Experts On Air - INTELLiVENT‑ASV webinar series

This depends on the patient's disease and lung condition, but for example in ARDS patients it is common to target a somewhat lower SpO2 of 90-94% to prevent excessive oxygen use and thus help prevent VILI. This usually results in a lower level of oxygen.

We used and still use APV-CMV (adaptive pressure ventilation), pressure controlled ventilation, pressure support ventilation and ASV.

When there is a measurement failure for any reason, INTELLiVENT-ASV will stop adjusting and freeze settings, and will generate alarms. If there is no solution at that time, i.e., SpO2 measurement due to poor perfusion, you can choose to set the controller to 'manual'.

ARDS due to COVID is not that different in terms of PEEP compared to other causes of ARDS. Some patients have good potential for recruitment and should be ventilated with PEEP above 10 cmH2O, while others have very low potential and should be ventilated with low PEEP. A P/V curve may help to assess recruitability using the same threshold as for ARDS.

This is not easy. For the moment, the ventilator does not provide this variable. So we have to calculate it manually using one of the simplified formula published. 

There are published data showing that the number of manual settings is decreased by half when using INTELLiVENT-ASV, as compared to conventionnal modes. How this translates in terms of workload, namely the time spent by caregivers to perform these tasks and the cognitive involvment, is unknown. Clinicians that use INTELLiVENT-ASV on a regular basis claim that it decreases workload, but we have no data to support it.

Speaker's response: The question is what you call 'protective' - usually it refers to tidal volume size and as explained in some patients, those with a relative normal compliance, ASV and INTELLiVENT-ASV 'use' or 'allow' larger tidal volumes; if you refer to protective ventilation as ventilation with a low driving pressure or low mechanical power, realize that ventilation per se is not a treatment: If lung conditions are such that driving pressure or mechanical power cannot be reduced, one should consider another lung-protective strategy, ECMO.

Interviewer's comments: The question is referring to manual controls. Actually, one of the purposes of advanced automation is avoiding possible human mistakes on manual controls. However, even INTELLiVENT-ASV includes some manual controls for fine-tuning, and Hamilton Medical ventilators provide all conventional modes with manual controls. All of them must have large ranges conceived to face uncommon needs or conditions, but this means (as you say) exposure to the risk of an inappropriate setting. The risk is considered to be largely mitigated by these controls being operated only by trained personnel, i.e. by the strength of education. Nevertheless, adding some GUI tool to force the user's attention to uncommon (possibly inappropriate) settings may be an idea to be considered.

Speaker's response: Correct, herein it follows the principles of Otis, and thus bases expiration (and thus inspiration) time on the expiratory time constant (RCexp): If high, a longer expiration time is needed, and thus the inspiration time will be shorter, and vice versa.

Interviewer's comments: This is an important observation. ASV pays much attention to an individualized setting of Ti, based on the best balance between the assigned expiratory time (Te) and the speed of exhalation as reflected by the expiratory time constant (RCexp). This typically results in an I:E of 1:1 in lung restriction, 2:3 or 1:2 in mechanically normal lungs, and lower than 1:2 in lung obstruction. This approach works effectively against air trapping and avoids unnecessary high inspiratory flow rates. Also, it mimics the natural human respiratory center response to deviation from normal respiratory mechanics.

Correct, when the compliance is relatively normal, it could be better to use a somewhat higher tidal volume in case the use of a lower tidal volume mandates a higher respiratory rate: The respiratory rate adds to the power of ventilation, so should better stay low.

(Editor's note: This question answered in the webinar; however, the speaker has provided a more comprehensive answer here.)
When the "Brain injury" patient condition is selected in I‑ASV, the ventilation controller responds only to PetCO2 and therefore the automatic %MinVol selection is not directly affected by changes in the spontaneous respiratory rate. However, if a pattern like a Cheyne‑Stokes starts, it has an effect, since it involves:

• A waving of PetCO2, with effects on the the I‑ASV ventilation controller
• Possibly, an average increase of ventilation and therefore a decrease of average PetCO2, with effects on the I‑ASV ventilation controller; in case of too high ventilation, I‑ASV will decrease %MinVol to the minimum setting of 70%
• A waving in both respiratory drive and frequency, with effects on the basic controllers of ASV (automatic selectors of Pinsp and Fr,mech)

Whether the dynamic responses of I‑ASV may provide any advantage (or disadvantages), compared to a more static ventilation mode, it is difficult to predict and the result may be different in the individual cases.
Intuitively, since the problem of periodical breathing stays in the patient's respiratory center, we can  harldy expect a really satisfactory solution to come from the mechanical ventilator. If clinically indicated, the fire should be extinguished by sedation or sedo-paralysis, and then I-ASV will take care of appropriate ventilation.

