Fluids and Pressure

Question 1

What is pressure?

Answer 1

Pressure = force applied per unit area

Measured in:

• N/m²
• Pa
• Bar
• Atm
• Psi
• Torr
• mmHg
• cmH2O

Question 2

What is a fluid?

Answer 2

Substance that continually deforms (or flows) under an applied shear stress.

“Fluid” can be used to describe different phases of matter that behave similarly under flow conditions.

• gases
• liquids
• plasma
• even some plastic solids

Question 3

What is fluid pressure?

Answer 3

The pressure at some point in a fluid.

May be static (when fluid not flowing) or dynamic (is flowing).

Question 4

What is flow?

Answer 4

Flow is the quantity of fluid (gas, vapour or liquid) passing a point per unit time. The units used are L/min.

Represented by the equation: Flow = Quantity/Time.

Question 5

What is laminar flow?

Answer 5

The molecules of a fluid pass smoothly and steadily, parallel to the walls of the vessel.

Question 6

What is turbulent flow?

Answer 6

The molecules of a fluid swirl into eddies

Question 7

What is viscosity?

Answer 7

The property of a fluid that causes it to resist flow.

Measured in pascal seconds (Pa.s) and denoted by the greek letter ‘η’ .

Fluids can be Newtonian - eg water, where viscosity is constant.

Or Non-Newtonian - eg blood, where viscosity is not constant.

Question 8

What is density?

Answer 8

The mass of substance per unit volume.

Measured in kg/m³ and denoted by greek letter ρ

Question 9

What is the pressure gradient?

Answer 9

It’s the fall in pressure across the tube length or orifice.

Denoted by ‘ΔP’ or ‘P1-P2’.

Question 10

What are the tube features?

Answer 10

Radius (diameter) and length.

Tube - wall length greater than annular diameter.

Orifice - wall length less than annular diameter.

Question 11

Which is the most important fluid factor that will affect laminar flow?

Answer 11

Viscosity

Question 12

What is the most important overall factor affecting laminar flow?

Answer 12

The tube radius

Question 13

What equation can be used to describe Laminar Flow?

Answer 13

The Hagen-Poiseuille equation which shows:

• flow is proportional to
  
  • the pressure gradient across the tube
  • the radius to the fourth power (most important factor)
• flow is inversely proportional to
  
  • the fluid viscosity
  • the length of the tube

Question 14

What kinds of fluid does the Hagen-Poiseuille equation apply to?

Answer 14

Not all fluids are Newtonian and the formula does not apply to them.

Eg blood (Non-Newtonian), viscosity decreases as flow increases

Question 15

Give 2 examples of laminar flow in clinical practice.

Answer 15

• crystalloid in a giving set
• air in the lower respiratory tree eg bronchioles

Question 16

What is the most important fluid factor for turbulent flow?

Answer 16

Density

Question 17

What is the formula for flow through an orifice?

Answer 17

Flow (Q) is inversely proportional to the square root of density (ρ = density).

Question 18

What examples of turbulent flow are there in clinical practice?

Answer 18

• at angle pieces or ET tube adapters eg Magill adapter
• in the upper airways (trachea/larynx) hence why heliox (79% helium and 21% O2) is used, because it has a lower density than room air and therefore improves gas flow in turbulent conditions

Question 19

What equation defines turbulent flow?

Answer 19

No equation for turbulent flow.

Reynold’s number (Re) can give an indication when flow may be turbulent. It is dimensionless.

Re < 2000 flow is likely laminar

Re > 2000 flow is likely turbulent

Question 20

What is resistance?

Answer 20

When a fluid flows through a tube/orifice, resistance opposes it. Resistance is constant and expressed by the following equation:

Question 21

How does the resistance of laminar flow compare to turbulent?

Answer 21

In laminar- flow is directly proportional to pressure. The gradient of the curve is resistance (a linear relationship).

In turbulent - the gradient of a curve of P against Q represents resistance. Such a curve is exponential rather than linear because of the multiple eddy currents that resist forward flow. So a greater change in pressure is required to produce the same change in flow.

Question 22

What is Bernoulli’s principle?

Answer 22

Due to the law of conservation of energy, as kinetic energy (flow) increases, potential energy (pressure) must decrease by an equal amount for total energy to remain constant.

