How is velocity of blood flow related to the cross-sectional area of a blood vessel or group of blood vessels?
- Velocity is distance per unit time
- flow refers to volume
- Flow = velocity x cross sectional area of a vessel
- Velocity is inversely related to cross-sectional area
V = Q/A
V: velocity
Q: flow
A: cross-sectional areal
*velocity is slower when area of a vessel is wider.
According to Ohm’s Law, what 2 factors determine blood flow though a blood vessel?
Ohm’s Law: relates to electrical potential (voltage drop, V), the resistance (R), and the current (I), in an electric circuit
(Though used to describe energy, also works with blood flow)
V = IR
Voltage = pressure gradient, driving pressure
I = V/R
Q = (PA-PV)/R
Define blood pressure
The force exerted by the blood agains any unit area of the vessel wall.
Pressure in the cv system are expressed in mmHg above atmospheric pressure
Flow is equal to what?
Cardiac output
CO = MAP-CVP
————-
SVR
What is the conversion factor from mmHg to cm H2O
One mmHg pressure = 1.36cm of H2O pressure
*said didn’t really need to know
What is cerebral perfusion pressure?
Cerebral Perfusion Pressure = MAP - CVP or ICP (whichever is higher)
What is Poiseuille’s Law? And what is the greatest impact on flow through a vessel?
Describes the flow rate of a fluid through a tube as a function of pressure difference, the radius of the tube, the length of the tube, and the viscosity of the fluid.
- Flow varies directly as the forth power of the radius
- Flow is directly proportional to the pressure gradient
- Flow is inversely proportional to the length of tube.
- Flow is inversely proportional to the viscosity of the fluid
*radius to the 4th power has the biggest impact
What is resistance?
- The impediment to blood flow in a vessel
- It can not be measured by any direct means
R = Pi - Po
———-
Q
-the initial pressure minus the second pressure
Q: flow
What is the formula for calculating Systemic Vascular Resistance?
(MAP-CVP)
SVR = ————— X80
CO
*normal = 700 - 1600
What is the formula for calculating Pulmonary Vascular Resistance?
(MPAP - PCWP)
PVR = ———————X80
CO
*normal = 100 - 300
How would you calculate resistance when blood vessels are arranged in series?
Rt = R1 + R2 + R3 + R4
How would you calculate resistance when blood vessels are arranged in parallel?
1 1 1 1
— = — + — + —
Rt R1 R2 R3
If you added another blood vessel in a parallel arrangement, would total vascular resistance increase or decrease? Why?
Decrease because the net total vascular resistance
What is laminar flow?
Can be modeled as a set of layers moving in the same direction, with no turbulence
What is turbulent flow?
Characterized by chaotic flow
What formula could you use to predict laminar vs turbulent flow?
Reynold’s Number
-looks at density, tube diameter, mean velocity, and viscosity
NR < 2000: usually laminar
NR = 2000 - 3000: variable
NR > 3000: usually turbulent
What is the relationship of hematocrit and blood viscosity?
The higher the hematocrit, the increased viscosity
As blood pressure increases within a vessel why does flow increase much greater than one would expect?
If change the driving pressure, will increase flow
not sure if this is correct*
What is the most important monitor in the OR?
We are!
Name a return-to-flow technique of blood pressure measurement.
- palpating
- pulse-ox
- indwelling arterial catheter
In a healthy individual with disease-free arteries does systolic BP increase or decrease in the leg relative to the proximal aorta?
It’s increased
Name several insertion sites for arterial catheters
- radial
- brachial
- femoral
- dorsalis pedis
- axillary
- ulnar
What is damping in the arterial line tracing?
After the “square wave test” (fast flushing line)
- waveform will rise up, plateau, drop off sharply.
- the “bounce’ after determines how dampened line is
- the time between oscillations will be short
- there should be at least one “bounce” oscillation (if doesn’t, too dampened)
- there should be no more than 2 oscillations (more than that, under dampened)
- there should be a distinct dicrotic notch. (This is first thing to disappear if becoming more dampened)
- If over dampened:
- may be clot at catheter tip of air bubbles in line.
- if under dampened:
- will over-estimate the systolic
MAP will remain the same in spite of dampening*
What is ringing in an arterial line tracing?
No clue
