[Music] thank you welcome to ICU primary prep class this is another primary snippet for this week and I'm joined by Dr Mike Clifford again from Melbourne welcome Mike thank you swapnil hello again last time we discussed about West zone so this week we are going to discuss about Airway resistance so can you please explain what are the physiological factors that affect Airway resistance okay excellent so this is a another fairly common SAQ and often repeated It's relatively straightforward it's nice core material there's two key words in this in this request one is physiological factors so by definition you don't have to talk about any pathology and then Airway resistance we're purely talking about the resistive element of the airway right so we don't have to worry about tissue or lung inertia or any of that sort of stuff we're just looking at the flow of gas in the airway so as can be expected Airway resistance is the the frictional resistance to gas flow in the airway and it's equal to the driving pressure divided by the flow okay and normal Airway resistance is fairly low it's about two centimeters of water per liter per second and that's largely due to resistance in the medium-sized bronchi there's a lovely graph that shows how resistance changes over the airway and it starts moderately high and goes up Peaks and then drops all the way down as you go down through your increased orders of branching in the airway so paradoxically despite the fact you've got these tiny little tubes you're breathing through because you've got so many millions of them it's actually very easy to breathe through them so your Peak resistance is actually occurring up in your higher order branching now that's that's true in the adult if you have a look at but infants their their resistance is peaks in the nasopharynx okay and that's one of the reasons why they have such an increased effort of breathing is because they're breathing through uh their nostrils and that's their highest area of resistance but qualities and purposes for the exam they all resistance peaks in your upper branches and then it drops away quite quickly and the resistance will depend as it does in any tube on whether the flow is laminar or turbulent and the resistance is significantly less when flow is laminar okay so how do we calculate that well we we rely on Hagen per sales equation when we're talking about laminar flow and that's the R equals eight n l over Pi R4 on the viscosity so you've got n equals viscosity L equals length R equals radius and where the flow is terminal or not will be predicted by Reynolds number and Reynolds number is the the function that relates gas density to tubular diameter and relates that to the velocity divided by the viscosity so you've got density multiplied by diameter multiplied by velocity divided by viscosity and the numbers to remember are two thousand and four thousand if your Reynolds number is less than two thousand you'll have laminar flow if your number is over four thousand you'll have turbulent flow and the reality is in the human Airway it's almost always a mixture and mostly turbulent because it takes quite a while for flow down a tube to reach the point where it has laminar flow so effectively in humans it's not until year 11 branching so that's quite well down the lung so to all intents and purposes all the gas flow in your nose and your trachea and your bronchi and your major bronchi and your middle order bronchi all that stuff you've got effectively turbulent flow you don't get laminate the flow till you're all the way down into the bronchioles and just before the rest of your bronchials down at sort of the 11th generation because you need quite a long piece of pipe before turbulent flow has a chance to become laminar and many of our respiratory pipes are quite short and the branches are coming off at various angles so it's actually quite a difficult trick for us to get laminar flow in our respiratory system but the important thing for your written answer is that you write that there is turbulent flow and laminar flow laminar flow you've got the Hagen possail equation the Reynolds number will predict what you've got and that you do get laminar flow in those much slower branches okay you can then answer your question about the physiological factors of influence that will be those that influence Airway diameter and the things that affect Airway Dam and are obviously lung volume so resistance is an inverse function of lung volume as you increase your lung volume you increase the radial traction on all of your Airways and all of your pipes so those pipes get larger and that has a significant effect on Resistance obviously we when you're breathing in you're tending to open up all those tubes and when you're breathing out you're tending to compress them so your Airways resistance increases during expiration and you can even end up with lung closure so you get even further reductions in along opening and increasing areas resistance the position of your your lung in space so in the Supine position breathing it volumes close to closing capacity will increase your Airways resistance and then things that can tend to narrow or compress your Airway that's largely your the balance between your sympathetic and your paracetic nerve system so the sympathetic tone on your bronchial smooth muscles causes bronchodilation and tends to open up those areas Airways that have smooth muscle in the wall and similarly parasympathetic activation tends to cause local constriction of those Airways and reduce diameter and increased Airways resistant things like foreign bodies mucus plugging and disease are not physiological factors so I wouldn't rush to put them in your answer I unless you've written all of the other factors and you're down at the bottom because you theoretically in this question you shouldn't necessarily score marks for writing pathological changes but things like local cellular mechanisms um cytokine reactions and changes the the role of nitric oxide are all they're all on that that balance between what is physiology and what is pathophysiology and there's no perfect answer to that but if they're they're the last bit of your answer rather than the first bit of your answer that's important so the big things to talk about are you know Airway size Airway opening Airway conductance then talk about Airway diameter down to the 11th order of branching and then after that the effects of parasympathetic and sympathetic nervous system on modulating the diameter of those Airways as the chief as the chief factors it's important to remember that things like Airway density and viscosity don't really come into it because we're not normally breeding anything other than air or oxygen and the differences between those two in terms of their viscosity or density is fairly minimal and doesn't affect resistance 20 grade degree and this isn't a question of pharmacology so we're not talking about helium so you can park that and not worry about it and just focus on the effects of Airway diameter as the chief problem thanks Mike it's often quite tempting for candidates to answer a lot of pathological factors and that's a common mistake they will talk about the drugs that cause bronchoconstrictions the pathology is like COPD asthma pausing and that's not really the focus of this question the focus is simply what are the physiological factors in normal a healthy adult patients that affect the airway resistance yeah and it's very tricky because almost all of our patients are coming to RCU are on some drug and particularly if they've got a respiratory problem they'll be on drugs it will be affecting the areas and many of the drugs we use will affect those those things but I think if you take a step back and say how can I link anything physiologically um it's all about what those drugs do to the sympathetic and the parasympathetic nervous system if the drugs don't have direct effect on The receptors of those systems um then the fact that propofol reduces your sympathetic nervous system and lets your Airways relax I mean that you really you're pushing to get a to get a link so if a question says what are the physiological effects try and stay very true to their that um that question and you'll get more marks than if you uh go off pissed and start describing pathology and pathophysiology and then pathopharmacology because you're you're not going to be scoring marks doing that so try and try and maintain core physiological descriptions in the body of your answer that's correct so thanks Mike thanks for explaining this quite nicely so that's the end of our today's snippet we'll be back with another snippet in week's time till then goodbye okay see you later [Music] thank you [Music]