Acute hypervolemia lengthens red cell pulmonary transit time during exercise in endurance athletes

Improving Endurance Exercise Performance at High Altitude: Traditional and Nontraditional Approaches

Acute exposure to terrestrial altitude (hypobaric hypoxia) causes decrements in endurance performance relative to sea level. Altitude acclimatization consistently results in partial attenuation of these decrements, but due to logistical challenges, it is not readily implemented. We discuss mechanisms and impact (or lack thereof) of other non-acclimatization interventions to improve endurance performance and provide suggestions for future research directions.

EXERCISE-INDUCED ARTERIAL HYPOXEMIA IN FEMALE MASTERS ATHTLETES

The purpose of the study was to characterize exercise induced arterial hypoxemia (EIAH) in female masters athletes (FMA). We hypothesized that FMA would experience EIAH during treadmill running. Eight FMA (48-57 years) completed pulmonary function testing and an incremental exercise test until exhaustion (V̇O₂max=45.7±6.5, range:35-54ml/kg/min). On a separate day, the participants were instrumented with a radial arterial catheter and an esophageal temperature probe. Participants performed three to four constant load exercise tests at 60-70, 75, 90, 95, and 100% of maximal oxygen uptake while sampling arterial blood and recording esophageal temperature. We found that FMA decrease their partial pressure of oxygen (86.0±7.6, range:73-108mmHg), arterial saturation (96.2±1.2, range:93-98%), and widen their alveolar to arterial oxygen difference (23.2±8.8, range:5-42mmHg) during all exercise intensities however, with variability in terms of severity and pattern. Our findings suggest that FMA experience EIAH however aerobic fitness appears unrelated to occurrence or severity (r=0.13, p=0.756).

Pulmonary gas exchange efficiency during exercise breathing normoxic and hypoxic gas in adults born very preterm with low diffusion capacity

Adults with a history of very preterm birth (<32 wk gestational age; PRET) have reduced lung function and significantly lower lung diffusion capacity for carbon monoxide (DLCO) relative to individuals born at term (CONT). Low DLCO may predispose PRET to diffusion limitation during exercise, particularly while breathing hypoxic gas because of a reduced O2 driving gradient and pulmonary capillary transit time. We hypothesized that PRET would have significantly worse pulmonary gas exchange efficiency [i.e., increased alveolar-to-arterial Po2 difference (AaDO2)] during exercise breathing room air or hypoxic gas (FiO2 = 0.12) compared with CONT. To test this hypothesis, we compared the AaDO2 in PRET ( n = 13) with a clinically mild reduction in DLCO (72 ± 7% of predicted) and CONT ( n = 14) with normal DLCO (105 ± 10% of predicted) pre- and during exercise breathing room air and hypoxic gas. Measurements of temperature-corrected arterial blood gases, and direct measure of O2 saturation (SaO2), were made prior to and during exercise at 25, 50, and 75% of peak oxygen consumption (V̇o2peak) while breathing room air and hypoxic gas. In addition to DLCO, pulmonary function and exercise capacity were significantly less in PRET. Despite PRET having low DLCO, no differences were observed in the AaDO2 or SaO2 pre- or during exercise breathing room air or hypoxic gas compared with CONT. Although our findings were unexpected, we conclude that reduced pulmonary function and low DLCO resulting from very preterm birth does not cause a measureable reduction in pulmonary gas exchange efficiency.

Pulmonary gas exchange and acid-base balance during exercise

As the first step in the oxygen-transport chain, the lung has a critical task: optimizing the exchange of respiratory gases to maintain delivery of oxygen and the elimination of carbon dioxide. In healthy subjects, gas exchange, as evaluated by the alveolar-to-arterial PO2 difference (A-aDO2), worsens with incremental exercise, and typically reaches an A-aDO2 of approximately 25 mmHg at peak exercise. While there is great individual variability, A-aDO2 is generally largest at peak exercise in subjects with the highest peak oxygen consumption. Inert gas data has shown that the increase in A-aDO2 is explained by decreased ventilation-perfusion matching, and the development of a diffusion limitation for oxygen. Gas exchange data does not indicate the presence of right-to-left intrapulmonary shunt developing with exercise, despite recent data suggesting that large-diameter arteriovenous shunt vessels may be recruited with exercise. At the same time, multisystem mechanisms regulate systemic acid-base balance in integrative processes that involve gas exchange between tissues and the environment and simultaneous net changes in the concentrations of strong and weak ions within, and transfer between, extracellular and intracellular fluids. The physicochemical approach to acid-base balance is used to understand the contributions from independent acid-base variables to measured acid-base disturbances within contracting skeletal muscle, erythrocytes and noncontracting tissues. In muscle, the magnitude of the disturbance is proportional to the concentrations of dissociated weak acids, the rate at which acid equivalents (strong acid) accumulate and the rate at which strong base cations are added to or removed from muscle.

