Download b. Ventricular stroke volume decrease

b. Ventricular stroke volume decrease
Stroke volume is the amount of blood being pumped
out the left ventricle (to the body) per heartbeat.
Cardiac output is the total volume leaving the left
ventricle per minute (or SV * HRM). The mechanical
performance of the ventricle is due to the FrankStarling mechanism. This mechanism shows that
stroke volume increases as the preload (or End
Diastolic Volume) increases. As we know from
the first point, blood volume decreases in
microgravity. With this decreased volume the
preload is not as great in the left ventricle
(Kotovskaya & Fomina, 2010). With this reduction in
preload the stretch of the ventricle is not as large,
and the force of contraction is not as great.
Therefore, the stroke volume is reduced. With the
reduction in stroke volume comes the reduction in
cardiac output (Carter & West, 2013).
The reduction of preload has been seen to decrease by 3-6% within the first month of flight. This number
increases to 8-10% by month 5-6 (Kotovskaya & Fomina, 2010). A decrease in stroke volume of 17% has
been seen after 31 days of flight. This percentage increased to 36% in a study after 171 days of flight
(Antonutto & di Prampero, 2003). It is important to note that although preload and stroke volume
decreased, the left ventricular ejection fraction (SV/EDV) stayed within a normal range (Kotovskaya &
Fomina, 2010). (Thigh cuffs are used as a way to reduce the change in stroke volume). The function and
myocardial contractility (pumping function) also remained within normal limits (Kotovskaya & Fomina,
2010; Schneider & Convertino). After return from flight the reduction in stroke volume is still apparent
(Carter & West, 2013).
In bed rest, cardiac compliance (the change in intraventricular pressure for a change in preload) was
shown to increase during the first few days of rest by showing that stroke volume remained higher at any
given filling pressure (Schneider & Convertino). Cardiac compliance is used as a defense mechanism to
maintain stroke volume even with a reduction in preload. Even with this change during bed rest, after
about two weeks ventricular remodeling was noted and cardiac compliance and preload decreased
(Schneider & Convertino).
The reduction of blood volume, stroke volume, and cardiac output all play a part in the reduced baroreflex
function and the ability for blood vessels to constrict in the lower extremity (Carter & West, 2013). When
the blood vessels are not as able to constrict, the blood flow is not as strong, and the venous return (to the
heart) is reduced. The reduction of blood getting to the heart also plays a part in the decrease of stroke
volume. There is a greater capacity for blood to pool due to the impaired constriction and also the
reduction in leg muscle mass (Schneider & Convertino). (The tissue mass normally provides structural
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b. Ventricular stroke volume decrease
support, but with the decreased size this may also play a part in the reduced ability to constrict). These
changes contribute to orthostatic intolerance (Hargens et al., 2013), hypovolemia (Schneider &
Convertino), and reduced aerobic capacity (Hargens et al., 2013).
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