Flow, Pressure, Tension
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the force applied or distributed over a surface as force per unit area
What is the equation for pressure?
What are the units of force in pressure equation?
Sources of force
- 1. Gravity:
- Force of gravity = 9.81 m s-2 =9.81N
- 2. Energy within atom or molecule (kinetic molecular theory)
- -The denser the substance, the greater the pressure exerted
What is used to determine atmospheric
Barometer! Atmospheric pressure is determined by height of column of mercury in the sealed glass tube. The downward pressure is balance by the outward atmospheric pressure
Mercury is ____times as dense as water
Mercury is 13.6x as dense as water. So force exerted by it’s weight is proportionally greater. So pressure that supports a column of 7.5mmHg will support a column 10.2cmH20.
What is the SI unit of pressure?
- The Pascal
- Such that 1 Pa is a pressure of 1 Newton
- N = kg m
- over an area of 1 square meter (m²)
What are the common units of pressure? (there are five of them)
PSI: pounds per square inch
mm Hg: millimeters of mercury
torr: named for Italian physicist Torricelli
Pa = Pascal (SI unit) = 101 kPa
What is the atmospheric pressure at sea level (in all the common units of pressure)?
- 1 atm =
- 760mmHg =
- 760 torr = 14.7psi = 101 kPa
1 mmHg = _____ cm water
1 mmHg = 1.36 cm water
Explain the Bier Block. ( IV regional technique)
- Start regular IV on the other side (sedation,
- emergency line if needed) then start small IV on the hand that will be operated used. Then put some Web roll over the upper arm, the put on a double tourniquet. Inflate/deflat A, then inflate/deflate B (set up to inflation points). Elevate arm to drain blood, then
- wrap an esmark from fingertips up, to drive blood to upper arm,once wrapped tight, sequentially inflate distal cuff then proximal cuff (inflate to 250-275mmHg). Unwrap esmark and lie arm on arm board, now injecting 50ml of 0.5% Lidocaine (MPF).
- 2 pressures going on: Got opposing pressure of tourniquet and pressure being generated
- by the size of the plunger on syringe (use
- 50cc syringe). In practice, we’ll keep both tourniquets inflated while SLOWLY injecting
- then once injected remove IV. The reason it works out well, once medication is injected, we can deflate distal cuff, allow local anesthetic to migrate under it,then re-inflate distal cuff, and deflate proximal. Now pressure of distal cuff is over local aestheticized site.
When would you use a Bier block?
Frequently used for hand surgery if it can be completed in 1hr or less. (Carpal tunnel syndrome)
Why is it so important to ensure the tourniquet is completely inflated during a bier block?
Tourniquet inflation is important because we’d be blasting an almost toxic level of Lidocaine.
Why do we use a 50ml syringe instead of a 5ml syringe for something like a PICC line?
Because we don't want to exert too much pressure on the line (could push clot somewhere!)
Explain this significance of this picture.
The pressure will vary according to area, (assume the force is constant).The pressure delivered will vary w/area (the cross sectional area of plunger of the syringe) If area goes up, the pressure goes down. Reverse is true, if area goes down, the pressure goes up.
Why do we use a 50ml syringe to give the lidocaine during a bier block?
If we use a smaller syringe we have more pressure being exerted. The goal is not to overcome the tourniquet pressure (275mmHg). So using a large syringe (50ml) will ensure that pressure will not be exceeded.
We use the pressure equation to explain the pressure points padded theory. Explain why it's important to pad these areas.
Part resting on bed is generating force then if you do out the pressure formula calculation, get a certain pressure (ex: 147mmHg)
Inanesthesia, BP decreases. Easy for BP to drop below 147mmHg, so a patient in position w/poor padding, can end up w/ischemic extremities.
Ischemia optic neuropathy for patients in head down position (prone), if the pressure generated enough to offset the pressure of circulation to optic nerve , patient can be blind after surgery
Explain how pressure relief valves work.
- Pressure exerted within anesthesia delivery system acts over area of disk, that’s opposed by a spring w/particular calibrated
- force above it. When pressure is enough to overcome forces exerting by spring, gas will escape.
In case of expiratory valve, light spring used, low pressures would be enough open valve in minimal setting. But we can adjust pressure by screwing down on this valve to tighten the force, so more pressure is required to release the valve.
When would you want to tighten the pressure relief valve?
Guage pressure vs absolute pressure.
What is absolute pressure?
