Quantitative measurement of oxygen consumption and forearm blood flow in patients with mitochondrial myopathies

Skeletal muscle mitochondrial capacity in patients with statin-associated muscle symptoms (SAMS)

OBJECTIVE

The objective of this article is to evaluate near-infrared spectroscopy (NIRS), a non-invasive technique to assess tissue oxygenation and mitochondrial function, as a diagnostic tool for statin-associated muscle symptoms (SAMS).

METHODS

We verified SAMS in 39 statin-treated patients (23 women) using a double-blind, placebo-controlled, cross-over protocol. Subjects with suspected SAMS were randomised to simvastatin 20 mg/day or placebo for 8 weeks, followed by a 4-week no treatment period and then assigned to the alternative treatment, either simvastatin or placebo. Tissue oxygenation was measured before and after each statin or placebo treatment using NIRS during handgrip exercise at increasing intensities of maximal voluntary contraction (MVC).

RESULTS

44% (n=17) of patients were confirmed as having SAMS (11 women) because they reported discomfort only during simvastatin treatment. There were no significant differences in percent change in tissue oxygenation in placebo versus statin at all % MVCs in all subjects. The percent change in tissue oxygenation also did not differ significantly between confirmed and unconfirmed SAMS subjects on statin (-2.4% vs -2.4%, respectively) or placebo treatment (-1.1% vs -9%, respectively). The percent change in tissue oxygenation was reduced after placebo therapy in unconfirmed SAMS subjects (-10.2%) (p≤0.01) suggesting potential measurement variability.

CONCLUSIONS

NIRS in the forearm cannot differentiate between confirmed and unconfirmed SAMS, but further research is needed to assess the usability of NIRS as a diagnostic tool for SAMS.

TRIAL REGISTRATION NUMBER

NCT03653663.

Forearm deoxyhemoglobin and deoxymyoglobin (deoxy[Hb + Mb]) measured by near-infrared spectroscopy (NIRS) using a handgrip test in mitochondrial myopathy

The purpose of this paper is to test whether peripheral oxygenation responses measured with near-infrared spectroscopy (NIRS) would differ between patients suffering from mitochondrial myopathy (MM) and healthy controls during an incremental handgrip exercise test. Two groups of subjects were studied: 11 patients with MM and 11 age- and gender-matched untrained healthy controls. A handgrip exercise until exhaustion protocol was used consisting of 2 min periods of work (½ Hz) at different intensities, separated by a 60 s rest period. The changes in deoxyhemoglobin and deoxymyoglobin (deoxy[Hb + Mb]) during each work step were expressed in percent to the maximum deoxy[Hb + Mb]-value measured during arterial occlusion in forearm muscles. A repeated measures analysis of variance was used to compare the increase in deoxy[Hb + Mb] between MM patients and controls with increasing intensity. Statistical analysis revealed a significant difference between both populations (P < 0.001) indicating that the increase in deoxy[Hb + Mb] showed a significantly different pattern in the two populations. In the post hoc analysis significant lower deoxy[Hb + Mb] -values were found for MM patients at every intensity. The results of this paper show significantly different skeletal muscle oxygenation responses, measured with an optical method as NIRS, between MM patients and age- and gender-matched healthy subjects at submaximal and maximal level during an incremental handgrip exercise. This optical method is thus a valuable tool to assess differences in peripheral oxygenation. Moreover, this method could be used as an evaluation tool for follow up in interventional pharmacological studies and rehabilitation programs.

Exertional fatigue in patients with CKD

BACKGROUND

Fatigue is one of the most prevalent symptoms in chronic kidney disease (CKD). However, fatigue mechanisms are poorly understood due in part to nonspecific definitions. This study investigates exertional fatigue during simulated activities of daily living, focusing on oxygen delivery and utilization.

STUDY DESIGN

"Explanatory" matched-cohort study.

PARTICIPANTS & SETTING

13 patients with CKD (stages 3b-4; mean age, 62 ± 13 [SD] years) and 13 healthy controls, mean matched for age, height, body mass and composition, and physical activity level. Participants completed an incremental cycle ergometer test to simulate energy expenditure of typical activities of daily living.

FACTOR

4 exercise intensities: 1, 1.8, 2.4, and 3.1 metabolic equivalent tasks (METs).

OUTCOMES

The primary outcome was exertional fatigue by rating of perceived exertion (RPE) on a 6-20 scale.

MEASUREMENTS

Other multidimensional measures of fatigue: UK Short Form Health Survey 36 (UK SF-36) Vitality and Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-Fatigue) subscales. Physiologic measures of the oxygen transport and utilization chain (expired gas analysis, cardiac output, and arterial oxygen content) and blood lactate.

RESULTS

RPE was increased in patients compared with controls at 2.4 (10.5 [ie, light] ± 2.7 vs 8.7 [very light] ± 1.7 units) and 3.1 (12.5 [somewhat hard] ± 2.6 vs 10.2 [light] ± 1.7 units) METs (interaction P = 0.03), which was consistent with higher chronic fatigue in patients by both the UK SF-36 Vitality (P = 0.01) and FACIT-Fatigue (P = 0.004) subscales. Arterial oxygen content was decreased in patients (P = 0.001), but cardiac output and oxygen extraction ratio were unchanged, decreasing oxygen delivery (P = 0.04). Respiratory exchange ratio (P = 0.004) and blood lactate production (P = 0.002) were increased.

LIMITATIONS

Those inherent to a matched-cohort study.

CONCLUSIONS

Using a novel application of the outcome measure RPE, patients with non-dialysis-dependent CKD reported considerable exertional fatigue during simulated activities of daily living. Poor compensation for mild anemia contributed to this symptom. In addition to anemia, the entire oxygen transport chain needs to be targeted to treat fatigue in patients with CKD.

