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Safe doubling of ventilator capacity: a last resort proposal for last resorts
Critical Care volume 24, Article number: 222 (2020)
The best way to ventilate two patients on a single ventilator is simply not to do it: The Authors
In light of the COVID-19 pandemic, this common-sense approach was recently clarified in a SCCM-ASA-AARC-AACN-ASPF-CHEST consensus statement on the Society of Critical Care Medicine (SCCM) website : ‘We recommend that clinicians do not attempt to ventilate more than one patient with a single ventilator while any clinically proven, safe, and reliable therapy remains available (ie, in a dire, temporary emergency)’ .
The current situation in several European countries and states in the USA is a ‘dire emergency’. Physicians have been, or may be, asked to make difficult choices in the face of ventilator shortages . Nevertheless, if you are faced with a decision to ventilate two patients at once, or deny care to one, we believe we can propose the next best way.
The SCCM recommendation  addresses a series of popular Internet concepts with multiple patients breathing in-parallel [3, 4]. In-parallel is a critical point, as inspiration and expiration all take place at the same time, so there is thus no change to respiratory rate (RR) and tidal volume or driving pressure are adjusted for the number of patients. All of these add risk over over-/under-ventilating patients and causing harm .
Instead, we recommend a multiplex in-series breathing approach to double (2-for-1) the patients on a ventilator (Fig. 1). In-series breathing means only 1 circuit volume (split between patients) is active at a time, but each patient’s inspiratory effort is singular. This approach addresses the limitations of shared, in-parallel breathing in the SCCM statement, where Table 1 addresses in detail each consensus statement concern .
Instead of the same RR and higher tidal volume or driving pressure, in-series breathing doubles the RR and keeps the other ventilator settings the same. One patient breathes in, while the other breathes out. With typical I:E ratios around 1:3, there is also shared expiration time when neither is breathing in (Fig. 1). As with other proposals , driving pressure can be modified by added resistors in the inspiratory circuit, and PEEP can be customised with in-line expiratory PEEP valves (Table 1).
This in-series approach ensures breath-by-breath ventilation parameters of each patient are displayed to ensure monitoring and safety are maintained. Staff can thus be assured more (or less) compliant patients are not over- (under-) ventilated, both of which could result in harm if ventilated in-parallel, where monitoring individual patients is not possible. Finally, one-way expiratory valves and filters prevent rebreathing and cross-contamination.
Clinically, we would suggest pressure-control modes, where driving pressures are easily customised per-patient with resistors, and are more commonly used, currently. This choice allows customised PEEP and driving pressure for each patient effectively as if they were ventilated separately.
This setup requires an active valve to switch between patients (Fig. 1), comprising a pressure sensor at the single end of a y-splitter on the inspiration circuit, and two active valves at the outlet. It uses measured inspiratory pressure to switch the inspiratory circuit from one patient to the other after inspiration (as pressure drops). The active y-splitter valve thus allows flow down only one inspiratory path per ventilator-supplied breath (at 2xRR). The components, sensors, and computation are low-cost and easily 3D printable by hospital bioengineers or others.
However, nothing is perfect. This approach is not suited for spontaneous, triggered breathing, which cannot be synchronised nor limited. In addition, it cannot be used if I:E < 1, or if a patient’s respiratory mechanics are such they will receive inadequate minute ventilation within the time for each allocated breath.
Availability of data and materials
SCCM, et al., Consensus Statement on Multiple Patients Per Ventilator. SCCM Website (https://www.sccm.org/Disaster/Joint-Statement-on-Multiple-Patients-Per-Ventilato), 2020. Accessed 13 Apr 2020.
Truog RD, Mitchell C, Daley GQ. The Toughest Triage - Allocating Ventilators in a Pandemic [published online ahead of print, 2020 Mar 23]. N Engl J Med. 2020;10.1056/NEJMp2005689. https://doi.org/10.1056/NEJMp2005689. Accessed 13 Apr 2020.
Pinson, H., A better way of connecting multiple patients to a ventilator, Medium.com, Editor. 2020, Medium.com: Medium.com (https://medium.com/@pinsonhannah/a-better-way-of-connecting-multiple-patients-to-a-single-ventilator-fa9cf42679c6). P. Blog Post by Hannah Pinson.
Differential Multiventilation International Working Group. Differential Multiventilation. Differential Multiventilation [https://www.differentialmultivent.org/] 2020 [cited 2020 April 13, 2020]; Available from: Differential Multiventilation.
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Chase, J.G., Chiew, Y., Lambermont, B. et al. Safe doubling of ventilator capacity: a last resort proposal for last resorts. Crit Care 24, 222 (2020). https://doi.org/10.1186/s13054-020-02945-z