Exercise-induced changes in plasma potassium and the ventilatory threshold in man

A new technique to analyse threshold-intensities based on time dependent change-points in the ratio of minute ventilation and end-tidal partial pressure of carbon-dioxide production

The aim of this study was to test the utility and effectiveness of an alternative computational approach to threshold-intensities based on time dependent change-points in minute ventilation divided by end-tidal partial pressure of CO₂ (V_E/P_ETCO₂) to reveal whether respiratory compensation point (RCP) is a third ventilatory threshold, or not. Ten recreationally active young adults and ten well-trained athletes volunteered to take part in this study. Following incremental ramp tests, gas exchange threshold (GET) and respiratory compensation point (RCP) were respectively evaluated by the slopes of VCO₂-VO₂ and V_E-VCO₂ using the Innocor system automatically. Respiratory threshold (RT) was analysed based on time dependent change-points in the V_E/P_ETCO₂ using binary segmentation algorithm. Additionally, those intersections were analysed independently by two experienced investigators using a visual identification technique in a double-blind design. According to the results, in the recreationally active group, there were the first (GET₁) and the second (GET₂) gas exchange thresholds which were identical with the RT₁ (139 W; 1.9 L⋅min^-1 of VO₂; 1.73 L⋅min^-1 of VCO₂; 49.9 L⋅min^-1 of V_E versus 139 W; 1.88 L⋅min^-1; 1.7 L⋅min^-1; 49 L⋅min^-1, respectively) and RT₂ (186 W; 2.39 L⋅min^-1 of VO₂; 2.44 L⋅min^-1 of VCO₂; 66 L⋅min^-1 of V_E versus 187 W; 2.41 L⋅min^-1; 2.49 L⋅min^-1; 65.7 L⋅min^-1, respectively). However, there were three threshold intensities which were determined by GET₁, GET₂, and RCP in well-trained athletes. Additionally, RT₁, RT₂, and RT₃ were determined as valid surrogates of the GET₁ (194 W; 2.56 L⋅min^-1 of VO₂; 1.99 L⋅min^-1 of VCO₂; 57.5 L⋅min^-1 of V_E versus 192 W; 2.61 L⋅min^-1; 1.99 Lmin^-1; 57.7 L⋅min^-1, respectively), GET₂ (267 W; 3.6 L⋅min^-1 of VO₂; 3.29 L⋅min^-1 of VCO₂; 94.5 L⋅min^-1 of V_E versus 266 W; 3.58 L⋅min^-1; 3.26 L⋅min^-1; 93.4 L⋅min^-1, respectively), and RCP (324 W; 4.05 L⋅min^-1 of VO₂; 4.13 L⋅min^-1 of VCO₂; 124 L⋅min^-1 of V_E versus 322 W; 4.02 L⋅min^-1; 4.07 L⋅min^-1; 122 L⋅min^-1, respectively) in well-trained athletes. There were high levels of agreements between the power outputs determined by traditional techniques and newly proposed change-points in RT. All markers were strongly correlated (p < 0.001). It was shown that RT technique can provide an accurate threshold determination. Furthermore, the RCP was observed as a third threshold-intensity for well-trained athletes but not for recreationally active young adults.

Lactate threshold by muscle electrical impedance in professional rowers

Lactate threshold (LT) is one of the physiological parameters usually used in rowing sport training prescription because it indicates the transitions from aerobic to anaerobic metabolism. Assessment of LT is classically based on a series of values of blood lactate concentrations obtained during progressive exercise tests and thus has an invasive aspect. The feasibility of noninvasive LT estimative through bioelectrical impedance spectroscopy (BIS) data collected in thigh muscles during rowing ergometer exercise tests was investigated. Nineteen professional rowers, age 19 (mean) ± 4.8 (standard deviation) yr, height 187.3 ± 6.6 cm, body mass 83 ± 7.7 kg, and training experience of 7 ± 4 yr, were evaluated in a rowing ergometer progressive test with paired measures of blood lactate concentration and BIS in thigh muscles. Bioelectrical impedance data were obtained by using a bipolar method of spectroscopy based on the current response to a voltage step. An electrical model was used to interpret BIS data and to derive parameters that were investigated to estimate LT noninvasively. From the serial blood lactate measurements, LT was also determined through Dmax method (LT_Dmax). The zero crossing of the second derivative of kinetic of the capacitance electrode (Ce), one of the BIS parameters, was used to estimate LT. The agreement between the LT estimates through BIS (LT_BIS) and through Dmax method (LT_Dmax) was evaluated using Bland-Altman plots, leading to a mean difference between the estimates of just 0.07 W and a Pearson correlation coefficient r = 0.85. This result supports the utilization of the proposed method based on BIS parameters for estimating noninvasively the lactate threshold in rowing.

