Double Benefit: Boost Your Fitness and Breathe More Easily

See also the article by Diaz et al in this issue.

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Breathing deeply while engaging in moderate to vigorous physical activity such as running, jumping jacks, burpees, and hopping in and out of squats builds cardiorespiratory endurance and maintains a healthy lifestyle free of cardiovascular and respiratory diseases. The cardiorespiratory fitness relies on the power of the circulatory and respiratory systems to transport oxygen from the atmosphere to the mitochondria of the skeletal muscles and meet the muscle oxygen requirement during sustained physical activity (1). This chain of processes includes intertwined function between the respiratory and cardiovascular systems, allowing the skeletal muscles to receive needed oxygen and communicate their metabolic demands back to the cardiovascular control center. Cardiorespiratory fitness is measured directly as maximal oxygen consumption and the most commonly used measurement is obtained indirectly from the peak work rate achieved on a treadmill (1).

Moreover, the objective quantification of cardiorespiratory fitness includes the metabolic equivalent of task (MET) as a ratio of the rate of the person’s energy expenditure relative to their body mass while performing an exercise (1,2). Mounting scientific evidence shows cardiorespiratory fitness as a survival benefit per MET (2). Whereas the unit 1 MET is considered energy expenditure at rest, studies reported that small (1 or 2) MET increments in cardiorespiratory fitness demonstrate 17% and 21% lower risk of mortality, respectively (3,4). Conversely, 20% lower survival was observed for every 1 MET decrement in cardiorespiratory fitness (5), meaning that a sedentary lifestyle triggers the ripple effect of low-grade systemic inflammation as a potential mechanism in developing cardiovascular and respiratory diseases (4–7). The premise of this hypothesis is supported by studies identifying the link between low levels of cardiorespiratory fitness and high levels of systemic blood inflammatory markers such as high-sensitivity C-reactive protein and interleukin-6 (6).

It is well established that low cardiorespiratory fitness is an independent and more powerful risk predictor of all-cause mortality than other combined traditional risk factors such as hypertension, diabetes, obesity, hyperlipidemia, and smoking (7). However, the précis of this evidence is encouraging low-fit individuals to engage in modest activity level. This activity level should be gradually uptitrated to establish exercise habits and increase physical activity to the level recommended by the guidelines set forth by the American Heart Association (8). These guidelines include minimum of 150 minutes per week of moderate or 75 minutes per week of vigorous intensity aerobic activity for adults to offset the risks associated with sedentary lifestyle. The amount and intensity of workouts should be increased gradually in adults to reach 300 minutes per week (8). Brisk walking, tennis (doubles), gardening, dancing, and biking are examples of moderate-intensity aerobic activities, whereas hiking, running, swimming, cycling 10 miles per hour, and tennis (singles) are examples of vigorous-intensity aerobic activities (8).

In addition, high-level cardiorespiratory fitness is associated with improved pulmonary function and decreased risk of chronic obstructive pulmonary disease including asthma (6). However, the benefit of high cardiorespiratory fitness on reducing the risk of other small airway structural remodeling caused by repeated episodes of inflammation such as bronchiectasis remains a clinical conundrum. Therefore, preventative strategies that focus on forestalling the presence of bronchiectasis on the basis of the maintained fitness level offer a patient-centric approach to offset decreased survival. The emphasis of this approach should be building cardiorespiratory endurance by engagement in daily aerobic activities.

In this issue of Radiology, Diaz et al (9) investigated the association between 20-year change in treadmill exercise duration or cardiorespiratory fitness and the presence of bronchiectasis on CT scans at year 25 of a 30-year follow-up. In addition, the authors investigated the potential mediation of systemic blood inflammatory markers on the relationship between cardiorespiratory fitness and bronchiectasis. For this purpose, the authors used 30-year longitudinal data from 2177 healthy participants aged 18–30 years enrolled in the Coronary Artery Risk Development in Young Adults, or CARDIA, study (9). The underpinning of their hypothesis, which is supported by the literature, is that high levels of high-sensitivity C-reactive protein and interleukin-6 were associated with low-level cardiorespiratory fitness (6). Repeated episodes of inflammation associated with low-level cardiorespiratory fitness are causing bronchial wall structural changes or remodeling, leading to bronchiectasis and symptoms of infection including difficulty breathing.

