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Biomedical subjects

A Vinet

Publications and source records attributed to A Vinet.

At least 37 records · Page 2Linked to original sources
PubMed2000

[Study of the reproducibility of cardiac output measurement during exercise in pre-pubertal children by doppler echocardiography and CO2 inhalation].

Non-invasive measurement of the cardiac output is essential in investigations of healthy children. However, the data concerning the reproducibility of the measurements are very limited. The aim of this study was to assess the reproducibility of the measurement of cardiac output during exercise by Doppler echocardiography and reinhalation of CO2 (extrapolation method). Fourteen pre-pubertal children underwent two similar tests at increasingly intense levels of exercise. The cardiac output was measured at rest and during the last minute of each stepwise increment of exercise. The results show no difference between the cardiac outputs of the two tests, whichever method was used and at all levels of exercise. They also demonstrate a better reproducibility of cardiac output measurement by Doppler echocardiography (coefficient of variation: 7.5% at rest and 5.2% at maximal effort) compared with reinhalation of CO2 (coefficient of variation: 16.8% at rest and 11.7% at maximal effort). Both methods showed better reproducibility on exercise, resulting from smaller variations in heart rate and stroke volume on effort than at rest. The authors conclude that Doppler echocardiography is very accurate and its simplicity makes it the method of choice in pre-pubertal children for measuring cardiac output during exercise.

Carbon Dioxide↗
PubMed1999

Reproducibility of the Adapted Leger and Boucher Test for wheelchair-dependent athletes.

STUDY DESIGN: This study analyzed the reproducibility of a field test. In a previous study, we showed that this test, the Adapted Leger and Boucher Test (ALBT), was progressive and maximal. The Leger and Boucher predictive equation for able-bodied subjects was not accurate for WD athletes, however, and a new predictive equation is needed. OBJECTIVES: To determine the reproducibility of an adapted incremental field test for wheelchair-dependent (WD) athletes. SETTING: France at Montpellier. METHODS: The proposed protocol was conducted on a 400 m track. Eight male paraplegics (mean age: 30.8+/-5.1 years) performed the test three times in the same conditions, ie same time of day, same wheelchair, same material. Maximal heart rate (HRmax) and maximal speed (Smax) were measured. RESULTS: We found no significant differences (P>0.05) between tests for either variable. The Bland and Altman graphic analyses showed a good reproducibility for both variables. Lastly, the reproducibility coefficients of HRmax and Smax were very low (2% and 1%, respectively). CONCLUSION: The ALBT is reproducible concerning measurements of HRmax and Smax. A valid predictive equation of maximal oxygen uptake from the Smax is now needed for WD athletes during this field test.

Adult↗
PubMed1999

Curvature effects on activation speed and repolarization in an ionic model of cardiac myocytes.

Reentry is a major mechanism underlying the initiation and perpetuation of many cardiac arrhythmias. Stimulated ventricular myocytes give action potential characterized by a fast upstroke, a long-lasting plateau, and a late repolarization phase. The plateau phase determines the action potential duration (APD) during which the system remains refractory, a property essential to the synchronization of the heart cycle. The APD varies much with prematurity and this change has been shown to be the main determinant of the dynamics in models of paced cells and cable, and during reentry in the one-dimensional loop. Curvature has also been shown to be an important factor for propagation in experimental and theoretical cardiac extended tissue. The objective of this paper is to combine both curvature and prematurity effects in a kinematical model of propagation in cardiac tissue. First, an approximation of the ionic model is used to obtain the effects of curvature and prematurity on the speed of propagation, the APD, and the absolute refractory period. Two versions of the ionic model are studied that differ in their rate of excitability recovery. The functions are used in a kinematical model describing the propagation of period-1 solutions around an annulus.

Action Potentials↗
PubMed1997

Aerobic metabolism and cardioventilatory responses in paraplegic athletes during an incremental wheelchair exercise.

