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Biomedical subjects

R L Hughson

Publications and source records attributed to R L Hughson.

At least 127 records · Page 7Linked to original sources
PubMed1991

Coarse-graining spectral analysis: new method for studying heart rate variability.

Heart rate variability (HRV) spectra are typically analyzed for the components related to low- (less than 0.15 Hz) and high- (greater than 0.15 Hz) frequency variations. However, there are very-low-frequency components with periods up to hours in HRV signals, which might smear short-term spectra. We developed a method of spectral analysis suitable for selectively extracting very-low-frequency components, leaving intact the low- and high-frequency components of interest in HRV spectral analysis. Computer simulations showed that those low-frequency components were well characterized by fractional Brownian motions (FBMs). If the scale invariant, or self-similar, property of FBMs is considered a new time series (x') was constructed by sampling only every other point (course graining) of the original time series (x). Evaluation of the cross-power spectra between these two (Sxx') showed that the power of the FBM components was preserved, whereas that of the harmonic components vanished. Subtraction of magnitude of Sxx from the autopower spectra of the original sequence emphasized only the harmonic components. Application of this method to HRV spectral analyses indicated that it might enable one to observe more clearly the low- and high-frequency components characteristic of autonomic control of heart rate.

Algorithms↗
PubMed1991

Time domain analysis of oxygen uptake during pseudorandom binary sequence exercise tests.

Pseudorandom binary sequence (PRBS) exercise tests involve repeated switching between two work rates (WR) according to a computer-generated pattern. This paper presents an approach to analysis of O2 uptake (VO2) in the time domain. First, the autocorrelation function (ACF) of the input WR was recognized to be a triangular-shaped pulse that can be taken to be equivalent to a ramp increase followed by a ramp decrease in WR. Then the cross-correlation function of the input (WR) and the output (VO2) was treated as if it were the response to a triangular-shaped pulse. The cross-correlation function was analyzed by fitting a linear summation of the ramp form of a two-component exponential function to this triangular pulse. VO2 responses of eight subjects were obtained from two different PRBS tests, as well as step changes in WR. The first PRBS test consisted of 15 units, each 30 s in duration. Its ACF had a base width of 60 s. The ramp increase-ramp decrease model fit the data throughout the range of response. The second PRBS test had 63 units, each 5 s in duration; thus its ACF base width was 10 s. Again, the ramp model fit adequately. The data from the second PRBS test could be fit by the impulse form of the two-component exponential equation, although the fit in the first 30 s tended to be poorer. The time constants of VO2 dynamics estimated from step and PRBS tests were not significantly different. PRBS tests can be analyzed in the time domain, and the indicators of system dynamics reflect physiological properties similar to those investigated during step changes in WR.

Adult↗
PubMed1991

Frequency domain analysis of ventilation and gas exchange kinetics in hypoxic exercise.

The kinetics of O2 up-take (VO2), CO2 output (VCO2), ventilation (VE), and heart rate (HR) were studied during exercise in normoxia and hypoxia [inspired O2 fraction (FIO2) 0.14]. Eight male subjects each completed 6 on- and off-step transitions in work rate (WR) from low (25 W) to moderate (100-125 W) levels and a pseudorandom binary sequence (PRBS) exercise test in which WR was varied between the same WRs. Breath-by-breath data were linearly interpolated to yield 1-s values. After the first PRBS cycle had been omitted as a warm-up, five cycles were ensemble-averaged before frequency domain analysis by standard Fourier methods. The step data were fit by a two-component (three for HR) exponential model to estimate kinetic parameters. In the steady state of low and moderate WRs, each value of VO2, VCO2, VE, and HR was significantly greater during hypoxic than normoxic exercise (P less than 0.05) with the exception of VCO2 (low WR). Hypoxia slowed the kinetics of VO2 and HR in on- and off-step transitions and speeded up the kinetics of VCO2 and VE in the on-transition and of VE in the off-transition. Frequency domain analysis confined to the range of 0.003-0.019 Hz for the PRBS tests indicated reductions in amplitude and greater phase shifts in the hypoxic tests for VO2 and HR at specific frequencies, whereas amplitude tended to be greater with little change in phase shift for VCO2 and VE during hypoxic tests.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1990

The effect of beta-adrenergic blockade on leg blood flow with repeated maximal contractions of the triceps surae muscle group in man.