Diaz et al stratified fitness changes in the participants into four groups on the basis of sex-specific median treadmill duration at two points, baseline (ie, year 0) and year 20 (9). The sustained low fitness group consisted of participants with fitness level below the median at both points, whereas the decreased fitness group exhibited fitness level below the median at the second point at year 20. Furthermore, the increased fitness group demonstrated increased fitness level at the second point in year 20 and the sustained high fitness group exhibited fitness level above the median for the two points (baseline and year 20). The authors showed that both men and women in the sustained high fitness group were significantly leaner, completed more years of education, and demonstrated lower levels of blood inflammatory biomarkers and higher pulmonary function than did the participants in the sustained low fitness group (9). The prevalence of bronchiectasis at year 25 was 9.6%, with similar distribution across men and women, and it affected predominantly lower lobes followed by the lingula, middle lobe, and upper lobes.

One of the most compelling findings in this study (9) was the longitudinal assessment of 20 years of cardiorespiratory fitness and its effect on the presence of bronchiectasis at year 25. Diaz et al showed that preservation of cardiorespiratory fitness or 1-minute–longer treadmill duration from year 0 to year 20 was associated with 12% decrease in the odds of developing bronchiectasis verified at CT (odds ratio, 0.88; 95% CI: 0.80, 0.98) (9). This association was independent of demographics, smoking history, or history of previous infections. The inference of their analysis was confirmed when clinical bronchiectasis defined as the presence of symptoms of phlegm and cough was used as a dependent variable instead of radiographic evidence of bronchiectasis. In addition, the authors showed that only interleukin-6, not high-sensitivity C-reactive protein, contributed 11% to the 20-year fitness change effect of the odds of bronchiectasis on CT scans at year 25 (9). Supporting studies second the correlation between low cardiorespiratory fitness and presence of asthma and pneumonia (6).

The corollary of these results suggests that high cardiorespiratory fitness decreases systemic and airway inflammation, improves the capacity and efficiency of cardiorespiratory systems with improved airway perfusion including mucociliary system, and therefore prevents the development of bronchiectasis. These results also add decreased risk of bronchiectasis to the myriad of health benefits achieved by staying fit. It may also have an important clinical impact on the use of preventative strategies to modify cardiorespiratory fitness to offset the development of bronchiectasis, a disease with frequent exacerbations that impacts quality of life. This study by Diaz et al showcases the need for personalized preventative treatment of patients with a sedentary lifestyle to engage in some level of modest aerobic activity recommended by the American Heart Association (8) to preserve the airway structure and therefore airway health.

There were several limitations including the small number of events (9.6%) despite the large sample of healthy participants (9). These limitations prevented generalizability of the findings. The second limitation was a lack of delving into the severity of bronchiectasis at year 25 and the effect on quality of life. The third limitation was the observational study design and lack of radiographic or clinical diagnosis of bronchiectasis at baseline, which further limited the determination of causality claim in either direction (9). This line of inquiry is ripe for future research and can serve as a hypothesis for further study of the relevance between cardiorespiratory fitness and bronchiectasis risk. The future line of inquiry will inform preventative patient-centered approaches to fight against a sedentary lifestyle and to promote lifestyle changes. Several questions remain. First, does cardiorespiratory fitness at baseline predict the presence of bronchiectasis at year 25? Second, what is the cutoff of minimum treadmill duration that protects against clinical and radiographic diagnosis of bronchiectasis? Finally, how does cardiorespiratory fitness change quality of life in patients with bronchiectasis? The answers to these relevant questions set the stage for a shift toward personalized medicine.

In summary, individuals with preserved fitness over 20 years have a lower risk of developing bronchiectasis. Conversely, individuals with decreased fitness over 20 years have a higher risk of developing bronchiectasis, a debilitating disease with frequent exacerbations and poor quality of life (10). The underlying premise is that improving cardiorespiratory fitness presents health professionals with unique opportunities to encourage lifestyle-based strategies designed to reduce the risk of respiratory diseases including bronchiectasis.

Stay fit by breathing deeply while exercising to be healthy!