This study was undertaken to provide a detailed description of daily physical activity (DPA) levels in patients with MDM-confirmed IPF and evaluate its association with conventionally used markers of IPF disease severity. We extended this analysis to report the association of DPA with several other relevant clinical and patient reported variables. The results confirmed our expectations that IPF patients have low levels of DPA and high sedentary time. Whilst disease severity (measured by DLCO) was an important determinant of DPA, other non-respiratory factors were also important, particularly age, BMI and functional exercise capacity.
Similar to previous studies (15–17), we found that daily step counts were markedly reduced compared to values that have been reported in healthy adults of a similar age(15).
In addition, we have shown that decreased DPA in patients with IPF is already present in early stages of physiologic impairment as defined by CPI, which has been identified as a more powerful prognostic marker of mortality then either lung function or alveolar arterial O2 gradient (23), and worsens as disease severity progresses, similar to patients with COPD (29). Troosters et al showed that patients with GOLD-stage II COPD had reduced DPA and this reduction worsened with increasing GOLD stage. A CPI score of > 41 has been demonstrated to have association with increased 3-year mortality(30). Our study demonstrated a significant reduction in steps walked per day in patients with a CPI score > 41 and between patient in GAP stage II and III compared with stage I. The modest association between 6MWD and DPA (Tables 4 and 5) further emphasises the need for direct assessment of daily functioning away from the hospital. The reduction in physical activity with increasing disease severity, without changes in total energy expenditure, highlights the need for improved understanding of contributors to energy expenditure and strategies to enhance physical activity in people with IPF.
Interestingly, despite FVC commonly being used in the pivotal IPF antifibrotic studies, it did not correlate well with DPA, with DLCO and 6MWD being better independent predictors of various DPA parameters. The association between spirometry and DPA has not been demonstrated in all IPF studies (15–17). While demonstrating a significant correlation with FVC % predicted and DPA on simple linear regression, this relationship was no longer significant in the multiple regression model. The 6MWD and DLCO % predicted were the only independent predictors of steps per day accounting for 42% of variability which is similar to results from Wallaert et al(15).
Overall, patients with IPF, irrespective of disease severity, were not spending significant periods performing higher intensity activity. Current American College of Sports Medicine (ACSM) recommendation for endurance exercise for healthy older adults is at least 30 to 60 minutes per day in bouts of at least 10 minutes each, to total of 150 to 300 minutes per week of moderate or 75 to 150 minutes of vigorous intensity activity per week (31). It is not clear whether these recommendations are appropriate or achievable for people with IPF. Similar to Nakayama et al (16) who demonstrated that patients spent only 6 minutes per day in activity with an intensity equivalent to running, our results demonstrate that patients spent 9 minutes performing moderate to high intensity activities. Only 18.4% of patients in this study performed moderate to vigorous activities for greater than 30-minute duration, consistent with activity guidelines. Time spent performing vigorous activity was predicted by 6MWD, a physiologically important prognostic predictor (32–34).
While the association between quality of life and DPA has been previously described in IPF (16, 17, 35), our study is the first to assess the impact of cough related quality of life; a relevant analysis given the high prevalence of this symptom in IPF patients. Both the overall LCQ score and the physical domain LCQ correlated with high intensity physical activity duration and steps per day. However, patient reported outcomes were not consistently associated with all parameters of DPA suggesting that how patients feel may not reflect on what they do.
The strengths of the study included a patient cohort that only included MDM-confirmed IPF, in whom SAB compliance was exceedingly high. Limitations of this study were; the SAB was not worn while performing wet activities such as showering and may have resulted in lowering the estimation of the DPA and other measures that might have contributed to impaired physical activity such as frailty scores were not collected.
Important progress has been made in the last two decades with regards to management of IPF through large clinical trials (36). These clinical trials have highlighted the strengths and limitations of study endpoints. They have also highlighted the need to develop validated surrogate endpoints that measure how a patient feels and functions, to ensure that outcomes are relevant to patients. Although clinicians responsible for the care of patients with IPF may assume that surrogate endpoints such as spirometry and six-minute walk test are reflective of DPA, the low to moderate correlation coefficients in this study demonstrate the complex multidimensional nature of physical, psychosocial and environmental factors which affect physical activity in patients. As such, direct and objective measures are required to fully understand and accurately quantify DPA.
This study has described various parameters of physical activity monitoring and their relationship with current markers of disease progression and patient reported outcomes. Longitudinal assessment of physical activity, how it changes over time and its ability to predict outcomes such as mortality will improve our understanding of whether these parameters will enhance clinical practice and monitoring of outcomes from clinical trials.