Use of contrast echocardiography for quantitative and qualitative evaluation of myocardial perfusion and pulmonary transit time in healthy dogs

OBJECTIVE

To evaluate reproducibility of ejection fraction (EF), myocardial perfusion (MP), and pulmonary transit time (PTT) measured in a group of dogs by use of contrast echocardiography and to examine safety of this method by evaluating cardiac troponin I concentrations.

ANIMALS

6 healthy dogs.

PROCEDURES

2 bolus injections and a constant rate infusion of contrast agent were administered IV. Echocardiographic EF was determined by use of the area-length method and was calculated without and with contrast agent. The PTT and normalized PTT (PTT/mean R-R interval) were measured for each bolus. Constant rate infusion was used for global MP evaluation, and regional MP was calculated by use of a real-time method in 4 regions of interest of the left ventricle. Cardiac troponin I concentration was analyzed before and after contrast agent administration. Intraoberserver and interobserver variability was calculated.

RESULTS

EF was easier to determine with the ultrasonographic contrast agent. For the first and second bolus, mean ± SD PTT was 1.8 ± 0.2 seconds and 2.1 ± 0.3 seconds and normalized PTT was 3.4 ± 0.3 seconds and 3.5 ± 0.3 seconds, respectively. A coefficient of variation < 15% was obtained for global MP but not for the regional MPs. No differences were detected between precontrast and postcontrast cardiac troponin I concentrations.

CONCLUSIONS AND CLINICAL RELEVANCE

Contrast echocardiography appeared to be a repeat-able and safe technique for use in the evaluation of global MP and PTT in healthy dogs, and it improved delineation of the endocardial border in dogs.

Pulmonary gas exchange does not worsen during repeat exercise in women

The purposes were to determine (1) if repeat exercise worsens pulmonary gas exchange in women, and, (2) if the level of pulmonary edema obtained in these same women is related to the gas exchange impairment during exercise. Fourteen women (27 +/- 4 yrs; maximal oxygen uptake = 3.12 +/- 0.42 L/min) with minimal arterial PO2 (PaO2) ranging from 76 to 104 mmHg with a maximal alveolar-arterial PO2 difference (AaDO2) ranging from 7 to 35 mmHg performed three bouts of near-maximal exercise on a cycle ergometer (236 +/- 27 W) for 5 min each with 10 min of rest between sets. Cardiorespiratory parameters and oxygenation were measured at rest, throughout exercise and recovery. Chest radiographs were obtained before and 30 min after the interval training session (see Respir Physiol Neurobiol, 153 (2006) 181-190). Repeat exercise did not affect pulmonary gas exchange between sets 1 and 3 (change in PaO2 = 3 +/- 2 mmHg; change in AaDO2 = 1 +/- 2 mmHg P > 0.05). Arterial PCO2 decreased by 4 +/- 2 mmHg (P < 0.05) between sets 1 and 2, which did not reduce further in set 3. The level of PaO2 or AaDO2 was not related to the change in edema score or the post-exercise edema score (P > 0.05). In conclusion, pulmonary gas exchange is not worsened in women during interval training despite the mild edema triggered by exercise.

Effect of acute increases in pulmonary vascular pressures on exercise pulmonary gas exchange

The purpose of this study was to determine the effect of acute increases in pulmonary vascular pressures, caused by the application of lower-body positive pressure (LBPP), on exercise alveolar-to-arterial Po2 difference (A-aDo2), anatomical intrapulmonary (IP) shunt recruitment, and ventilation. Eight healthy men performed graded upright cycling to 90% maximal oxygen uptake under normal conditions and with 52 Torr (1 psi) of LBPP. Pulmonary arterial (PAP) and pulmonary artery wedge pressures (PAWP) were measured with a Swan-Ganz catheter. Arterial blood samples were obtained from a radial artery catheter, cardiac output was calculated by the direct Fick method, and anatomical IP shunt was determined by administering agitated saline during continuous two-dimensional echocardiography. LBPP increased both PAP and PAWP while upright at rest, and at all points during exercise (mean increase in PAP and PAWP 3.7 and 4.0 mmHg, respectively, P < 0.05). There were no differences in exercise oxygen uptake or cardiac output between control and LBPP. Despite the increased PAP and PAWP with LBPP, A-aDo2 was not affected. In the upright resting position, there was no evidence of shunt in the control condition, whereas LBPP caused shunt in one subject. At the lowest exercise workload (75 W), shunt occurred in three subjects during control and in four subjects with LBPP. LBPP did not affect IP shunt recruitment during subsequent higher workloads. Minute ventilation and arterial Pco2 were not consistently affected by LBPP. Therefore, small acute increases in pulmonary vascular pressures do not widen exercise A-aDo2 or consistently affect IP shunt recruitment or ventilation.

