Ventilatory oscillations at exercise: effects of hyperoxia, hypercapnia, and acetazolamide

Carotid-body modulation through meditation in stage-I hypertensive subjects: Study protocol of a randomized and controlled study

Adjunctive therapy for hypertension is in high demand for clinical research. Therefore, several meta-analyses have provided sufficient evidence for meditation as an adjunct therapy, without being anchored on reliable physiological grounds. Meditation modulates the autonomic nervous system. Herein, we propose a hierarchical-dependent effect for the carotid body (CB) in attenuating blood pressure (BP) and ventilatory variability (VV) fine-tuning due to known nerve connections between the CB, prefrontal brain, hypothalamus, and solitary tract nucleus. The aim of this exploratory study was to investigate the role of CB in the possible decrease in BP and changes in VV that could occur in response to meditation. This was a prospective, single-center, parallel-group, randomized, controlled clinical trial with concealed allocation. Eligible adult subjects of both sexes with stage 1 hypertension will be randomized into 1 of 2 groups: transcendental meditation or a control group. Subjects will be invited to 3 visits after randomization and 2 additional visits after completing 8 weeks of meditation or waiting-list control. Thus, subjects will undergo BP measurements in normoxia and hyperoxia, VV measurements using the Poincaré method at rest and during exercise, and CB activity measurement in the laboratory. The primary outcome of this study was the detection of changes in BP and CB activity after 8 weeks. Our secondary outcome was the detection of changes in the VV at rest and during exercise. We predict that interactions between hyperoxic deactivation of CB and meditation; Will reduce BP beyond stand-alone intervention or alternatively; Meditation will significantly attenuate the effects of hyperoxia as a stand-alone intervention. In addition, VV can be changed, partially mediated by a reduction in CB activity. Trial registration number: ReBEC registry (RBR-55n74zm). Stage: pre-results.

Exercising in Hypoxia and Other Stimuli: Heart Rate Variability and Ventilatory Oscillations

Periodic breathing is a respiratory phenomenon frequently observed in patients with heart failure and in normal subjects sleeping at high altitude. However, until recently, periodic breathing has not been studied in wakefulness and during exercise. This review relates the latest findings describing this ventilatory disorder when a healthy subject is submitted to simultaneous physiological (exercise) and environmental (hypoxia, hyperoxia, hypercapnia) or pharmacological (acetazolamide) stimuli. Preliminary studies have unveiled fundamental physiological mechanisms related to the genesis of periodic breathing characterized by a shorter period than those observed in patients (11~12 vs. 30~60 s). A mathematical model of the respiratory system functioning under the aforementioned stressors corroborated these data and pointed out other parameters, such as dead space, later confirmed in further research protocols. Finally, a cardiorespiratory interdependence between ventilatory oscillations and heart rate variability in the low frequency band may partly explain the origin of the augmented sympathetic activation at exercise in hypoxia. These nonlinear instabilities highlight the intrinsic "homeodynamic" system that allows any living organism to adapt, to a certain extent, to permanent environmental and internal perturbations.

[Adaption to chronic hypoxaemia by populations living at high altitude]

Permanent life at high altitude induces important physiological stresses linked to the exposure to chronic hypoxia. Various strategies have been adopted by diverse populations living in the Andes, Tibet or East Africa. The main mechanism is an increase in red blood cell production, more marked in Andeans than in Tibetans or Ethiopians. Other changes are observed in the cardiovascular or respiratory systems, as well as in the utero-placental circulation. Sometimes, a de-adaptation process to hypoxia develops, when erythrocytosis becomes excessive and leads to haematological, vascular and cerebral complications (Monge's disease or chronic mountain sickness). Pulmonary hypertension may also appear. Therapeutic options are available but not sufficiently used. Genetic studies have recently been undertaken to try to better understand the evolution of the human genome in populations living in various high altitude regions of the world, as well as the genetic risk factors for chronic diseases. A new model has appeared, intermittent chronic hypoxia, due to the development of economic activities (mainly mining) in desert regions of the Altiplano.

Low-frequency ventilatory oscillations in hypoxia are a major contributor to the low-frequency component of heart rate variability

PURPOSE

Heart rate variability (HRV) may be influenced by several factors, such as environment (hypoxia, hyperoxia, hypercapnia) or physiological demand (exercise). In this retrospective study, we tested the hypothesis that inter-beat (RR) intervals in healthy subjects exercising under various environmental stresses exhibit oscillations at the same frequency than ventilatory oscillations.

METHODS

Spectra from RR intervals and ventilation ([Formula: see text]E) were collected from 37 healthy young male subjects who participated in 5 previous studies focused on ventilatory oscillations (or periodic breathing) during exercise in hypoxia, hyperoxia and hypercapnia. Bland and Altman test and multivariate regressions were then performed to compare respective frequencies and changes in peak powers of the two signals.

RESULTS

Fast Fourier analysis of RR and [Formula: see text]E signals showed that RR was oscillating at the same frequency than periodic breathing, i.e., ~ 0.09 Hz (11 s). During exercise, in these various conditions, the difference between minimum and maximum HRV peak power was positively correlated to the same change in ventilation peak power (P < 0.05). Low-frequency (LF) peak power was correlated to tidal volume (P < 0.01) and breathing frequency (P < 0.001).

CONCLUSIONS

This study suggests that low-frequency ventilatory oscillations in hypoxia are a major contributor to the LF band power of heart rate variability. CLINICAL TRIAL REG. NO.: NCT02201875.

Systemic blood pressure at exercise in hypoxia in hypertensive and normotensive patients

OBJECTIVES

The current study aimed to determine whether acute hypoxia exposure in laboratory conditions associated with exercise induces an increase in systemic blood pressure (BP) in normotensive and hypertensive patients, and whether hypertensive patients are more prone to develop severe acute mountain sickness (sAMS). Finally, to determine if BP changes at exercise in acute hypoxia in hypertensive patients are predictive factors for sAMS.

