Flow
Quantity of fluid passing a point in unit time
F = QT
Laminar flow
Flowing liquid made up of a series of parallel laminae
Flow greatest in centre (x2 mean flow), and least at walls where approaches zero
∝
Pressure drop
Radius4
1/Length
1/Viscosity (the property of a fluid that prevents flow)
Hagen poiselle
F = ∏ P r4 / 8 n l
Applies to newtonian fluids (water, not blood)
Turbulent flow
Critical flow is the flow above which flow becomes turbulent
Turbulence becomes likely when reynold’s no >2000
Re = velocity x density x diameter / viscosity
∝
√P
r2
1/√length
1/√density
Clinical implications
Gas
- Turbulent flow increases resistance
- Minimise angles and constrictions in breathing systems
- Decreased calibre of airways may critically impair respiration
- Fluid flow through a cannula will double if driving pressure doubled or length halved and x16 if internal diameter doubled
Flow measurement
Liquid
Syringe driver
Drop counter
Blood flow
US Doppler
- Change in observed frequency when the source moves towards or away from the observer
- US from crystal reflects off RBCs and sensed by a second crystal
- Frequency of reflected waves increases ∝ to the velocity of the flow towards the probe
- Cold fluid injected and temp measured at thermistor in PA catheter or PICCO A-line
- Conc time curve generated from which CO worked out
- Blood flow = rate of uptake or excretion / AV difference
Gas
- Constant orifice / variable pressure
Senses the pressure difference across a fixed resistance
Flow made laminar so pressure ∝ flow
- Constant pressure / variable orifice
Tapered glass tube containing a bobbin with angled grooves so it spins
Laminar at low flow; turbulent at high flow
Peak flow meter
- Variable pressure variable orifice
Gas through a tube with holes immersed in water
Variable orifice due to no of holes
Variable pressure due to depth
- Other
Heated wire in gas flow
↑flow → ↑current needed to maintain constant temp
Volume of gas per unit time
Respirometer
Spirometer