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Biomedical subjects

W W Stringer

Publications and source records attributed to W W Stringer.

15 recordsLinked to original sources
PubMed2001

Carbon dioxide pressure-concentration relationship in arterial and mixed venous blood during exercise.

To calculate cardiac output by the indirect Fick principle, CO(2) concentrations (CCO(2)) of mixed venous (Cv(CO(2))) and arterial blood are commonly estimated from PCO(2), based on the assumption that the CO(2) pressure-concentration relationship (PCO(2)-CCO(2)) is influenced more by changes in Hb concentration and blood oxyhemoglobin saturation than by changes in pH. The purpose of the study was to measure and assess the relative importance of these variables, both in arterial and mixed venous blood, during rest and increasing levels of exercise to maximum (Max) in five healthy men. Although the mean mixed venous PCO(2) rose from 47 Torr at rest to 59 Torr at the lactic acidosis threshold (LAT) and further to 78 Torr at Max, the Cv(CO(2)) rose from 22.8 mM at rest to 25.5 mM at LAT but then fell to 23.9 mM at Max. Meanwhile, the mixed venous pH fell from 7.36 at rest to 7.30 at LAT and to 7.13 at Max. Thus, as work rate increases above the LAT, changes in pH, reflecting changes in buffer base, account for the major changes in the PCO(2)-CCO(2) relationship, causing Cv(CO(2)) to decrease, despite increasing mixed venous PCO(2). Furthermore, whereas the increase in the arteriovenous CCO(2) difference of 2.2 mM below LAT is mainly due to the increase in Cv(CO(2)), the further increase in the arteriovenous CCO(2) difference of 4.6 mM above LAT is due to a striking fall in arterial CCO(2) from 21.4 to 15.2 mM. We conclude that changes in buffer base and pH dominate the PCO(2)-CCO(2) relationship during exercise, with changes in Hb and blood oxyhemoglobin saturation exerting much less influence.

Adult↗
PubMed2001

The role of passive immunization in hiv-positive patients : a case report.

An HIV-positive patient presented with pulmonary tuberculosis as her AIDS-defining diagnosis in 1993 and was effectively treated with 12 months of standard antituberculosis medications (isoniazide, rifampin, and pyrazinamide for 2 months). She received zidovudine for 6 weeks at the time of her diagnosis; however, because of patient preference, she has not received subsequent standard HIV medications (7 years). Her CD4 count at the time of diagnosis (1993) was 297/microL. Monthly passive immunotherapy was administered (fresh frozen plasma from HIV-negative blood donors with a significant titer for the anti-vasoactive intestinal peptide [VIP]/NTM antibody) from December 1993 to June 1994. Her CD4 count increased to > 400/microL during the passive immunotherapy and has remained stable for the past 6 years. The rational for the use of anti-VIP/NTM antibodies preparations in HIV, the possible mode of action of anti-VIP/NTM antibodies, the use of Ig preparations, and the role of exercise as a natural source of anti-VIP/NTM antibodies are discussed. This case report supports the potential therapeutic use of anti-VIP antibodies for treatment of HIV disease.

Antibodies↗
PubMed2000

Mechanisms of exercise limitation in HIV+ individuals.

Cardiopulmonary exercise testing (CPX) is an important diagnostic tool for both clinical and research purposes in HIV positive (HIV+) individuals. Important information can be obtained from the gas exchange responses that can elucidate heart, lung, peripheral vascular, pulmonary vascular, and muscle abnormalities in this population. A large percentage of these patients are deconditioned, respond well to exercise training, and have no intrinsic limitation to exercise. Results of a progressively increasing CPX can be used to: 1) design an exercise prescription for aerobic training in HIV+ individuals; 2) identify and refer patients with subtle abnormalities of gas exchange for further diagnostic studies to exclude early infectious complications: and 3) evaluate the improvements in maximal oxygen uptake, lactic acidosis threshold, and gas exchange kinetics as result from an aerobic exercise training program. Gas exchange kinetic analysis of constant work rate tests can provide similar information in a nonmaximal, reproducible, readily obtainable format. Both progressively increasing and constant work rate CPX tests provide important information on the changes in oxygen flow from the environment to the exercising muscle that occur with aerobic exercise training. Finally, a case study involving the exercise prescription for HIV+ individuals is reviewed as well as the risk of transmission of HIV during competitive sports competition events.

Exercise↗
PubMed2000

Comparison of exercise cardiac output by the Fick principle using oxygen and carbon dioxide.

