Effects of enhanced human chemosensitivity on ventilatory responses to exercise

Ventilatory acclimatisation is beneficial for high-intensity exercise at altitude in elite cyclists

AIM

The aim of this study was to examine the relationship between ventilatory adaptation and performance during altitude training at 2700 m.

METHODS

Seven elite cyclists (age: 21.2 ± 1.1 yr, body mass: 69.9 ± 5.6 kg, height 176.3 ± 4.9 cm) participated in this study. A hypoxic ventilatory response (HVR) test and a submaximal exercise test were performed at sea level prior to the training camp and again after 15 d at altitude (ALT15). Ventilation (VE), end-tidal carbon-dioxide partial pressure (PETCO2) and oxyhaemoglobin saturation via pulse oximetry (SpO2) were measured at rest and during submaximal cycling at 250 W. A hill climb (HC) performance test was conducted at sea level and after 14 d at altitude (ALT14) using a road of similar length (5.5-6 km) and gradient (4.8-5.3%). Power output was measured using SRM cranks. Average HC power at ALT14 was normalised to sea level power (HC%). Multiple regression was used to identify significant predictors of performance at altitude.

RESULTS

At ALT15, there was a significant increase in resting VE (10.3 ± 1.9 vs. 12.2 ± 2.4 L·min(-1)) and HVR (0.34 ± 0.24 vs. 0.71 ± 0.49 L·min(-1)·%(-1)), while PETCO2 (38.4 ± 2.3 vs. 32.1 ± 3.3 mmHg) and SpO2 (97.9 ± 0.7 vs. 94.0 ± 1.7%) were reduced (P < .05). Multiple regression revealed ΔHVR and exercise VE at altitude as significant predictors of HC% (adjusted r(2) = 0.913; P = 0.003).

CONCLUSIONS

Ventilatory acclimatisation occurred during a 2 wk altitude training camp in elite cyclists and a higher HVR was associated with better performance at altitude, relative to sea level. These results suggest that ventilatory acclimatisation is beneficial for cycling performance at altitude.

Modified ventilatory response characteristics to exercise in breath-hold divers

Specific adjustments to repeated extreme apnea are not fully known and understood. While a blunted ventilatory chemosensitivity to CO2 is described for elite breath-hold divers (BHDs) at rest, it is unclear whether specific adaptations affect their response to dynamic exercise. Eight elite BHDs with a previously validated decrease in CO2 chemosensitivity, 8 scuba divers (SCDs), and 8 matched control subjects were included in a study where markers of ventilatory response, Fowler's dead space, partial pressure of carbon dioxide (pCO2), and blood lactate concentrations during cycle exercise were measured. Maximal power output did not differ between the groups, but lactate threshold (θL) appeared at a significantly lowered respiratory compensation point (RCP) and at a higher VO2 for the BHDs. End-tidal (petCO2) and estimated arterial pCO2 (paCO2) were significantly higher in BHDs at θL, the RCP, and maximum exhaustion. BHDs showed a significantly (P < .01) slower breathing pattern in relation to a given tidal volume at a specific work rate. In summary, BHDs presented signs of a metabolic shift from aerobic to anaerobic energy supply, decreased chemosensitivity during exercise, and a distinct ventilatory-response pattern during cycle exercise that differs from SCDs and controls.

Intermittent hypoxia does not affect endurance performance at moderate altitude in well-trained athletes

In this study, we examined the effects of a pre-acclimatization programme on endurance performance at moderate altitude using a resting intermittent hypoxia protocol. The time-trial performance of 11 cyclists was determined at low altitude (600 m). Athletes were randomly assigned in a double-blind fashion to the hypoxia or the control group. The pre-acclimatization programme consisted of seven sessions each lasting 1 h in normobaric hypoxia (inspired fraction of oxygen of 12.5%, equivalent to approximately 4500 m) for the hypoxia group (n = 6) and in normoxia (inspired fraction of oxygen of 20.9%) for the control group (n = 5). The time-trials were repeated at moderate altitude (1970 m). Mean power output during the time-trial at moderate altitude was decreased in the hypoxia group (-0.26 +/- 0.11 W x kg(-1)) and in the control group (-0.13 +/- 0.04 W x kg(-1)) compared with at low altitude but did not differ between groups (P = 0.13). Our results suggest that the applied protocol of intermittent hypoxia had no positive effect on endurance performance at moderate altitude. Whether different intermittent hypoxia protocols are advantageous remains to be determined.