The aims of the present study were: (1) to assess aerobic metabolism in paraplegic (P) athletes (spinal lesion level, T4-L3) by means of peak oxygen uptake (VO2peak) and ventilatory threshold (VT), and (2) to determine the nature of exercise limitation in these athletes by means of cardioventilatory responses at peak exercise. Eight P athletes underwent conventional spirographic measurements and then performed an incremental wheelchair exercise on an adapted treadmill. Ventilatory data were collected every minute using an automated metabolic system: ventilation (l x min[-1]), oxygen uptake (VO2, l x min[-1], ml x min[-1] x kg[-1]), carbon dioxide production (VCO2, ml x min[-1]), respiratory exchange ratio, breathing frequency and tidal volume. Heart rate (HR, beats x min[-1]) was collected with the aid of a standard electrocardiogram. VO2peak was determined using conventional criteria. VT was determined by the breakpoint in the VCO2 - VO2 relationship, and is expressed as the absolute VT (VO2, ml x min[-1] x kg[-1]) and relative VT (percentage of VO2peak). Spirometric values and cardioventilatory responses at rest and at peak exercise allowed the measurement of ventilatory reserve (VR), heart rate reserve (HRr), heart rate response (HRR), and O2 pulse (O2 P). Results showed a VO2peak value of 40.6 (2.5) ml x min(-1) x kg(-1), an absolute VT detected at 23.1 (1.5) ml x min(-1) x kg(-1) VO2 and a relative VT at 56.4 (2.2)% VO2peak. HRr [15.8 (3.2) beats min(-1)], HRR [48.6 (4.3) beat x l(-1)], and O2 P [0.23 (0.02) ml x kg(-1) x beat(-1)] were normal, whereas VR at peak exercise [42.7 (2.4)%] was increased. As wheelchair exercise excluded the use of an able-bodied (AB) control group, we compared our VO2peak and VT results with those for other P subjects and AB controls reported in the literature, and we compared our cardioventilatory responses with those for respiratory and cardiac patients. The low VO2peak values obtained compared with subject values obtained during an arm-crank exercise may be due to a reduced active muscle mass. Absolute VT was somewhat comparable to that of AB subjects, mainly due to the similar muscle mass involved in wheelchair and arm-crank exercise by P and AB subjects, respectively. The increased VR, as reported in patients with chronic heart failure, suggested that P athletes exhibited cardiac limitation at peak exercise, and this contributed to the lower VO2peak measured in these subjects.

Adult↗
PubMed1997

Underdetection of ventricular tachycardia using a 40 ms stability criterion: effect of antiarrhythmic therapy.

Inappropriate shocks can complicate cardioverter defibrillator therapy. Among solutions proposed to avoid oversensing are algorithms to reduce inappropriate detection of atrial fibrillation (AF) or sinus tachycardia. In patients not on antiarrythmic drugs, an interval stability criterion of 40 ms has been validated with the Medtronic PCD to discriminate ventricular tachycardia (VT) from AF. With this algorithm, VT is considered stable if no interval varies from one of the three preceding intervals by more than 40 ms. If an interval does not fulfill this criterion, the VT event counter is reset to zero. The aim of this study was to investigate the incidence of underdetection when this criterion is applied in patients treated with antiarrhythmic drugs. We studied 132 sustained monomorphic VTs induced in 42 patients during 101 electrophysiological studies (EPS). EPS were performed without treatment (group I, 24 patients, 44 VTs); on Class Ia drug (group II, 17 patients, 24 VTs); Class Ic drug (group III, 22 patients, 39 VTs); or sotalol (group IV, 17 patients, 25 VTs). The endocardial electrogram of all VT episodes was digitized and the stability algorithm was applied. The reset arrhythmias were distributed among no delay, small, moderate (< 10 s) and important (> 15 s) delay in VT detection. The relation between drug use and reset was analyzed. Reset was found in 86 (65%) of induced VTs. No difference in heart rate or induction mode was shown between reset and nonreset VTs. There was a significative association between drug use and reset probability (Chi2 significantly different, P < 0.05). In patients treated with Class Ic drugs, the probability of finding an important delay in VT detection was 12.5% versus 0% in nontreated patients or in patients treated with sotalol. We conclude that a stability criterion of 40 ms is probably safe in nontreated patients but should be used with caution in patients treated with antiarrhythmics, especially in the presence of Class Ic drugs.

Adult↗
PubMed1997

Modulation of electrical heterogeneity by compensated hypertrophy in rat left ventricle.

