Increased Ventilatory Response To Submaximal Exercise After 5 Weeks Of Head-down Bed Rest

Abstract

Previously, ventilation (VE) during incremental exercise was found to be elevated after 7 days of horizontal bed rest (FASEB J 1991, 5:A1477). We investigated the effects of 5 weeks of head-down bed rest (HDBR) on the exercise ventilatory response (EVR), defined as the slope of the relationship between VE and CO2 production (VCO2), to submaximal constant load exercise. To further explore the effects of HDBR, we examined short-term modulation (STM) of the EVR before and after HDBR. STM is defined as a neural mechanism where EVR is augmented in the presence of increased respiratory dead space (DS; J Appl Physiol 2008, 104: 244-252). PURPOSE: To examine the effects of 5 weeks of HDBR on EVR and the capacity for STM. METHODS: 25 subjects (3 female) spent 5 weeks in HDBR. As part of a larger study, subjects were assigned to 3 groups, 2 of which undertook exercise training during HDBR (semi-recumbent rowing 3 days/wk and horizontal strength training 2 days/wk) while the third remained inactive. The EVR was measured at rest and during upright cycle exercise (30W) without (Control) and with 400ml added external DS, before and after HDBR. The main effects of group and HDBR were analyzed with a two-way repeated measures ANOVA. RESULTS: The main effect of group was not significant for any of the variables tested. The physical characteristics for the 25 subjects were: age 33±11 yr, height 175±10 cm, weight 74±12 kg and BMI 24±2 kg/m2. EVR both with and without added DS was elevated post HDBR (Control: 27.9±4.5 and 30.3±5.6; DS: 31.4±7.2 and 36.0±8.1; pre and post HDBR, respectively; p<0.05). EVR was significantly augmented with DS vs. Control both pre and post (p<0.05), indicating STM of the EVR, and this capacity to augment the EVR was unaffected by HDBR. The increase in EVR was due to a significant decrease in VCO2 (Control: 0.79±0.15 and 0.73±0.11; DS: 0.75±0.11 and 0.70±0.10 l/min; at 30W pre and post HDBR, respectively; p<0.05), with no affect of HDBR on VE during exercise. CONCLUSION: HDBR appears to alter the EVR mainly by a reduction in VCO2, which we speculate might be associated with a change in substrate metabolism. However, HDBR does not appear to affect the capacity for STM to augment the EVR with added DS. Supported by NASA grant CA00701, the King Charitable Foundation Trust, Cain Foundation.