Experimental Physiology最新文献

Cheyne-Stokes respiration (CSR), a rhythmic rise and fall in ventilation often experienced by patients with heart failure during sleep, is typically accompanied by an oscillation in heart rate (HR) at the same frequency. The mechanisms responsible for this oscillation are still debated. In this study, we used the experimental model of the transient hypoxia test (i.e., a laboratory test that mimics the transient nature of the cyclic desaturations that occur during hyperpnoeic phases of CSR) to assess accurately the temporal relationship between the HR response to transient hypoxia and the tidal volume response in six heart failure patients. The same relationship was assessed during CSR using polysomnographic signals. We hypothesized that this relationship would provide important insights into the key mechanisms contributing to the HR response. During transient hypoxia, HR started to increase around the onset of tidal volume increase but continued to increase after the peak of the latter had been reached. The time delay between the two peaks (HR vs. tidal volume) was 7.9 ± 4.8 s. The same delay during hyperpnoeic phases of CSR was 1.0 ± 0.9 s. In addition, the increases in lung volume were much greater than those found in the laboratory tests. Based on the known dynamics of vagal and sympathetic control of HR, we speculate that the HR response to transient hypoxia might be attributable predominantly to the sympathetically mediated tachycardic effect of the increased central inspiratory drive, whereas the fast, vagally mediated pulmonary inflation reflex might be the predominant mechanism during CSR.

To investigate the acute effects of hypoxia applied during discrete work and recovery phases of a perceptually regulated, high-intensity interval exercise (HIIE) on external and internal loads in inactive overweight individuals. On separate days, 18 inactive overweight (28.7 ± 3.3 kg m^-2; 31 ± 8 years) men and women completed a cycling HIIE protocol (6 × 1 min intervals with 4 min active recovery, maintaining a perceived rating of exertion of 16 and 10 during work and recovery, respectively, on the 6-20 Borg scale) in randomized conditions: normoxia (NN), normobaric hypoxia (inspired O₂ fraction ∼0.14) during both work and recovery (HH), hypoxia during recovery (NH) and hypoxia during work only (HN). Markers of external (relative mean power output, MPO) and internal load (blood lactate concentration, heart rate and tissue saturation index (TSI)) were measured. MPO was lower in HH compared to NN, NH and HN (all P < 0.001), with HN also being lower than NN (P < 0.001) and NH (P < 0.023). Heart rate was higher in HN than NN, HH and NH (all P < 0.001). Blood lactate response was higher in NN than HH (P = 0.003) and NH (P = 0.008). Changes in the TSI area above the curve were greater in HN relative to NN, HH and NH (all P < 0.001). Hypoxia applied intermittently during the work or recovery phases may mitigate the declines in mechanical output observed when exercise is performed in continuous hypoxia, although hypoxia implemented during the work phase resulted in elevated heart rate and lactate response. Specifically, exercise performance largely comparable to that in normoxia can be achieved when hypoxia is implemented exclusively during recovery.

Hyperthermia can cause intestinal injury, facilitating endotoxin translocation and an inflammatory response that has been associated with heat illness. However, the potential occurrence of these responses has been incompletely reported during passive hyperthermia, and the independent effect of hyperthermia is equivocal. Furthermore, passive hyperthermia is a feature of heat therapy interventions, with mechanistic understanding developing. This experiment quantified the changes in intestinal fatty acid binding protein (iFABP), a marker of intestinal injury, and cytokine, chemokine and growth factor responses during three different prolonged passive hyperthermia protocols. Eight healthy males visited the laboratory on four counterbalanced occasions to undertake 2.5 h of rest (CON), one-leg heating (OLH), two-leg heating (TLH) and whole-body heating (WBH) via a garment circulating water at 50°C. Plasma concentrations of iFABP and 38 cytokines, chemokines and growth factors were quantified periodically, and core temperature (T_core) was measured continuously. The T_core increased from baseline in OLH, TLH and WBH (+0.4°C ± 0.2°C, +0.7°C ± 0.2°C and +2.3°C ± 0.4°C, respectively; P < 0.05) but remained unchanged in CON. iFABP increased from baseline in WBH only (∆587 ± 651 pg ml^-1) and was different from CON and OLH in WBH after 2 h (P < 0.05). Increased iFABP (∆1085 ± 572 pg ml^-1) was observed in 50% of participants at the end of WBH, with the other 50% demonstrating no change (∆89 ± 19 pg ml^-1). All chemokines, cytokines and growth factors were unchanged in all protocols. These data indicate that passive whole-body hyperthermia, but not lower-limb hyperthermia, can cause intestinal injury in some individuals without a systemic inflammatory response.

During exercise stress, heart rate (HR) increases to support cardiac output, which also reduces ventricular filling time. Although echocardiography is widely used to assess cardiac function, studies display conflicting data on the dynamic changes in the healthy trained and untrained heart during rest and acute exercise stress. To address these discrepancies, we tested a new echocardiography exercise protocol on two groups with significant differences in cardiorespiratory fitness. Ten untrained individuals with maximal oxygen uptake of 38 ± 8 ml/kg/min and 10 endurance-trained athletes matched for body surface area but with higher maximal oxygen uptake (71 ± 5 ml/kg/min) were evaluated at rest, during semi-recumbent cycling at 25 and 75 W and at a relative workload intensity eliciting a HR of 140 beats/min (HR140). Stroke volume was 36% higher in the trained at rest, and this difference increased during exercise to 42% at 25 W, 46% at 75 W and 63% at HR140 (all P < 0.05). In contrast, no group differences were found in markers of myocardial function (ventricular contraction and relaxation velocities) or other traditional echocardiographic measures of ventricular function at rest or exercise for a given HR. However, while similar at rest, diastolic and systolic function provided limited insight into differences between less fit and highly fit individuals. The new exercise echocardiography protocol improves the ability to uncover differences in dynamic changes in diastolic filling capacity that explain the previously reported higher end-diastolic compliance in endurance-trained athletes.

The current understanding of crew health maintenance is founded upon decades of physiological research conducted in terrestrial spaceflight analogues and in low Earth orbit, particularly on the International Space Station. However, as we progress towards the Lunar Gateway and interplanetary missions, it is imperative that the tools employed to maintain crew health are redefined, including the utilisation of exercise countermeasures. The successful implementation of exercise countermeasures for deep space missions must address a number of challenges, including those posed by new environments with elevated levels of cosmic radiation and solar particle events, extended mission durations and constrained space availability. In this Topical Review, the authors address points that are important (and sometimes critical), but often ignored, in order to define future exercise countermeasures for long-duration space missions. Multi-organ countermeasures, countermeasure enjoyment, time-dependent load variability, the relationship between nutrition and the success of exercise countermeasures, and the individual variability in response to a given countermeasure are presented and discussed. The aim of this article is to raise awareness of important aspects that can profoundly influence the efficacy of exercise countermeasures, thereby affecting the health of the crew and the success of the mission during prolonged spaceflight.