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Flashcards in Limb Circulation Deck (15)

Determinants of Blood Viscosity

1. Hematocrit
increased Hct = increased Viscosity
e.g., increased Hct from 40 to 60% doubles viscosity

2. Temperature
decreased Temp = increased Viscosity
e.g., hypothermia

3. Flow Velocity (Shear Rate)
decreased Flow Velocity = increased Viscosity
e.g., circulatory shock


Flow and Resistance Equations

F = Partery - Pvein / R

R = nL / r^4


Viscosity and Hematocrit

Reducing blood viscosity by decreasing hematocrit can significantly enhance blood flow, although it may reduce oxygen delivery

Fluid load them with saline – can help with ischemia by reducing R, but trade off of diluting formed elements and O2 carrying capacity which isn’t exactly the best

Pentoxifylline: inhibitor of phosphodiesterase so blood cells can be more flexible (changes membrane) so can go through capillaries with reduced R to increase flow without reducing O2 carrying capacity


Decreasing Diameter Proximal to Tibial Arteries with Pressure Effects

MAP is not controlled in the leg (no sensors in the leg) and baroreceptors are centrally acting that regulate BP located in carotid body and aortic arch. These mechanisms are not sampling in the leg, thus pressure will not increase proximally

Only time you get an increase BP is if brain BP decreases then pressure everywhere increases

The pressure drop along a distributing artery increases with increased blood flow or resistance

Increased resistance of an arterial segment reduces arterial pressure in distal segments


Ankle-Brachial Pressure Index

Measure systolic arterial pressure at ankle and at upper arm (brachial artery)

ABI is normally greater than 1.0 (1.0 - 1.1)
Systolic arterial pressure increases as blood flows from thoracic aorta into large distributing arteries
Mean arterial pressure, however, decreases a small amount



Active vs. Reactive Hyperemia

Active: Increased tissue blood flow associated with increased metabolic activity (e.g., increased muscle blood flow during exercise)
Active contractions = increased metabolites in blood and increase blood flow

Reactive: Increased tissue blood flow following a brief period of ischemia (e.g., after removing a tourniquet)


Autoregulation of R and PP

Distal vasodilation (autoregulatory response) can help maintain normal blood flow when perfusion pressure is reduced by proximal arterial stenosis


Sympathetic Nerves and Circulating Hormones

Sympathetic adrenergic nerves
The neurotransmitter norepinephrine acts on a-adrenoceptors to contract vascular smooth muscle (Gq-protein/IP3 pathway)

Circulating factors
Angiotensin II (vasoconstrictor [AT1])
Epinephrine (vasodilator [b2]/vasoconstrictor [a1, a2])
Vasopressin (vasoconstrictor [V1])


Local Mechanisms vs. Sympathetic/Neural Influences

Local vasodilator mechanisms can override sympathetic and neural vasoconstrictor influences

Skeletal muscle contraction causes the release of vasodilator metabolites that decrease vascular resistance and increase blood flow


Single Artery Stenotic Lesion

Stenotic lesions causes reduction in ABI, but not much affect on resting blood flow
Say at rest with stenotic lesion, the drop is 25mmHg (proximal pressure is normal aka 93 and distal pressure is 68mmHg) which is normal
When start exercising there is an increase in flow by 3x for example causing 25 to be 75mmHg causing a drop down to 18mmHg distally, which is very bad and can cause ischemia if persistent

Reduction of blood F can be tolerated at rest, but once exercise you increase pressure drop and leave distal pressure to be little and even lose the pedal pulses

A single arterial stenotic lesion reduces maximal distal blood flow


Multiple Artery Stenotic Lesions

Measure ABI, it will be even further reduced at the ankle
At rest, some reduction in flow (very little) at rest, but exercise is even worse reduction and below resting levels

Hemodynamic basis for vascular steal: Increased flow in one vascular bed reduces the perfusion pressure to a parallel vascular bed


Vascular Steal in Leg

What is it?
Reduced blood flow to one region of the limb when blood flow increases to another region

What causes it?
Requires multiple lesions with one parallel vascular bed, supplied by distal stenotic artery, maximally dilated (or nearly so)
Increased flow across proximal stenosis reduces distal pressures to a distal parallel vascular bed, thereby reducing flow
Occurs during exercise or when vasodilator drugs are administered


Critical Stenosis

Mild to moderate stenosis of large arteries has little effect on resting flow
Large arteries normally comprise 60% to be clinically significant


Hemodynamics of Venous Thrombosis

Large veins, like large arteries, contribute only a small fraction to total vascular resistance

Depending on the location of an occluded vein, total limb resistance (and therefore total limb flow) may or may not change significantly

Venous occlusion (e.g., thrombosis), however, leads to increases in proximal capillary pressure, which can cause regional edema and ischemia


Limb Edema

Edema (tissue swelling) occurs when:
Filtration > Reabsorption + Lymph Flow

Three Causes of Edema
1) Increased capillary pressure e.g. venous thrombosis
2) Increased capillary permeability e.g., trauma, infection/inflammation
3) Lymphatic obstruction/damage