pressure, flow, and tension
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What is pressure?
force applied or distributed over a surface as force per unit area
What is the formula for pressure?
What is a Newton?
unit of force, N= kg m / s2 =units of accel / units of time
What are sources of force?
- 1) gravity
- 2)energy within an atom or molecule (kinetic molecular theory)
Will a more dense substance exert more or less pressure?
How much more dense is Hg than water?
13.6 x more dense than water, so it exerts more force
What is the SI unit of pressure?
How to pascals and newtons relate?
1 Pa = 1 N
What are the different units of pressure and their conversions?
1 atm = 760 mmHg = 760 torr = 14.7 psi = 101 kPa
1 mmHg = ___ cm water.
1 mm Hg = 1.36 cm water
Explain how a Bier block works.
If the force is constant and the area increases, what will happen to the pressure?
It will decrease.
What is the toxic dose of lidocaine when NOT used with epi?
4 mg / kg
When talking about pressure, force, and area with a syringe, what constitutes the "area"?
Area is the cross sectional area of the syringe plunger
If exerting the same amount of force, would a 5 cc or a 10 cc syringe exert more pressure?
the 5 cc syringe
What can occur if pressure points aren't padded and why?
- Ischemia to extremities
- If pressure is greater than SBP (remember BP likely to be low during surgery), no blood flow
What is the gravitational force?
What is ischemic optic neuropathy?
Pt prone during laminectomy (for example), pt can loose circulation to optic nerve and become blind.
What is gauge pressure?
Pressure on the gauge.
What is absolute pressure?
gauge pressure + atmospheric pressure
What represents the kinetic energy of fluids?
flow; kinetic energy is energy of motion
Is pressure considered potential or kinetic energy?
What is the total mechanical energy of a moving fluid?
sum of kinetic energy (flow) + potential energy (pressure)
What is the equation to calculate flow?
F= Q/ t
- F= mean flow
- Q= quantity (mass or volume)
- t= time
What is laminar flow?
Occurs in smooth tubes, low flow rates, needs a pressure gradient to have flow
In laminar flow where is flow greatest and where is it least?
- greatest in center of tube
- least at the walls
What is the relationship btw pressure and flow in laminar flow?
What is the key factor in laminar flow?
viscosity (resistance to flow)
What is turbulent flow?
- Nonparallel paths of travel, eddy currents
- occurs at points of constriction
How are flow and pressure related in turbulent flow?
flow is proportional to square root of the pressure
What is a key factor in turbulent flow?
At the same flow rate where is resistance higher, laminar or turbulent flow?
turbulent flow has greater resistance
What is Reynold's Number?
determines if flow is laminar or turbulent
What is the formula for Re #?
Re = 2rvd / n
- r= radius
- v= velocity
- d= density
- n= viscosity
What does Re > 2000 mean?
What does Re < 2000 mean?
What is critical flow?
- the change from laminar to turbulent flow;
- depends on velocity of gas which depends on volume of flow and tube diameter, depends on the gas, and on the T (as this can change density)
Why are anesthetic gases likely to have laminar flow for longer?
Anesthetic gases warmed as enter the airway, as T increases, density decreases, and critical flow increases (less likely for Re # to reach 2000 and for critical flow to occur)
In what vessels would turbulent flow be likely to occur in?
PA and aorta which benefit from pulsatile flow and high velocity blood flow
Does turbulent flow usually occur in small vessels?
No as the Reynold's Number is almost never high enough to cause turbulence
What are clinical examples of laminar flow?
quiet breathing, blood flow except at bifurcations
What are clinical examples of turbulent flow?
coughing, speaking, deep breathing, increased secretions, carotid stenosis (manifests as bruit)
What is the formula for Poiseuille's equation?
(Q) flow = Pi (P1-P2) r4
/ 8 nl
- n= viscosity
- l= length
- r= radius
What does the Hagan-Poiseuille's equation tell us?
that changes in vessel size, airway, and IV catheter size can influence flow significantly
Poiseuille's equation only applies to laminar flow, T or F?
What happens to blood flow when radius doubles?
Flow rate increases 16 fold!
What is another name for flow rate?
What is the driving force moving blood thru the vessel? What opposes it?
What is Ohm's Law?
- R = change in P / F
- resistance = change in P / flow
What is a clinical application of Ohm's Law?
- Calculating CO or SVR
- CO = MAP-CVP / SVR
What is a benefit of using Heliox?
- Uses density and viscosity to decrease resistance to flow
- When giving He mixed with O2, it decreases the pressure needed to deliver the O2
When might you use Heliox?
- subglottic edema
- foreign bodies
- tracheal tumor
Why can Heliox effectively deliver O2 in cases of severe turbulent flow?
It has a very low density / viscosity ratio
What is Bernoulli's Law?
applies to a neb or venturi mask; at the point of constriction, forward velocity is greatest and pressure is least, negative pressure is created, and this allows it to draw air or fluid in (RA is entrained)
What is jet ventilation?
Aimed into side channel of laryngoscope, provides high flow, RA is entrained; it is an example of Bernoulli's Principle
What is tension? What law defines it?
- a tangential force in N/ m acting on a length of a wall
- tension is part of the structure of a blood vessel, is caused by smooth muscle and elastic tissue
- Law of LaPlace
What happens when there is an imbalance btw tension and fluid pressure inside tube?
It becomes distended or collapses
What is the equation for the Law of La Place for a cyllinder?
- P= T/ R
- pressure = tension / radius
What is the equation for the Law of La Place for a sphere?
- P= 2 T / R
- Pressure = 2 tension / radius
What force does tension oppose?
It opposes the hydrostatic pressure inside the vessel
How does tension change as shape changes from cylinder to sphere?
What physiologic process resembles this?
tension decreases due to law of laplace
an enlarging aneurysm
How does the Law of LaPlace apply to the heart?
As distended heart fails, the radius increases and the pressure falls unless the muscle contracts more forcefully
What law explains how both the aorta (very muscular vessel) and capillaries (only 1 layer of cells) can both withstand pressures high as 100 mm Hg?
Law of LaPlace, smaller radii means less resistance
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