Effects of gas exchange on acid-base balance

This paper describes the interactions between ventilation and acid-base balance under a variety of conditions including rest, exercise, altitude, pregnancy, and various muscle, respiratory, cardiac, and renal pathologies. We introduce the physicochemical approach to assessing acid-base status and demonstrate how this approach can be used to quantify the origins of acid-base disorders using examples from the literature. The relationships between chemoreceptor and metaboreceptor control of ventilation and acid-base balance summarized here for adults, youth, and in various pathological conditions. There is a dynamic interplay between disturbances in acid-base balance, that is, exercise, that affect ventilation as well as imposed or pathological disturbances of ventilation that affect acid-base balance. Interactions between ventilation and acid-base balance are highlighted for moderate- to high-intensity exercise, altitude, induced acidosis and alkalosis, pregnancy, obesity, and some pathological conditions. In many situations, complete acid-base data are lacking, indicating a need for further research aimed at elucidating mechanistic bases for relationships between alterations in acid-base state and the ventilatory responses.

Relationship of potassium ions and blood lactate to ventilation during exercise

Ventilatory control during exercise is a complex network of neural and humoral signals. One humoral input that has received little recent attention in the exercise literature is potassium ions [K(+)]. The purpose of this study was to examine the relationship between [K(+)] and ventilation during an incremental cycle test and to determine if the relationship between [K(+)] and ventilation differs when blood lactate [lac-] is manipulated. Eight experienced triathletes (4 of each sex) completed 2 incremental, progressive (5-min stages) cycle tests to volitional fatigue: 1 with normal glycogen stores and 1 with reduced glycogen. Minute ventilation was measured during the final minute of each stage, and blood [lac(-)] and [K+] were measured at the end of each exercise stage. Minute ventilation and [K(+)] increased with exercise intensity and were similar between trials (p > 0.5), despite lower [lac-] during the reduced-glycogen trial. The concordance correlations (R(c)) between [lac(-)] and minute ventilation were stronger for both trials (R(c) = approximately 0.88-0.96), but the slopes of the relationships were different than the relationships between [K(+)] and minute ventilation (R(c) = approximately 0.76-0.89). The slope of the relationship between [lac-] and minute ventilation was not as steep during the reduced-glycogen trial, compared with the normal trial (p = 0.002). Conversely, the slope of the relationships between [K(+)] and minute ventilation did not change between trials (p = 0.454). The consistent relationship of minute ventilation and blood [K(+)] during exercise suggests a role for this ion in the control of ventilation during exercise. Conversely, the inconsistent relationship between blood lactate and ventilation brings into question the importance of the relationship between lactate and ventilation during exercise.

[Response of tidal volume to inspiratory time ratio during incremental exercise]

OBJECTIVE

There is some debate about the participation of the Hering-Breuer reflex during exercise in human beings. This study aimed to investigate breathing pattern response during an incremental exercise test with a cycle ergometer. Participation of the Hering-Breuer reflex in the control of breathing was to be indirectly investigated by analyzing the ratio of tidal volume (VT) to inspiratory time (tI).

