Read on Twitter
I think it’s time for a thread on the “Three F’s” of gas delivery in anaesthesia.
Filters, Fresh gas flows and…er…humidiFication.
It’s not as simple as I once thought, so buckle up!
Filters, Fresh gas flows and…er…humidiFication.
It’s not as simple as I once thought, so buckle up!
For a long time, I just referred to these things as “HME filters”, “HMEFs”, or sometimes just “HMEs”.
I didn’t give them much thought and figured they all did pretty much the same thing. Filter, humidify and warm inspired gases.
A few years ago I discovered I was wrong!
I didn’t give them much thought and figured they all did pretty much the same thing. Filter, humidify and warm inspired gases.
A few years ago I discovered I was wrong!
They are in fact 4 different devices, with different specs, & their own sets of pros & cons. The devices are:
1.Pleated mechanical filter
2.Pleated mechanical filter + HME component
3.Electrostatic filter
4.Electrostatic filter + HME component
See: https://doi.org/10.1111/j.1365-2044.2010.06563.x">https://doi.org/10.1111/j...
1.Pleated mechanical filter
2.Pleated mechanical filter + HME component
3.Electrostatic filter
4.Electrostatic filter + HME component
See: https://doi.org/10.1111/j.1365-2044.2010.06563.x">https://doi.org/10.1111/j...
This image from Wards Anaesthesia Equipment gives a good visual representation of the physical differences between these devices.
But what about performance differences? 1st up, filtration. When tested as per ISO 23328 with NaCl particles, mechanical filters tend to outperform electrostatic ones. Particularly relevant if reusing circuits for multiple patients, which is common these days.
They also perform very differently when wet. With electrostatic filters, there is the risk that water can penetrate through the filter medium and contaminate the circuit.
This is bad for the patient, or perhaps more to the point, subsequent patients (and maybe even yourself!), but it does at least allow you to make coffee if ever you get desperate. http://www.freshgasflow.com/filter_coffee/ ">https://www.freshgasflow.com/filter_co...
So what about the HME bit, Fresh gas flows, and humidiFication? This is where it gets even more complicated, but first of all, why is humidiFication (and warming, you need both) important in the first place?
Well, here’s a description of a piglet’s tracheal & bronchial epithelium following 10 hours of anaesthesia & ventilation with cold, dry gas at Fresh gas flows of 6 L/min. Pretty brutal (as is the conclusion!), but I do wish more journals would allow descriptive prose like that!
But we breathe cold dry air so why doesn’t it happen to us?
Because we have a nose!
It’s the workhorse of heating & humidifying gases in the upper airways, so by the time they reach the carina they are close to body temperature & nearly fully saturated with water vapour.
Because we have a nose!
It’s the workhorse of heating & humidifying gases in the upper airways, so by the time they reach the carina they are close to body temperature & nearly fully saturated with water vapour.
But if you bypass the nose (ie with an ETT), this “isothermal saturation point” is pushed further down the respiratory tract. So now the trachea and bronchi will lose more heat and water to the inspired gas, increasing the risk of a desiccated desertscape as above.
This is where HME devices come into play. That might be as a separate unit such as the “Swedish nose" often used by tracheostomy patients…
…or combined with a filtration medium to create an “HMEF” as mentioned above.
Complicating the issue though is the fact that filtration media themselves have some degree of intrinsic HME function! This means Filters can contribute to airway humidification, even without an added HME component. Mechanical filters more so than electrostatic ones.
Obviously it won’t be as much as a dedicated HME, but it can be a significant contributor. So at one level, referring to Filters without an HME component as “HME filters” is, technically, “not wrong”.
But comparing humidiFication performance between devices is further complicated due to variations in how it’s reported. The ISO-9360 standard measures “moisture loss” from a standardised test rig. It can then be mathematically converted to a “moisture output” of the device.
A lower moisture loss, or a higher moisture output, indicate better HME performance. Sometimes you’ll see both reported, sometimes one or the other, and sometimes neither! And if you look closely, you’ll see there is also variation in the tidal volume used during measurement.
This is important because tidal volume affects HME performance. HME’s perform better at lower tidal volumes. You know what else affects performance?
Fresh gas flows!
You remember those piglets?
Fresh gas flows!
You remember those piglets?
Well, there was another group who were also ventilated for 10 hours. Below is a description of their respiratory epithelia. The description’s not quite as emotive as the earlier one, but their epithelium is clearly in better nick!
So what was altered between the two groups that resulted in this difference. Yes, you guessed it! While the desertscape piglets were ventilated with Fresh gas flow of 6L/min, these ones had a FGF of 0.5L/min.
A FGF of only 0.5L/min in a circle system will result in significant rebreathing of expired gases. But before it’s rebreathed, it’s passed through a CO2 absorbent, and as can be seen from the reactions below, this adds both heat and moisture to the gases.
This means the gases in the inspiratory limb of the circuit are no longer cold and dry, but warm and wet! Not surprisingly, the performance of HMEs in terms of moisture loss/output is improved if it’s machine side is also hot and moist! 
https://abs.twimg.com/emoji/v2/... draggable="false" alt="😳" title="Errötetes Gesicht" aria-label="Emoji: Errötetes Gesicht">
So, bringing these “Three F’s” together, we can see that while humidiFication is important, there’s some flexibility in how we achieve the recommended absolute humidity level of inspired gases during anaesthesia of 20 grams of H2O per litre.
So, one option available to us is using low flows. We can then prioritise choosing a Filter with good filtration performance ie mechanical. Because even if its humidiFication performance isn’t ideal, we should still be able to reach that 20g H2O/L level.
Alternatively, if we want to run high flows to save some money as per Zhong et al ( https://doi.org/10.1016/j.bja.2020.07.043),">https://doi.org/10.1016/j... we can look for a Filter with good filtration AND humidiFication performance. This graph shows it’s possible, but some digging may be needed to find the info you need!
To end, some take-home points:
-Humidification is important.
-So is filtration.
-Not all HMEs & filters are created equal.
-High flows? Use a filter with good HME *&* filtration capacity.
-Low flows? Humidification is probably not an issue, so just make sure your filter is good!
-Humidification is important.
-So is filtration.
-Not all HMEs & filters are created equal.
-High flows? Use a filter with good HME *&* filtration capacity.
-Low flows? Humidification is probably not an issue, so just make sure your filter is good!
This thread was prompted by some recent discussion around the ideal FGF to use during #TIVA. You can find some of the background and context here: http://drgetafix.com/2020/10/19/fresh-gas-flows-filters-hmes-and-humidification/">https://drgetafix.com/2020/10/1...
