Heart rate and oxygen uptake kinetics obtained from continuous measurements with wearable devices during outdoor walks of patients with COPD: Validation study (Preprint)

BACKGROUND Continuous physiological measurements during a laboratory-based exercise test can provide physiological biomarkers, such as heart rate (HR) and oxygen uptake (V̇O2) kinetics, that carry clinically relevant information. In contrast, it is not yet clear how continuous data generated during daily life routines by wearable devices could provide meaningful biomarkers that, in turn, could improve telemonitoring applications. OBJECTIVE To determine whether valid HR and V̇O2 kinetics can be obtained from measurements with wearable devices during outdoor walks in patients with chronic obstructive pulmonary disease (COPD). As a secondary objective, we aimed to determine whether the amount of valid kinetics and kinetic model performance was comparable between outdoor walks and a conventional, laboratory-based exercise test. METHODS Eight patients with COPD measured HR (Polar Belt) and V̇O2 (METAMAX 3B) during three outdoor walks of different intensities and a six-minute walk test. For every patient and walk/test, HR and V̇O2 data were extracted after the following physical activity transitions: (i) starting a walk/test, (ii) finishing a walk/test and (iii) walking upstairs. An additional, averaged HR and V̇O2 response during outdoor walks was generated for each type of transition of every patient. Kinetic models were used to describe every separate and averaged HR and V̇O2 response when participants started a walk, finished a walk and walked upstairs. HR and V̇O2 kinetics were considered valid if the response magnitude and model fit were adequate, and model parameters were reliable. Kinetic model performance was assessed by model fit and standard errors of the parameter estimates. RESULTS Most HR kinetics were valid when starting (range 75% to 100% for separate walks; 100% for the averaged response) or finishing (range 63% to 88% for separate walks; 100% for the averaged response) an outdoor walk, but not when walking upstairs (≤29%). Many V̇O2 kinetics were valid when finishing (range 63% to 100% for separate walks; 100% for the averaged response), but not when starting an outdoor walk (range 38% to 50% for separate walks; 88% for the averaged response) or when walking upstairs (0%). The amount of valid kinetics, and kinetic model performance (P>.05), when starting/finishing an outdoor walk was comparable to starting (HR: 100%; V̇O2: 50%) or finishing (HR: 88%; V̇O2: 75%) a laboratory-based six-minute walk test. CONCLUSIONS Continuous measurements with wearable devices can provide valid HR kinetics when starting or finishing, and valid V̇O2 kinetics when finishing, an outdoor walk in patients with COPD. The amount of valid kinetics and kinetic model performance was comparable between outdoor walks and a conventional, laboratory-based exercise test. We envision that telemonitoring applications for patients with COPD could be improved by incorporating regular assessments of HR (and possibly V̇O2) kinetics, as obtained from continuous measurements with wearable devices during outdoor walks.

Spleen Emptying Does Not Correlate With Faster Oxygen Kinetics During a Step-Transition Supine Cycling

This manuscript quantified spleen volume changes and examined the relationship between those changes and V̇O2 kinetics during supine cycling. Ten volunteers (age=22±3), completed three step-transitions from 20 W to their power output at 90% gas exchange threshold. Ultrasonic measurements of the spleen were performed each minute. The largest spleen volume reduction was 105 mL (p=.001). No associations existed between: i) spleen volumes at rest ii) spleen volume changes (%) and τV̇O2p. Larger resting spleen volume and greater emptying do not correlate with a faster τV̇O2p. Novelty: • Greater splenic contractions do not augment τV̇O2p, irrespective of spleen emptying and subsequent erythrocyte release.

Effects of Normobaric Hypoxia on Matched-severe Exercise and Power-duration Relationship

We investigated the effects of hypoxia on matched-severe intensity exercise and on the parameters of the power-duration relationship. Fifteen trained subjects performed in both normoxia and normobaric hypoxia (FiO2=0.13, ~3000 m) a maximal incremental test, a 3 min all-out test (3AOT) and a transition from rest to an exercise performed to exhaustion (Tlim) at the same relative intensity (80%∆). Respiratory and pulmonary gas-exchange variables were continuously measured (K5, Cosmed, Italy). Tlim test’s V̇O2 kinetics was calculated using a two-component exponential model. V̇O2max (44.1±5.1 vs. 58.7±6.4 ml.kg-1.min-1, p<0.001) was decreased in hypoxia. In Tlim, time-to-exhaustion sustained was similar (454±130 vs. 484±169 s) despite that V̇O2 kinetics was slower (τ1: 31.1±5.8 vs. 21.6±4.7 s, p<0.001) and the amplitude of the V̇O2 slow component lower (12.4±5.4 vs. 20.2±5.7 ml.kg-1.min-1, p<0.05) in hypoxia. CP was reduced (225±35 vs. 270±49 W, p<0.001) but W’ was unchanged (11.3±2.9 vs. 11.4±2.7 kJ) in hypoxia. The changes in CP/V̇O2max were positively correlated with changes in W’ (r = 0.58, p<0.05). The lower oxygen availability had an impact on aerobic related physiological parameters, but exercise tolerance is similar between hypoxia and normoxia when the relative intensity is matched despite a slower V̇O2 kinetics in hypoxia.

