Differences in vascular and central cardiac responses to LBNP among healthy, young adults

Lower body negative pressure (LBNP) is a well-established technique for assessing cardiovascular function by shifting central blood volume and increasing sympathetic outflow via baroreceptor unloading. While methods like the cold pressor test demonstrate considerable interindividual variability, the variability during LBNP remains less explored. Purpose: To investigate interindividual differences in central cardiovascular and peripheral hemodynamic responses to LBNP. Methods: Fifty participants (mean age: 23 ± 3 years; 23 females tested during the early follicular phase) completed two sessions (1 familiarization, 1 experimental). Participants underwent a 5-minute supine baseline, followed by 2 minutes of LBNP at -20 mmHg. Forearm blood flow (FBF, Doppler ultrasound), mean arterial pressure (MAP) and stroke volume (SV) via finger photoplethysmography, and heart rate (HR, 2-lead ECG) were measured continuously. Cardiac output (CO, HR x SV), and forearm vascular conductance (FVC = [FBF / MAP] x 100 mmHg) and total peripheral resistance (TPR = MAP / CO) were calculated Results: Data are mean ± SD. Participants were categorized as dilators (n = 8, 6 females; 34 ± 21 Δ% FVC) or constrictors (n = 42, 17 females; -36 ± 16 Δ% FVC) based on their FVC response to LBNP (p < 0.001, d = 4.1). MAP decreased in dilators (Baseline: 102 ± 12 mmHg vs. LBNP: 99 ± 14 mmHg; p = 0.01, d = 1.2), but remained unchanged in constrictors (Baseline: 101 ± 11 mmHg vs. LBNP: 101 ± 11 mmHg; p = 0.7, r = 0.1). MAP Δ% differed between groups (dilators: -3.3 ± 2.7%; constrictors: -0.3 ± 4.2%; p = 0.02, d = 0.7). CO decreased in both groups: dilators (4.3 ± 0.9 L/min vs. 4.1 ± 0.7 L/min; p = 0.02, d = 1.0) and constrictors (5.7 ± 1.6 L/min vs. 5.1 ± 1.2 L/min; p < 0.001, r = 0.8). HR remained unchanged in dilators (61 ± 6 vs. 62 ± 9 BPM; p = 0.48, d = 0.3) but increased in constrictors (62 ± 7 vs. 66 ± 12 BPM; p = 0.02, d = 0.4). SV decreased in both groups (dilators: 76 ± 15 vs. 69 ± 12 ml, p = 0.04, d = 0.8; constrictors: 92 ± 19 vs. 80 ± 19 ml, p < 0.001, d = 1.3). TPR remained unchanged in the dilators (24 ± 7 vs. 25 ± 6 mmHg·min/L; p = 0.17, d = 0.5) but increased in constrictors (19 ± 5 vs. 20 ± 5 mmHg·min/L; p < 0.001, r = 0.6). Conclusion: Significant interindividual variability exists in hemodynamic responses to LBNP. A subset of participants exhibited a limb dilatory response (increased FVC) and no change in HR, suggesting differential sympathetic outflow in dilators compared to constrictors, which manifests to an inability to maintain MAP during -20mmHg LBNP. These findings add to the literature heterogeneity of cardiovascular regulation in young healthy humans and highlight the importance of screening participants to identify responders to LBNP in vascular research. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.