Exercise induced arterial hypoxemia: the role of ventilation-perfusion inequality and pulmonary diffusion limitation

Many apparently healthy individuals experience pulmonary gas exchange limitations during exercise, and the term "exercise induced arterial hypoxemia" (EIAH) has been used to describe the increase in alveolar-arterial difference for oxygen (AaDO2), which combined with a minimal alveolar hyperventilatory response, results in a reduction in arterial PO2. Despite more than two decades of research, the mechanisms of pulmonary gas exchange limitations during exercise are still debated. Using data in 166 healthy normal subjects collated from several previously published studies it can be shown that approximately 20% of the variation in PaO2 between individuals can be explained on the basis of variations in alveolar ventilation, whereas variations in AaDO2 explain approximately 80%. Using multiple inert gas data the relative contributions of ventilation-perfusion ("VA/Q") inequality and diffusion limitation to the AaDO2 can be assessed. During maximal exercise, both in individuals with minimal (AaDO2 < 20 Torr, x = 13 +/- 5, means +/- SD, n = 35) and moderate to severe (AaDO2= 25-40 Torr, x = 33 +/- 6, n = 20) gas exchange limitations, VA/Q inequality is an important contributor to the AaDO2. However, in subjects with minimal gas exchange impairment, VA/Q inequality accounts for virtually all of the AaDO2 (12 +/- 6 Torr), whereas in subjects with moderate to severe gas exchange impairment it accounts for less than 50% of the AaDO2 (15 +/- 6 Torr). Using this framework, the difficulties associated with unraveling the mechanisms of pulmonary gas exchange limitations during exercise are explored, and current data discussed.

Intra-pulmonary shunt and pulmonary gas exchange during exercise in humans

In young, healthy people the alveolar-arterial P(O(2)) difference (A-aDO(2)) is small at rest, but frequently increases during exercise. Previously, investigators have focused on ventilation/perfusion mismatch and diffusion abnormalities to explain the impairment in gas exchange, as significant physiological intra-pulmonary shunt has not been found. The aim of this study was to use a non-gas exchange method to determine if anatomical intra-pulmonary (I-P) shunts develop during exercise, and, if so, whether there is a relationship between shunt and increased A-aDO(2). Healthy male participants performed graded upright cycling to 90% while pulmonary arterial (PAP) and pulmonary artery wedge pressures were measured. Blood samples were obtained from the radial artery, cardiac output was calculated by the direct Fick method and I-P shunt was determined by administering agitated saline during continuous 2-D echocardiography. A-aDO(2) progressively increased with exercise and was related to (r = 0.86) and PAP (r = 0.75). No evidence of I-P shunt was found at rest in the upright position; however, 7 of 8 subjects developed I-P shunts during exercise. In these subjects, point bi-serial correlations indicated that I-P shunts were related to the increased A-aDO(2) (r = 0.68), (r = 0.76) and PAP (r = 0.73). During exercise, intra-pulmonary shunt always occurred when A-aDO(2) exceeded 12 mmHg and was greater than 24 l min(-1). These results indicate that anatomical I-P shunts develop during exercise and we suggest that shunt recruitment may contribute to the widened A-aDO(2) during exercise.

NaHCO3 does not affect arterial O2 tension but attenuates desaturation of hemoglobin in maximally exercising Thoroughbreds

The objective of the present study was to examine the effects of preexercise NaHCO3 administration to induce metabolic alkalosis on the arterial oxygenation in racehorses performing maximal exercise. Two sets of experiments, intravenous physiological saline and NaHCO3 (250 mg/kg iv), were carried out on 13 healthy, sound Thoroughbred horses in random order, 7 days apart. Blood-gas variables were examined at rest and during incremental exercise, leading to 120 s of galloping at 14 m/s on a 3.5% uphill grade, which elicited maximal heart rate and induced pulmonary hemorrhage in all horses in both treatments. NaHCO3 administration caused alkalosis and hemodilution in standing horses, but arterial O2 tension and hemoglobin-O2 saturation were unaffected. Thus NaHCO3 administration caused a reduction in arterial O2 content at rest, although the arterial-to-mixed venous blood O2 content gradient was unaffected. During maximal exercise in both treatments, arterial hypoxemia, desaturation, hypercapnia, acidosis, hyperthermia, and hemoconcentration developed. Although the extent of exercise-induced arterial hypoxemia was similar, there was an attenuation of the desaturation of arterial hemoglobin in the NaHCO3-treated horses, which had higher arterial pH. Despite these observations, the arterial blood O2 content of exercising horses was less in the NaHCO3 experiments because of the hemodilution, and an attenuation of the exercise-induced expansion of the arterial-to-mixed venous blood O2 content gradient was observed. It was concluded that preexercise NaHCO3 administration does not affect the development and/or severity of arterial hypoxemia in Thoroughbreds performing short-term, high-intensity exercise.