Absolute pressure = Gauge pressure + Atmospheric pressure
If a full oxygen cylinder has a pressure of 2000 psi (gauge), what is the absolute pressure?
What is the bourdon gauge?
Bourdon gauge: flexible coil, one end exposed to needle connector, the other to gas, as pressure gets higher, the tube uncoils, the needle points by pressure reflected by tank.
When an oxygen cylinder is empty, it reads zero. What is the absolute pressure?
So empty cylinder when gauge reads zero, still at ambient atmospheric pressure (unless we apply vacuum to it, can’t get all gas out)
So the absolute pressure would be 14.7PSI.
What is the definition of flow?
the quantity of a fluid (gas or liquid) passing a point in unit time
What is the equation for flow?
F = Q/t
F = mean flow
Q = quantity (mass or volume)
t = time
True or False.
Flow represents kinetic energy of a fluid.
Pressure represents potential energy of a fluid.
Can pressure and flow can change
- Can have high pressure but low flow d/t vasoconstriction (high SVR) likewise septic patient can have low BP but high CO (septic shock).
The total mechanical energy of moving fluid is sum of what?
of kinetic energy (flow) and potential energy (pressure).
TRUE or FALSE
Laminar flow happens in smooth tubes with low flow rates.
In laminar flow, flow is _____ at the center and approaches ____ as the wall.
flow is greatest at the center and approaches zero at the wall.
What is the key factor in laminar flow?
In Laminar flow, there is a linear relationship between what?
Pressure and flow. (Pressure increases, flow increases)
TRUE OR FALSE.
In Laminar flow, you don't need a pressure gradient across the ends of the tube
FALSE.There must be a pressure gradient across the ends of the tube in laminar flow.
The____ the viscosity the more the layers (think arrows in picture) will interact with each other. The ___ the viscosity the less flow there is for a specific or given pressure gradient.
The HIGHER the viscosity the more the layers (think arrows in picture) will interact with each other. The HIGHER the viscosity the less flow there is for a specific or given pressure gradient.
Viscosity = (definition)
Viscosity =a fluid’s resistance to flow
In what kind of flow do molecules travel in nonparallel paths leading to eddy currents?
When does turbulent flow happen?
Occurs at points of constriction →↑ fluid velocity
In turbulent flow, flow is approximately proportional to the _______________?
- Flow is approximately proportional to the
- square root of the pressure
TRUE or FALSE
In turbulent flow, the pressure & flow have a linear relationship.
FALSE. It is non-linear
What is the most important property in turbulent flow?
TRUE or FALSE
In turbulent flow, there is > resistance than for laminar flow at same flow rate.
If rate of blood flow becomes to great,
or constriction, or obstruction, or a bifurcation, or a rough surface, the flow becomes ....
turbulent & there’s more resistance.
What determines if flow is turbulent or laminar?
Reynold's number relates a ratio of what two things?
Reynold’s number relates ratio of the density of a gas to its viscosity
What is the equation for the Reynold's number?
- Re= 2rvd
- where: d=density, v=average velocity, r=radius, n=viscosity
Re >2000 =
Re < 2000 =
Re>2000 = turbulent flow
Re < 2000 = laminar flow
In what vessels are there a number of conditions conducive to turbulent
The aorta and pulmonary artery. high velocity of blood flow, there is pulsatile flow, & there is a large vessel diameter
In ___ vessels, Re# is almost never high enough to cause turbulence.
What is critical flow?
It's a change from laminar to turbulent flow
- -Depends on velocity of gas which….
- -Depends on volume flow & diameter of tube
Critical flow is defined specifically for what two things?
Defined specifically for the gas & temperature that it’s at
With warming of anesthetic gases as they
enter the airway, the critical flow rises due to the ___density from __ temperature
With warming of anesthetic gases as they enter the airway, the critical flow rises due to the ↓ density from ↑ temperature
Warming of anesthetic gases, Flow will stay laminar longer, takes higher flow rate for Re# to get to 2000 with increase in temperature
Give two clinical examples of laminar flow.
- Most Blood flow (except at bifurcations, large
- vessels, etc.)
Give three clinical examples of turbulent flow.
Coughing, speaking, deep breath
What is Poiseuille’s equation.
- Flow= (pie) (P1- P2) r4
- 8 nl
- ΔP =pressure gradient/ driving pressure
- r =radius
- n =viscosity
- l = length
TRUE or FALSE.
Pouseuille's equation applies only to Laminar flow
- doubling the radius, we get 16fold increase in flow rate.
