A Cardiorespiratory Fitness Test for Acute Illness: Validity, Reliability, and Clinical Utility of Functional Incremental Stepping-in-Place Test

Acutely hospitalized patients are at risk for hospital-acquired functional decline (HAD). This decline increases the likelihood of developing physical disability after discharge, particularly in those with older age, multimorbidity, pre-existing functional impairment and life support exposures (i.e. mechanical ventilation). Identifying patients at increased risk provides important prognostic data to inform planning strategies for both in-hospital and post-acute intervention.

Common physiological consequences associated with HAD are diminished skeletal muscle function and cardiorespiratory fitness (CRF). Historically, the six-minute walk test (6MWT) has been utilized as a chief outcome measure in the acute hospitalization and post-discharge phases to examine functional exercise capacity, estimate maximal capacity, and prescribe exercise intervention. The 6MWT is a submaximal field test that allows patients to self-select their walking pace and may be influenced by several factors including motivation. The internally-selected pace coupled with the lack of incremental increases in intensity can underrepresent CRF. In addition, the test requires a 30-meter straight course, which can be challenging to find in the hospital environment. Thus, to address the limitations of submaximal field tests, our team developed the Functional-Incremental Stepping-in-place Test (F-IST).

Designed to objectively examine CRF across the variety of patients encountered in clinical practice, the F-IST is an incremental, externally-paced, symptom-limited stepping-in-place test. While maintaining the graded components of gold standard cardiopulmonary exercise testing (CPET), it has high clinical utility since it requires minimal equipment (UE support allowed), minimal space (performed at bedside), and short administration time (< 10 min). Also, the incremental increase in intensity is individualized to perceived exertion and heart rate responses during test stages, which ultimately enables precise exercise prescription. Preliminary data on F-IST from the ongoing randomized controlled trial (NCT05218083) demonstrate potential for strong construct validity when compared to 6MWT (n = 9, spearman rho = 0.65, p = 0.05). However, before implementation in clinical practice, a new test requires the study of its measurement properties to assure that its selection is evidence-based.

Findings from this study will demonstrate that F-IST is a robust valid and reliable assessment of CRF that can be implemented on the frontline of acute care physical therapy. The F-IST addresses important limitations of field tests with an enhanced rigor by maintaining principles of gold-standard CPET. The F-IST will enable clinicians and researchers to provide precision exercise prescription to optimize patient function.