ABC | Volume 110, Nº1, January 2018

Original Article Liporaci et al Effects of tilt test in healthy subjects Arq Bras Cardiol. 2018; 110(1):74-83 attached to the tilt table, which acts as a support for the whole back of the body. Therefore, during the HUT, what we find is a greater EMG activity for the ESs than during the AS, possibly due to what was discussed above. In light of the role of muscles in postural control and venous return, we found it reasonable to analyze the COP sway bymeans of a force platform in attempt to understand when cardiovascular changes and themuscle action in response to these hemodynamic changes would affect oscillations of the body. It is worth pointing out the statistical relevance of TD and TMV at around the second VM, in which oscillation had higher displacement and velocity values as compared with previous time points. By dividing the total time the patient stayed on the force platform during the AS into three parts – pre-VM, VM and post‑VM– considering VM as the period in which the three VMs were performed, we found that TD of COP on the platform and TMV were significantly higher in the VM as compared with the pre-VM. In addition, TMV significantly decreased in the post-VM period. Gatev et al. 23 reported that activity of the lateral gastrocnemius muscle was positively related to the COP displacement. Also, the authors found that this oscillation, especially the anteroposterior motions of the gastrocnemius is in accordance with the “climbing hill” theory for balance maintenance, that states that muscle contracts when tensioned and decreases its activity when it loses its tension. 24 Thus, in case of the ankle, the activity of anterior and posterior muscles increases as the COP displacement increases over time. In our context, MG showed a concomitant increase in EMG activity with the increase in TD and mean velocity of TD. This also occurred for the TA, which suggests that, although we did not analyze the displacement direction, both MG and TA may follow the same trend as reported by Gatev et al. 23 The fact that cardiovascular changes were more relevant during the period when the VMs were performed suggests that the VM exerts not only a hemodynamic stress, but its effects also affect body motion, which can result in increased muscle activity to maintain orthostatic and hemodynamic balance. This, in individuals with syncope, who may have impaired venous return by the muscle pump system, this oscillation may be even greater until presyncope symptoms or even syncope per se occurs. Claydon&Hainsworth 25 observed that cardiovascular changes affect orthostatic tolerance that alters the movement of lower limbs for compensation. The authors reported that patients with postural syncope have impairedmuscle response to compensate for their smaller reflex responses, which may contribute to the episodes of fainting. These findings are in agreement with our concept of the role of muscles on the COP displacement. Conclusions Results of the present study obtained under the experimental conditions corroborate with our initial hypotheses, as we showed that, during the active postural maneuver, postural oscillation and the electrical activity of muscles associated with postural maintenance revealed a progressive change in the response pattern of biomechanical variables and cardiac variables, augmented by repeated VMs. For the passive postural maneuver, muscle activity was qualitatively and quantitatively different. Study limitations The study has some limitations that should be considered. The number of participants may have been a limiting factor for the magnitude of the changes reported. This may be caused by the relatively complex design of the study, in which each volunteer underwent two test sessions with approximately 2-hour duration on different days. In addition, we selected patients with no history of syncope, which makes the inclusion of a larger number of patients to the study protocol difficult, since a considerable part of the population has experienced syncope. However, aiming to achieve homogenized responses to the tests and higher consistency of the results, we decided to select only volunteers with no history of syncope. Changes in systemic arterial pressure were monitored by manual sphygmomanometer. Continuous measurement of blood pressure using the Finapres monitor (Ohmeda, Denver, Colorado) would be interesting, since this instrument allows that both blood pressure and heart rate be measured continuously. Nevertheless, for technical reasons, our Finapres device could not be used during the pilot data collection and, since we used less accurate devices, blood pressure data were not included in this study. Clinical implications and future studies The present study seeks to consolidate the proposal of NCS diagnostic tests that would require a shorter period of patient exposure, thereby increasing the number of patients examined per session, and to analyze a test that does not require a tilt table, which is not available in all cardiology clinics. Studies comparing active and passive protocols in NCS subjects are the next step to clarify whether the changes observed in healthy individuals in the present study cause syncope. This is essential before we can present the protocols analyzed in this study as an alternative for the study of NCS by the professional community of interest. Author contributions Conception and design of the research: Liporaci RF, SaadMC, Credência JC, Marques F, Bevilaqua-Grossi D, Gallo-Júnior L; Acquisition of data: Liporaci RF, SaadMC, Credência JC; Analysis and interpretation of the data: Liporaci RF, Saad MC; Writing of themanuscript: Liporaci RF; Critical revision of themanuscript for intellectual content: Liporaci RF, Marques F, Bevilaqua-Grossi D, Gallo-Júnior L. Potential Conflict of Interest No potential conflict of interest relevant to this article was reported. Sources of Funding This study was funded by CNPq. Study Association This article is part of the thesis of master submitted by Rogerio Ferreira Liporaci, from Universidade de São Paulo. 81