METHODS

From 2012 to 2015, 852 normotensive and 106 hypertensive patients went through an acute hypoxia exercise test before a sojourn at high altitude. A subgroup of 228 normotensive was selected to match age, sex ratio, body weight and BMI and compared with the hypertensive group.

RESULTS

In normotensive and hypertensive patients, for a given workload, BP was higher in hypoxia than in normoxia, whereas, for a given heart rate, it was lower in hypoxia than in normoxia. Hypertensive patients treated by beta-blockers showed lower arterial oxygen saturation (vs. other treatments) and blunted cardiac and ventilatory responses to hypoxia at exercise. Based on questionnaires filled out at high altitude, hypertensive patients were not more prone than normotensive patients to develop sAMS. During the laboratory acute hypoxic exercise test, hypertensive patients suffering from sAMS, although taking acetazolamide showed similar BP than hypertensive patients without sAMS and without acetazolamide.

DISCUSSION AND CONCLUSION

We hypothesize that acute hypoxia with exercise in laboratory conditions induces a peripheral vasodilation that balances vasoconstriction and tachycardia centrally induced through the adrenergic system. Hypertensive and normotensive patients behave similarly during exercise in acute hypoxia. Acute hypoxia does not exacerbate the exercise-induced increase in BP. BP variation, during the acute hypoxia exercise test, is not a useful predictor of intolerance to high altitude. Based on laboratory tests in acute hypoxia, hypertensive patients may not be at higher risk to develop sAMS at high altitude.

Highs and lows of hyperoxia: physiological, performance, and clinical aspects

Molecular oxygen (O₂) is a vital element in human survival and plays a major role in a diverse range of biological and physiological processes. Although normobaric hyperoxia can increase arterial oxygen content ([Formula: see text]), it also causes vasoconstriction and hence reduces O₂ delivery in various vascular beds, including the heart, skeletal muscle, and brain. Thus, a seemingly paradoxical situation exists in which the administration of oxygen may place tissues at increased risk of hypoxic stress. Nevertheless, with various degrees of effectiveness, and not without consequences, supplemental oxygen is used clinically in an attempt to correct tissue hypoxia (e.g., brain ischemia, traumatic brain injury, carbon monoxide poisoning, etc.) and chronic hypoxemia (e.g., severe COPD, etc.) and to help with wound healing, necrosis, or reperfusion injuries (e.g., compromised grafts). Hyperoxia has also been used liberally by athletes in a belief that it offers performance-enhancing benefits; such benefits also extend to hypoxemic patients both at rest and during rehabilitation. This review aims to provide a comprehensive overview of the effects of hyperoxia in humans from the "bench to bedside." The first section will focus on the basic physiological principles of partial pressure of arterial O₂, [Formula: see text], and barometric pressure and how these changes lead to variation in regional O₂ delivery. This review provides an overview of the evidence for and against the use of hyperoxia as an aid to enhance physical performance. The final section addresses pathophysiological concepts, clinical studies, and implications for therapy. The potential of O₂ toxicity and future research directions are also considered.

Effect of dead space on breathing stability at exercise in hypoxia

Recent studies have shown that normal subjects exhibit periodic breathing when submitted to concomitant environmental (hypoxia) and physiological (exercise) stresses. A mathematical model including mass balance equations confirmed the short period of ventilatory oscillations and pointed out an important role of dead space in the genesis of these phenomena. Ten healthy subjects performed mild exercise on a cycloergometer in different conditions: rest/exercise, normoxia/hypoxia and no added dead space/added dead space (aDS). Ventilatory oscillations (V˙E peak power) were augmented by exercise, hypoxia and aDS (P<0.001, P<0.001 and P<0.01, respectively) whereas V˙E period was only shortened by exercise (P<0.001), with an 11-s period. aDS also increased V˙E (P<0.001), tidal volume (VT, P<0.001), and slightly augmented PETCO₂ (P<0.05) and the respiratory frequency (P<0.05). These results confirmed our previous model, showing an exacerbation of breathing instability by increasing dead space. This underlines opposite effects observed in heart failure patients and normal subjects, in which added dead space drastically reduced periodic breathing and sleep apneas. It also points out that alveolar ventilation remains very close to metabolic needs and is not affected by an added dead space. Clinical Trial reg. n°: NCT02201875.

Heart failure and sleep disorders

Awareness of the importance of sleep-related disorders in patients with cardiovascular diseases is growing. In particular, sleep-disordered breathing, short sleep time, and low sleep quality are frequently reported by patients with heart failure (HF). Sleep-disordered breathing, which includes obstructive sleep apnoea (OSA) and central sleep apnoea (CSA), is common in patients with HF and has been suggested to increase the morbidity and mortality in these patients. Both OSA and CSA are associated with increased sympathetic activation, vagal withdrawal, altered haemodynamic loading conditions, and hypoxaemia. Moreover, OSA is strongly associated with arterial hypertension, the most common risk factor for cardiac hypertrophy and failure. Intrathoracic pressure changes are also associated with OSA, contributing to haemodynamic alterations and potentially affecting overexpression of genes involved in ventricular remodelling. HF treatment can decrease the severity of both OSA and CSA. Indeed, furosemide and spironolactone administration, exercise training, cardiac resynchronization therapy, and eventually heart transplantation have shown a positive effect on OSA and CSA in patients with HF. At present, whether CSA should be treated and, if so, which is the optimal therapy is still debated. By contrast, more evidence is available on the beneficial effects of OSA treatment in patients with HF.