Principles of Mechanical Ventilation 9: Pressure Volume relationship in VC

[@meloyellow6488]

Very informative ! Love your videos

[@beccahuff2481]

OMG! Thank you SO much for explaining this!!!

[@karenchung4601]

In the last example you gave (where you set a higher limit of 40cmH20) is the trigger now pressure instead of volume?

[@RespiratoryReview]

A 'trigger' variable is what causes the inspiration to begin. The example i gave of 40cmH2O causing inspiration to end is an example of 'high pressure cycling'. Reaching that pressure (40cmH2O) causes inspiration to end (cycle). It doesn't affect what triggers the breaths. In AC VC the trigger variable would either be time (based on the RR) or patient effort (flow, or pressure). The high pressure cycling is a different concept. Check out the Principles of Mechanical Ventilation 6: Phase variables video it should help clear that up.

[@briandeluca6908]

You need to do something to make the sound louder on your videos.. its low

[@twistedoracle1144]

Thank you How will the pressure increase? During inspiration, the diaphragm contracts, causing the volume of the thorax to increase. As lung volume increases, the pressure in the lungs must decrease. I understood everything except why the pressure increase? Is not the decrease of pressure is the driving force of the volume to increase. I am confused

[@RespiratoryReview]

The pressure will increase because we are delivering positive pressure to the lungs with the ventilator. In spontaneous ventilation (not on a ventilator), the relative drop in pressure in the intrapleural space leads to gas flowing down the pressure gradient into the lungs, so you are right in your thinking. But remember that this video is talking about positive pressure ventilation, where we are essentially forcing the gas into the lungs with a machine, so the mechanics of it are different. Hope that helps.

[@edlalita4495]

@@RespiratoryReview First I wish to compliment you on your well-presented material. Please let me know if I’m on the right track. I believe the point of confusion may be a result of using Boyles' law to predict the pressure at the end of inspiration. [I typed this up with subscripts for V1 and P1, and pasted it here, but apparently KZbin does not provide subscript functionality] Boyles' law is one of several special cases of the ideal gas law (see Wikipedia link* below): The ideal gas law equation is: PV = nRT where: P = Pressure V= Volume n = the the amount of gas R = (ideal) gas constant T = Temperature Boyles' law is the special case where both the the amount of gas (n) and Temperature are constant and R is already a constant; therefore nRT is a constant: i.e.: P1 V1 = nRT = P2V2 If you try to use Boyles' law you would get an incorrect result of a predicted lower pressure in the lung at the end of inspiration as compared to that at the start of inspiration. However, in the case of inflating a set of lungs, the amount of gas is not constant; it would start at whatever is in the lung at the start of inspiration as a small amount left in the lung at the end of the previous breath*, and at end of inspiration as that plus the amount of additional gas pumped into the lung. Assume for example,we need to put a specific amount of gas into a lung such that at the end of inspiration is 100 times the amount that is in the lung at the start of inspiration (whether we are measuring the amount of gas in terms of the number of moles,the number of molecules,its mass, or whatever***). The actual volume of the lungs at start of inspiration will depend on whatever is leftover at the end of expiration of the previous breath** and its compliance. Therefore, substituting for n: P1 V1 = RT (at start of inspiration), and P2V2 = 100RT (at end of inspiration) or (P1 V1 ) = RT and (P1 V1 )/100 = RT So: (P1 V1 ) = (P2 V2 )/100 (since both sides = RT) Solving for P2 : (P2 V2 ) = 100(P1 V1 ) P2 = (100P1 V1 )/ V2 Now we are confronted as to what to use for V1 and V2 , and that in turn will depend on lung compliance. Looking at two (2) extreme cases: 1. For a lung with an extremely low compliance, to the point of being completely rigid, V1 and V2 would be equal so P2 = 100P1 . i.e.: the pressure at end of inspiration would be 100 times that at the start of inspiration. 2. For a lung with an extremely high compliance to the point of being completely floppy with no elasticity whatsoever, P2 = P1 since the lungs would offer no resistance whatsoever to any attempt to increase the pressure. i.e.: V2 = 100 V1. So the pressure at end of inspiration would be the same as that at the start of inspiration. For a normal lung P2 would end up somewhere in range from P1 to100 P1 . Based on your subsequent video could we then add a compliance term CL such that: P2 = (nCLP1 V1 )/ V2 An Issue: CL would need to have units compatible with n, but n is just a unitless number, so I am not sure where that would lead. *Wikipedia Link: en.wikipedia.org/wiki/Ideal_gas_law **perhaps left as a result of the PEEP at the end of the previous breath. *** or even perhaps what its volume would be if converted to standard temperature and pressure (but that would confuse things with an extra volume term thrown into the mix).

[@MrAldosteron]

Thanks a lot