Muscles of CF Youth Consume Oxygen Faster Than Peers, Affecting Exercise

Poorer oxygen uptake and transport may underlie skeletal muscle differences that affect exercise tolerance in children and adolescents with cystic fibrosis (CF) relative to healthy peers, a recent study suggests.

The study, “Tissue oxygenation in peripheral muscles and functional capacity in cystic fibrosis: A cross‐sectional study,” was published in the journal Experimental Physiology.

Problems in the functioning of peripheral, or skeletal, muscles are known in CF patients, leading to exercise intolerance, or a poorer ability to perform physical activity.

Many factors contribute to this dysfunction, such as abnormalities related to the cystic fibrosis transmembrane conductance regulator (CFTR) protein (the protein defective in CF patients), the metabolism of muscle cells, and mitochondria (the power-houses of cells where energy is produced).

The question of how muscle oxygenation and levels of physical conditioning in young CF patients compare with healthy age-matched peers, however, remains open.

Researchers in Brazil measured and compared muscle oxygen saturation, along with several measures of cardio-respiratory function, between these two groups of children and teenagers, matched for age and sex.

Their study enrolled 124 youth (median age of 10), in equal groups of 62 people with or without CF.

Over 40% of the CF patients had one F508del mutation, nearly 39% had two F508del mutations, and 20.97% had other mutations.

Participants’ muscle oxygenation and exercise capacity was evaluated through spirometry, a common test of the speed and volume of air moving in and out of the lungs. Researchers also used  the modified shuffle test (MST), in which a person walks or runs at increasing speeds repeatedly over a 10-meter course, largely until either reaching the maximum distance of 1,500 meters or being no longer able to continue.

Throughout the MST, muscle oxygenation was measured using near-infrared spectroscopy (NIRS) sensors placed on the calf muscle. NIRS is a non-invasive tool that monitors muscle function by measuring the oxygen content of hemoglobin, the protein that transports oxygen in circulating blood.

Other vital parameters related to cardiopulmonary function, including heart rate, respiratory rate, and blood pressure, were measured before MST, immediately after, and one minute after finishing.

Overall, MST scores were lower among CF patients compared with healthy peers.

NIRS measures showed that the hemoglobin of CF youth carried less oxygen, leading to reduced tissue oxygen saturation. This caused them to consume oxygen more quickly than their peers, and to recover from oxygen consumption more slowly, the researchers wrote.

As a result, CF patients tended to walk shorter distances with less efficiency. They also showed lower respiratory and heart rates following the MST, and needed longer times for their heart rates to return to normal.

Youth with CF also had higher levels of dyspnea, or shortness of breath, as measured by the modified Borg scale for dyspnea.

Neither disease severity nor genetic mutations appeared to affect patients’ performance.

Children and adolescents with cystic fibrosis in this study “consumed more oxygen, more quickly and exhibited slower recovery, demonstrating that there may have been deficiencies in oxygen supply related to both oxygen uptake and oxygen transport,” the researchers wrote.

Physical fitness, they added, is a strong indicator of quality of life and mortality in CF. Tools that help in monitoring functional capacities, such as NIRS, may be useful in establishing physical activity programs for patients.