Cerebral hypoperfusion during hypoxic exercise following two different hypoxic exposures: independence from changes in dynamic autoregulation and reactivity

We examined the effects of exposure to 10–12 days intermittent hypercapnia [IHC: 5:5-min hypercapnia (inspired fraction of CO2 0.05)-to-normoxia for 90 min ( n = 10)], intermittent hypoxia [IH: 5:5-min hypoxia-to-normoxia for 90 min ( n = 11)] or 12 days of continuous hypoxia [CH: 1,560 m ( n = 7)], or both IH followed by CH on cardiorespiratory and cerebrovascular function during steady-state cycling exercise with and without hypoxia (inspired fraction of oxygen, 0.14). Cerebrovascular reactivity to CO2 was also monitored. During all procedures, ventilation, end-tidal gases, blood pressure, muscle and cerebral oxygenation (near-infrared spectroscopy), and middle cerebral artery blood flow velocity (MCAv) were measured continuously. Dynamic cerebral autoregulation (CA) was assessed using transfer-function analysis. Hypoxic exercise resulted in increases in ventilation, hypocapnia, heart rate, and cardiac output when compared with normoxic exercise ( P < 0.05); these responses were unchanged following IHC but were elevated following the IH and CH exposure ( P < 0.05) with no between-intervention differences. Following IH and/or CH exposure, the greater hypocapnia during hypoxic exercise provoked a decrease in MCAv ( P < 0.05 vs. preexposure) that was related to lowered cerebral oxygenation ( r = 0.54; P < 0.05). Following any intervention, during hypoxic exercise, the apparent impairment in CA, reflected in lowered low-frequency phase between MCAv and BP, and MCAv-CO2 reactivity, were unaltered. Conversely, during hypoxic exercise following both IH and/or CH, there was less of a decrease in muscle oxygenation ( P < 0.05 vs. preexposure). Thus IH or CH induces some adaptation at the muscle level and lowers MCAv and cerebral oxygenation during hypoxic exercise, potentially mediated by the greater hypocapnia, rather than a compromise in CA or MCAv reactivity.

Respiratory control during air-breathing exercise in humans following an 8 h exposure to hypoxia

Hypoxic exposure lasting a few hours results in an elevation of ventilation and a lowering of end-tidal PCO2(PETCO2) that persists on return to breathing air. We sought to determine whether this increment in ventilation is fixed (hypothesis 1), or whether it increases in proportion to the rise in metabolic rate associated with exercise (hypothesis 2). Ten subjects were studied on two separate days. On 1 day, subjects were exposed to 8 h of isocapnic hypoxia (end-tidal PO2 55 Torr) and on the other day to 8 h of euoxia as a control. Before and 30 min after each exposure, subjects undertook an incremental exercise test. The best fit of a model for the variation in PETCO2 with metabolic rate gave a residual squared error that was ∼20-fold less for hypothesis 2 than for hypothesis 1 (p < 0.005, F-ratio test). We conclude that the alterations in respiratory control induced during early ventilatory acclimatization to hypoxia better reflect those associated with hypothesis 2 rather than hypothesis 1.

Control of ventilation in humans following intermittent hypoxia

Exposure to chronic or intermittent hypoxia produces alterations in the ventilatory response to hypoxia. These adaptations can differ depending on the severity of the hypoxic stimulus, its duration, its pattern, and the presence or absence of other chemical stimuli. As such, there are significant differences between the responses to intermittent versus continuous hypoxia. Intermittent hypoxia (IH) has been shown to elicit significant changes in the peripheral chemoresponse, but the functional implications of these changes for resting and exercise ventilation are not clear. We summarize the impact of IH on resting chemosensitivity and discuss the use of IH to better understand ventilatory control during exercise. We also suggest future directions for this relatively young field, including potential clinical applications of IH research.

Long-term intermittent hypoxia increases sympathetic activity and chemosensitivity during acute hypoxia in humans

We determined the effects of 10 daily exposures of intermittent hypoxia (IH; 1 h day(-1); oxyhaemoglobin saturation = 80%) on muscle sympathetic nerve activity (MSNA, peroneal nerve) and the hypoxic ventilatory response (HVR) before, during and after an acute 20 min isocapnic hypoxic exposure. We also assessed the potential parallel modulation of the ventilatory and sympathetic systems following IH. Healthy young men (n = 11; 25 +/- 1 years) served as subjects and pre- and post-IH measures of MSNA were obtained on six subjects. The IH intervention caused HVR to significantly increase (pre-IH = 0.30 +/- 0.03; post-IH = 0.61 +/- 0.12 l min(-1) %S(aO(2)) (-1)). During the 20 min hypoxic exposure sympathetic activity was significantly greater than baseline and remained above baseline after withdrawal of the hypoxic stimulus, even though oxyhaemoglobin saturation had normalized and ventilation and blood pressure had returned to baseline levels. When compared to the pre-IH trial, burst frequency increased (P < 0.01), total MSNA trended towards higher values (P = 0.06), and there was no effect on burst amplitude (P = 0.82) during the post-IH trial. Following IH the rise in MSNA burst frequency was strongly related to the change in HVR (r = 0.79, P < 0.05) suggesting that these sympathetic and ventilatory responses may have common central control.