Hypovolemia explains the reduced stroke volume at altitude

Area of Science:

• Physiology
• Cardiovascular Research
• Altitude Medicine

Background:

• During acute altitude exposure, cardiac output (Q) increases via tachycardia to maintain oxygen delivery.
• Upon acclimatization, Q normalizes due to an unexplained reduction in stroke volume (SV).
• The role of reduced plasma volume (PV) and central blood volume (CBV) in this SV reduction is unclear.

Purpose of the Study:

• To investigate if reduced plasma volume (PV) and central blood volume (CBV) cause the altitude-mediated reduction in stroke volume (SV).
• To assess cardiovascular responses to altered CBV using head-down (HDT) and head-up (HUT) tilting at sea level (SL) and high altitude.

Main Methods:

• Seven lowlanders underwent passive whole body tilting (HUT/HDT) at SL and after 25/26 days at 3454 m.
• Plasma volume (PV) was normalized via infusions before altitude testing.
• Cardiovascular responses were monitored using pulse contour analysis.

Main Results:

• At altitude, PV and blood volume decreased significantly (9% and 6%, respectively).
• Stroke volume (SV) was reduced at altitude compared to SL across all HUT angles.
• PV expansion restored SV to SL levels during HUT and increased it further during HDT, indicating SV is dependent on CBV.

Conclusions:

• Reduced stroke volume (SV) at altitude is primarily caused by hypovolemia-induced reduction in central blood volume (CBV).
• The dependence of SV on CBV remains unchanged at altitude.
• Restoration of PV effectively mitigates the altitude-induced reduction in SV.