Both Heart and Muscle Factors Limit Exercise Capacity in Mitochondrial Disease

Reduced exercise capacity in people with mitochondrial diseases is linked to both a heart that works inefficiently and muscles that inadequately extract oxygen, according to a study that pinned down the physiological contributions that limit a patient’s ability to perform physical activity.

These mechanisms differ between patients with mitochondrial diseases and those with other chronic conditions, researchers found, suggesting their insights might help tailor interventions that aim to improve exercise capacity, and so, improve patients’ quality of life.

Since exercise capacity is linked to improved health outcomes in both healthy people and those with chronic diseases, knowing how to best target limitations might be crucial to improving exercise capacity, researchers at the Newcastle University in the U.K figured.

In healthy people, exercise capacity is governed by the ability of the cardiovascular system to supply muscles with oxygen along with the ability of muscles to use that oxygen.

The study, “Pathophysiology of exercise intolerance in chronic diseases: the role of diminished cardiac performance in mitochondrial and heart failure patients,” figured that it was most likely a poor heart performance that limited performance across various types of patients.

Published in the journal Open Heart, the study showed that the reality was more complex.

Researchers recruited patients with heart failure, stroke, mitochondrial disease, type 2 diabetes, and healthy controls, whom they followed between September 2012 and January 2016.

Patients with mitochondrial disease had either MIDD (maternally inherited diabetes and deafness), a so-called myopathic phenotype, or MELAS — all related to two known mitochondrial mutations.

Extensive measurements of heart function and oxygen use showed that only patients with heart failure and mitochondrial disease had a poorer heart performance. Those with mitochondrial conditions had, on average, 26% lower heart performance than healthy people in the study.

They also showed a 24% reduction in the ability of muscles to extract oxygen, which was more extensive than that found in patients with stroke and diabetes.

Cardiac output — the volume of blood that leaves the heart in each heartbeat — explained 51% of the variability in peak oxygen consumption in mitochondrial patients, while the extraction of oxygen by muscles explained 48% of the variability, researchers said.

With this, it became clear that exercise capacity in patients with mitochondrial disease is limited by both heart and muscle factors — insights that might help healthcare staff to better manage these patients’ condition.

An optimally designed intervention could improve these patients’ stress tolerance and quality of life, the researchers concluded.