BACKGROUND AND STUDY OBJECTIVE: Theoretically, cardiac output (CO) calculated by the Fick principle should be the same using O(2) (CO[O2]) or CO2 (CO[CO2]) as the test gas. However, agreement depends on the accuracy of gas exchange and blood gas measurements and the validity of the equations to convert measured variables into blood gas contents. Considering the widespread use of indirect estimates of pulmonary artery blood PCO2 and CO2 content to measure Fick principle CO during exercise, we wished to determine whether CO[O2] and CO[CO2] were equal during exercise and whether CO[CO2] could be accurately and precisely determined using direct measures of pulmonary artery blood. PREPARATION AND METHODS: Five healthy young nonsmoking volunteer men performed incremental exercise from rest to peak exercise on two separate occasions with intervening rest. Catheters were placed in brachial and pulmonary arteries to allow repeated blood sampling every minute during concurrent breath-by-breath gas exchange measurements from rest to peak exercise. CO[O2] was compared with CO[CO2] at multiple levels of exercise. Using standard equations, arterial and mixed venous O2 contents were calculated from hemoglobin concentration (Hb), oxyhemoglobin saturation (SO2), and PO2, whereas CO2 contents were calculated from PCO2, pH, Hb, and SO2. Blood gas analyzers were used for measurement of pH, PCO2, and PO2, and a co-oximeter was used for measurement of Hb and SO2. Initial calculations suggested that exercise CO[CO2] was 14% higher than CO[O2] and helped disclose small systematic measurement errors in PCO(2) for values > 45 mm Hg detected by proficiency testing surveys and documented with blood tonometry in the blood gas analyzer. RESULTS: After correcting PCO2 for the small systematic measurement error found, the measures and equations used to calculate arterial and mixed venous O2 and CO2 contents were adequate to provide mean CO values that are reasonably similar. However CO[CO2] values were more than twice as variable as CO[O2]. CONCLUSIONS: The increased variability of Fick principle CO[CO2] compared with CO[O2] is attributable to the much lower extraction ratio for CO2 and the greater complexity in calculation of blood CO2 than O2 contents. These results raise concerns about the accuracy and precision of estimating CO and stroke volume using CO2 as a test gas, even with direct measurement of blood CO2 contents in normal subjects.

Adult↗
PubMed1999

HIV and aerobic exercise. Current recommendations.

Aerobic exercise training is an important therapy to offer individuals who are HIV positive (HIV+). Six to 12 weeks of moderate exercise sessions (3 times per week for 1 hour) significantly improves aerobic capacity (VO2max and lactic acidosis threshold), apparently without detrimental effects on the immune system. In addition, small but significant beneficial effects on the immune system (skin test reactivity) and quality of life are obtained. The effects on lean body mass with aerobic exercise training have been less marked, but are clearly less costly than androgenic hormone administration in this population. Aerobic exercise training can be recommended with confidence in HIV+ patients.

Disease Progression↗
PubMed1998

The effect of exercise training on aerobic fitness, immune indices, and quality of life in HIV+ patients.

PURPOSE: Thirty four HIV+ patients participated in a 6-wk aerobic exercise training program to determine whether exercise improved aerobic fitness, immune indices, and quality of life. METHODS: Subjects were assigned to three groups: control (no regular aerobic exercise), moderate exercise, and heavy exercise training. At study entry and exit (in each subject) we evaluated aerobic function with a symptom limited cardiopulmonary exercise test, immune indices with CD4 counts and Candida skin tests, viral replication with plasma HIV RNA measurements, and quality of life with a HIV+ population validated questionnaire. RESULTS: Aerobic fitness increased significantly in both exercise groups relative to the control group; immune indices changed very little among all three groups; however, the Candida skin tests (mm2) increased significantly in the moderate group; viral replication was essentially unchanged in all three groups; quality of life (QOL) markers improved in both exercising groups but not the control group. There were no opportunistic infections during the study. CONCLUSIONS: Exercise training resulted in a substantial improvement in aerobic function while immune indices were essentially unchanged. Quality of life markers improved significantly with exercise. Exercise training is safe and effective in this patient group and should be promoted for HIV+ patients.

Adult↗
PubMed1997

Effect of acute bicarbonate administration on exercise responses of COPD patients.