Aerobic trainability

The key parameters of aerobic fitness are arguably peak oxygen uptake (V̇O2), pulmonary V̇O2 kinetics, blood lactate/ventilatory gas exchange thresholds, and exercise economy. The effects of training on these parameters are well-established in adults but, with the exception of peak V̇O2, data from children and adolescents are sparse and confounded by methodological and ethical issues. It has been hypothesized that children lack trainability due to the existence of a maturation threshold that must be surpassed before training adaptations manifest themselves. While a persuasive theoretical argument exists regarding the reality of a maturation threshold, there is no compelling empirical evidence to support it. The extrapolation of a training-induced increase in aerobic fitness to enhanced youth sport performance is complex, and sport-specific research models need to be developed and implemented. To determine the mechanisms underpinning aerobic trainability during youth, rigorous investigations utilizing recent advances in non-invasive technologies are required.

Aerobic fitness

Peak oxygen uptake (V̇O2) is the criterion measure of young people's aerobic fitness, and blood lactate accumulation (BLA) is a useful indicator of aerobic fitness with reference to the ability to sustain submaximal exercise. In sport and in everyday life it is the pulmonary (p)V̇O2 kinetics of the non-steady state which best assess the integrated responses of the oxygen delivery system and the metabolic demands of the exercising muscle. Data analysis using sophisticated modelling techniques has enhanced understanding of sexual dimorphism and the independent effects of chronological age, body size, and biological maturity on peak V̇O2 and BLA. The extant data on young people's pV̇O2 kinetic responses to step changes in exercise intensity are sparse, but describe intriguing chronological age and sex differences across exercise domains. However, independent effects of biological maturation are yet to be revealed.

Regulation of V̇o2 kinetics by O2 delivery: insights from acute hypoxia and heavy-intensity priming exercise in young men

This study examined the separate and combined effects of acute hypoxia (Hypo) and heavy-intensity “priming” exercise (Hvy) on pulmonary O2 uptake (V̇o2p) kinetics during moderate-intensity exercise (Mod). Breath-by-breath V̇o2p and near-infrared spectroscopy-derived muscle deoxygenation {deoxyhemoglobin concentration [HHb]} were monitored continuously in 10 men (23 ± 4 yr) during repetitions of a Mod 1-Hvy-Mod 2 protocol, where each of the 6-min (Mod or Hvy) leg-cycling bouts was separated by 6 min at 20 W. Subjects were exposed to Hypo [fraction of inspired O2 (FiO2) = 15%, Mod 2 + Hypo] or “sham” (FiO2 = 20.9%, Mod 2-N) 2 min following Hvy in half of these repetitions; Mod was also performed in Hypo without Hvy (Mod 1 + Hypo). On-transient V̇o2p and [HHb] responses were modeled as a monoexponential. Data were scaled to a relative percentage of the response (0–100%), the signals were time-aligned, and the individual [HHb]-to-V̇o2 ratio was calculated. Compared with control (Mod 1), τV̇o2p and the O2 deficit (26 ± 7 s and 638 ± 144 ml, respectively) were reduced ( P < 0.05) in Mod 2-N (20 ± 5 s and 529 ± 196 ml) and increased ( P < 0.05) in Mod 1 + Hypo (34 ± 14 s and 783 ± 184 ml); in Mod 2 + Hypo, τV̇o2p was increased (30 ± 8 s, P < 0.05), yet O2 deficit was unaffected (643 ± 193 ml, P > 0.05). The modest “overshoot” in the [HHb]-to-V̇o2 ratio (reflecting an O2 delivery-to-utilization mismatch) in Mod 1 (1.06 ± 0.04) was abolished in Mod 2-N (1.00 ± 0.05), persisted in Mod 2 + Hypo (1.09 ± 0.07), and tended to increase in Mod 1 + Hypo (1.10 ± 0.09, P = 0.13). The present data do not support an “O2 delivery-independent” speeding of τV̇o2p following Hvy (or Hvy + Hypo); rather, this study suggests that local muscle O2 delivery likely governs the rate of adjustment of V̇o2 at τV̇o2p greater than ∼20 s.