Effect of changing frequency on HFOV + VG

[sujatad]

I need help to clear a doubt on mechanics of HFOV with VG.

We use Fabian ventilator with HFOV. It is  known  that increasing frequency on HFOV decreases CO2 washout ( although that's not a change we prefer as most of us alter the amplitude for a desired change in CO2). 

However occasionally we use the VG mode on HFOV ( HFOV + VG). It was recently pointed out by a colleague that if VG is on with HFOV, increasing the frequency has the opposite effect on CO2 wash out as compared to HFOV without VG. 

Since I was not aware of this I searched for guidelines on this but could not find any literature which clarifies this concept for me. On working out theoretically I did come to the conclusion that this could be true since targeted tidal volume remains constant on HFOV+ VG so increasing the frequency increases DCO2 and therefore facilities CO2 washout ( unlike the case in HFOV without VG). 

I am aware that this topic is very theoretical as in practice most of us like to alter the amplitude ( HFOV without VG) or target tidal volume ( HFOV + VG) for a desired change in CO2 and rarely alter the frequency. However, getting this doubt cleared is important for me and I would be grateful for clarification on this by experts on this group.

Thanks 

[Stefan Johansson]

Hi and thanks for posting this very interesting question. I must admit I am having any good answer, was trained and used Sensormedics (actually, a lot!) in my level-3 years, so the times before there was VG and tidalvolumes to be measured in HFV.

As you write, the traditional reasoning around HFV frequence was that a higher frequence reduced CO2 washout. We sometimes utilised this feature with Sensormedics but for the opposite purpose, i.e. we reduced frequence to increase CO2 washout in tricky cases.

If the VG-function during HFV manages to keep a defined tidal volume, then it also shifts the "mechanics of thinking". I mean, in that case frequence and CO2 should correlate and change as during "normal mechanical ventilation", i.e. higher frequence -> more CO2 washout and vice-versa.

But, as said, I have limited experience using HFV on new / more sophisticated machines, so lets hope more people join this discussion. I ping some experts I know are around here: @Francesco Cardona@Martin.Keszler  @Padkaerand @Vicky Payne

[anja stein]

you may find this brochure helpful: https://www.draeger.com/Library/Content/jane-pillow-hfov-br-9102693-en.pdf

[juan carlos vidal]

hi sujatad. i tell you from my experience, in the mode of hfov + vg in drager ventilator. in this mode the idea is to set frequency (according to compliance) and Mean airway pressure, the ventilator varies the amplitude. to reach a preset tidal volume (1. 5 ml/kg), ventilation in this modality is equal to fr * vt (amplitude)², where both frequency and amplitude increase the minute volume, but as the amplitude is exponential there is a moment where the frequency increase shortens the inspiratory time so much that the amplitude does not reach the set value, and at that moment it is more efficient to lower the respiratory frequency. you see this because the dco2 decreases and it is not modified with amplitude increase. So, in very premature patients with short time constant, you can use this modality as lung protection, as long as you agree to tolerate permissive hypercapnia, since with a minute volume between 1.5 and 2 /kg it will be just right. you start with the highest frequency that allows you to maintain a constant tidal volume (approx 1.5 ml /kg) with the lowest amplitude. each ventilator has a different power, in the drager you can use frequencies in very small preemies from 12 to 15 hz. Therefore, it is a relative truth, since there will be times when the increase of the Fr lowers the PCO2 and other times when it increases it.

 

[gadelca]

"VOLUME GUARANTEE COMBINED WITH HIGH-FREQUENCY OSCILLATORY
VENTILATION—A NEW ERA
VThf is proportional to the DP generated in each cycle and the length of the inspiratory
time and is inversely proportional to the oscillation frequency; thus, traditionally, an increase in DP or a decrease in frequency to finally increase VThf has been used to
improve CO2 clearance. Also, the frequency has an important role in CO2 removal and has an independent effect on the distribution of the gas within the air-
ways and to changes in the volume when oscillating at or near the resonant frequency of the respiratory system.
Measurement of the VThf gives an important advantage, because there is a close correlation of the VThf and the CO2 washout, described as the diffusion coefficient of CO2 (DCO2) which is related to the VThf2 and the frequency, as follows:
DCO2 =VThf2 x fr

Traditionally, HFOV devices did not measured the VThf, so looking at the transmission of the oscillation to the thorax of the patient has been used to clinically control VThf. Today it is possible not only to measure but also to control and fix the VThf (vol-
ume guarantee [VG]) to maintain it as constant, similar to the VG described for conventional ventilation (Fig. 1), enabling an independent adjustment of the VThf and the frequency. The ventilator, using this new technology, modifies DP to maintain the VThf at the setting value (Fig. 2).
When the VThf is fixed, any change in the frequency does not affect the VThf generated (Fig. 3) but directly modifies DCO2 and CO2 washout in the same direction as in
conventional ventilation.

High-frequency Ventilation
Manuel Sánchez-Luna

Clin Perinatol 48 (2021) 855–868
https://doi.org/10.1016/j.clp.2021.08.003

[Yinghua Li]

Hi doctors, everything you are talking about is right, especially theoretically about how changing of frequency affects tidalvolym in HFOV+ VG mode.

In my clinical experience it doesn't work in either Leoni plus,  Dräger babylog 500, or Servo N. 

You have to increase frequency in order to increase the tidalvolym although I HFOV+VG mode in those ventilators.

Sensomedics is an old machine with higher/highest power for elimination of CO2 but without VG function 

Best regards 

Yinghua Li

 

 

 

 

[Martin.Keszler]

I can't comment about how well HFOV VG works with other devices, but it works remarkably well with the Draeger VN500 (and presumably the VN800 as well.

We completed a 225 subject multicenter safety clinical trial to provide data for an eventual approval by the USA Food and Drug Administration and found that the measured TV was within 0.1ml of set TV  in 93% of cycles and 96% when user error was excluded. See attached.

And as several posts above indicated, with VG, the TV is fixed, so a change in frequency has no impact on TV (unless reaching the limit of TV in a large baby ventilated at an inappropriately high frequency). So, increasing frequency will DECREASE PaCO2 , the opposite of what you see with HFOV without VG. The easy way to remember is that it now behaves like conventional ventilation: faster rate = more CO2 is removed.

2019 DOVE VG Does it work Poster_ Final.pdf

[sujatad]

Thank you all for clarifying this concept for me. All replies were helpful.