On moving from supine to erect posture there is a large gravitational shift of blood away from the chest to the distensible venous capacitance system below the diaphragm. This shift is estimated to total ½–1 L of thoracic blood with most of the volume shift occurring in the first 10 s of upright posture. In addition, with prolonged standing, the high capillary transmural pressure in dependent parts of the body causes filtration of protein-free fluid into the interstitial spaces. It is estimated that this results in about a 15–20% (700 ml) decrease in plasma volume in 10 min in healthy humans. As a consequence of this gravitationally induced blood pooling and the superimposed decline in plasma volume, the return of venous blood to the heart is reduced; the result is rapid diminution of cardiac filling pressure and a consequent decrease in stroke volume. However, normally, despite decreased cardiac output, hypotension is prevented by compensatory vasoconstriction of the resistance and the capacitance vessels in the splanchnic, musculo-cutaneous, and renal vascular beds.
Tilt Testing Venous Cannulation High Positivity Rate Good Diagnostic Accuracy Cardiac Filling Pressure
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