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W L Beaver

Publications and source records attributed to W L Beaver.

11 recordsLinked to original sources
PubMed1992

Factors affecting the components of the alveolar CO2 output-O2 uptake relationship during incremental exercise in man.

The VCO2-VO2 (alveolar CO2 output-alveolar O2 uptake) relationship (V-slope) during increasing work rate (ramp) cycle ergometer exercise has two approximately linear components: a lower component slope (S1) with a value of about 0.95 and a steeper, upper component (S2). We examined the effect of muscle glycogen depletion (protocol 1) and the rate of increase in work rate (ramp rate) without muscle glycogen depletion (protocol 2) on S1 and S2. In protocol 1, ten healthy men with a mean age of 31.4 years (S.D. 6.2) were studied on each of 3 days (days 1 and 3 were control days). They performed a ramp exercise test to maximum tolerance and steady-state tests at rest, during unloaded pedalling and at two constant work rates below their anaerobic threshold (AT). To deplete muscle glycogen before the test on day 2, the subjects performed 2 h of very heavy cycle exercise on the preceding day and fasted overnight. S1 was reduced on day 2 (0.79 compared with 0.95, P less than 0.001), as was the VCO2-VO2 slope derived from steady-state measurements (0.81 compared with 0.99, P less than 0.001), but AT and the slope difference (S2 - S1) were unchanged. In protocol 2, seven healthy men with a mean age of 20.6 years (S.D. 2.4) performed ramp tests at three different rates of increasing work rate (15, 30 and 60 W min-1), each ramp rate being performed twice in random sequence. The ramp rate did not affect S1 but S2 was steeper with the faster rates of work rate increase (1.27, 1.43 and 1.63, respectively, P less than 0.01). Our findings support the concept that the lower component of the V-slope plot (below AT) represents muscle substrate respiratory quotient (RQ) while the difference between S1 and S2 reflects 'excess CO2' derived from bicarbonate buffering of lactic acid.

Adolescent↗
PubMed1990

Evidence supporting the existence of an exercise anaerobic threshold.

In this paper, we provided evidence to support the concept that work above the anaerobic threshold, measured by the V-slope method, is, in fact, performed partially anaerobically. In contrast, work performed below the anaerobic threshold is totally aerobic (Figure 2, 4 and 5). The VO2 at the transition from aerobic to partial anaerobic metabolism must depend on cardiovascular performance since it regulates the capillary PO2 level needed for O2 diffusion transport into the mitochondria (Figure 1). At high work rates, the capillary PO2 needed for the oxygen requirement might not be met by the cardiovascular oxygen supply. This would result in the oxygen consumed being less than the oxygen required by the working tissue (Figure 2), with the oxygen equivalent difference necessarily coming from anaerobic metabolism. The consequences are increased lactate formation and metabolic acidosis, and the physiological and biochemical disturbances which result from the latter.

Adolescent↗
PubMed1990

Gas exchange theory and the lactic acidosis (anaerobic) threshold.

The physiological requirements of performing exercise above the anaerobic threshold are considerably more demanding than for lower work rates. Lactic acidosis develops at a metabolic rate that is specific to the individual and the task being performed. Although numerous pyruvate-dependent mechanisms can lead to an elevated blood lactate, the increase in lactate during muscular exercise is accompanied by an increase in lactate/pyruvate ratio (i.e., increased NADH/NAD ratio). This is typically caused by an inadequate O2 supply to the mitochondria. Thus, the anaerobic threshold can be considered to be an important assessment of the ability of the cardiovascular system to supply O2 at a rate adequate to prevent muscle anaerobiosis during exercise testing. In this paper, we demonstrate, with statistical justification, that the pattern of arterial lactate and lactate/pyruvate ratio increase during exercise evidences threshold dynamics rather than the continuous exponential increase proposed by some investigators. The pattern of change in arterial bicarbonate (HCO3-) and pulmonary gas exchange supports this threshold concept. To estimate the anaerobic threshold by gas exchange methods, we measure CO2 output (VCO2) as a continuous function of O2 uptake (VO2) (V-slope analysis) as work rate is increased. The break-point in this plot reflects the obligate buffering of increasing lactic acid production by HCO3-. The anaerobic threshold measured by the V-slope analysis appears to be a sensitive index of the development of metabolic acidosis even in subjects in whom other gas exchange indexes are insensitive, owing to irregular breathing, reduced chemoreceptor sensitivity, impaired respiratory mechanics, or all of these occurrences.

Acidosis, Lactic↗
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

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↗
PubMed1977

Ventilatory and gas exchange dynamics in response to sinusoidal work.

The dynamic relationships between ventilation and gas exchange variables during exercise were determined utilizing frequency analysis techniques. Five subjects exercised on a cycle ergometer for 30 min at work rates which fluctuated sinusoidally between 25 W and 80% of the anaerobic threshold at sinusoidal periods of 0.7, 1,2,4,6, and 10 min. VE, VCO2, VO2, and HR were computed and displayed breath-by-breath. From these and steady-state response data, digital computer routines extracted amplitude and phase relations between each variable and the perturbing work load. These response characteristics were well described by first-order linear dynamics with time constants for VE, VCO2, VO2, and HR averaging 1.4, 1.2, 0.8, and 0.8 min, respectively. The time constants of VE and VCO2 were strongly correlated among subjects (r = 0.97). Further, there was no evidence that neural afferents from the exercising limbs induced fast components in the ventilatory response to these forcings. These results are consistent with the hypothesis that exercise hyperpnea is linked to metabolism via carbon dioxide production.

Adult↗
PubMed1977

Pseudo-tricuspid stenosis: a rare entity.

A case is presented in which features of constrictive pericarditis and a pericardial pseudotumor coexisted. Preoperative cardiac catheterization revealed a gradient across the tricuspid valve. Exploratory surgery revealed a blood-filled cavity within the thickened pericardium overlying the right atrium and right ventricle and a normal tricuspid valve. After pericardiectomy, right cardiac catheterization revealed normal pressures and no gradients.

Fibroma↗
PubMedDate not supplied

State-of-the-art in two-dimensional ultrasonic transducer array technology.

Attention is given to means of sensing ultrasonic energy distributions over an area. Under the restraints appropiate to real-time imaging of deep body organs, piezoelectric arrays offer the most promising method. Adaptation of integrated circuit techniques to array assembly permits very large arrays of small elements to be batch fabricated. Further, special semiconductor switching devices specifically designed for addressing the array have been produced and applied. These permit both the passage of weak received signals and the application of voltage and current levels sufficient for transmission without significant disturbance of the basic piezoelectric element properties. Emphasis is placed on the complete data acquisition, processing, and display flexibility that arises from an array capability. Operation becomes possible in any of the presently used A or time motion (TM), in real time B or C scan, or in novel scan patterns adapted to specific organs. Further development with acoustic and electronic focusing, taking advantage of the unique possibilities obtained with the bidirectional array structure, is discussed.

Transducers↗