Vascular Technology Lecture 23: Venous Hemodynamics HHHoldorf

Venous physiology and Hemodynamics Venous Resistance Peripheral venous and arterial resistances are similar Both arteries and veins carry same amount of blood This Paradox is explained by the “collapsible” nature of the venous wall. Flattened shape offers more flow resistance than circular shape

Pressure/volume Relationships When distended, cross-sectional area of the vein is about 3-4 times that of the corresponding artery. The extra-pulmonary veins carry about 2/3’s of the blood in the body Shape of veins determined by transmural pressure (distension pressure), e.g., the pressure within the veins versus pressure outside the vein

Low transmural pressure: Low volume of blood results in dumbbell shape High transmural pressure: high volume results in circular shape

Small pressure changes required to expand or distend vein from normal dumbbell shape to a circular one Once completely distended, greater pressure ranges required to accommodate further increases in volume

Hydrostatic Pressure (HP) Equivalent to the weight of a column of blood pressing against the vessels of the body; uses the heart as a reference point (HP is zero at the heart level). HP = pgh P = specific gravity of blood g = acceleration due to gravity h = distance from the heart

HP is added to the existing circulatory pressure and is related to position: Supine: HP exerted on veins and arteries negligible, assumed to be zero (Pressure (P) measured at all levels = actual circulatory P)

Standing HP gradually increases from level to level down the body, reaching approximately 100 mmHg at the ankle.

Body part above heart: Negative HP. Measured pressure less than circulatory P.

Factors Affecting Venous Flow Venous/Skeletal muscle pump /’venous heart’ Muscle contraction squeezes vein propelling blood toward the heart

Effective calf muscle pump Blood moves from superficial system (S) to deep system (D) Competent valves prevent reflux Venous volume and pressure decreases Venous return to heart increases

Ineffective calf muscle pump Incompetent valves cause reflux Venous volume and pressure increases Results in venous pooling and ambulatory venous hypertension

Respiration Inspiration Decrease in intra-thoracic pressure Increases blood flow from upper extremities Increase in intra-abdominal pressure Decreases blood flow from lower extremities

Exhalation Increase in intra-thoracic pressure Decreases blood flow from upper extremities Decrease in intra-abdominal pressure Increases blood flow from lower extremities

Valsalva Maneuver Patient takes in deep breath and holds it, then bears down as if having a bowel movement Intra-thoracic and intra-abdominal pressure increases significantly All venous return is halted This maneuver equates with proximal compression while performing Doppler assessment of the lower extremities

Additional Notes: When distended, cross-sectional area of the vein is about 3-4 times that of he corresponding artery. Excluding pulmonary veins, extra-pulmonary veins carry about 2/3’s of the blood in the body- because they can STRETCH. Hydrostatic pressure (HP) Standing Heart 0 mm Hg Ankles 100+ mm Hg Arm raised -50 mmHg

Homework Textbook SDMS assignments Chapter 25 Venous Hemodynamics Pages 277 – 280 SDMS assignments