(Editor's note: The question refers to previous mention of HME filters made by the speaker.)
Due to its position at the Y‑piece of the ventilator circuit, an HME is a kind of parasite for the efficiency of ventilation, and hence of CO2 elimination. Adding an HME means to increase the system deadspace exactly by the internal volume of the HME. For any given minute volume, adding an HME will involve some increase in PaCO2 that can be offset only by increasing the minute volume. The other way around, the opposite takes place if an HME is removed and replaced by a hot‑water humidifier, the latter being positioned within the inspiratory limb of the ventilator circuit and thus never involving an increase in deadspace.
For these reasons we should avoid using HMEs whenever we believe it is clinically important to reduce the amount of ventilation needed for good control of PaCO2 and arterial pH.
All this is important not only during passive ventilation, but even more during active breathing.
The same principles should be applied to any other piece placed between the Y-piece and the ET-tube: whatever unnecessary should be considered a parasite to be removed.

(Editor's note: This was answered in part during the webinar, the speaker has added more information here).
I discussed for acute brain injury a PaO2 target of 80‑120 mmHg (with no less than 70 and no more that 200 mmHg). Aiming to this target excludes any permission for even mildly reduced arterial oxygenation, while adding the difficulty of simultaneously avoiding any releval hyperoxia. In terms of functional hemoglobin saturation, I think that these targets match well with an SpO2 of 98%, that is obviously higher than the oxygenation target we would consider in pure ARDS. 
How to achieve this SpO2 when ABI is combined with ARDS?

1. FiO2. By manual control of FiO2, it may be not so easy in ARDS to find and track the FiO2 setting that achieves an SpO2 as high as 98%, while simultaneously avoiding any relevant exposure to hyperoxia. For that purpose, in theory the automatic Oxygen controller of I‑ASV should provide an enormous advantage.
2. PEEP. An effective PEEP level is necessary with ARDS. Unfortunately (and differently from FiO2) there is no simple cookbook for selecting the PEEP level in ABI + ARDS. In my opinion the choice should be inspired by more caution than in pure ARDS and supported by strong neuro‑monitoring.

At the moment I focus most on bedside teaching about the theory itself. I explain, when adjusting the ventilator, that MP is a balance of all parameters, like tidal volume and respiratory rate, and that for reaching the best balance (the lowest possible MP) a certain combination of settings is the best combination.

I think in future trials it is interesting to use a calculation where MP is normalized to the compliance of the lung.

(Editor's note: This question was partly answered in the webinar; the speaker has provided additional information here.)

I selected the chronic hypercapnia condition because the patient's ABG showed a Bicarbonate HCO3- of 30 mmol/L, suggesting a renal compensation for a chronic elevation of PaCO2. Under the patient condition Chronic hypercapnia, the EtCO2 target is shifted automatically to a higher level, which can further be increased manually.

(Editor's note: This question was answered during the webinar; the speaker has provided additional information here.)

In INTELLIVENT-ASV %MinVol maximum is 200%. If you wish to increase it because EtCO2 (confirmed by a blood gas measurement of PaCO2) is still too high, you can increase %MinVol manually up to 300%.

There are three conditions in which you have to change %MinVol from automatic to manual.

• A) In the case your target minute ventilation is not reached by %MinVol at 200%.
• B) In the case of asynchronies due to a to high Pinsp (%Min/Vol).
• C) In the case of a strong respiratory drive with a Vt > 10 ml/kg IBW.

To be in full operational capability it took almost 6 months, starting with a small physician staff (just 8 doctors and 12 nurses work in our HEMS) who already was familiar with ASV.

For about 2 years and again, the staff was small and we fly a lot with many critical patients.