Hemodynamics

Hemodynamics

General Anatomy

  • Large artery (aorta) is elastin rich. The more elastin, the more elastic the vessel is.
  • Medium vessels (extremities) is collagen rich.
  • Arterioles are mostly smooth muscles.
  • Small vessels such as arterioles and capillaries make up 60-70% of peripheral arterial resistance. Large and medium-sized vessels (iliac, femoral, popliteal) account only for 15-20%.
  • Approximately 50% of diameter reduction in a vessel or 75% of area reduction is considered critical stenosis, although less degree of stenosis may become critical as flow increases (exercise ABI)
  • Most of the energy loss is at entrance and exit of a st

Multiple stenoses in series

Multiple stenoses in series has a larger effect on blood pressure drop compared to a single long segment stenosis. This is because most of the frictional energy losses happen during post-stenotic turbulence. As a result, multiple short segment stenotic lesions create more turbulence overall and hence the drastic pressure drop.

Physics

Poiseuille’s Law

Poiseuille’s Law characterizes the frictional energy loss of a viscous fluid over a segment of tube (artery). Pressure gradient across a segment of artery is as follows:

ΔP = Q( 8Lμ/πr4)

This can further simply into ΔP = QR where R is signify the resistance across the arterial segment. An easy way I remember this is equate this equation to electrical potential, where the voltage is equal to current multiplied by resistance, or V = IR. In this case, the ΔP is the voltage (gradient), Q is the current (flux), and R is the resistance.

Whether you memorize it or not, for the purposes of taking tests, the resistance across an artery can be summarized as below:

R = Q( 8Lμ/πr4)

As you can see, the resistance is proportional to the length of the artery that the blood needs to travel, and inversely proportional to the radius of the artery. This is intuitive as we know the longer the artery is, the more resistance the blood has to overcome. Also, as the diameter of the vessel gets smaller, the pressure gradient goes up (stenosis). Note that r is raised the the 4th power, so the radius has exponentially more effect on total resistance. For example, doubling the length L increases resistance by 2-folds, but if you halve the radius, the resistance goes up by 16 times (1/2)4 = 1/16 at the denominator.

Tangential Wall Stress

Tangential wall stress τ is expressed as:

τ = P(r/δ)

Where P is fluid pressure, r is the internal tube radius, and δ is the wall thickness. The tangential stress is directly proportional to the fluid pressure and the radius, and inversely proportional to the wall thickness. When you think about it intuitively in aneurysms, the higher the pressure, thinner the wall, and bigger the raidus (or expressed alternatively as diameter), the higher the tangential wall stress and higher the chance of rupture.

 

Venous Hemodynamics

  • The principle venous return mechanism of the lower extremity is the muscle pump, which accounts for 90% of the venous return. The calf muscles are the most efficient, with an ejection fraction of 60%. Thigh muscles only has an EF of 15%.
  • After ambulating, the venous pressure in the calf drops to less than 30 mmHg, which is termed ambulatory venous pressure (AVP). Venous refilling time (VRT) is the time it takes to refill to 90% of resting venous pressure. AVP is elevated and VRT is reduced in patients with reflux disease.
  • Ambuatory venous pressure (AVP) is measured by sticking a needle into a dorsal foot vein and measure pressure after 10 tiptoe movements. Less than 30 mmHg is normal, and greater than 90 mmHg is associated with venous ulcers.
  • In venous reflux disease, the reflux component is more important than obstructive component.
  • Femoral venous return ceases during inspiration due to diaphragmatic breathing secondary to increase in intraabdominal pressure (not due to diaphragmatic compression of inferior vena cava). Femoral venous return is augmented during expiration.
  • Inferior vena cava and common iliac veins usually have no valves. External iliac veins and common femoral veins may have 1 valve. Great saphenous vein has around 6 valves, and the small saphenous vein has 7-10 valves. Valve numbers increase as veins become smaller and more distal into the leg/calf.