Patients with severe chronic obstructive pulmonary disease (COPD) are limited in their exercise tolerance by the level of ventilation (VE) they can sustain. We determined whether acutely increasing blood bicarbonate levels decreased acid stimulation to the respiratory chemoreceptors during exercise, thereby improving exercise tolerance. Responses were compared with those obtained during 100% O2 breathing (known to reduce VE in these patients) and to the responses of healthy young subjects. Participants were six patients with severe COPD (forced expired volume in 1 s = 31 +/- 11% predicted) but without chronic CO2 retention and 5 healthy young subjects. Each subject performed three incremental cycle ergometer exercise tests: 1) control, 2) after ingestion of 0.3 g.kg-1 of sodium bicarbonate and 3) while breathing 100% O2. During these tests VE was measured continuously and arterialized venous blood (patients) or arterial blood (healthy subjects) was sampled serially to assess acid base variables. Bicarbonate loading increased standard bicarbonate by 4-6 mmol.L-1 and this elevation persisted during exercise. In both groups, bicarbonate loading resulted in a substantially higher arterial pH; arterial PCO2 was either unchanged (healthy subjects) or mildly (averaging 5 torr) higher (COPD patients). However, in neither group did bicarbonate loading result in an altered VE response to exercise or an increase in exercise tolerance. In contrast, superimposing hyperoxia on bicarbonate ingestion yielded, on average, 24% reduction in VE and 50% increase in peak work rate in the patients (but not in the healthy young subjects). We conclude that acute bicarbonate loading is not an ergogenic aid in patients with severe COPD.

Adult↗
PubMed1997

Cardiac output estimated noninvasively from oxygen uptake during exercise.

Because gas-exchange measurements during cardiopulmonary exercise testing allow noninvasive measurement of oxygen uptake (Vo2), which is equal to cardiac output (CO) x arteriovenous oxygen content difference [C(a-vDo2),] CO and stroke volume could theoretically be estimated if the C(a-vDo2) increased in a predictable fashion as a function of % maximum Vo2 (Vo2max) during exercise. To investigate the behavior of C(a-vDo2) during progressively increasing ramp pattern cycle ergometry exercise, 5 healthy subjects performed 10 studies to exhaustion while arterial and mixed venous blood were sampled. Samples were analyzed for blood gases (pH, Pco2, Po2) and oxyhemoglobin and hemoglobin concentration with a CO-oximeter. The C(a-vDo2) (ml/100 ml) could be estimated with a linear regression [C(a-vDo2) = 5.72 + 0.105 x % Vo2max; r = 0.94]. The CO estimated from the C(a-vDo2) by using the above linear regression was well correlated with the CO determined by the direct Fick method (r = 0.96). The coefficient of variation of the estimated CO was small (7-9%) between the lactic acidosis threshold and peak Vo2. The behavior of C(a-vDo2), as related to peak Vo2, was similar regardless of cardiac function compared with similar measurements from studies in the literature performed in normal and congestive heart failure patients. In summary, CO and stroke volume can be estimated during progressive work rate exercise testing from measured Vo2 (in normal subjects and patients with congestive heart failure), and the resultant linear regression equation provides a good estimate of C(a-vDo2).

Adult↗
PubMed1997

Mechanism of the exercise hyperkalemia: an alternate hypothesis.

A progressive hyperkalemia is observed as exercise intensity increases. The current most popular hypothesis for the hyperkalemia is that the Na+-K+ pump cannot keep pace with the K+ efflux from muscle during the depolarization-repolarization process of the sarcolemmal membrane during muscle contraction. In this report, we present data that suggest an alternate hypothesis to those previously described. Because phosphocreatine (PCr) is a highly dissociated acid and creatine is neutral at cell pH, the concentration of nondiffusible anions decreases, and an alkaline reaction takes place when PCr hydrolyzes. This creates a state of cation (K+) excess and H+ depletion in the cell. To examine the balance of K+ and H+ for exercising muscle during the early period of exercise when PCr changes most rapidly, catheters were inserted into the brachial artery and femoral vein (FV) in five healthy subjects who performed two 6-min cycle ergometer exercise tests at 40 and 85% of peak oxygen uptake. FV blood was sampled every 5 s during the first 2 min, then every 30 s for the remaining 4 min of exercise and the first 3 min of recovery, and then less frequently for the next 12 min. Arterial sampling was every 30 s during exercise and simultaneous with FV sampling during recovery. Arterial K+ concentration ([K+]) increase lagged FV [K+] increase. The hyperkalemia observed during early exercise results from K+ release from skeletal muscle. FV [K+] increased by 5 s of the start of exercise and followed the rate of H+ loss from the FV blood for the first 30 s of exercise. FV lactate and Na+ kinetics differed from K+ kinetics during exercise and recovery. As predicted from the PCr hydrolysis reaction, the exercising limb took up H+ and released K+ at the start of exercise (first 30 s) at both exercise intensities, resulting in a FV metabolic alkalosis. K+ release was essentially complete by 3 min, the time at which oxygen uptake (and, presumably, PCr) reached its asymptote. These findings lead us to hypothesize that the early K+ release by the cell takes place with H+ exchange and that the major mechanism for the exercise hyperkalemia is the reduction in nondiffusible intracellular anions in the myocyte as PCr hydrolyzes.

