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Biomedical subjects

B J Whipp

Publications and source records attributed to B J Whipp.

At least 109 records · Page 6Linked to original sources
PubMed1987

Coupling of ventilation and CO2 production during exercise in children.

The purpose of this study was to determine how ventilation (VE) and CO2 production (VCO2) in response to exercise change during the growth process in children and teenagers. Dynamic gas exchange responses were measured in two types of studies: 128 healthy children ranging in age from 6 to 18 yr performed progressive exercise tests ("ramp" type protocol) for measurement of the slope of the relationship between VE and VCO2--delta VE/delta VCO2; and the response characteristics of VE and VCO2 in the transition between rest and exercise were measured in 11 teenagers and 11 younger children. Gas exchange was measured breath by breath. We found a small but significant decrease in delta VE/delta VCO2 with increasing body weight (r = -0.46, p less than 0.05), height, or age (mean slope of 27 in the youngest in 21 in the oldest subjects). The response characteristics of VE and VCO2 (measured as the time constant of the best-fit exponential response) were longer than for VO2 in both younger children and teenagers; but the time constants for VE and VCO2 were each approximately 30% faster in younger children compared to teenagers. In addition, end-tidal PCO2 during exercise was significantly lower in the younger subjects (mean value of 39.6 torr) compared to the teenagers (mean value of 43.5 torr). The results suggest that the process of respiratory control in exercise matures to a small degree during childhood in that PCO2 may be regulated at lower levels in younger children and there may be growth-related differences in the relative amounts of CO2 that can be stored in tissues.

Adolescent↗
PubMed1987

Dynamics of pulmonary gas exchange.

Power outputs that are below the anaerobic threshold (theta an) may be sustained for prolonged durations, whereas power outputs that are greater than theta an result in a significant reduction in the tolerable duration to fatigue. The theta an may therefore be considered to demarcate exercise intensity into moderate (below) and heavy (above) domains. O2 uptake (VO2) responds with linear first-order dynamics for sub-theta an power outputs with a time constant of approximately equal to 25 to 35 sec and a "delay" of 15 to 20 sec. A steady state is therefore normally achieved within 3 min. For supra-theta an exercise an additional, slower component of VO2 delays the steady state (if attainable). This slow phase of the VO2 response causes the VO2 to rise to values above the steady-state level attainable by fitter subjects at that work rate. The magnitude of this "excess" VO2 correlates highly with the increased arterial blood lactate [L-] and becomes marked when [L-] exceeds 4 to 5 meq/liter. The theta an may therefore be considered a crucial index for sustainable physical activity that is not--or is modestly--fatiguing.

Carbon Dioxide↗
PubMed1986

Influence of inspired oxygen concentration on the dynamics of the exercise hyperpnoea in man.

In order to determine the role of the carotid bodies on the ventilatory control characteristics during the non-steady-state phase of exercise in man, six normal males performed cycle ergometry with four repetitions of a 6 min, constant-load work bout at inspired O2 fractions (FI,O2) of 0.12, 0.15, 0.21, 0.30 and 1.00. Each test began with unloaded pedalling; this was followed by a constant load which was 90% of the subject's anaerobic threshold at FI,O2 = 0.12. Ventilation (VE), CO2 output (VCO2) and O2 uptake (VO2) were determined breath-by-breath during the test and the time constants of response (tau VE, tau VCO2 and tau VO2) were established by least-squares techniques, following interpolation (1 s), temporal alignment and averaging of the four responses. In each subject, tau VE and tau VCO2 increased as functions of increasing FI,O2, and were inverse functions of the proportional contribution to VE of peripheral chemoreceptor drive (as estimated from hyperoxic-transition or 'Dejours' tests). tau VE averaged 40 s at FI,O2 = 0.12 and 112 s at FI,O2 = 1.00, each response being well fitted by a single exponential. However, tau VO2 was not significantly affected by the alterations in FI,O2. Although there was no discernible peripheral chemosensitivity at FI,O2 = 0.30 or 1.00, the tau VE increased appreciably between these inspirates. We therefore conclude that the peripheral chemoreceptors are important, but not exclusive determinants of the exponential response characteristics during the non-steady-state phase of the exercise hyperpnoea in man. This supports the contention of a component of the control being humorally mediated even during moderate exercise.

