I ruminated a consideration on this proposal for the last couple of days as Germany is in anticipation of the surge.
To be clear: This is also just a theoretical exercise and – if applied - would count as a desperate measure in the case of patients otherwise not being ventilated at all.
What if the mode of ventilation chosen would not be conventional BIPAP but APRV/inverse ratio PCV?
The control parameters correspond to conventional ventilator settings as follows: P-high (P-insp), P-low (PEEP), T-high (insp. Time), T-low (exp. time) and FiO2. Note that implementing this mode of ventilation with conventional BIPAP, which is possible in some devices, will in some setups require calculation of respiratory rate, which is 60/(Thigh+Tlow). The slightly different terminology as well as specific algorithms for implementation have been characterized and properly published by experts in the field – which I am NOT. There have been promising studies in ARDS which I will attach below.
If one assumes that for two (or more) patients the key treatment goal would be oxygenation with a more liberal approach to CO2 removal, it should be possible to choose P-high, FiO2 and to some degree T-low to achieve safe oxygenation margins for both patients.
This would still allow for some inhomogeneity in compliance between the patients as the lung is splinted to a rather high point in the P-V curve by P-high resulting in a high resting volume. This is the proposed “open-lung” aspect of this ventilation mode. Nevertheless, due to the sparse and brief passive pressure relief phases it is believed to go without higher frequency repeated expansion like in conventional PCV, resulting in less energy deposition into the tissue. The repeated overinflation/atelectasis due to inhomogeneos P-V dependencies had been my main concern when thinking about PCV in a shared circuit and a possible result of volutrauma/atelectotrauma in the patients.
In case of inhomogeneous severity between the patients/different disease progression the treatment goal could be set for the more severe patient accepting hyperoxia in the “more well” patient.
In terms of CO2 removal a “common ground” target for the patients could be set in wider range of acceptable values, opting for permissive hypercapnia.
As the passive release of the lung volume to exhalation in T-low will generate outflow which is cropped at a certain level of peak flow (by setting T-low) when conventionally setting up/adjusting this mode, it might be difficult to assess this control parameter for the individual patient in a shared circuit.
APRV does even allow for spontaneous breathing on P-high in weaning approaches which have been proposed as well.
If flow for the device serving two patients would be dialed high enough to compensate for potential maximum inspiratory peak flow generated by both patients in spontaneous breathing, it could be fathomable to even conduct weaning with two spontaneously breathing patients sharing the same circuit.
I write this as I feel we are all staring in a potential abyss, which might force us to make choices that neither of us ever wanted to make.
Hence, this is a THEORETICAL CONSIDERATION and not a recommendation. Also I do not take any legal responsibility if patient harm ensues. I hope everybody understands that.
Also probably someone already thought of this.
But I thought: Sharing is caring.
Take care and stay strong.
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