DISCUSSION AND CONCLUSION
Up to 50% - 80% of patients with peripheral arterial occlusive disease (PAOD) are symptomatic, and half of these demonstrate complaints of IC (1). A resting ABI measurement with additional exercise testing using a standard treadmill test is a simple, inexpensive and effective non-invasive diagnostic method for IC with a high sensitivity (79-95%) and specificity (95-100%) (7). However, some patients are not able to adequately perform such testing, occasionally due to fear or (temporary) orthopedic or neurological causes. In the present trial, 7.5% of the patients were not able to safely/reliably complete a treadmill test. Other studies reported that 22-45% of the eligible patients were unable to undertake a treadmill test, mainly because of cardiac and pulmonary problems (8). An alternative stress test may allow for a proper identification of IC in these patients. The present study was designed to validate two protocols of different intensity using a bicycle ergometer as an alternative diagnostic tool in patients suspected of IC. Findings of this study indicate that data obtained from bicycle testing are closely correlated with data after treadmill testing. Moreover high sensitivities for the diagnosis IC were obtained. Both a submaximal as well as a maximal
bicycle test appeared valid methods and may serve as an alternative to treadmill testing in patients with IC.
Several theoretical advantages associated with a bicycle test may be identified. The method is 'non-weightbearing' allowing the testing of patients with orthopedic disease or with equilibrium problems. Bicycles can be used in patients who do not feel confident with treadmill testing. As in treadmill testing, diagnostic bicycle tests may also serve as a screening tool for determining cardiovascular risk (9, 10). For instance, a 10-year mortality rate due to cardiovascular disease of 62% for men with PAOD compared to 17% in the population of men without was found (5). Others already demonstrated that bicycle testing is predictive of cardiovascular events and all-cause death. Furthermore, bicycling testing may have additional advantages in a subgroup of patients with severe cardiopulmonary disease or with very mild IC as the workload is more easily administered and monitored. These issues are particularly relevant in cyclists suffering from IC at high intensity exercise due to vascular flow limitation in the iliac arteries (11). Furthermore, other characteristics of bicycle testing such as pedal forces are potentially useful in quantifying IC or PAOD. One could hypothesize that diminished blood flow in affected arteries will result in declining muscle strengths. As a consequence, pedal forces may also decline after pain symptoms are reported. Therefore, a bicycle may serve as a tool in several areas including the diagnostic, therapeutic and research arena
A fair correlation between ABI after walking tests and bicycling tests was earlier found in a small IC population (r=0.51-0.86)(12). The present study confirmed in a twice as large group that ABI after walking and bicycling were strongly correlated (r=0.88-0.90). Bland-Altman plots, traditionally considered a robust reflection of agreement between different two tests, demonstrated minimal differences between both exercise methods. However, a substantial dispersion was observed indicating a considerable variability. This inconsistency may possibly be explained by an error in reproducibility. Earlier research suggested that an up to 19% difference between two consecutive ABI values may just be attributed to intra-observer errors (13). A comparable magnitude of error was also observed in the present study as ABIs of 23% of the measured legs demonstrated an absolute ABI difference >0.15 prior to testing.
It is doubtful whether ABI drops after a cycling test reflect alterations obtained after a treadmill test. Although expected, one group of researchers did not find a significant correlation between absolute fall in ABI after both types of exercises (12). It was hypothesized that this absent correlation was possibly due to a different haemodynamic response in the lower extremity after either cycling or walking. Motivation, experience and joint stiffness may also have added to the remaining variability in performance ability (14). Cycling was deemed less useful as a diagnostic tool. In contrast the present study found a significant correlation (ICC=0.66, p<0.001, figure 4) between ABI falls after either challenge. It may well be that systemic and local factors may play a less important role than previously suggested. Moreover, this finding
also contributes to the confidence we have in the potential of a bicycle test as a useful diagnostic tool.
Which bicycle test may be preferred as an alternative diagnostic tool in future IC populations? The present study tested two different protocols. The first was a 'submaximal' regimen characterized by identical energy expenditure and mode of administration (direct and continuous submaximal exercise). By using such an exercise design, a possible confounding effect of workload or differences in energy expenditure was minimized. A second 'maximal test' was introduced to maximize these effects. As sensitivity of both bicycle tests were high (98%) whereas the SBT demonstrated a higher specificity, it is advised to standardly use a submaximal regimen. Moreover, the risk on a cardiovascular event during submaximal testing may be reduced.
In conclusion, the present study indicates that bicycle exercise testing is a valid alternative tool in diagnosing patients with intermittent claudication (IC). Bicycle testing confers additional advantages including incorporation of a cardiovascular screening program. Patients suspected of having IC harbouring (temporary) contraindications for treadmill testing (neurological, orthopedic, fear) may safely be diagnosed using a bicycle. Submaximal testing is preferred to maximal testing because of optimal sensitivity and specificity whereas a risk on (cardiovascular) complications is lowest.