A.E. Aubert

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.

Nonlinear heart rate dynamics : circadian profile and influence of age and gender

Heart rate variability (HRV) is used as a marker of autonomic modulation of heart rate. Nonlinear HRV parameters providing information about the scaling behaviour or the complexity of the cardiac system were included. In addition, the chaotic behaviour was quantified by means of the recently developed numerical noise titration technique. 24h Holter recordings of a large healthy population (N=276, 141 males, 18-71 years of age) were available. The goal was to investigate the influence of gender, age and day-night variation on these nonlinear HRV parameters. Numerical titration yielded similar information as other nonlinear HRV parameters do. However, it does not require long and cleaned data and therefore applicable on short (5min) noisy time series. A higher nonlinear behaviour was observed during the night (NLdr; day: 50.8±19.6%, night: 59.1±19.5%; P<0.001) while nonlinear heart rate fluctuations decline with increasing age (NLdr; Pearson correlation coefficient r between -0.260 and -0.319 dependent on gender and day or night, all P<0.01). A clear circadian profile could be found for almost every parameter, showing in particular which changes occur during the transition phases of waking up and going to sleep. Our results support the involvement of the autonomic nervous system in the generation of nonlinear and complex heart rate dynamics.

Effect of long term physical training on heart rate variability

The purpose of this study was to investigate the influence of physical training on the autonomic nervous system in healthy subjects. The authors compared 28 trained athletes and 28 sedentary subjects and divided them into two age groups: 18 to 34 and 35 to 55 years of age. ECG was recorded during 2 minutes while the subjects were breathing periodically under metronome control. RR tachograms were obtained and heart rate variability (HRV) was calculated. In the younger subgroup (18-34 years of age) only the standard deviation of mean RR interval and the frequency content were significantly different. In the older subgroup (35-55 years of age), all measured parameters were significantly different. A clear peak around 0.083 Hz, in coherence with respiratory rate, was observed in athletes and was present to a much smaller amplitude in sedentary subjects. These results show that regular physical training enhances the synchronization between heart rate and respiratory frequency, with a more pronounced effect at a higher age. The fact that physical training has a distinct impact on HRV in healthy subjects, infers that exercising may be of value in the modification of cardiac autonomic activity in cardiac patients.

Laser method for recording displacement of the heart and chest wall

Several non-invasive methods are in use for recording mechanocardiograms. In this paper a new laser technique will be presented to measure heart motion, chest wall displacement and other displacement curves of cardiovascular structures. Principles of the laser displacement technique are described. The measurement range within which displacement is sensed, is 32 mm with a detector to object distance of 25 cm and a resolution of 8 micron (digital output) or 16 micron (analogue output). The specific surface of which motion is sensed is 1 mm2. The sensitivity of the system is 156 mV/mm at a frequency bandwidth of 0-2 kHz. Assessment of the laser displacement technique was carried out during 6 dog experiments on the closed chest, on the exposed heart, on blood vessels and also on the chest wall of 5 normal subjects. Displacement of the chest wall at the apical site ranges between 0.3-0.8 mm and of the exposed heart between 3-10 mm.

Heart rate variability during gravity transitions

During parabolic flight short periods of microgravity and hypergravity are created. During standing position time domain analysis showed higher vagal modulation of the autonomic nervous system in microgravity compared to hypergravity. We hypothesised that this behaviour could better be unravelled by analysis of frequency domain heart rate variability (HRV) techniques. During parabolic flights a subject is exposed to 20-25 sec periods of microgravity (at the top of the parabola), preceded and followed by 20 sec duration episodes of hypergravity. No significant differences were found in the frequency parameters in supine position for each of the phases. In standing position higher values for Total Power, high frequency (HF), low frequency (LF), LF/HF and HF% in 0 g phase were found in comparison with the 3 other phases. LF% was significant lower in standing position at 0g compared with 1.8g. These results show that despite the time window limitations, frequency analysis is still possible on ultra short data segments.

