Jiexin Liu

Steep fall in cardiac output is main determinant of hypotension during drug-free and nitroglycerine-induced orthostatic vasovagal syncope

BACKGROUND How much of the hypotension occurring during postural syncope is cardiac output-mediated and how much can be ascribed to a fall in systemic vascular resistance are unknown. The contribution of both determinants may be influenced by the use of vasoactive drugs. OBJECTIVE The purpose of this study was to assess the determinants of hypotension during drug-free and nitroglycerine (NTG)-induced vasovagal presyncope in routine tilt table testing. METHODS In this retrospective study, a total of 56 patients (37 female; age 36 +/- 19 years) with suspected vasovagal syncope and a positive tilt test at two clinical centers were selected. In 29 patients, presyncope was provoked by 0.4 mg sublingual NTG, administered in the 60 degrees head-up tilt position. In the other 27 patients, presyncope was provoked by passive tilt alone. Finger arterial pressure was monitored continuously, and left ventricular stroke volume was computed from pressure pulsations. RESULTS After NTG administration, heart rate rose, and peak heart rate was similar in all patients. Use of NTG did not affect circulatory patterns precipitating a vasovagal response. On average in all patients, marked hypotension was mediated by an approximately 50% fall in cardiac output, whereas systemic vascular resistance was well maintained until presyncope. CONCLUSION Hypotension during routine tilt testing is cardiac output-mediated, and the mechanism appears independent of the use of 0.4 mg sublingual NTG. The study data challenge the conventional idea of systemic vasodilation as the overriding cause of hypotension during postural syncope.

Adaptation of heart rate and blood pressure to short and long duration space missions

To what extent does going to space affect cardiovascular function? Although many studies have addressed this question, the answer remains controversial. Even for such primary parameters as heart rate (HR) and blood pressure (BP) contradictory results have been presented. The purpose of this investigation was to evaluate HR and arterial BP in 11 male astronauts who each took part in nine different space missions aboard the International Space Station (ISS), for up to 6 months. Pre-flight HR and BP readings were obtained in both the standing and supine positions on Earth and were taken as reference values. Our results show that HR and arterial BP in space equal pre-flight supine values. In all subjects, HR and mean arterial BP (MAP) were lower in space compared with pre-flight standing (both p<0.05). HR in space was well maintained at pre-flight supine level for up to 6 months in all astronauts while MAP tended to adapt to a level in between the ground-based standing and supine positions. Also pulse pressure (PP) decreased over the course of long duration spaceflight. In conclusion, our data indicate that weightlessness relaxes the circulation in humans for an extended duration of up to 6 months in space.

Respiratory modulation of cardiovascular rhythms before and after short-duration human spaceflight.

Aim:  Astronauts commonly return from space with altered short‐term cardiovascular dynamics and blunted baroreflex sensitivity. Although many studies have addressed this issue, post‐flight effects on the dynamic circulatory control remain incompletely understood. It is not clear how long the cardiovascular system needs to recover from spaceflight as most post‐flight investigations only extended between a few days and 2 weeks.

Complexity of cardiovascular regulation in small animals

Oscillations of heart rate and blood pressure are related to the activity of the underlying control mechanism. They have been investigated mostly with linear methods in the time and frequency domains. Also, in recent years, many different nonlinear analysis methods have been applied for the evaluation of cardiovascular variability. This review presents the most commonly used nonlinear methods. Physiological understanding is obtained from various results from small animals.

Is Autonomic Modulation Different between European and Chinese Astronauts

Purpose The objective was to investigate autonomic control in groups of European and Chinese astronauts and to identify similarities and differences. Methods Beat-to-beat heart rate and finger blood pressure, brachial blood pressure, and respiratory frequency were measured from 10 astronauts (five European taking part in three different space missions and five Chinese astronauts taking part in two different space missions). Data recording was performed in the supine and standing positions at least 10 days before launch, and 1, 3, and 10 days after return. Cross-correlation analysis of heart rate and systolic pressure was used to assess cardiac baroreflex modulation. A fixed breathing protocol was performed to measure respiratory sinus arrhythmia and low-frequency power of systolic blood pressure variability. Results Although baseline cardiovascular parameters before spaceflight were similar in all astronauts in the supine position, a significant increase in sympathetic activity and a decrease in vagal modulation occurred in the European astronauts when standing; spaceflight resulted in a remarkable vagal decrease in European astronauts only. Similar baseline supine and standing values for heart rate, mean arterial pressure, and respiratory frequency were shown in both groups. Standing autonomic control was based on a balance of higher vagal and sympathetic modulation in European astronauts. Conclusion Post-spaceflight orthostatic tachycardia was observed in all European astronauts, whereas post-spaceflight orthostatic tachycardia was significantly reduced in Chinese astronauts. The basis for orthostatic intolerance is not apparent; however, many possibilities can be considered and need to be further investigated, such as genetic diversities between races, astronaut selection, training, and nutrition, etc.

Orthostatic Intolerance Is Independent of the Degree of Autonomic Cardiovascular Adaptation after 60 Days of Head-Down Bed Rest.

Spaceflight and head-down bed rest (HDBR) can induce the orthostatic intolerance (OI); the mechanisms remain to be clarified. The aim of this study was to determine whether or not OI after HDBR relates to the degree of autonomic cardiovascular adaptation. Fourteen volunteers were enrolled for 60 days of HDBR. A head-up tilt test (HUTT) was performed before and after HDBR. Our data revealed that, in all nonfainters, there was a progressive increase in heart rate over the course of HDBR, which remained higher until 12 days of recovery. The mean arterial pressure gradually increased until day 56 of HDBR and returned to baseline after 12 days of recovery. Respiratory sinus arrhythmia and baroreflex sensitivity decreased during HDBR and remained suppressed until 12 days of recovery. Low-frequency power of systolic arterial pressure increased during HDBR and remained elevated during recovery. Three subjects fainted during the HUTT after HDBR, in which systemic vascular resistance did not increase and remained lower until syncope. None of the circulatory patterns significantly differed between the fainters and the nonfainters at any time point. In conclusion, our data indicate that the impaired orthostatic tolerance after HDBR could not be distinguished by estimation of normal hemodynamic and/or neurocardiac data.

Cardiovascular Function and Gravity Transitions during Parabolic Flight

Cardiovascular function and gravity transitions during parabolic flight.