Adult↗
PubMed1986

Bicarbonate buffering of lactic acid generated during exercise.

The pattern of decrease in arterial bicarbonate concentration ([HCO3-]) during progressive incremental exercise was compared with that of the rise in arterial lactate ([La-]) to determine the degree of buffering of lactic acid by bicarbonate. A mathematical model was derived for the change in [HCO3-] beyond the lactate threshold. This was based on a log-log transformation of the data, a model previously found to provide a very good fit to the [La-]-O2 consumption (VO2) relationship. The results of the analysis of incremental exercise data from 10 subjects show that the decrease in [HCO3-] very nearly matches the increase in [La-]. However, it was found by comparing regression models that the correspondence between [HCO3-] and [La-] could be improved by assuming that the [HCO3-] decrease was delayed until the arterial lactate level had increased by approximately 0.4 meq/l. This result is compatible with the existence of buffering mechanisms in the cell which buffer the initial increase of lactic acid. Beyond this initial buffering, lactic acid appears to be buffered almost entirely by the bicarbonate buffer system.

Adult↗
PubMed1986

A new method for detecting anaerobic threshold by gas exchange.

Excess CO2 is generated when lactate is increased during exercise because its [H+] is buffered primarily by HCO-3 (22 ml for each meq of lactic acid). We developed a method to detect the anaerobic threshold (AT), using computerized regression analysis of the slopes of the CO2 uptake (VCO2) vs. O2 uptake (VO2) plot, which detects the beginning of the excess CO2 output generated from the buffering of [H+], termed the V-slope method. From incremental exercise tests on 10 subjects, the point of excess CO2 output (AT) predicted closely the lactate and HCO-3 thresholds. The mean gas exchange AT was found to correspond to a small increment of lactate above the mathematically defined lactate threshold [0.50 +/- 0.34 (SD) meq/l] and not to differ significantly from the estimated HCO-3 threshold. The mean VO2 at AT computed by the V-slope analysis did not differ significantly from the mean value determined by a panel of six experienced reviewers using traditional visual methods, but the AT could be more reliably determined by the V-slope method. The respiratory compensation point, detected separately by examining the minute ventilation vs. VCO2 plot, was consistently higher than the AT (2.51 +/- 0.42 vs. 1.83 +/- 0.30 l/min of VO2). This method for determining the AT has significant advantages over others that depend on regular breathing pattern and respiratory chemosensitivity.

Adult↗
PubMed1986

Ventilatory responses to partial cardiopulmonary bypass at rest and exercise in dogs.

We determined the role of blood flow-induced changes in CO2 load to the lungs on ventilatory control, at rest and in the steady-state of electrically induced exercise, in the anesthetized dog. A portion of the vena caval blood was diverted to the descending aorta following "arterialization" through an extracorporeal gas exchanger. Ventilation typically decreased, both at rest and during exercise (i.e., at 2 different levels of mixed venous CO2), in proportion to the CO2 loss; arterial PCO2 was consequently regulated. There were concomitant increases of the pulmonary and peripheral vascular resistance. Bilateral cervical vagosympathectomy markedly attenuated the ventilatory response at rest, thus disrupting arterial PCO2 homeostasis, but not so during exercise. The results therefore provide evidence for and support the suggestion of CO2 flow-related hyperpnea both at rest and during muscular exercise.

Animals↗
PubMed1986

Dynamics of oxygen uptake during exercise in adults with cyanotic congenital heart disease.

The dynamic increase in oxygen uptake (VO2) at the start of exercise reflects the circulatory adjustments to metabolic changes induced by the exercise. Because VO2 measured at the lungs is the product of pulmonary blood flow and arteriovenous oxygen difference, pathologic conditions affecting the capacity of these factors to change would be expected to alter VO2 kinetics. To determine whether measurement of VO2 kinetics can detect conditions in which the pulmonary blood flow response to exercise is abnormal, VO2 was measured, breath-by-breath, during the transition from rest to exercise in 13 adults with cyanotic congenital heart disease (central venoarterial shunting) and in nine normal subjects. The increase in VO2 above baseline during the first 20 sec of exercise (phase I), reflecting the immediate increase in pulmonary blood flow, was diminished in the patients compared with that in normal subjects (14.8 +/- 10.9 vs. 49.8 +/- 19.2 ml of oxygen) (p less than .001). The patients' phase I responses correlated with their reported physical activity tolerance (p less than .01). In addition, the second phase of the VO2 response kinetics was prolonged in patients compared with normal subjects (half-time = 63 +/- 13 vs 15 +/- 13 sec) (p less than .001). We conclude that striking disturbances in VO2 kinetics occur in patients with cyanotic congenital heart disease and that these measurements provide a useful noninvasive means of evaluating the degree to which the increase in pulmonary blood flow is constrained in response to exercise.