Recognition of ventricular fibrillation tachycardia from electrogram analysis

Recognition of ventricular fibrillation (VF) and tachycardia (VT) and differentiating life-threatening situations from physiological high rhythms remains a difficult problem. In order to improve detection ability, electrogram analysis was performed using three different methods: the autocorrelation function (ACF), the probability-density function (PDF) and the power-spectrum (fast Fourier transform). During the implantation of an automatic defibrillator in 11 patients, electrograms were obtained from a bipolar electrode in the right ventricle, from an intracardiac spring-epicardial patch, and the ECG. Aortic pressure (AP) was also recorded. VT and VF were induced using AC current or programmed stimulation. Each sinus rhythm (SR) and VF episode could be properly identified using either analysis method. The frequency spectra during VF (peak in the band 4-6 Hz) were significantly different from SR (peaks at heart rate and harmonics). Some VT were missed using ACF and/or PDF. Including AP in the detection algorithm allowed a 100% recognition. The results indicate that all electrogram analysis methods perform well for reliable discrimination between SR and VF but are not very successful in differentiating physiologic VT from life-threatening rhythms.<<ETX>>

Automated contour detection of the left ventricle in short axis view and long axis view on 2D echocardiograms

An algorithm is presented to automatically delineate the endocardium in echocardiograms. The algorithm uses an optimization technique to overcome the poor image quality of existing algorithms and to use dynamic information. The information used in the process consists of grey level gradient, texture gradient, smoothness of the contour, and deviation of the present contour with respect to one of the previous video frames. The optimization algorithm locates the best compromise between image information and continuity in space and time. The automatic delineation is used for wall motion analysis and calculation of volumes and volume changes of the left ventricle (LV). An assessment of the accuracy of the volume calculations and indirectly the delineation was made by comparing the actual and calculated volume of the LV of excised dog hearts filled with a known amount of water. It is demonstrated that the dynamic programming technique is promising in detecting the myocardial borders.<<ETX>>

Comparison of angiographic methods for the assessment of the extent of experimental anterior myocardial infarction in dog hearts

Infarction was provoked in the dog by introducing a copper coil into the left anterior descending coronary artery. Eight groups, each of 6 dogs, were studied which received various treatments and were evaluated after 24 hours or 1 week. The anatomical extent of infarction was measured by the triphenyltetrazoliumhydrochloride method; size was evaluated angiographically by the centerline, chord and radial method and by ejection fraction. The extent of infarction was similar in groups studied after 24 hours and 1 week. Angiographical changes were most marked after 24 hours, and the ejection fraction was related to the extent of infarction (r = -0.5; P less than 0.02). Some parameters of the radial and centerline method showed also a relation to the extent of infarction (r = 0.47 to 0.57; P less than 0.05). After one week, the ejection fraction was no longer correlated to the extent of infarction. The correlation between the parameters of the chord, radius and centerline method and the extent of infarction improved to r-values of 0.58 to 0.63 (P less than or equal to 0.01). There was no difference between the 3 methods.

Application of numerical noise titration during autonomic blockade

The influence of pharmacological autonomic nervous system interventions on nonlinear cardiovascular regulatory indices is nowadays still not clear. The present study assesses the effects of autonomic blockade (alpha- and beta-adrenoceptor and cholinergic) on cardiovascular function by heart rate variability (HRV) and blood pressure variability (BPV) in rats using the nonlinear numerical noise titration technique. The results show that the alpha-adrenoceptor pathways seem to have an important contribution in the generation and control of nonlinear HR and especially BP fluctuations, while beta-adrenoceptor and cholinergic interventions have a negligible contribution to the nonlinear dynamical control.

Non-linear heart rate control in orthostatic tolerant cosmonauts after short-duration spaceflight

Spaceflight causes changes in the cardiovascular system. These changes contribute to the occurrence of orthostatic intolerance after spaceflight. The study of heart rate variations (HRV) provides a non-invasive means to study the autonomic modulation of heart rate. Non-linear control mechanisms have been suggested to be involved in normal heart rate variations. Non-linear control would be helpful in maintaining healthy cardiovascular values. However, these methods have not yet been used in combination with data after spaceflight. In this study we examined the non-linear response of heart rate to standing before and after spaceflight. Methods: ECG was measured for at least 10 minutes in supine and standing position 30 days before launch; and at 1, 4, 9, and 25 days after return to earth. These measurements were performed in 5 cosmonauts who were in the ISS for 10 days. The Approximate entropy of HRV was calculated. Results: The non-linear stand response was slightly altered immediately after spaceflight but was restored after 25 days. Conclusion: Nonlinear control of heart rate was affected immediately after short duration spaceflight, but not to the extent that the astronauts experienced orthostatic intolerance.