Impairment of muscle energy metabolism in patients with sleep apnoea syndrome

The Heart Rate Response to Exercise Is Blunted in Patients with Sleep-Related Breathing Disorder

BACKGROUND

This study addressed the hypothesis that cardiovascular reactivity during graded bicycle exercise is influenced by a coexisting sleep-related breathing disorder (SRBD).

METHODS

Blood pressure and heart rate were assessed at rest and during graded exercise (110 W and maximal load) in 1,149 patients of a Sleep Disorders Center. The degree of SRBD, and potential confounders, including blood gases and pulmonary function, were determined.

RESULTS

Maximal exercise capacity tended to decrease with SRBD activity (p = 0.07). The heart rate reactivity was blunted and reduced by -0.07 and -0.09 beats/min for every SRBD unit at 110 W (p < 0.001) and at maximal load (p < 0.001), respectively. Systolic and diastolic blood pressure reactivity at maximal load were positively associated with SRBD severity. These associations were independent of possible confounders.

CONCLUSION

SRBD is associated with reduced physical working capacity and a modified hemodynamic response to exercise.

Skeletal muscle changes in patients with obstructive sleep apnoea syndrome

Previous studies have shown that chronic hypoxia leads to changes in skeletal muscle structure (fibre size and type) and activities of several bioenergetic enzymes. Whether this occurs also in conditions characterised by intermittent hypoxia, such as the obstructive sleep apnoea syndrome (OSAS), is unknown. To explore this possibility, we obtained a needle biopsy of the quadriceps femoris in 12 consecutive stable outpatients with severe OSAS (52 +/- 9 year, apnoea-hypopnoea index 70 +/- 14 h(-1)) (x +/- SD) and in six healthy volunteers (49 +/- 8 year), where we quantified fibre type, size and protein content, as well as phosphofructo-kinase (PFK) and cytochrome oxidase (CytOx) activities. We found that fibre-type distribution was similar in patients and controls. In contrast, the diameter of type II fibres (74 +/- 10 microm vs. 56 +/- 11 microm, P < 0.05) and protein content (100 +/- 14 vs. 88 +/- 8 microg/mg) was higher in patients with OSAS. Likewise, we observed upregulation of CytOx (0.93 +/- 0.38 vs. 0.40 +/- 0.22 microkat/mg protein, P < 0.01) and PFK activities (5.35 +/- 4.8 vs. 1.3 +/- 1.3 microkat/ mg protein, P < 0.05) in patients with OSAS. These results show that, paralleling which occurs in conditions characterised by continuous hypoxia, patients with OSAS (and intermittent hypoxia) also show structural and bioenergetic changes in their skeletal muscle.

Maximal exercise and muscle energy metabolism after recovery from exercise hyperthermia syndrome

Muscle energy metabolism was studied in 30 subjects after recovery from exercise hyperthermia syndrome (EHTS subjects) and 15 healthy men with identical physical activities. Blood lactate, free fatty acid (FFA), serum creatine kinase activity (CK), and glycerol and the temperature in the auditory duct (T(c)) and on the thumb pad (T(sk)) were measured at rest and during and after maximal exercise on a cycloergometer. The EHTS subjects had a limitation of physical performance, with lowered values for maximal oxygen uptake (VO(2max), P < 0.0005), maximal workload (P < 0.05), and ventilatory threshold (V(t), P < 0.0005). The discrepancy between high plasma concentrations of FFA and the lack of decrease in respiratory ratio (RR) suggests that, in EHTS subjects, a very active release of FFA was not balanced by a proportional increase in catabolism. The increased skin temperature was smaller in EHTS subjects (P < 0.05 at 180 and 200 W). At the end of exercise, auditory duct temperature increase was higher in EHTS subjects than in control subjects (P < 0.05). This study thus showed an impairment of muscle metabolism and an abnormality of thermoregulatory mechanisms. These results may provide insight into the underlying physiopathological disturbance.

Invited review: Adaptive responses of skeletal muscle to intermittent hypoxia: the known and the unknown

Intermittent hypoxia (IH) describes conditions of repeated, transient reductions in O2 that may trigger unique adaptations. Rest periods during IH may avoid potentially detrimental effects of long-term O2 deprivation. For skeletal muscle, IH can occur in conditions of obstructive sleep apnea, transient altitude exposures (with or without exercise), intermittent claudication, cardiopulmonary resuscitation, neonatal blood flow obstruction, and diving responses of marine animals. Although it is likely that adaptations in these conditions vary, some patterns emerge. Low levels of hypoxia shift metabolic enzyme activity toward greater aerobic poise; extreme hypoxia shifts metabolism toward greater anaerobic potential. Some conditions of IH may also inhibit lactate release during exercise. Many related cellular phenomena could be involved in the response, including activation of specific O2 sensors, reactive oxygen and nitrogen species, preconditioning, hypoxia-induced transcription factors, regulation of ion channels, and influences of paracrine/hormonal stimuli. The net effect of a variety of adaptive programs to IH may be to preserve contractile function and cell integrity in hypoxia or anoxia, a response that does not always translate into improvements in exercise performance.

Selected Contribution: Improved anoxic tolerance in rat diaphragm following intermittent hypoxia

Intermittent hypoxia (IH), associated with obstructive sleep apnea, initiates adaptive physiological responses in a variety of organs. Little is known about its influence on diaphragm. IH was simulated by exposing rats to alternating 15-s cycles of 5% O2 and 21% O2 for 5 min, 9 sets/h, 8 h/day, for 10 days. Controls did not experience IH. Diaphragms were excised 20-36 h after IH. Diaphragm bundles were studied in vitro or analyzed for myosin heavy chain isoform composition. No differences in maximum tetanic stress were observed between groups. However, peak twitch stress (P < 0.005), twitch half-relaxation time (P < 0.02), and tetanic stress at 20 or 30 Hz (P < 0.05) were elevated in IH. No differences in expression of myosin heavy chain isoforms or susceptibility to fatigue were seen. Contractile function after 30 min of anoxia (95% N2-5% CO2) was markedly preserved at all stimulation frequencies during IH and at low frequencies after 15 min of reoxygenation. Anoxia-induced increases in passive muscle force were eliminated in the IH animals (P < 0.01). These results demonstrate that IH induces adaptive responses in the diaphragm that preserve its function in anoxia.

Effects of short-term PAP treatment on endurance exercise performance in obstructive sleep apnea patients

Objective: To measure the effects of 4 weeks of nasal positive airway pressure therapy (PAP) on exercise performance in obstructive sleep apnea patients (OSA).Background: Little published research is available which describes the effects of OSA on exercise tolerance or upon the potential of exercise testing to evaluate the outcomes of PAP therapy.Methods: Exercise testing was performed on an electronic cycle ergometer with continuous ramping to allow collection of numerous data points for each subject, up to a vigorous terminal intensity. Linear regression established each subject's pre-treatment scores for the dependent variables at 60% of estimated peak power (W(60%)). Responses at the pre-treatment W(60%) test were used to quantify and compare to responses at the same power output after treatment.Results: OSA by nocturnal polysomnography was moderately severe in this group; the respiratory distress index was 48+/-22 (mean+/-SD; n=9). Exercise heart rates after PAP therapy averaged 10.2 bt/min less at W(60%) (P<0.05). Other variables were lower but non-significantly so, further suggesting a lower cardiorespiratory exercise demand after treatment, i.e. oxygen consumption ( downward arrow7.6%), and Rating of Perceived Exertion ( downward arrow8.8%).Conclusion: Brief treatment with PAP therapy improves objective markers of aerobic exercise performance.