- substantial change if radius changes up or down
What is the importance of Pouseuille's equation in the clinical setting?
Explains importance of airway narrowing, vessel size or IV catheter size.
Choice of catheter size or vessel to place it in, flow can go up or down dramatically.
Explain this picture
The effect of vessel diameter on blood flow. So if slight change in diameter of the vessel it will cause tremendous change in the vessels ability to conduct blood when the flow is streamlined. So here we have three vessels with relative diameters of 1, 2, and 4 but all have the same pressure difference of 100mmHg. Even thought the diameters only increase 4-fold, the respective flow rates increase dramatically. The flow rate (or the conductance through the vessel) increases the 4th power of the radius.
What does conductance mean?
the measure of blood flow through a vessel for a given pressure difference
What does resistance mean?
the impediment to blood flow
What is the clinical equation of Ohm's law?
What is the basic equation (math vs the clinical application) of Ohm's law?
How do we use Ohm's law?
to describe the flow of fluid (blood) through a tube (blood vessels) even through vessels are dynamic rather than static (Dynamic, smooth muscle can contract and radius can change).
We use Heliox in the clinical setting by relating density & viscosity to ↓ resistance to flow. What type of clinical scenarios would we use Heliox?
- Used in acute, severe airway obstruction
- -Subglottic edema
- -Foreign bodies
- -Tracheal tumors
Why is Heliox so much better than 100% oxygen?
Highly improved ratio of Helium compared with 100% oxygen( & much more favorable than oxygen and N20). So if we were using small ETT (ENT surgery to do resection they need small ETT to do resection) the pressure needed to ventilate patientt can be cut in ½ when 80/20 mixture is used. Heliox is not a cure but temporary measure while more definitive treatment is being done.
What is BERNOULLI’S LAW?
When liquids flow through a tube, a pressure is exerted against the sides of the tube. The faster the flow rate, the less side pressure is generated. If the pathway is varied in cross-sectional area, forward velocity is fastest & side pressure is least at the point of greatest constriction
Describe a clinical use of Bernoulli's law
- Nebulizer or Venturi. (or laryngoscope/jet vent)
- Meant to show that as the variable width of tube, where there is the greatest constriction the forward velocity is greatest and the pressure will be less, (so in the case of negative pressure can draw in fluid from neb) or the case of a Venturi mask, based on size of orifice will draw in more air to dilute out 100% oxygen. Where constriction occurs, the flow is greatest, kinetic energy increases, potential energy (pressure) goes down.
TRUE or FALSE.
Total energy is constant
- Total energy = kinetic + potential.
- (if one goes up the other must go down to stay constant).
What is the definition of tension.
A tangential force (in units of Newtons/meter) acting on a length of the wall
Tension is the pressure caused by the ___ ____ & ____ ____.
Tension is the pressure caused by the smooth muscle & elastic tissue
TRUE or FALSE
There mut be a balance between tension & the fluid pressure inside the tube to prevent it from collapsing or distending
Law of LaPlace for a tube (blood vessel)=
Law of La Place for a sphere (heart) =
Clinically, how do we use the concept of LaPlace's law?
Use this concept to explain how aorta (radius of 1cm) can sustain a pressure of 100mmHg with elastin, collagen and smooth muscle. And a capillary (one endothelial layer and radius is much smaller) can still sustain a pressure of 100mmHg. T = P xR, if pressure is constant at 100mmHg. And radius is high in Aorta, then consequently is also high (have structures). Capillaries, radius is now down, don’t need as much tension.
What happens if wall tension exceeds a certain point in vessel?
If wall tension exceeds a certain point the vessel collapses and there would be no flow
nWhat happens to the wall tension required
to sustain a developing aortic aneurysm when the radius increases to 2 cm, then
3 cm, and so on?
P = T
Tension ↑ as radius ↑. The wall tension required to hold a relatively constant arterial pressure in an expanding aneurism increases and any reach a point where available vessel wall structures are incapable of sustaining the pressure and the aneurysm ruptures.
Why does an aneurysm initially minimize tension?
- Expanded area of aneurysm initially resembles a sphere, this minimizes tension because Laplace's law for sphere is
- Example: T= PR
- T= 100mmHg x 4 = 400
- T = 100mmHg x8 = 400
- T = 100mmHg x12 = 600
- (tension exceeds what wall pressure tolerates and it bursts)
Explain LaPlace law and BP measurement
- As the distended heart fails the radius increases & the pressure falls unless the muscle contracts proportionally more forcefully
- T= PR
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