The effect of beta-adrenergic blockade on torque output and leg blood flow was examined in seven healthy young men during repeated maximal isometric voluntary contractions of the triceps surae muscle group. Exercise was performed in either a bent- or straight-leg position during each of four drug treatments: placebo, propranolol, metoprolol, oxprenolol. Contractions were sustained for 5 s with 5 s relaxation for a total of 10 min followed by a 10-min recovery. Leg blood flow was measured during the 5 s relaxation separating contractions using strain gauge plethysmography. Torque output decreased during the 10-min contractions with no differences between the four drug treatments. Leg blood flow was lower with beta-blockade during the initial stages of exercise and recovery in the bent-leg position but no differences were observed after 3 min exercise or recovery. Leg blood flow in the straight-leg position was not different between any of the four drug treatments, but it was significantly less than in bent-leg exercise. The lower blood flows during the initial stages of exercise in the beta-blocked conditions probably reflect a slowing of the central cardiovascular response because of beta 1-receptor blockade of the heart rather than on the beta 2-receptors effects on peripheral vascular resistance. It is concluded that local vasodilator substances released from the working muscle may play a more important role than beta 2-receptor stimulation of smooth muscle in skeletal muscle resistance vessels in regulating local muscle blood flow during maximal exercise of the triceps surae muscle group.

Adrenergic beta-Antagonists↗
PubMed1990

Effect of beta-adrenergic blockade on VO2 kinetics during pseudorandom binary sequence exercise.

The effect of beta-adrenergic blockade on the dynamic response characteristics of oxygen uptake (VO2) was examined in seven healthy men. Cycle ergometer exercise consisted of a pseudorandom binary sequence (PRBS) with 15 units per sequence, each unit lasting 15 s, for a total period of 225 s. Work rate was either 25 W or 125 W. Six identical sequences were performed continuously in a single exercise test. Each subject was studied on three occasions after 48 h treatment of placebo (CON); a nonselective beta-blocker, propranolol (PROP); and a beta 1-selective blocker, metoprolol (MET). VO2 was measured breath-by-breath. The data were analyzed in the frequency domain by Fourier analysis to yield amplitude and phase shift coefficients for the relationship between the input work rate and output VO2. Time domain analysis was performed on the cross-correlational analysis of VO2 (output).work rate (input) to determine the total lag time. The amplitude of the VO2.W-1 relationship was lower (P less than 0.0022) and the phase shift was greater (P less than 0.0039) with beta-blockade compared to CON; no differences were found between PROP and MET. The total lag time was longer (P less than 0.0053) in PROP (34.3 +/- 2.2 s) and MET (34.4 +/- 1.6 s) compared to CON (29.8 +/- 1.3 s). These data indicate that beta-adrenergic blockade causes a significant slowing of kinetics of the O2 transport system as determined by both frequency and time domain analysis. In addition, the PRBS exercise test is a sensitive indicator of change in function of the O2 transport system.

Adrenergic beta-Antagonists↗
PubMed1990

Investigation of VO2 kinetics in humans with pseudorandom binary sequence work rate change.

The dynamic response of oxygen uptake (VO2) was investigated with two different cycle ergometer tests in which the work rate changed as a pseudorandom binary sequence (PRBS). One sequence had 15 units, each of 30-s duration for a total of 450 s (PRBS1). The second had 63 units, each of 5-s duration for a total of 315 s (PRBS2). The useful range of frequencies available for investigation of the dynamic characteristics of the VO2 response as described by their bandwidth were 0.002-0.013 Hz for PRBS1 and 0.003-0.089 Hz for PRBS2. Eight subjects each completed both PRBS tests. Data from four or five consecutive sequences were ensemble averaged to reduce the biological noise. A Fourier analysis was then conducted, with the range of frequencies investigated spanning those of the bandwidth for PRBS2. This was up to the 28th harmonic. For PRBS1, the VO2 response could be adequately reconstructed by including Fourier coefficients only up to the 5th harmonic. In contrast, for PRBS2, there was still a clear pattern in the residuals at the 5th harmonic. The data were not adequately reconstructed until higher-frequency components up to the 28th harmonic were included. Evidence for this came from analysis of the mean square error. The mean square error at the 28th harmonic was reduced to 83 +/- 8% of the mean square error at the 5th harmonic for PRBS1 and to 31 +/- 3% for PRBS2 (P less than 0.0001). These data obtained by Fourier analysis and reconstructed for comparison with the original VO2 response indicate the presence of a high-frequency component that was not apparent when a test with a smaller bandwidth was used as the work rate forcing.

Exercise↗
PubMed1990

Effect of hypoxia on VO2 kinetics during pseudorandom binary sequence exercise.

