What the curriculum thinks you need to know:
PC_BK_31 Bernoulli principle
PC_BK_32 Venturi effect and entrainment devices
What you need to know (The theory):
The Bernoulli Principle
An increase in velocity of a moving fluid results in simultaneous decrease in its pressure (and visa versa)
So, is we introduce a constriction into a pipe with a certain flow rate of fluid running through it, the velocity of the fluid has to increase as the pipe constricts to maintain that flow rate. Think of a motorway, if we close two of the three lanes, the cars would all have to speed up dramatically to all get through and keep the overall number of cars passing per second the same. This also explains those annoying speed limit signs way before road works. Limiting the inward flow allows to maintain the flow of traffic.
The side effect of this increase in kinetic energy (from the increase in velocity) is a decrease in the amount of potential energy. This is due to the first law of thermodynamics (energy cannot be made or destroyed, only changed from one form to another). The result of this decrease in potential energy is a decrease in pressure.
We’ve all sprayed people with a hose pipe when we were kids by squeezing the end. You may have been told that its because it increases the pressure of the water… Except it actually does the opposite. Just remember this in the exam and don;t get mixed up. Pressure will DROP when a fluid flows through a constriction, its the velocity that increases.
The Venturi Effect
We all know the word ‘Venturi’ from a Venturi mask. Unfortunately Giovanni Venturi Didn’t invent the Venturi mask! He ‘merely’ found the principle behind it.
A Venturi valve consists of an open ended constriction in a tube. This results in a high velocity jet coming from the end of the constriction. This jet has a high velocity, but a low pressure as described above. If this pressure is less than that of the gas surrounding it, the surrounding gas will be ‘entrained’ in to equalize out the pressure.
The Coanda effect
Now this isn’t technically on the curriculum, but it has come up. I’m sure you’ve noticed this before…
Next time you have a bag of fluid running fast into a patient, look at the chamber in the giving set. Gently tip it to one side and as the flow of fluid gets closer to the wall of the chamber you’ll notice it seems to jump and ‘stick’ to the wall. This is the Coanda effect.
Its caused by a similar principle to the Venturi effect. The flowing fluid has a high velocity and a low pressure so entrains surrounding gas into it. Thats ok on the side which isnt close to the chamber wall, but on the chamber wall side this ‘attempted’ entrainment lowers the pressure and ‘sucks’ the fluid against the wall.
What you need to know (How it works in practice):
The Bernoulli Principle
This is how pressure restriction valves (the type on the inlet to anaesthetic machines) work. They take the 4 Bar gas supply pressure, put it through a restriction in the pipe and this increases the velocity of gas flow, but drops the pressure to something more manageable.
The Venturi Effect
In a Venturi valve this is used to decrease the delivered concentration of concentration, usually in patients with COPD who are at risk of loosing their hypoxic drive if given too high concentrations of oxygen.
The valve itself states the eventual concentration of oxygen that will be achieved if the stated flow rates are put through it. When these flow rates are achieved, the surrounding room air will be entrained at an appropriate rate and achieve the end concentration desired.
Now, its a common misconception that you HAVE to have the oxygen flow at this rate or you don’t get the oxygen concentration you want. This is only kind of true (but mostly a load of rubbish!).
If you increase the flow to double the stated rate, you will still get the same eventual oxygen concentration at the end. Why? Because the surrounding air is entrained at twice the rate. But, the flow rate out of the end of the valve is also doubled. The table below shows this but with some more usable increased flows.
|Venturi Valve||Oxygen %||Flow rate Setting||Eventual Flow rate||High Flow Setting||Eventual High flow rate|
So why does this ‘myth’ get perpetuated? Its got more to do with misunderstanding. If you stick a mask with a 28% venturi valve on, the flow rate of gas reaching the patient is 45L/Min. Now what if your patient’s peak inspiratory flow rate exceeds this? Then the required inspiratory flow is obtained by entraining air around the mask. This will dilute down your oxygen concentration delivered to the patient rendering the device ineffective.
You need to be very cautious with the 40 and 60% valves as their flow rates to the patient are actually quite low. A hyperventilating hypoxic patients PIFR can easily exceed the 24L/min provided, leading to delivery of waaaay less oxygen than you think.
The solution? Turn up the oxygen flow. This is the idea of high flow oxygen supplies on ITU.
Note that this may be difficult with the higher % valves as your required flow quickly gets more than your wall rotameter will provide (usually 15L/min max). With very high flows you may find it hard to keep the oxygen tubing attached to the valve!!
The Coanda effect
The Coanda effect is commonly used for ‘fluid logic’ systems. One example of this is the good old Penlon Nuffield 200 ventilator. This uses a valve similar to the one below to move gas flow from one limb of a circuit to another with no moving parts.
Other clinical examples of the Coanda effect include the unequal blood flow at a bifurcation of a blood vessel due to an atheroma and the unequal filling of a pair of alveoli due to mucus plugs in the conducting airways.
Random Exam factoids (i.e. the things the college like asking):
- Make sure you have Bernoulli and Venturi the right way around in your head. Its easy to get them mixed up when asked about definitions.
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