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Coping with COVID‐19: ventilator splitting with differential driving pressures using standard hospital equipment

Summary The global COVID‐19 pandemic has led to a worldwide shortage of ventilators. This shortage has initiated discussions on how to support multiple patients with a single ventilator (ventilator splitting). Ventilator splitting is incompletely tested, experimental and the effects have not been fully characterised. This study investigated the effect of ventilator splitting on system variables (inspiratory pressure, flow and volume) and the possibility of different ventilation targets for each limb using only standard hospital equipment. Experiments were conducted on two test lungs with different compliances (0.02 l.cmH2O−1 and 0.04 l.cmH2O−1). The ventilator was used in both pressure and volume control modes and was set to ventilate the low compliance lungs at end‐tidal volumes of 500 ± 20 ml. A flow restrictor apparatus consisting of a Hoffman clamp and tracheal tube was connected in series to the inspiratory limb of the high compliance test lungs and the resistance modified to achieve end‐tidal volumes of 500 ± 20 ml. The restriction apparatus successfully modified the inspiratory pressure, minute ventilation and volume delivered to the high compliance test lungs in both pressure control (27.3–17.8 cmH2O, 15.2–8.0 l.min−1 and 980–499 ml, respectively) and volume control (21.0–16.7 cmH2O, 10.7–7.9 l.min−1 and 659–498 ml, respectively) ventilation modes. Ventilator splitting is not condoned by the authors. However, these experiments demonstrate the capacity to simultaneously ventilate two test lungs of different compliances, and using only standard hospital equipment, modify the delivered pressure, flow and volume in each test lung.


Introduction
The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus and its associated disease, coronavirus disease (COVID-19), have caused a global pandemic. presents many challenges, including: ventilator-patient dyssynchrony; cross-infection from inter-patient gas exchange; inability to set individual patient tidal volume, oxygen concentration, positive end-expiratory pressure; and lack of monitoring of individual tidal volume, flow and pressure [4,5]. The ventilator splitting system proposed by Neyman and Irvin [2] exposes both patients to the same ventilation dynamics throughout the respiratory cycle. This limits the utility of the design when ventilating two patients with dissimilar lungs [6], due to either pathology, such as acute respiratory distress syndrome or differing patient lung variables.
This paper outlines the use of an inspiratory flow restrictor with ventilator splitting, a novel approach to applying differing driving pressures to two patients connected to the same ventilator [7], thus compensating for differing target tidal volumes and differing lung compliance.
Several design concepts have been proposed that use parts repurposed from fluid or gas plumbing [8] or 3Dprinted fixed-flow restrictors [7]. This study aims to investigate ventilator splitting with flow restriction using only commonly available medical components.

Methods
The experimental setup is shown in Figs. 1-4. Standard 22mm connection breathing circuits were used except where otherwise stated. In-line heat and moisture exchangers (HME) were used in place of viral filters to allow pressure monitoring lines to be connected at key points (Fig. 1). The pressure drop across the HME was found to be within 1.36 cmH 2 O of viral filters, meaning the induced circuit resistances are comparable.
The inspiratory limb of the ventilator (Hamilton C6S, Hamilton Medical, Bonaduz, Switzerland) was connected by 50 cm of breathing circuit to a humidifier (FP950, Fisher and Paykel Healthcare, Auckland, New Zealand), with an inline HME. The in-circuit humidifier was not turned on or filled with water for these experiments. A length of breathing circuit (120 cm) was run from the humidifier to a standard Y-connector splitting into two inspiratory limbs. NY, USA) which was then connected to a 3D printed Y-connector (design previously described [7]) before reentering the ventilator expiratory limb (Fig. 4). The 3D printed connector allowed the side-by-side placement of HMEs, which facilitated sidestream pressure sensors (P3, P7, With the flow restrictor in place, the ventilator was split (as described above) to support both the low compliance test lung and an additional high compliance test lung  Table 1. Subsequently, the flow restrictor (with no clamp applied) was included to determine the effect on measured variables. In order to test the effect of the flow restrictor, the high compliance lung was connected via the splitter assembly. Due to the different targets in volume control ventilation, the target volume on the ventilator was doubled (from 480 to 960 ml) and the peak flow rate adjusted to maintain the inspiratory/ expiratory ratio. Initial baseline data were recorded for both test lungs without the Hoffman clamp. The flow resistance to the high compliance lung was then adjusted via the Hoffman clamp until the end-tidal volume was within the desired ranges of 500 AE 20 ml. Finally, the ventilator flow meter was returned to the inspiratory limb of the low compliance lung to ensure that parameters remained within the target ranges.
All pressures were recorded via a LabJack U6 Pro (LabJack, Lakewood, CO, USA) at a sampling rate of 10 Hz.   Pressure data were averaged over three breaths, whereas flow and volume data were averaged using the built-in filters within the ventilator.

Results
In pressure control ventilation mode, minimal differences in measured ventilator pressure, volume and flow were recorded after adding the unclamped flow restrictor section ( Table 2) Table 3).
Positive end-expiratory pressure was unaffected by the splitting configuration. Flow and volume were measured on the high compliance lungs, and the Hoffman clamp resistance was increased until the measured volume was within the desired range of 500 AE 20 ml. At this point, peak inspiratory pressure, volume and flow were tested again on the low compliance lung, and a match between the pairs of test lungs was demonstrated ( Table 3).
As seen with pressure control ventilation, the addition of the unclamped flow restrictor had minimal effect on the measured variables in volume control ventilation (Table 4).  This experiment demonstrates that, in order to deliver a safe tidal volume and airway pressure, a resistance mechanism is required on one inspiratory limb of the circuit.   control ventilation, the pressure drop across the flow resistor was more sensitive to adjustment in volume control ventilation than when using the ramped flow waveform of pressure control ventilation. Further research on static inline flow restrictors may take advantage of this property.
The addition of one-way valves on the expiratory limbs of the circuit was found to be crucial to avoid inspiratory backflow in the presence of an inspiratory flow restrictor.
The need for these valves may be a function of this particular ventilator's design. In these experiments, a custom 3Dprinted Y-splitter was used at the end of the expiratory limb.
In clinical practice, this could be replaced by standard Y or T breathing circuit connectors.
In this study, a single disposable flow sensor was available. We suggest the use of two disposable flow sensors to enable intermittent measurement of flow,  In conclusion, we have demonstrated in this bench study that it is possible to achieve ventilation of two test lungs with differential driving pressures using standard medical equipment. Pressure control and volume control