NIRS and Exercise Intensity in Patients With FLIA

A professional cyclist covers approximately 25,000 km a year and flexes the hip 8,000,000 times in a year, while leg blood flow is in the range of 10-15 litres per minute. This poses a substantial hemodynamic load on the iliac artery. As a result, a proportion of endurance athletes develop a limitation in leg circulation due to arterial narrowing in this iliac artery. An early 'Lancet' study of the department of Sports Medicine of Máxima Medical Centre (MMC) found that 20% of professional cyclists were suffering from such a sport-related Flow Limitation in the Iliac Artery (FLIA) necessitating treatment. The incidence in recreational cyclists is unknown, but with 849,000 recreational cyclists in the Netherlands cycling over 3,000 km a year with an impressive 1,000,000 hip flexions, many of them travel similar distances as a professional cyclist, incurring similar risks for developing FLIA. If untreated, FLIA may have a pronounced impact on quality of life. Professional athletes may have to end their careers prematurely. In a substantial subset of cyclists, abnormalities may even lead to complete occlusion and/or thrombosis, with severe symptoms in daily life.

Clinical experience suggests that early detection and treatment leads to better outcomes. If diagnosed at a late stage, conservative management including changes in training behaviours and body position, or least-invasive surgical repair options will no longer suffice. The only options remaining would be to cease participation in the provocative activities altogether, or to undergo extensive and risky reconstructive vascular surgery. Understanding the early pathogenesis in order to improve detection is thus of paramount importance. Unfortunately, early detection is often missed due to the non-specific presentation of symptoms and the high level of specialisation required for clinical evaluation. There is a wide range of differential diagnoses that could contribute to the non-specific symptoms observed in the early stages of FLIA, including common musculoskeletal and tendinous injuries, mechanical or neurogenic pain referred from the low back or SI joint, hip acetabular labral tear, chronic exertional compartment syndrome, or fibromuscular dysplasia.8 Currently available diagnostic evaluations can have low sensitivity for an athletic population.

There is no single gold-standard evaluation for diagnosing FLIA. The current consensus suggests that the best single functional test is a provocative maximal exercise test on a cycle ergometer, followed by measuring blood pressure at the ankle and brachial arteries (ankle-brachial blood pressure index; ABI) in a competitive posture. In the rare case that the problem is unilateral, the sensitivity is 73%. If the problem is bilateral, the sensitivity is only 43%. Imaging techniques, including echo-Doppler examination, magnetic resonance angiogram (MRA), and computed tomography (CT) scan are more sensitive, but they are more expensive, less accessible, and not part of primary care evaluation, instead being typically reserved for investigation of more severe or complex presentations, and to guide surgical repair.

Near-infrared Spectroscopy (NIRS) is an innovative technique that measures relative oxygenation in the muscle, as the balance of oxygenated and deoxygenated haemoglobin and myoglobin. Impaired arterial leg circulation, such as observed in peripheral vascular disease (PVD) has been shown to produce a drop in oxygen saturation of skeletal muscle tissue relative to workload or exercise performance, and delays in reoxygenation kinetics after exercise and ischemic vascular occlusion tests (VOT). Consequently, NIRS may be able to detect alterations in oxygenation that are associated with the level of arterial insufficiency. We recently reported proof of concept studies regarding the potential diagnostic role of both power output and NIRS in patients with diagnosed sport-related FLIA.

Complaints reported in the early stages of FLIA are powerlessness and pain in the leg muscles when cycling near maximal exertion, which rapidly disappear with rest. Traditionally, incremental ramp cycling exercise to maximal exercise tolerance has been used as a provocative functional test, after which clinical outcome measures including ABI are tested. As the condition progresses however, symptoms can occur earlier during exercise at a lower intensity and take longer to resolve during recovery. Multi-stage exercise protocols are commonly used to understand metabolic responses related to submaximal exercise intensity. Therefore, a progressive multi-stage cycling protocol with brief recovery intervals between work intervals will be introduced. This protocol is designed to allow for multiple opportunities to evaluate work and recovery responses in an intensity-dependent manner. Subjective symptoms, performance impairments (including limitations to cycling power output) and muscle oxygenation kinetic delays will be evaluated across submaximal workloads including after maximal intensity.

Understanding the onset of symptoms and objective signs of flow limitation with progressive exercise intensity will improve understanding of severity and progression of this condition. These outcome measures will be compared to healthy subjects, in order to develop normative values related to healthy performance, compared to pathological impairment. The use of a common multi-stage performance assessment protocol will improve the applicability of using this approach for screening and early detection of FLIA outside of a specialised vascular clinic.

It has been suggested that altered vascular function and structure may contribute to the appearance of symptoms in patients in which obvious stenosis or intraluminal disease is not apparent on imaging. In addition to standard clinical evaluation of the aortoiliac tract with echo-Doppler ultrasound, vascular flow velocity will be recorded for later offline analysis of pulse wave velocity as a measurement of arterial stiffness.