SUBJECTS AND METHODS

The 9 active subjects who participated the study followed an incremental protocol on a cycle ergometer until peak criteria were reached. During exercise, VT/ti can be described in 2 phases, separated by activation of the Hering-Breuer reflex (inspiratory off-switch threshold). In phase 1, ventilation increases because VT increases, resulting in a slight decrease in tI, whereas, in phase 2, increased ventilation is due to both an increase in VT and a decrease in tI.

RESULTS

The mean (SD) inspiratory off-switch threshold was 84.6% (6.3%) when expressed relative to peak VT (mean, 3065 [566.8] mL) and 48% (7.2%) relative to the forced vital capacity measured by resting spirometry. The inspiratory off-switch threshold correlated positively (r=0.93) with the second ventilatory threshold, or respiratory compensation point.

CONCLUSIONS

The inspiratory off-switch threshold and VT/ti are directly related to one another. The inspiratory off-switch threshold was related to the second ventilatory threshold, suggesting that the Hering-Breuer reflex participates in control of the breathing pattern during exercise. Activation of the reflex could contribute by signaling the respiratory centers to change the breathing pattern.

Respiratory compensation point during incremental exercise as related to hypoxic ventilatory chemosensitivity and lactate increase in man

The pulmonary ventilation-O2 uptake (VE-VO2) relationship during incremental exercise has two inflection points: one at a lower VO2, termed the ventilatory threshold (VT); and another at a higher VO2, the respiratory compensation point (RCP). The individuality of RCP was studied in relation to those of the chemosensitivities of the central and peripheral chemoreceptors, which were assessed by resting estimates of hypercapnic ventilatory response (HCVR) and hypoxic ventilatory response (HVR), respectively, and the rate of lactic acid increase during exercise, which was estimated as a slope difference (delta slope) between a lower slope of VCO2-VO2 relationship (VCO2:CO2 output) obtained at work rates below VT and a higher slope at work rates between VT and RCP. Twenty-two male and sixteen female subjects underwent a 1 min incremental exercise test until exhaustion, in which VT, RCP and delta slope were determined. All measures were normalized for body surface area. In the males, the individual difference in RCP was inversely correlated with those of HVR and delta slope (p < 0.05), and in the females, similar tendencies persisted, while the correlation did not reach statistically significant levels (0.05 < p < 0.1). There was no significant correlation between RCP and HCVR in either sex. A multiple linear regression analysis showed that 40 to 50% of the variance of RCP was accounted for by those of HVR and delta slope, both of which were related linearly and additively to RCP, this relation being manifested in the males but not in the females without consideration of the menstrual cycle. These results suggest that the individuality of RCP depends partly on the chemosensitivity of the carotid bodies and the rate of lactic acid increase during incremental exercise.

Potassium and breathing in exercise

The increase in ventilation caused by exercise is controlled by a combination of neural and chemical events, although the precise contribution and relative importance of these signals is still debated. It is generally agreed that the genesis of exercise hyperpnoea lies within the central nervous system and that peripheral reflexes, both chemical and neural, modulate central drive. Recently, attention has once again focused on the idea that circulating factors, in particular potassium, may play an important role in this modulation by stimulating known areas of peripheral chemoreception. Arterial chemoreceptors, muscle chemoreflex and slowly adapting pulmonary stretch receptors are all excited by hyperkalaemia. When potassium is raised to mimic exercise concentrations it increases ventilation in anaesthetised animals. This response is abolished by surgical denervation of the arterial chemoreceptors and is markedly reduced by chemical denervation with hyperoxia. Hypoxia enhances the ventilatory response to hyperkalaemia, and the stimulatory effects of potassium are further increased when combined with lactic acid or raised concentrations of noradrenaline. Hyperkalaemia can also increase the hypoxic sensitivity of the arterial chemoreflex in exercise. There is a close temporal relationship between potassium and ventilation during exercise, but changes in potassium are not proportionally related to changes in ventilation. When all data are taken together, there is good evidence that potassium has a supporting role in the control of exercise hyperpnoea, predominantly through modulation of the arterial chemoreflex.