Adult↗
PubMed1995

Comparison of arterial potassium and ventilatory dynamics during sinusoidal work rate variation in man.

1. The mechanisms underlying the exercise hyperpnoea have been difficult to define. Recently it has been suggested that exercise ventilation (VE) changes in proportion to changes in arterial potassium concentration ([K+]a). Similar VE and [K+]a time courses following work rate changes have been cited as supporting evidence. This study compared [K+]a and VE dynamics during moderate exercise in man. 2. We observed VE and gas exchange responses in five healthy men to sinusoidal work rate variation between 25 and approximately 105 W. Tests of approximately 30 min duration were performed at sinusoidal periods of 9, 6 and 3 min and in the steady state. In each test, during two or three sine periods, arterial blood was sampled (24 per test) and analysed for [K+] and blood gases. Response amplitude and phase (relative to work rate) were determined for each variable. 3. [K+]a fluctuated in response to sinusoidal work rate forcing with mean-to-peak amplitude averaging 0.15 mmol 1(-1). However, among tests, VE amplitude and phase were not highly correlated with [K+]a (r = 0.36 and 0.67, respectively). Further, average [K+]a amplitude in the 9 and 6 min sinusoidal studies tended to exceed the steady-state amplitude, while average VE amplitude fell progressively with increasing forcing frequency. The dissimilar dynamics of [K+]a and VE seem inconsistent with a major role for [K+]a as a proportional controller of ventilation during non-steady state moderate exercise in man. 4. Among tests, VE and CO2 output (VCO2) amplitude and phase were closely correlated (r = 0.87 and 0.94, respectively). Further, arterial CO2 pressure (Pa,CO2) and arterial pH(pHa) did not fluctuate significantly in ten of twenty and thirteen of twenty studies, respectively. In tests where sinusoidal fluctuation was detected, amplitude averaged 1.1 mmHg and 0.008 units, respectively. Thus VE demonstrated a close dynamic coupling to CO2 output, with consequent tight regulation of Pa,CO2 and pHa.

Adult↗
PubMed1994

Determination of the anaerobic threshold by gas exchange: biochemical considerations, methodology and physiological effects.

This paper explains the physiological and biochemical basis of the anaerobic threshold (AT), achieved during physical exercise. The lactate concentration is approximately the same at rest in relatively fit adults, in normal sedentary subjects in adult patients with heart disease. But during exercise, the increase of lactate is inversely related to the physical fitness of the individual. During incremental work, the lactate concentration increases initially very little until a distinct metabolic rate (VO2 AT) is reached at which lactate starts to increase steeply (anaerobic threshold/AT; VO2 AT). Above the anaerobic threshold, accelerated glycolysis increases muscle lactic acidosis. This acidosis is buffered primarily by bicarbonate. The bicarbonate-derived CO2 causes an increased alveolar CO2 output relative to O2 uptake. Oxygen uptake is increased virtually linearly with work rate in healthy subjects with a slope of approximately 10 ml O2/min/Watt. VCO2 starts to increase more steeply in the mid-work-rate range after an initial linear behavior. This steepening is caused by an increased CO2 production from the HCO3-buffering of lactic acid for the range of work rates above the AT. Below the AT, the slope of increase in VCO2 is 1 or slightly less, averaging 0.95. Above the AT, it is greater than 1. The submaximal exercise protocol for the determination of AT includes a period of 2-3 min of unloaded cycling, a ramp program with x Watt increase/minute and a recovery period of 2 min. X is the rate of work rate increase per min, so that the incremental period of the exercise test lasts 8-10 min, stressing the patient for only a short time. The anaerobic threshold can be determined during the ramp program using the following four parameters: 1) steeper increase of VCO2 as compared to VO2 (V-slope-method); 2) respiratory exchange ratio = 0.95; 3) PETO2 increase; 4) VE/VO2 increase. The V-slope-method can be successfully applied, not only in healthy volunteers, but also in patients suffering from cardiac and/or pulmonary (breathing abnormalities) diseases. The so far published data show that the anaerobic threshold in healthy people and patients is a highly reproducible, accurately measurable, securely achievable parameter for the non-invasive evaluation of the individual cardiopulmonary exercise capacity.

Acid-Base Equilibrium↗