Modulation of the regional distribution of the action potential by left ventricular hypertrophy and the role of the L-type Ca2+ current (I(Ca)) and transient outward current (I(to)) in the action potential duration (APD) were investigated in normal and hypertrophied rat ventricular myocytes from the apex (A), septum (S) and left ventricular free wall (FW) by using whole cell current- and voltage-clamp techniques. Hypertrophy was induced by abdominal aortic constriction. In control cells, the APD measured at 20% repolarization (APD20) assumed the shortest values in the A and the longest in the S, whereas FW cells showed intermediate values. Hypertrophy significantly prolonged the APD20 and increased APD variability within the A and FW regions but did not modify the APD in S cells. Analysis of the APD, I(Ca), and I(to) at the instant of 20% repolarization in the same cell showed that in control cells the shortest APD20 was associated with a prominent I(to) in the A and FW, whereas the long APD20 was identified with a lower I(to) in S myocytes. Hypertrophy-induced prolongation ofAPD20 was associated with a reduction in the I(to) in the A and FW. Significant correlations could be established between the APD20 and the "net current," defined as the algebraic addition of I(to) and I(Ca) in the A and FW control groups but not in the control S or hypertrophied cells whatever their origin. Our results indicate that interregional APD heterogeneity is lost while intraregional APD variability is increased in the A and FW during the hypertrophic process. These effects are largely due to a change in the balance between the I(Ca) and I(to), which is a major contributing factor to the heterogeneity of the initial phase of repolarization in the normal rat ventricle.

Action Potentials↗
PubMed1996

Cycle length dynamics and spatial stability at the onset of postinfarction monomorphic ventricular tachycardias induced in patients and canine preparations.

BACKGROUND: The aim of this study was to determine whether cycle length (CL) variations at the onset of monomorphic ventricular tachycardias follow distinctive patterns. METHODS AND RESULTS: We retrospectively analyzed 59 monomorphic ventricular tachycardias induced in 40 patients in whom intraoperative mapping was performed with 63 epicardial and 64 endocardial electrograms recorded simultaneously. Activation times and CL were determined at each electrode site over several beats (36+/-10 beats, mean+/-SD) starting with the first after programmed stimulation. In the majority of the tachycardias, CL variations were accounted for by fitting to an exponential function: CL=CLs+Ae-b/tau, where CLs is the stable CL, b is beat number, tau is the time constant (in beat number), and A is the magnitude of CL relaxation. A decelerating trend (with reference to rate) (negative A) accounted for 21 tachycardias, an accelerating trend in rate (positive A) accounted for 12 tachycardias, and 4 others displayed a double dynamic behavior, with an initial acceleration followed by a decelerating trend in rate. Among the ventricular tachycardias that were not fitted to exponential models, 12 showed a constant trend and 10 others showed irregular CL fluctuations. The monomorphic character of the tachycardias was established by principal-component analysis, which also indicated that CL dynamics associated with the accelerating and decelerating trends may be related to shortening and prolongation of activation times, respectively, occurring in equal proportion at all recording sites. In canine preparations in which reentry circuits could be mapped with high resolution, CL showed an accelerating trend in rate when circus movement of excitation occurred around a transmural scar in muscle generating unipolar electrograms with relatively high -dV/dtmax, and a decelerating trend in rate occurred when functional reentry occurred in muscle generating unipolar electrograms with depressed -dV/dtmax. CONCLUSIONS: Beat-to-beat CL variations may occur at the onset of sustained monomorphic ventricular tachycardia as a result of uniform acceleration or deceleration of activation times while the overall activation pattern remains constant. The associated initial trends in the rate of sustained monomorphic ventricular tachycardia follow typical patterns that might provide "signatures" corresponding to reentry substrates with distinctive functional properties.

Adult↗
PubMed1996

Validation of an incremental field test for the direct assessment of peak oxygen uptake in wheelchair-dependent athletes.

