and nonlinear indices, and although PRV and HRV were altered in a similar manner when there was a change in BP, PRV seemed to be more sensitive to these changes. Bland–Altman analysis, correlation analysis, and Friedman rank sum tests were used to compare HRV and PRV in each state, and PRV and HRV indices were compared among BP states using Kruskal–Wallis tests. Several time-domain, frequency-domain, and nonlinear indices were obtained from these signals. Using the MIMIC III database, 5 min segments of PPG and ECG signals were used to extract PRV and HRV, respectively. This study aimed to assess the relationship between PRV and HRV under different BP states: hypotension, normotension, and hypertension. Several studies have found that PRV is not entirely valid as an estimation of HRV and that several physiological factors, including the pulse transit time (PTT) and blood pressure (BP) changes, may affect PRV differently than HRV. Pulse rate variability (PRV) utilizes photoplethysmography (PPG) and recently has been used as a surrogate for HRV. Heart rate variability (HRV) utilizes the electrocardiogram (ECG) and has been widely studied as a non-invasive indicator of cardiac autonomic activity.