Adaptation of the cardiovascular system to weightlessness: Surprises, paradoxes and implications for deep space missions

Abstract

Weightlessness in space induces a fluid shift from the dependent to the cephalad parts of the body leading to distension of the cardiac chambers and an accumulation of blood in the veins of the head and neck. Surprisingly, central venous pressure (CVP) during the initial hours of spaceflight decreases compared to being horizontal supine on the ground. The explanation is that the thorax is expanded by weightlessness leading to a decrease in inter‐pleural pressure (IPP), which exceeds the measured decrease in CVP. Thus, transmural CVP (TCVP = CVP − IPP) is increased indicating an augmented cardiac preload. Simultaneously, stroke volume and cardiac output (CO) are increased by 18%‐26% within the initial weeks and more so by 35%‐56% during the subsequent months of flight relative to in the upright posture on the ground. Mean arterial pressure (MAP) is decreased indicating a lower systemic vascular resistance (MAP/CO). It is therefore a surprise that sympathetic nerve activity is not suppressed in space and thus cannot be a mechanism for the systemic vasodilation, which still needs to be explored. Recent observations indicate that the fluid shift during long duration (months) flights is associated with increased retinal thickness that sometimes leads to optical disc oedema. Ocular and cerebral structural changes, increases in left atrial size and decreased flows with thrombi formation in the left internal jugular vein have also been observed. This is of concern for future long duration deep space missions because the health implications are unknown.

Vascular and Microvascular Dysfunction Induced by Microgravity and Its Analogs in Humans: Mechanisms and Countermeasures

“…In most HDBR and dry immersion studies, systemic vascular resistance is increased (Pavy-Le Traon et al, 2007;Navasiolava et al, 2011). By contrast, in inflight studies, systemic vascular resistance seems to be decreased as indirectly deduced from a measured decrease in blood pressure and increase in cardiac output, despite preserved or even increased sympathetic nervous activity (Norsk, 2019). Mechanisms for this unexpected systemic vasodilation remain to be identified.…”

“…Mechanisms for this unexpected systemic vasodilation remain to be identified. Proposed contributors are a headward fluid shift-induced decrease in lower body vessel stretch (with reduced myogenic tone), cardiac distention (with release of cardiac vasodilatory natriuretic peptides), and increased core body temperature (Norsk, 2019).…”

“…Vascular morphology and function, especially for smooth muscle cells, are sensitive to transmural pressure, which itself depends on blood and hydrostatic pressure. Systemic blood pressure is preserved or slightly decreased inflight (Norsk, 2019) and is not significantly modified by long-term bed rest (Fortney et al, 1996;van Duijnhoven et al, 2010a,b). However, hydrostatic pressure should be taken into account for vascular remodeling related to actual or simulated microgravity.…”

Effectiveness of nutritional countermeasures in microgravity and its ground-based analogues to ameliorate musculoskeletal and cardiopulmonary deconditioning–A Systematic Review

“…When exposed to μg the hydrostatic gradient associated with upright posture in Earth's gravity is lost, and blood is redistributed towards the head and neck [19]. As a result, on return to Earth astronauts can experience orthostatic intolerance, the incidence rate of which increases with mission duration [20][21][22].…”

The thrombotic risk of spaceflight: has a serious problem been overlooked for more than half of a century?

“…2 The first astronaut was anticoagulated according to current ESC guidelines for the management of deep vein thrombosis 11 adapted to the unique spaceflight context. 1 Possibly, venous congestion of the head and lung embolism, driven by a headward shift of 2000 mL blood and extracellular volume in weightlessness, 12 could pose a serious health risk in professional and recreational astronauts. The high impact forces imposed on the returning capsule at ground contact could conceivably dislodge preformed venous thrombi, thus, provoking embolic events.…”