Therefore, an increase in velocity of an ideal fluid undergoing laminar flow is accompanied by a simultaneous reduction in it’s pressure.

This gives rise to the venturi effect.

Question 23

What is the Venturi effect?

Answer 23

When fluid flows through a constriction in a tube, the velocity increases across that narrowing.

Due to the Bernoulli principle, there’s a consequent reduction in pressure. That reduction is used to entrain a second fluid into the main fluid flow. The degree of entrainment can be expressed by the entrainment ratio

Question 24

What devices use the Venturi effect?

Answer 24

• venturi O2 masks (driving O2 entrains room air to dilute the 100% O2)
• nebulizers
• sanders injector for ventilation
• suction

Question 25

What is a vitalograph?

Answer 25

Gas flow measurement.

Single expired breath acts on bellows attached to a scribe, which marks volume on paper that moves at constant speed - therefore flow can be calculated

Question 26

What is a bell spirometer?

Answer 26

Gas glow measurement.

Gas is expired into an inverted bell that is immersed in water, which rises and falls on a pulley according to gas volume - when measured over time, flow can be calculated

Question 27

What is a rotameter?

Answer 27

Gas flow measurement.

Have variable orifice and constant pressure and are accurate to within 2% of the flow rate shown

Question 28

What is a pneumotachograph?

Answer 28

Gas flow measurement device. Works on Poiseuille’s law.

May consist of a large number of smaller tubes. Have variable pressure and constant orifice - use various pneumotachograph heads that cause a pressure drop across a differential pressure transducer, which is proportional to flow.

Will be affected by gas viscosity and will give an erroneous reading in the presence of condensed water vapour.

Question 29

What is a Vane meter?

Answer 29

Gas flow measurement device.

e.g. Wright’s Spirometer. Gas flow moves a turbine that is linked to a dial from which maximum flow rate can be measured

Question 30

What is a mechanical flow transducer?

Answer 30

Gas flow measurement device.

A small disc in the gas flow is bent backwards against a strain gauge, the force is dependent on gas flow

Question 31

What is a hot-wire anemometer?

Answer 31

Gas flows over heated wires and the degree of cooling is related to gas flow.

Question 32

What is an ultrasonic flow meter?

Answer 32

When gas flows in the same direction as an ultrasound signal (transmitted within gas flow) an increase in signal velocity occurs; the reverse is also true

Question 33

What are methods to measure liquid flow?

Answer 33

Infusion pumps

• syringe drivers
  
  • when a syringe of known size and volume is attached to the driver, the plunger is driven inwards by a lead screw. Flow is determined by the known volume in the syringe and the rate at which the lead screw moves
• peristaltic pumps
  
  • rollers squeeze a fluid-filled tube in a ‘peristaltic’ manner and force fluid in the direction of the peristaltic wave. Flow is determined by the bore diameter of the tube and the rate of peristalsis

IV drip counters

• Photo-sensors are attached to the drip chamber to sense drip rate
• Drip rate - the number of fluid drips per minute
• The tubing drip factor - number of drips per ml. The setting on the drip regulator (fully closed, fully open or somewhere between)

Question 34

How do the bobbins work in a rotameter?

Answer 34

When the valve is opened, flow rate of gas acts on the bobbin to push it up to a position where the force of the gas pushing it up is matched by gravity pulling it down.

At low flow- the gas is flowing through a tube, (bobbin is at narrow end of tube, obstruction is of greater length than diameter) so flow is laminar.

At high flow - the gas is flowing through an orifice, (obstruction caused by the bobbin has greater diameter and reduced length) so flow is turbulent.

Question 35

Why are rotameters gas specific?

Answer 35

Because different gases have different densities and viscosities which will affect flow.

Question 36

What factors may affect the rotameter’s accuracy?

Answer 36

• warmer gas has lower density and viscosity, so can cause over reading
• reduced atmospheric pressure reduces density of gas, which can cause over-reading

Question 38

What kind of flow is always through an orifice?

Answer 38

Turbulent

Question 39

How is flow determined in laminar flow?

Answer 39

Flow is determined by the pressure gradient divided by resistance. Resistance is constant and independent of flow.