Skeletal muscle reoxygenation after high-intensity exercise in mitochondrial myopathy

This study addressed whether O(2) delivery during recovery from high-intensity, supra-gas exchange threshold exercise would be matched to O(2) utilization at the microvascular level in patients with mitochondrial myopathy (MM). Off-exercise kinetics of (1) pulmonary O(2) uptake VO(2P) (2) an index of fractional O(2) extraction by near-infrared spectroscopy (Δ[deoxy-Hb + Mb]) in the vastus lateralis and (3) cardiac output (Q'(T)) by impedance cardiography were assessed in 12 patients with biopsy-proven MM (chronic progressive external ophthalmoplegia) and 12 age- and gender-matched controls. Kinetics of VO(2P) were significantly slower in patients than controls (τ = 53.8 ± 16.5 vs. 38.8 ± 7.6 s, respectively; p < 0.05). Q'(T), however, declined at similar rates (τ = 64.7 ± 18.8 vs. 73.0 ± 21.6 s; p > 0.05) being typically slower than [Formula: see text] in both groups. Importantly, Δ[deoxy-Hb + Mb] dynamics (MRT) were equal to, or faster than, τVO(2P) in patients and controls, respectively. In fact, there were no between-group differences in τVO(2P)MRTΔ[deoxy-Hb + Mb] (1.1 ± 0.4 vs. 1.0 ± 0.2, p > 0.05) thereby indicating similar rates of microvascular O(2) delivery. These data indicate that the slower rate of recovery of muscle metabolism after high-intensity exercise is not related to impaired microvascular O(2) delivery in patients with MM. This phenomenon, therefore, seems to reflect the intra-myocyte abnormalities that characterize this patient population.

Lactate increase and oxygen desaturation in mitochondrial disorders – Evaluation of two diagnostic screening protocols

BACKGROUND

Mitochondrial disorders are characterized by an accumulation of lactate and an insufficient oxygen extraction from blood during exercise. Therefore, both parameters (lactate and oxygen saturation) can be used as screening tests in mitochondrial disorders. However, conflicting results regarding sensitivities and specifities of both tests have been reported.

METHOD

We examined 27 patients with genetically defined mitochondrial disorders (single deletions n = 15, multiple deletions n = 5, A3243G mutation n = 7), patients with other neuromuscular disorders, and healthy controls. In the first test subjects performed intermittent isometric handgrip exercise (0.5 Hz) at 80% (3 minutes) and 30% (3 and 15 minutes) of maximal contraction force (MCF). Oxygen saturation and partial pressure in cubital venous blood from the exercising arm were measured. In the second test subjects underwent cycle ergometry at 30 W for 15 minutes. Venous lactate at rest, during and 15 minutes postexercise was determined.

RESULT

Both tests showed specificities of 92-96%. Sensitivities for changes of venous oxygen partial pressure and oxygen saturation ranged from 21-26% at 80% MCF for 3 minutes to 47-58% at 30% MCF for 15 minutes. Sensitivities for venous resting, peak, and post-exercise lactate was 33%, 58%, and 67%, respectively. The degree of deoxygenation, however,was independent of the intensity and duration of the applied forces. Oxygen desaturation and lactate increase in patients with mitochondrial disorders were not different in patients with and without clinical symptoms of myopathy. There were significant correlations between the heteroplasmy and both the degree of oxygen desaturation and lactate increase in patients with single deletions. In patients who performed both protocols (n = 16) a combination of both tests increased sensitivity up to 87%.

CONCLUSION

Oxygen desaturation in forearm exercise tests and lactate increase in cycle ergometry tests show a high specifity but only moderate sensitivity. Combination of the two screening test clearly increases the sensitivity.

Mitochondrial dysfunction measured in vivo

AIMS

Mitochondria are responsible for meeting the majority of the energetic demand of most tissues. They also play a major role in regulating cell survival. These dual roles of mitochondria place them at the centre of many pathologies leading to tissue degeneration and disruption of energy balance. The prominent role of mitochondria in ageing and disease has led to a tremendous growth in mitochondrial research at the cellular and molecular level. We describe below a new non-invasive approach to measure mitochondrial function that will bridge the gap between our understanding of mitochondrial function in vitro and that in the intact organism.

METHODS AND RESULTS

This approach uses optical and magnetic resonance spectroscopy to measure in vivo O2 consumption and ATP synthesis rates, respectively, from skeletal muscle. These values lead to a quantitative assessment of the mitochondrial ATP/O2 or P/O. The P/O represents the efficiency of coupling between phosphorylation and oxygen consumption in the mitochondria, which is a measure of mitochondrial dysfunction.

CONCLUSIONS

This work represents a significant advance in research on the role of mitochondria in degenerative disease and ageing because it allows a quantitative measure of mitochondrial pathology in vivo. The non-invasive nature of this approach also enables repeated measures of mitochondrial function on the same individual, thereby making this a potentially useful diagnostic technique. The results from this work have led to insights into the coupling of ATP synthesis to oxidation and the regulation of oxidative phosphorylation by intracellular PO2.

Chronic progressive external ophthalmoplegia

Chronic progressive external ophthalmoplegia (CPEO) is a descriptive term for a heterogenous group of disorders characterized by chronic, progressive, bilateral, and usually symmetric ocular motility deficit and ptosis. Significant pain, proptosis, or pupil involvement are not features of CPEO and should prompt evaluation for alternative etiologies. Mitochondrial DNA mutations are increasingly being recognized as the etiology for CPEO syndromes. Clinicians should recognize the specific syndromes associated with CPEO, characterized by variable systemic, neurologic, or other findings. Treatment is limited, but newer therapies are being investigated.