In the present study, HRs at VT1 and VT2 have been compared to fixed HR percentages, i.e., of 70%HRpeak, 70%Wpeak, and 60%HRR in patients with ILD, ILD subgroups and an age-matched healthy control group. Our findings demonstrate differences in performance characteristcs and the related scattering of fixed HR percentages when compared to the individual VT1 and VT2. Patients with ILD had lower exercise capacity (VO2peak and Wpeak) and lower cardorespiratory responses (HRpeak and SpO2peak) to maximal exercise than controls. Comparisons between ILD types revealed higher VO2peak (%pred) and peak HRs in patients with sarcoidosis compared to those with CTD, which is in agreement with other studies (2,3,31). Scattering of fixed HR percentages is rather small for HRs at 70%Wpeak and 60%HRR but comparatively large for HR at 70%HRpeak (figure 1). In contrast to the control group, HR at 70%HRpeak in ILD is at or slightly below the HR at VT1. However, the scatter range is probably too large to generate optimal individual training effects, because exercise intensity may be below VT1 in some ILD patients or above VT1 in others.
Assessment of appropiate exercise intensities in patients with chronic diseases becomes
more and more important. It has been suggested that people with ILD may need more careful planning and modification of their exercise prescription than healthy subjects or even patients with COPD . Compared to the number of studies including COPD patients, clinical studies dealing with pulmonary rehabilitation in ILD is relatively small . Priciples of pulmonary rehabilitation are similar for both groups of diseases. However, exercise-induced desaturation and related complications occur more frequently in ILD patients, emphasizing the importance of proper training intensity selections .
Generally, VTs derived from CPET ensure individual physiological adaptations to exercise and can help to find the optimal training “zones” . VT1 and VT2 form boundaries for the determination of 3 training zones (from low to high) successfully applied in athletes and patients as well [21, 28]. Whereas in athletes the largest proportion of the training volume is performed at intensities below VT1 , in patients suffering from lung diseases, including ILD, intensities above VT1 are preferentially applied in rehabilitation [12, 16, 29]. This is at least partly based on the early study by Casaburi et al., who evaluated effects of various training intensities in COPD patients. These authors found reduced ventilatory requirements and improved exercise tolerance after training at intensities above VT1, due to metabolic adaptations within the working muscles resulting in lower blood lactate concentration, diminished carbon dioxide production and associated lower exercise ventilation .
The individual application of training intensities based on CEPT is particularly needed by patients suffering from different diseases. For instance, several training studies in chronic heart failure patients implicated the VT1 as an useful and valid method for individual training prescription [27, 30, 31]. In those patients, the proper assessment of training intensity was emphasized because of the high inter-patient variance. Similarly, intensity prescription based on HR identification at the VT was also highlighted for patients with left-ventricular dysfunction (LVDF) . Even in healthy subjects it was shown that exercising according to a fixed HRR for 12 weeks, VO2peak was increased in only 42% of the total group when compared to a significantly improved VO2peak in all individuals exercising according to the range between VT1 and VT2 [23, 32]. It was also suggested that due to the heterogenity of ILD patients, i.e., those suffering from sarcoidosis, modification and program adjustment of the standard pulmonary rehabilitation format, including individual prescription of training intensity, are required . Our findings confirm the large variability of heart rate responses to exercise (CPET) and the considerable scattering of fixed HR percentages in comparison to HRs at the individual VTs in ILD patients. Thus, as claimed for cardiac rehabilitation , or even more important, the approach of fixed HR percentages may be inaccurate in a large proportion of ILD patients undertaking rehabilitation and should be replaced by individual VTs determined by CPET.
To the best of our knowledge, this is the first study comparing HRs at the individual VTs and fixed HR percentages. Thus, the presented findings derived from a relatively large cohort of ILD patients not only highlight the importance of CEPT but also provide valuable basis for training intensity prescription for those patients.
This study may be limited by the inter-observer variability in the determination of ventilatory threshold. In order to minimise the bias the ventilatory thresholds were determined and cross-checked by two different observers. The patients in our study were only mild to moderately limited, which explain on one side that the VT2 could be asessed in all but 2 patients and on the other hand the relatively mild impairement in exercise capacity, which was nevertheless significantly lower compared to the control group.