The fantastic Prof Tom Barnes gave us a talk today on the PHYSICS of mechanical ventilation. This is his material via @ESICM #ESICMphysio in the form of a Tweetorial :) #FOAMed THREAD BELOW --> (continued...)
and so we start... Physics can tell us an awful lot about the way our bodies work - but it's SO different from clean 'physics' - this is applied physics. All patients lungs are different. These concepts however are vital to understanding key issues in ventilation. 
Lung damage can amplify from the smallest of initial injuries. Positive feedback ++. Damage begets damage. This is why we try to mitigate pressures in our vent settings. #ESICMphysio Question is tho... how can we do it better?
Let's look at a simple ventilator and some simple vent mechanics. We a patient, connected to a flow or pressure generator, with a valve thrown in there for good measure ;) #ESICMphysio
Key components from this vent actually depend on CONSTANT FLOW or CONSTANT PRESSURE... they're quite different and this becomes important when we look further at modes of ventilation. #ESICMphysio
If we simplify a patient down into an 'electrical model' - with resistors in place you can see that we have so many elements of resistance arising from the airway and different components of anatomy within the chest wall (and beyond...) #ESICMphysio #FOAMed
As we know - Voltage is analogous to pressure and Current is analogous to flow. #ESICMphysio
Here we can see examples of PV loops at different stages of the tubing and the patient - in PRESSURE and VOLUME modes. Any thoughts on the different modes? #ESICMphysio #FOAMed
Small amounts of energy, subtly applied can have massive consequences in heterogenous lungs. Energy dissipation can occur to the tissue, but also to surrounding tubes, machinery. When damage occurs > than the healing rate --> spreads --> VILI --> badness. #ESICMphysio #FOAMed
Flow --> Energy Dissipation --> Damage.... so what to do? #ESICMphysio
We need to try and eliminate 'power pulses' - how to achieve constant flow? We need to control it to be constant in both inspiration and expiration... #ESICMphysio
You can see the effect of controlling flow in both exp and insp here.... bear this is mind for the next bit... #ESICMphysio
THEORY: If resistance is constant.... for a given Vt over a given cycle time... FLOW should be constant in order to minimise energy dissipation. #ESICMphysio #FOAMed
This theory works because HIGHER flows require more pressure across the airway resistance to inject or expel gases. ALSO - we know that resistance is less when the RR(freq) is higher anyway.... not that that can be applied easily clinically... #ESICMphysio
You can see the example of these phases of increased pressures required by comparing the two pictures.. #ESICMphysio
Literature to support the theory that there is reduced damage with flow control... #ESICMphysio
So.... how do we apply this constant flow throughout the entire cycle of ventilation? Watch out for some research to be published soon, on this very topic... using pig lungs! #ESICMphysio #FOAMed