The aim of this study was to validate an incremental field test performed by wheelchair-dependent (WD) athletes. Nine male paraplegic subjects (mean age 28.9 +/- 4.2 years) performed an incremental field test (FT) and a comparable laboratory test (LT) with their own usual wheelchairs. Both tests started with an initial speed of 4 km.hr(-1) and increased by increments of 1 km.hr(-1) every minute until volitional exhaustion. The FT was an adapted Léger and Boucher test (ALBT) and was conducted on a 400 m tartan field marked-off every 50 m with pylons. Ventilatory data were collected every 15 s using a portable telemetric system (Cosmed K2, JFB International, Italy). The LT was performed on an adapted treadmill (Sopur, Germany) and ventilatory data were collected every minute using a breath-by-breath automated system (CPX, Medical Graphics, MN, USA). The LT and the FT were not significantly different for duration (8 min 50 +/- 1 min 24 vs 9 min 55 +/- 29 s), percentage of maximal heart rate (HR, 86.2 +/- 3.9 vs 89.7 +/- 5.3%), maximal minute ventilation (VE, 101.6 +/- 28.5 vs 96.8 +/- 28.2 1.min(-1)) and peak oxygen uptake (VO2 peak, 39.7 + 7.3 vs 36.1 + 5.8 ml.kg(-1).min(-1) assessed with the CPX and the K2, respectively. We concluded that the FT proposed in the present study is a valid test for direct VO2 peak assessment in wheelchair athletes using a portable VO2 telemetric system. Nonetheless, the Léger and Mercier model equation did not accurately predict VO2 max and further investigation is needed to determine a valid VO2 max prediction equation for these subjects during the FT.

Adult↗
PubMed1994

Excitability and repolarization in an ionic model of the cardiac cell membrane.

The main objective of this study was to investigate the possibility of expressing the activation and repolarization processes of a realistic ionic model of the myocyte membrane in terms of simplified dynamic equivalents. The modified Beeler-Reuter model (MBR) of the ventricular membrane was selected for this purpose because its action potential upstroke, plateau and selected for this purpose because its action potential upstroke, plateau and repolarization phase occur along sufficiently well separated timescales. The information on the MBR model dynamics was obtained by premature stimulation at various coupling intervals, under stable conditions of regular pacing at different cycle lengths. A general method was developed to study the threshold behavior of the system. As a first step, a pair of complementary threshold criteria was defined in terms of peak ionic current and time to repolarization in order to reliably distinguish between classes of sub-threshold and supra-threshold responses. Of the main conclusions is that the activation of the MBR model by short-duration stimuli (< 5 msec) can be accurately represented by a one-variable or a two-variable dynamic equivalent. In addition, because of the large surge of Na+ current at threshold, the recovery of excitability is essentially independent of the conditioning action potential waveform (no threshold-memory effect). Another major result pertains to the higher complexity of the repolarization process, stressing the critical role played by the activation dynamics of the secondary inward current. There is a substantial dependence of the action potential duration (APD) on the conditioning action potential waveform (APD-memory effect), and at least a three-variable model is necessary for a reasonable approximation.

Action Potentials↗
PubMed1994

Analysis of an iterative difference equation model of the cardiac cell membrane.

An iterative difference equation (DE) model based on a small set of functional characteristics derived from the modified Beeler-Reuter ionic model (MBR) of a space-clamped membrane of the ventricular myocyte was studied. These characteristics were expressed as functions of a common independent variable, the diastolic interval (DIA), to define threshold, Thr (DIA), latency, L (DIA), action potential duration, APD (DIA), and delayed excitability recovery, R(DIA), owing to a sub-threshold response. Memory effects, in the form of a dependence on the immediately proceeding action potential waveform, were not included in the current DE formulation. The entrainment bifurcation structure of the DE model was found to be very sensitive to the form of APD (DIA) but, with a suitable choice of parameter values, the model reproduced the successive stable rhythms of synchronization (1:1-->2:2-->2:1-->4:2) found in the MBR model with decreasing basic cycle lengths. The fine details of the transition between stable rhythms were seen to depend on the specific forms of APD (DIA) and R (DIA) chosen, and on the initial conditions at the onset of pacing. The minor deficiencies of the DE R (DIA) chosen, and on the initial conditions at the onset of pacing. The minor deficiencies of the DE model are attributable to the memory-free APD (DIA) used, and to uncertainties in the description of the MBR model behavior owing to the slow convergence toward a given stable rhythm. Overall, the iterative DE formulation based on the transient properties of an ionic model, as derived by premature stimulation, it is also adequate to describe the stable entrainment patterns of the ionic model.