Adult↗
PubMed1986

Mechanisms and patterns of blood lactate increase during exercise in man.

The close balance between the O2 requirement to perform exercise and the O2 supply was analyzed. A non-uniform capillary PO2 can result in anaerobic metabolism in some muscle fibers despite an apparently adequate mean capillary PO2. The pattern of lactate increase for constant work rates and incremental exercise is described. Lactate increases without an increase in pyruvate at a threshold work rate above which the lactate/pyruvate ratio increases. The latter decreases immediately at the start to recovery. From simultaneous measurements of arterial lactate and pyruvate during exercise and recovery, we conclude that the lactate increase at the lactate threshold is consequent to a change in redox state rather than a mass action effect.

Adult↗
PubMed1986

Effect of anaerobiosis on the kinetics of O2 uptake during exercise.

The anaerobic threshold is an O2-related threshold of metabolic acidemia of which the chief metabolic acid is lactic acid. As such, it is a crucial parameter of aerobic function. For power outputs that are below the anaerobic threshold, the dynamics of O2 uptake (VO2) is well characterized as a linear first-order exponential process. The system time constant for leg exercise in humans has been shown to be congruent to 25-35 s with a "delay" of 15-20 s. Steady states are therefore normally achieved within 3 min at this work intensity. Above the anaerobic threshold a second, slower component of VO2 becomes evident that delays the steady state (if attainable). Consequently, the difference in VO2 between the third and the sixth minute of exercise is zero if the work rate is subthreshold and becomes progressively greater, the higher the increment above this parameter; this also correlates highly with the increment of arterial blood lactate, [L-]. This slow phase of the VO2 kinetics results in "excess" VO2, in that the VO2 rises to values above those attained by fitter subjects. This excess VO2 correlates highly with the increased [L-] (and possibly other factors), although its magnitude increases even more rapidly at work rates for which the increase in [L-] exceeds 4-5 meq/liter.

Anaerobiosis↗
PubMed1985

Lactate, pyruvate, and lactate-to-pyruvate ratio during exercise and recovery.

The pattern of lactate increase and its relation to pyruvate and lactate-to-pyruvate (L/P) ratio were studied during exercise and early recovery in 10 normal subjects for incremental exercise on a cycle ergometer. Gas exchange was measured breath by breath. Lactate and pyruvate were measured by enzymatic techniques. Lactate and log lactate changed only slightly at low levels of O2 uptake (VO2) but both began to abruptly increase at approximately 40-55% of the maximal VO2. However, the point of abrupt increase in pyruvate occurred at higher work rates and the rate of increase was not as great as that for lactate. Thus L/P ratio increased at the same VO2 as the log lactate increase. Following the exercise, pyruvate continued to increase steeply for at least the first 5 recovery min, whereas at 2 min lactate increased only slightly or decreased. Thus arterial L/P ratio reversed its direction of change and decreased toward the resting value by 2 min of recovery. Lactate, as well as L/P ratios, decreased in all subjects by 5 min. This study demonstrates that lactate and pyruvate concentrations increase slightly at low levels of exercise without a change in L/P ratio until a threshold work rate at which lactate abruptly increases without pyruvate. The resulting increase in L/P ratio is progressive as work rate is incremented and abruptly reverses when exercise stops.

Adult↗
PubMed1985

Improved detection of lactate threshold during exercise using a log-log transformation.