Dangerous curves. A perspective on exercise, lactate, and the anaerobic threshold

A number of general observations can be made from these recent studies. Lactate is a ubiquitous substance that is produced and removed from the body at all times, even at rest, both with and without the availability of oxygen. It is now recognized that lactate accumulates in the blood for several reasons, not just the fact that oxygen supply to the muscle is inadequate. Lactate production and removal is a continuous process; it is a change in the rate of one or the other that determines the blood lactate level. Rather than a specific threshold, there is most likely a period of time during which lactate production begins to exceed the body's capacity to remove it (through buffering or oxidation in other fibers). It may be appropriate to replace the term "anaerobic threshold" to a more functional description, since the muscles are never entirely anaerobic nor is there always a distinct threshold ("oxygen independent glycolysis" among others has been suggested) Lactate plays a major role as a metabolic substrate during exercise, is the preferred fuel for slow-twitch muscle fibers, and is a precursor for liver gluconeogenesis. The point at which lactate begins to accumulate in the blood, causing an increase in ventilation, is important to document clinically. Irrespective of the underlying mechanism or specific model that describes the process, the physiologic changes associated with lactate accumulation have significant import for cardiopulmonary performance. These include metabolic acidosis, impaired muscle contraction, hyperventilation, and altered oxygen kinetics, all of which contribute to an impaired capacity to perform work. Thus, any delay in the accumulation of blood lactate which can be attributed to an intervention (drug, exercise training, surgical, etc) may add important information concerning the efficacy of the intervention. A substantial body of evidence is available demonstrating that lactate accumulation occurs later (shifting to a higher percentage of Vo2max) after a period of endurance training. In athletes, the level of work that can be sustained prior to lactate accumulation, visually determined, is an accurate predictor of endurance performance. Presumably, these concepts have implications related to vocation/disability among patients with cardiovascular and pulmonary disease, but few such applied studies have been performed outside the laboratory. Blood lactate during exercise and its associated ventilatory changes maintain useful and interesting applications in both the clinical exercise laboratory and the sport sciences. However, the mechanism, interpretation, and application of these changes continue to rely more on tradition and convenience than science.

Effects of potassium and lactic acid on chemoreceptor discharge in anaesthetized cats

Both increasing [K+]a and falling pHa stimulate ventilation through an action on the peripheral chemoreceptors. We have examined the effect on afferent carotid chemoreceptor discharge, of intravenous infusion of lactic acid alone, KCl alone, and both combined at constant PETCO2 in anaesthetized, artificially ventilated cats. Infusions of lactic acid alone and KCl alone caused similar increases in both the mean and amplitude of oscillation of chemoreceptor discharge. In the case of the lactic acid alone infusion the increase in the amplitude of oscillation could be accounted for by the resultant increase in carbon dioxide production. Simultaneous infusion of KCl and lactic acid caused an increase in the mean and amplitude of the discharge which was greater than either given alone, although the combined effect was less than additive. The alterations in mean and amplitude of oscillation of discharge during infusion of both agents together may be completely accounted for by a combined effect of increased carbon dioxide production and elevated [K+]a.

A review of the control of breathing during exercise

During the past 100 years many experimental investigations have been carried out in an attempt to determine the control mechanisms responsible for generating the respiratory responses observed during incremental and constant-load exercise tests. As a result of these investigations a number of different and contradictory control mechanisms have been proposed to be the sole mediators of exercise hyperpnea. However, it is now becoming evident that none of the proposed mechanisms are solely responsible for eliciting the exercise respiratory response. The present-day challenge appears to be one of synthesizing the proposed mechanisms, in order to determine the role that each mechanism has in controlling ventilation during exercise. This review, which has been divided into three primary sections, has been designed to meet this challenge. The aim of the first section is to describe the changes in respiration that occur during constant-load and incremental exercise. The second section briefly introduces the reader to traditional and contemporary control mechanisms that might be responsible for eliciting at least a portion of the exercise ventilatory response during these types of exercise. The third section describes how the traditional and contemporary control mechanisms may interact in a complex fashion to produce the changes in breathing associated with constant-load exercise, and incorporates recent experimental evidence from our laboratory.