Animals↗
PubMed1992

Certified sick leave as a non-specific morbidity indicator: a case-referent study among nurses.

A case-referent study assessed the association between medically certified sick leave from work and some occupational characteristics--namely, two work load indices (nurse patient ratio and patients' average duration of stay) and hospital and care unit. Study participants were nurses from seven general hospitals in Québec City who had been employed for at least six months at the time of study. Cases (n = 184) experienced at least one episode of medically certified sick leave for a diagnosis "most likely to be related to work load" between 1 January 1984 and 31 May 1987. Referents (n = 1165) were chosen from subjects who had no such leave, whatever the medical reason, and were matched to cases by the incidence density sampling method. Occupational data were collected from employment records and administrative files. Analysis was by multiple logistic regression. Significant associations were found between sick leave and nurse-patient ratio among head nurses, patient's duration of stay, and one hospital. Sick leave was more frequent among full time permanent nurses and among those on night and evening schedules. These associations were independent of age, duration of service in this hospital or in the actual job assignment, and care unit. This study supports the relevance of using sick leave as a non-specific indicator of health out-comes.

Absenteeism↗
PubMed1992

Health problems of women employed in jobs involving psychological and ergonomic stressors: the case of garment workers in Québec.

Health problems of women whose jobs involved intense time pressure were evaluated in a study of 800 sewing-machine operators employed in Québec between 1976 and 1985. Information on workers' occupational characteristics were obtained from public records. Symptoms of anxiety and depression, use of medication and disability status were determined by interview. Garment workers had an increased prevalence of slight, moderate and severe disability and higher levels of symptoms of anxiety and depression when compared to workers in other occupations. Garment workers paid piecework rates took medication for stomach problems in greater proportion than workers paid an hourly wage. Furthermore, workers who spend 5-9, 10-14, 15-19 and 20 or more years in piecework had an increased prevalence of severe disability compared to the baseline category 0-4 years, with adjusted risk ratios of 2.2 (95% CI = 1.14-4.6), 3.3 (95% CI = 1.5-6.9), 3.6 (95% CI = 1.5-8.4 and 2.3 (95% CI = 0.8-6.6) respectively, independent of age, smoking habits, education, type of task and total length of employment. These findings suggest that short-term, non-disabling conditions associated with time pressure by previous authors may have more important long-term sequelae then had been previously documented.

Aged↗
PubMed1992

Certified sick leave and work load. A case referent study among nurses.

A case-referent study assessed the association of medically certified sick leave from work with some occupational characteristics: hospital, care unit, and two work-load indices (patients' average length-of-stay, and nurse-patient ratio). Study participants were nurses from seven general hospitals in Quebec City, who had been employed for at least 6 months at the time of study. Cases (n = 1165) experienced at least one episode of medically certified sick leave between January 1, 1984 and May 31, 1987. Referents (n = 1165) were chosen from subjects who had no such leave and were matched to cases on the basis of dates on which sick leave occurred. Occupational data were collected from employment records and administrative files. Analysis was conducted using multiple logistic regression. Statistically significant associations were found between sick leave and one hospital, two care units, and nurse-patient ratio among head nurses. These odds ratios were independent of length of service in the hospital or in the actual job assignment. The study supports the pertinence of using certified sick leave as a nonspecific indicator of health outcomes.

Absenteeism↗
PubMed1991

Phase resetting and entrainment of pacemaker activity in single sinus nodal cells.

The phase-resetting and entrainment properties of single pacemaker cells were studied using computer simulations in a model of the rabbit sinus nodal cell, as well as using the whole-cell patch-clamp (current-clamp) technique in isolated rabbit sinus nodal cells. Spontaneous electrical activity in the cell model was reconstructed using Hodgkin-Huxley-type equations describing time- and voltage-dependent membrane currents. In both simulations and experiments, single subthreshold current pulses (depolarizing or hyperpolarizing) were used to scan the spontaneous cycle of the cells. Such pulses perturbed the subsequent discharge, producing temporary phasic changes in pacemaker period, and enabled the construction of phase response curves. On the basis of these results, we studied entrainment characteristics of the cells. For example, application of repetitive pulses allowed for phasic changes in the spontaneous cycle and resulted in stable 1:1 entrainment at a range of basic cycle length around the spontaneous cycle, or a 2:1 pattern at basic cycle length values about half the spontaneous cycle length. Between the two entrainment zones, complex Wenckebach-like patterns (e.g., 5:4, 4:3, and 3:2) were observed. The experimental data from the isolated cell were further analyzed from a theoretical perspective, and the results showed that the topological characteristics of the phase-resetting behavior accounts for the experimentally observed patterns during repetitive stimulation of the cell. This first demonstration of phase resetting in single cells provides the basis for phenomena such as mutual entrainment between electrically coupled pacemaker cells, apparent intranodal conduction, and reflex vagal control of heart rate.

