(P278) INVASIVELY MEASURED HEMODYNAMIC RESPONSES TO THE ACTIVE STAND TEST: A PILOT EXPERIENCE

Background: The active stand test is recommended to evaluate syndromes of orthostatic intolerance (OI) such as postural orthostatic tachycardia syndrome or orthostatic hypotension. This test is a non-invasive assessment of heart rate (HR) and blood pressure (BP) during supine rest followed by unsupported standing, which replicates stressors that provoke symptoms experienced by patients with OI. Diagnostic criteria for OI are dependent on both magnitude and timing of changes in HR and BP. The response to orthostasis is complex, involving changes to cardiac filling and stroke volume (SV) for which HR and BP alone offer incomplete insight. There are few published data of comprehensive invasive hemodynamic evaluation during active stand tests, possibly due to requirement of trained personnel, and procedural risks involved. Since OI is highly burdensome, impairs functional capacity, and may be increasingly prevalent following COVID-19 infection, we conducted a pilot experience examining the feasibility of repeated measurements of intracardiac pressures, cardiac output (CO) and SV during invasive active stand tests.

METHODS AND RESULTS: We prospectively recruited 3 study groups including healthy volunteers (n=5; age: 60 ± 5), patients participating in a study of Long COVID syndrome (n=3; age 46 ± 29), and patients referred for evaluation of dyspnea of unknown origin (DUO) (n=12; age: 57 ± 15). Participants completed an active stand test (5-min supine, 10-min unsupported stand) during right heart catheterization and recorded simultaneous pressure-time waveforms from the right atrium (RAP), pulmonary artery (PAP), and intermittently in the pulmonary capillary wedge (PCWP) position. Pressures and mixed-venous oxygen saturations (SvO2) were sampled during supine and at 1, 3, 5 and 10 minutes of unsupported standing. Thermodilution CO and derived SV were measured at supine and 10 minutes of standing. Additionally, participants completed the Vanderbilt Orthostatic Symptom Score questionnaire for 1 minute, immediately following standing to assess orthostatic symptom burden.

For all groups, we noted HR increases with concomitant decreases in SV and SvO2 from supine to standing. RAP and PCWP decreased non-significantly in healthy volunteers from supine to standing, whereas in DUO and Long COVID patients, these pressures decreased significantly. Healthy participants reported no orthostatic symptoms, while DUO and Long COVID patients reported mild nausea and dyspnea (3/10 Borg units) following the stand.

Conclusion: We successfully completed multiple repeated hemodynamic measurements with invasive active stand testing in our laboratory in healthy and patient populations. This data and further studies may provide valuable insights into the pathophysiology of OI, with the benefit of healthy controls.