The pattern of arterial lactate concentration ([La-]) increase during incremental exercise was studied using a transformation defined by plotting log([La-]) vs. log(VO2), where VO2 is O2 uptake. A plot of this function exhibits a phase of very slow increase followed by a phase of rapid increase, defining a transition in the underlying relationship between [La-] and VO2. These phases were found to be linear on the log-log plot; linear regression analysis may therefore be used to locate the transition between them (lactate threshold). A transition point was found at a lower VO2 on the semilog plot of log([La-]) vs. VO2. Consideration of the data from all studies led to the conclusion that a more accurate result is provided by the log-log transformation. This analysis shows lactate to have a small, but significant, increase before the transition and to increase beyond it with a power law having an exponent of about 2.9. This result shows that, during incrementally increasing work rate tests, arterial lactate exhibits a threshold behavior, i.e., an abrupt transition from a phase of slow increase to a phase of rapidly accelerating increase.

Adult↗
PubMed1984

Aerobic parameters of exercise as a function of body size during growth in children.

To examine the relationship between body weight in children and aerobic parameters of exercise, we determined the anaerobic threshold (AT), maximum O2 uptake (VO2max), work efficiency, and response time for O2 uptake (RT-VO2) in 109 healthy children (51 girls and 58 boys, range 6-17 yr old) using a cross-sectional study design. Gas exchange during exercise was measured breath by breath. The protocol consisted of cycle ergometry and a linearly increasing work rate (ramp) to the limit of the subject's tolerance. Both AT and VO2max increased systematically with body weight, whereas work efficiency and RT-VO2 were virtually independent of body size. The ratio of AT to VO2max decreased slightly with age, and its mean value was 60%. AT scaled to body weight to the power of 0.92, not significantly different from the power of 1.01 for VO2max. Thus both the AT and the VO2max increase in a highly ordered manner with increasing size, and as judged by AT/VO2max, the onset of anaerobic metabolism during exercise occurred at a relatively constant proportion of the overall limit of the gas transport system. We conclude that in children cardiorespiratory responses to exercise are regulated at optimized values despite overall change in body size during growth.

Adolescent↗
PubMed1984

The ventilatory stress of exercise in obesity.

The effect of obesity as a factor limiting exercise tolerance is described and can be attributed to three mechanisms: (a) the increased metabolic and therefore ventilatory requirement to perform a given work task; (b) the increased metabolic cost of breathing because of interfering chest wall and abdominal obesity and high breathing frequency; and (c) pulmonary insufficiency in response to the high breathing work and lung atelectasis. The role of support and posture minimizing the effect of obesity should be considered when attempting to improve the work tolerance of obese patients. One cannot predict accurately the O2 cost of exercise in the obese patient from the ergometer load because of uncertainties of distribution of the adipose tissue, the uncertain effects on breathing work, and often reduced motor efficiency or skill. The metabolic cost of exercise in the obese patient, therefore, needs to be directly measured; only then can one establish the degree of normality of the physiologic responses to exercise, e.g., heart rate, blood pressure, cardiac output, and minute ventilation.

Humans↗
PubMed1984

Growth-related changes in oxygen uptake and heart rate during progressive exercise in children.

Although body size and muscle mass increase considerably during growth in children, certain aerobic responses to exercise appear to be regulated so that the delivery of oxygen to muscle is maintained at optimized levels. We proposed that the relationship between oxygen uptake, (VO2) and heart rate (HR) was one of the regulated responses. We further hypothesized that the increase in VO2 per increase in HR during progressive exercise would differ in subjects of different size, but when normalized to body weight would be constant since changes in muscle mass are highly correlated to changes in body mass. To test this, we performed a cross-sectional study of 107 normal children, 50 girls and 57 boys ranging in age from 6 to 17 years. The protocol consisted of a continuously increasing work rate on a cycle ergometer, to the limit of the child's tolerance (ramp forcing function). Gas exchange was measured breath-by-breath for the determination of VO2, and heart rate was measured beat-by-beat. We used linear regression techniques to determine M, the slope, and B, the y intercept of the equation: VO2 = M X HR - B. In both boys and girls, M increased significantly with body weight, but when normalized for body weight (M/kg), there was no systematic change with increasing weight or age, the mean value being 0.33 +/- 0.10 ml/min/kg (SD). The mean value for the boys was 0.37 +/- 0.10 which was significantly greater than that of the girls (0.29 +/- 0.08, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