Animals↗
PubMed1990

A model study of stability and oscillations in the myocardial cell membrane.

As a step towards an improved understanding of cardiac arrhythmias caused by abnormal automaticity, we perform a stability analysis of a Hodgkin-Huxley model of the myocardial cell membrane (modified Beeler-Reuter, MBR). The bifurcation structure of the model is obtained as a function of three parameters: the intensity of an applied constant current; the potassium equilibrium potential representing the accumulation of K+ ions in the external medium; and the maximum conductance of the slow inward current mimicking the local application of catecholamines on the membrane. For a range of parameter values, the model exhibits either stable automaticity or bistability between two quiescent states or between a quiescent state and an oscillatory state. These transformations of the bifurcation structure are shown to depend on the interrelationship between three elements: the activation of the slow inward current, the region of high slope conductance of the time-independent potassium current functions, and the slow variables controlling the activation of the potassium current and the inactivation of the slow inward current. Reduced two- and three-dimensional models are shown to reproduce the main stability properties of the full MBR model and to facilitate the understanding of its dynamic behavior. The onset of instability and the oscillatory features of the MBR model are in good agreement with relevant experimental results, and possible sources of disagreement on certain points are discussed.

Arrhythmias, Cardiac↗
PubMed1990

Irregular dynamics of excitation in biologic and mathematical models of cardiac cells.

Excitation and impulse propagation in cardiac tissues are dependent on the heart rate and can occur in extremely complex patterns. In this chapter we present the results of Purkinje fiber experiments and of computer simulations using an ionic (Beeler & Reuter) model for the ventricular cell. We have studied the global rate-dependent behavior of cardiac cells through a systematic analysis of their response to single as well as repetitive depolarizing stimuli, and determined the role of nonlinearity in the mechanism(s) of their behaviors. To this end, we devised an analytical difference equation model of cardiac cell excitation which could be used to predict simple as well as chaotic behavior of both the Purkinje fiber and the Beeler & Reuter cell, depending on the stimulation rate. Both experimental and modeling results suggest that the presence of supernormal recovery in cell excitability establishes sufficient nonlinearity so that, during repetitive stimulation, the dynamics of cell response may be regular and predictable when the stimulus magnitude is either very small or very large, or they may be chaotic and very unpredictable when the stimulus magnitude is intermediate. The overall results suggest that the application of nonlinear systems theory to electrophysiology may have importance in the understanding of cardiac rhythm and conduction disturbances, and may have clinical implications as well.

Animals↗
PubMed1990

Nonlinear dynamics of rate-dependent activation in models of single cardiac cells.

Recent studies in isolated cardiac tissue preparations have demonstrated the applicability of a one-dimensional difference equation model describing the global behavior of a driven nonpacemaker cell to the understanding of rate-dependent cardiac excitation. As a first approximation to providing an ionic basis to complex excitation patterns in cardiac cells, we have compared the predictions of the one-dimensional model with those of numerical simulations using a modified high-dimensional ionic model of the space-clamped myocyte. Stimulus-response ratios were recorded at various stimulus magnitudes, durations, and frequencies. Iteration of the difference equation model reproduced all important features of the ionic model results, including a wide spectrum of stimulus-response locking patterns, period doubling, and irregular (chaotic) dynamics. In addition, in the parameter plane, both models predict that the bifurcation structure of the cardiac cell must change as a function of stimulus duration, because stimulus duration modifies the type of supernormal excitability present at short diastolic interval. We conclude that, to a large extent, the bifurcation structure of the ionic model under repetitive stimulation can be understood by two functions: excitability and action potential duration. The characteristics of these functions depend on the stimulus duration.

Action Potentials↗