The dynamic response characteristics of the oxygen uptake (VO2) response were investigated during upright cycle ergometer exercise in six healthy male volunteers. The exercise test consisted of a pseudorandom binary sequence (PRBS) with 15 units per sequence, each unit 15 s long, for a total period of 225 s. Six identical sequences were completed in a single test session. Each subject exercised under both normoxic and hypoxic (FIO2 = 14%) conditions. VO2 was measured breath-by-breath. The data were analyzed in the frequency domain by Fourier analysis to yield amplitude and phase shift coefficients for the relationship between the input work rate and the output responses of VO2 and heart rate (HR). The amplitude of the VO2/work rate was significantly reduced by hypoxia compared to normoxia over a wide range of frequencies. The mean VO2 was not different between hypoxia and normoxia. The phase shift for the VO2/work rate response was significantly greater for hypoxia than normoxia. The amplitude of the HR/work rate relationship was not significantly altered by hypoxia; however, the mean HR was higher during hypoxia. The phase shift of the HR/work rate response was significantly different between hypoxia and normoxia only at certain frequencies. These data indicate that the effects of hypoxia on the cardiorespiratory response to exercise can be characterized by the use of PRBS exercise and Fourier analysis techniques. A significant reduction in the ability of the cardiorespiratory system to adapt to changes in work rate appears to be caused by a reduction in the arterial O2 content.

Adult↗
PubMed1990

Cardiovascular response to 4 hours of 6 degrees head-down tilt or of 30 degrees head-up tilt bed rest.

The cardiovascular responses to 4 h of 6 degrees head-down tilt (HDT) were compared to those of 4 h of 30 degrees head-up tilt (HUT) following a period of 1 h baseline in the 30 degrees HUT position. Eight healthy males completed each tilt position. Immediately on assuming HDT, heart rate decreased slightly from baseline, but did not differ from HUT. Stroke volume and cardiac output both increased significantly by as much as 54% and 26%, respectively, in the first minute of HDT. The difference between HDT and HUT was no longer present after 30 min. Mean arterial blood pressure was unchanged throughout 4 h or HUT or HDT. The ratio of pre-injection period to left ventricular ejection time was significantly decreased across all 4 h of HDT. Plasma volume was slightly elevated over the 4 h of HDT, while plasma hemoglobin concentration was significantly reduced. No evidence of a diuresis was found with 4 h HDT. Plasma catecholamines were not different between HDT and HUT. The present results show that the immediate transition from a HUT to a HDT position causes a dramatic change in cardiovascular variables. These changes are generally transient with baseline values resumed by many variables within 30 min of exposure to 6 degrees HDT.

Adult↗
PubMed1990

Oxygen uptake and heart rate responses to treadmill and water running.

The relationship between heart rate (HR) and oxygen uptake (VO2) was assessed during treadmill running and during running-like motions in water without ground support. Athletes self-selected easy, moderate and hard efforts during water running. While there was a linear relationship between HR and VO2 in both exercise conditions, there was considerable between-subject variability of the slope and intercept of the linear regressions. These data are taken to indicate that HR for a given VO2 during water compared to land running is influenced in large part by the skill of the athlete in the water running activity.

Adult↗
PubMed1990

Exploring cardiorespiratory control mechanisms through gas exchange dynamics.

The rate of increase in oxygen uptake (VO2) at the onset of a step change in work rate can be studied to provide information about the physiological mechanisms that control this process. Several systems must interact to produce the total response. These can be grouped into oxygen transport and oxygen utilization mechanisms. In this paper, the hypothesis that one or the other of these mechanisms limits the adaptation of VO2 to a change in work rate will be examined. In addition to the traditional approach with step changes in work rate, the responses to other work rate forcing functions will be reported. These include ramp, impulse, and pseudorandom binary sequence work rate changes. The evidence that is accumulating from studies involving transitions from different baseline levels of exercise, as well as studies of the effects of hypoxia and beta-adrenergic receptor blockade, has led to the conclusion that oxygen transport mechanisms limit the rate of increase in VO2. However, the dynamic response of VO2 in the presence of adequate oxygen is not much different from that of oxygen limited conditions.

Exercise↗
PubMed1989

Ramp work tests with three different beta-blockers in normal human subjects.

