Effects of Age, Sex and Isometric Exercise on Ventricular-Vascular Interactions During Cardiac Unloading

Orthostatic intolerance becomes prevalent with advancing age (≤5% below age 50 but ~20% above age 70), and females are 3 to 5% more susceptible. Orthostatic intolerance is associated with syncope, falls, cardiovascular disease, and mortality in older adulthood. The characteristic excessive fall in blood pressure (BP) underlies a complex dysregulation of heart rate (HR), BP, and flow responses to postural changes. When standing, blood pools in the lower limbs, decreasing venous return, stroke volume (SV), and ultimately BP. The resultant unloading of the heart upregulates the sympathetic arm of the baroreflex to increase HR and peripheral vasoconstriction, but this counter-regulation to restore normal BP appears absent or reduced in people suffering from orthostatic intolerance.

Cardiac unloading, which can be accomplished experimentally via lower body negative pressure (LBNP), produces changes in arterial and ventricular loads while attempting to maintain adequate cardiac output. Heart-vessel interactions are, thus, key to understanding BP regulation during cardiac unloading, but have been ignored by research on orthostatic intolerance. The ventricular-vascular coupling framework based on pressure-volume loops describes how heart contractility (i.e., ventricular elastance (Ees)) and arterial loads (i.e., arterial elastance (Ea)), respond to changing loading conditions. With aging, females show greater increases in Ees to match increased Ea, caused by aortic stiffening and high BP. Hence, the coupling ratio (Ea/Ees) is reduced in females, but is preserved in males. However, whether such changes at rest impact the ventricular-vascular coupling response to acute cardiac unloading is unknown, and may be an important mechanism of orthostatic intolerance in females. Notably, the ventricular-vascular coupling framework disregards the wave reflection phenomenon underlying pulsatile pressure-flow relationships. Early reflection of pressure waves, which is characteristic of aging, increases cardiac afterload and thus has important implications for heart-vessel interactions. Such pulsatile arterial load is expected to rise in response to cardiac unloading due to peripheral vasoconstriction, but this remains untested. Importantly, defining heart-vessel interactions during cardiac unloading will shed light on whether plausible ventricular-vascular mismatch contributes to orthostatic intolerance and may help to develop countermeasures to alleviate the symptoms and consequences of intolerance.

Isometric exercise attenuates the cardiac unloading effects via increases in both cardiac output and BP. The pressor response elicited by isometric handgrip exercise increases both heart and arterial load. While isometric handgrip exercise holds potential to counteract BP reduction during cardiac unloading, the augmented work of the heart may limit tolerance. Thus, defining the effects of isometric handgrip exercise on ventricular-vascular interactions under cardiac unloading and the impact of age and sex on these responses will be key to determining its feasibility as an orthostatic intolerance countermeasure.

Therefore, our specific aims are to determine the impact of age (Aim 1), sex (Aim 2), and isometric handgrip exercise (Aim 3) on ventricular-vascular interactions during LBNP-induced cardiac unloading.

• Aim 1's working hypothesis is that older, compared to young healthy adults will exhibit a more pronounced ventricular-vascular mismatch due to greater increases in Ea and Ew not matched by Ees during presyncope- limited LBNP.
• Aim 2's working hypothesis is that older but not younger healthy females compared to males will exhibit a more pronounced ventricular-vascular mismatch due to smaller increases in Ea and Ew and higher Ees during presyncope-limited LBNP.
• Aim 3's working hypothesis is that isometric handgrip exercise will offset BP reductions, but increase ventricular and arterial load during presyncope- limited LBNP.