
What is pressure?
force applied or distributed over a surface as force per unit area

What is the formula for pressure?
P=F/A

What is a Newton?
unit of force, N= kg m / s^{2 =}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?
more

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?
the Pascal

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?
9.81 N

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?
potential

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?
linear

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?
density

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?
turbulent flow

What does Re < 2000 mean?
laminar flow

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 (P1P2) r ^{4 }/ 8 nl
 n= viscosity
 l= length
 r= radius

What does the HaganPoiseuille'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?
T!

What happens to blood flow when radius doubles?
Flow rate increases 16 fold!

What is another name for flow rate?
Conductance

What is the driving force moving blood thru the vessel? What opposes it?
Pressure gradient
Resistance

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 = MAPCVP / 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
 asthma

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

