key: cord-1004482-19djulgt
authors: Clarke, A. L.; Stephens, A. F.; Liao, S.; Byrne, T. J.; Gregory, S. D.
title: Coping with COVID‐19: ventilator splitting with differential driving pressures using standard hospital equipment
date: 2020-04-25
journal: Anaesthesia
DOI: 10.1111/anae.15078
sha: 804264af323810082c1754e9d73b34836b5736d3
doc_id: 1004482
cord_uid: 19djulgt

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.cmH(2)O(−1) and 0.04 l.cmH(2)O(−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 cmH(2)O, 15.2–8.0 l.min(−1) and 980–499 ml, respectively) and volume control (21.0–16.7 cmH(2)O, 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.

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.cmH 2 O À1 and 0.04 l.cmH 2 O À1 ). 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 AE 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 AE 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 cmH 2 O, 15.2-8.0 l.min À1 and 980-499 ml, respectively) and volume control (21.0-16.7 cmH 2 O, 10.7-7.9 l.min À1 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.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus and its associated disease, coronavirus disease (COVID-19), have caused a global pandemic.

Patients with COVID-19 may develop progressive viral pneumonitis leading to severe respiratory failure, requiring mechanical ventilation in up to 17% of cases [1] . The combination of unprecedented disease burden and global supply chain disruption has resulted in worldwide shortages of medical equipment.

Ventilators are technically complex, expensive devices and rapidly increasing their production has proven to be difficult. To address the need to immediately increase the number of available ventilators, the concept of ventilating multiple similar patients' lungs with one ventilator (ventilator splitting) has been previously described [2] and subsequently tested in adult human-sized sheep [3] . The basic principle of ventilator splitting is simple; two or more patients are connected to one ventilator and both are exposed to the same circuit dynamics. However, this 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.

The experimental setup is shown in Figs Pressure data were averaged over three breaths, whereas flow and volume data were averaged using the built-in filters within the ventilator.

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. 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 

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We thank Dr D. Duke for his feedback and input into the