Arterial desaturation during exercise in man: implication for O2 uptake and work capacity

Exercise-induced arterial hypoxaemia is defined as a reduction in the arterial O2 pressure (PaO2) by more than 1 kPa and/or a haemoglobin O2 saturation (SaO2) below 95%. With blood gas analyses ideally reported at the actual body temperature, desaturation is a consistent finding during maximal ergometer rowing. Arterial desaturation is most pronounced at the end of a maximal exercise bout, whereas the reduction in PaO2 is established from the onset of exercise. Exercise-induced arterial hypoxaemia is multifactorial. The ability to maintain a high alveolar O2 pressure (PAO2) is critical for blood oxygenation and this appears to be difficult in large individuals. A large lung capacity and, in turn, diffusion capacity seem to protect PaO2. A widening of the PAO2-PaO2 difference does indicate that a diffusion limitation, a ventilation-perfusion mismatch and/or a shunt influence the transport of O2 from alveoli to the pulmonary capillaries. An inspired O2 fraction of 0.30 reduces the widened PAO2-PaO2 difference by 75% and prevents a reduction of PaO2 and SaO2. With a marked increase in cardiac output, diffusion limitation combined with a fast transit time dominates the O2 transport problem. Furthermore, a postexercise reduction in pulmonary diffusion capacity suggests that the alveolo-capillary membrane is affected. An antioxidant attenuates oxidative burst by neutrophilic granulocytes, but it does not affect PaO2, SaO2 or O2 uptake (VO2), and the ventilatory response to maximal exercise also remains the same. It is proposed, though, that increased concentration of certain cytokines correlates to exercise-induced hypoxaemia as cytokines stimulate mast cells and basophilic granulocytes to degranulate histamine. The basophil count increases during maximal rowing. Equally, histamine release is associated with hypoxaemia and when the release of histamine is prevented, the reduction in PaO2 is attenuated. During maximal exercise, an extreme lactate spill-over to blood allows pH decrease to below 7.1 and according to the O2 dissociation curve this is critical for SaO2. When infusion of sodium bicarbonate maintains a stable blood buffer capacity, acidosis is attenuated and SaO2 increases from 89% to 95%. This enables exercise capacity to increase, an effect also seen when O2 supplementation to inspired air restores arterial oxygenation. In that case, exercise capacity increases less than can be explained by VO2 and CaO2. Furthermore, the change in muscle oxygenation during maximal exercise is not affected when hyperoxia and sodium bicarbonate attenuate desaturation. It is proposed that other organs benefit from enhanced O2 availability, and especially the brain appears to increase its oxygenation during maximal exercise with hyperoxia.

Preexercise hypervolemia does not affect arterial hypoxemia in Thoroughbreds performing short-term high-intensity exercise

It is reported that preexercise hyperhydration caused arterial O(2) tension of horses performing submaximal exercise to decrease further by 15 Torr (Sosa-Leon L, Hodgson DR, Evans DL, Ray SP, Carlson GP, and Rose RJ. Equine Vet J Suppl 34: 425-429, 2002). Because hydration status is important to optimal athletic performance and thermoregulation during exercise, the present study examined whether preexercise induction of hypervolemia would similarly accentuate the arterial hypoxemia in Thoroughbreds performing short-term high-intensity exercise. Two sets of experiments (namely, control and hypervolemia studies) were carried out on seven healthy, exercise-trained Thoroughbred horses in random order, 7 days apart. In resting horses, an 18.0 +/- 1.8% increase in plasma volume was induced with NaCl (0.30-0.45 g/kg dissolved in 1,500 ml H(2)O) administered via a nasogastric tube, 285-290 min preexercise. Blood-gas and pH measurements as well as concentrations of plasma protein, hemoglobin, and blood lactate were determined at rest and during incremental exercise leading to maximal exertion (14 m/s on a 3.5% uphill grade) that induced pulmonary hemorrhage in all horses in both treatments. In both treatments, significant arterial hypoxemia, desaturation of hemoglobin, hypercapnia, acidosis, and hyperthermia developed during maximal exercise, but statistically significant differences between treatments were not found. Thus preexercise 18% expansion of plasma volume failed to significantly affect the development and/or severity of arterial hypoxemia in Thoroughbreds performing maximal exercise. Although blood lactate concentration and arterial pH were unaffected, hemodilution caused in this manner resulted in a significant (P < 0.01) attenuation of the exercise-induced expansion of the arterial-to-mixed venous blood O(2) content gradient.