The effects of beta-blockade on the responses of oxygen uptake (VO2), heart rate (HR) and blood lactate (La-) were examined during ramp cycle ergometer tests (50 W.min-1 ramp slope) in 8 healthy male volunteers. Each subject took placebo, or one of four different doses of three different beta-blockers (propranolol, metoprolol or oxprenolol) 2 h prior to each test for a total of 15 exercise tests. VO2 was measured breath-by-breath, HR was sampled once per breath, and La- was obtained every minute. Linear regression analysis was applied to VO2 and HR data to obtain the kinetic parameter total lag time (TLT) and a slope value. La- was analyzed by a continuous exponential model with the lactate slope index (LSI) being derived from the individual response curves. Submaximal exercise HR was significantly depressed at the baseline as well as during the ramp tests by beta-blockade. TLT for HR was significantly affected by beta-blockade, with a dose dependent shift from a placebo value of 16 to 26 s with placebo to a value of -40 to -60 s at the highest dose. Slope of HR was significantly depressed relative to placebo. VO2 kinetics assessed by TLT were not significantly affected by beta-blockade. This slope of the VO2 vs work rate relationship was significantly less than placebo only at the highest dose of beta-blocker. The LSI was not significantly affected by beta-blockade. In contrast with the clear impairment of HR response to exercise during beta-blockade, both the VO2 and La- responses appear to be relatively unaffected by beta-blockade during ramp exercise tests.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1989

The effect of citrate loading on exercise performance, acid-base balance and metabolism.

Nine subjects (VO2max 65 +/- 2 ml.kg-1.min-1, mean +/- SEM) were studied on two occasions following ingestion of 500 ml solution containing either sodium citrate (C, 0.300 g.kg-1 body mass) or a sodium chloride placebo (P, 0.045 g.kg-1 body mass). Exercise began 60 min later and consisted of cycle ergometer exercise performed continuously for 20 min each at power outputs corresponding to 33% and 66% VO2max, followed by exercise to exhaustion at 95% VO2max. Pre-exercise arterialized-venous [H+] was lower in C (36.2 +/- 0.5 nmol.l-1; pH 7.44) than P (39.4 +/- 0.4 nmol.l-1; pH 7.40); the plasma [H+] remained lower and [HCO3-] remained higher in C than P throughout exercise and recovery. Exercise time to exhaustion at 95% VO2max was similar in C (310 +/- 69 s) and P (313 +/- 74 s). Cardiorespiratory variables (ventilation, VO2, VCO2, heart rate) measured during exercise were similar in the two conditions. The plasma [citrate] was higher in C at rest (C, 195 +/- 19 mumol.l-1; P, 81 +/- 7 mumol.l-1) and throughout exercise and recovery. The plasma [lactate] and [free fatty acid] were not affected by citrate loading but the plasma [glycerol] was lower during exercise in C than P. In conclusion, sodium citrate ingestion had an alkalinizing effect in the plasma but did not improve endurance time during exercise at 95% VO2max. Furthermore, citrate loading may have prevented the stimulation of lipolysis normally observed with exercise and prevented the stimulation of glycolysis in muscle normally observed in bicarbonate-induced alkalosis.

Acid-Base Equilibrium↗
PubMed1989

Blood lactate responses in incremental exercise as predictors of constant load performance.

Seven trained male cyclists (VO2max = 4.42 +/- 0.23 l.min-1; weight 71.7 +/- 2.7 kg, mean +/- SE) completed two incremental cycling tests on the cycle ergometer for the estimation of the "individual anaerobic threshold" (IAT). The cyclists completed three more exercises in which the work rate incremented by the same protocol, but upon reaching selected work rates of approximately 40, 60 and 80% VO2max, the subjects cycled for 60 min or until exhaustion. In these constant load studies, blood lactate concentration was determined on arterialized venous ([La-]av) and deep venous blood ([La-]v) of the resting forearm. The av-v lactate gradient across the inactive forearm muscle was -0.08 mmol.l-1 at rest. After 3 min at each of the constant load work rates, the gradients were +0.05, +0.65* and +1.60* mmol.l-1 (*P less than 0.05). The gradients after 10 min at these same work rates were -0.09, +0.24 and +1.03* mmol.l-1. For the two highest work rates taken together, the lactate gradient was less at 10 min than 3 min constant load exercise (P less than 0.05). The [La-]av was consistently higher during prolonged exercise at both 60 and 80% VO2max than that observed at the same work rate during progressive exercise. At the highest work rate (at or above the IAT), time to exhaustion ranged from 3 to 36 min in the different subjects. These data showed that [La-] uptake across resting muscle continued to increase to work rates above the IAT. Further, the greater av-v lactate gradient at 3 min than 10 min constant load exercise supports the concept that inactive muscle might act as a passive sink for lactate in addition to a metabolic site.

Adult↗
PubMed1989

A study of cardiorespiratory dynamics with step and ramp exercise tests in normoxia and hypoxia.

The ability of the cardiorespiratory system to adapt to the demands of increased exercise intensity was studied under the effects of hypoxia with two different submaximal cycling exercise protocols. A step transition in work rate from 25 W to 105 W and a ramp increase in work rate from a baseline of 25 W with a slope of 40 W.min-1 were used. Eight healthy male subjects each completed maximal exercise tests to exhaustion, as well as six repetitions of the step and the ramp, during normoxia and during hypoxia (FIO2 = 14%, balance N2). VO2 was measured breath by breath and cardiac output (Q) was estimated beat by beat by impedance cardiography. Kinetic analyses of the VO2 and Q data were performed to obtain the gain and the total lag time. With hypoxia, the gain of the VO2 response was no different from normoxia, at 10.6(SEM 0.1) ml.min-1.W-1, but total lag time increased from 23.0(2.2) s to 32.5(2.6) s (p less than 0.05). In contrast, with ramp exercise there was a significant reduction in the gain or slope of the VO2 response from 9.4(0.2) ml.min-1.W-1 in normoxia to 9.0(0.1) ml.min-1.W-1 in hypoxia, while total lag time was no different between normoxia and hypoxia, at 19.7(3.3) v 21.4(2.9) s. Both the gain and total lag time of the step tests were significantly different from the ramp test. Kinetics of Q during the step transition did not differ between normoxia and hypoxia, at 26.5(2.3) v 31.0(4.9) s, respectively (NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1989

Continuous increase in blood lactate concentration during different ramp exercise protocols.

The applicability of a continuous model description of the blood lactate concentration [( La-]) vs. O2 uptake (VO2) relationship was studied in nine healthy male volunteers during three different ramp exercise protocols. The work rate was increased at either 8, 15, or 50 W/min. The continuous model for [La-] = â + b exp(ĉVO2) was compared statistically with a previously proposed log-log transformation model for the [La-] and VO2 variables. It was found that the mean square error was significantly less for the continuous as opposed to the log-log model (P less than 0.01) by analysis of variance pooled across all three ramp slopes. The mean square errors from the individual ramp slopes were also significantly less for the continuous model by paired t test (P less than 0.05). It was observed that the major contributor to the increased error of the log-log model was at VO2's at or above the intersection point (lactate threshold) of the two linear log-transformed segments. The log-log transformation does not appear to relate to any physiological process. The lactate slope index, taken as the point where the slope of the relationship between [La-] and VO2 (i.e., d[La-]/dVO2) equaled 1, occurred at a mean VO2 of 2.25 and 2.37 l/min for the 15- and 8-W/min ramp slopes, respectively, but at 2.76 l/min for the 50-W/min ramp (P less than 0.05). It is concluded that [La-] increases as a continuous function with respect to VO2 across a wide range of ramp work rate slopes.

Adult↗
PubMed1989

Mechanical and metabolic alterations in rat diaphragm during electrical stimulation.

To investigate the hypothesis that the rate of fatigue development is not influenced by the absolute duration of contraction (train duration) and relaxation (off-phase of duty cycle) at constant duty cycle, strips of the diaphragm from 36 male adult rats (mean +/- SD wt 152 +/- 21 g) were stimulated directly for periods of 180, 250, and 320 ms at a constant duty cycle of 50%. The frequency of stimulation was adjusted to produce 40% of maximal tetanic tension at supramaximal voltages. After 30 min of stimulation, analysis of twitch characteristics between control and experimental groups indicated a prolongation of contraction time of 9% (P less than 0.05), an increase in relaxation time of 75% (P less than 0.05), and a decrease in twitch tension by 78% (P less than 0.05). Similarly, reductions (P less than 0.05) in isometric force output at high stimulation frequency (100 Hz) of 58% and at low frequency (20 Hz) of 67% were also noted. These changes were accompanied by an approximately 60% reduction in the maximal velocity of shortening. No difference was observed for any of the mechanical measures between experimental conditions. After 30-min stimulation, decreases of between 43 and 46% were noted for ATP (P less than 0.05) and increases of between three- and fourfold noted for IMP (P less than 0.05). No changes were found for either ADP or AMP. Total adenine nucleotide concentrations declined (P less than 0.05) an average of 24%. As with the mechanical data, no differences were found between the different stimulation conditions. It is concluded that for the conditions studied, fatigue mechanisms become manifest early in the stimulation period and are only minimally altered by the duration of specific contractions provided the relaxation period is of equal duration.

Animals↗