With COVID19-induced ventilator shortages in Italy and New York, supporting multiple patients on a single ventilator was attempted as an apparently simple solution. The anecdotal use of this approach was also attempted in the 2017 Las Vegas mass shooting event. However, despite these widely publicized attempts, a [joint statement](https://www.apsf.org/wp-content/uploads/news-updates/2020/Multiple-Patients-Single-Vent-Statement.pdf) by the Society of Critical Care Medicine (SCCM), American Association for Respiratory Care (AARC), American Society of Anesthesiologists (ASA), Anesthesia Patient Safety Foundation (APSF), American Association of Critical‐Care Nurses (AACN), and the American College of Chest Physicians (CHEST) all recommended **against** the practice based on safety concerns.
The anecdotal procedure requires "matching" patients, but it remains unclear what this really means. How well do they have to be matched before the benefits to both patients outweigh the risks, especially in rapidly changing pulmonary conditions typical of patients suffering from COVID-19?
***Fundamentally, splitting a ventilator between patients is risky.*** However, should it be absolutely necessary, a potentially reasonable procedure is provided in the following example.
To illustrate the effect of progressive mis-matching, we ventilated two mechanical lungs on a ventilator (840, Puritan Bennett) using pressure control-mode and ARDS-compatible settings (Ppeak=20-30 cmH2O; R =20 bpm; Ftotal=24 L/min; I=1.5 sec; PEEP=8 cmH2O). While keeping patient B at a constant pulmonary compliance (0.03 L/cmH2O), we let patient A progressively deteriorate in compliance from 0.06 to 0.01 L/cmH2O and finally created a maximum mismatch between the patients as shown in the figure.
One-way valves on both inspiratory and expiratory limbs ensured unidirectional flow, which both reduces functional dead-space, risk of cross-contamination between patient A and B and seemingly also facilitated stable ventilation of B as A deteriorated.
*Figure: Patient A (red line) and B (blue line) pressure curve and average tidal volumes during positive pressure ventilation, pressure control mode mechanical ventilation at 20 and 30 cmH2O and PEEP of 8 cmH2O. One-way valves ensured stable tidal volumes of B while A deteriorated, however, a severe mismatch between the patients could lead to fatal simultaneous hyper- and hypo-ventilation with triggering of alarms.*
Importantly though, **simultaneous and opposite changes in compliance made it possible to fatally hypo-ventilate one patient and hyper-ventilate the other and do so without triggering alarms**, highlighting the danger of splitting a ventilator.
As ventilator alarms are triggered only by changes in the sum of pressure/volume of both patients on the circuit, we instead recommend a narrow alarm range (e.g. Vtidal ±200 mL; Ftotal ±1 L; Ppeak ±5 cmH2O). The one-way valves on each expiratory limb prevented back-flow but introduced a risk of competing exhalation: a slightly earlier or more forceful expiration from A can (partly) impair B and worsen breath-staggering, particularly at higher respiration rates.
Frequent or constant monitoring of patients and shuffling when a mismatch arises is recommended. Asthma or COPD may increase the rate of which fatal mismatch evolves, making the method even more unpredictable. Finally, each class of ventilators requires a specific set up; if the method is considered, use the calm before the patient-surge to familiarize yourself with the ventilators and ameliorate the many risks associated with sharing a ventilator.
This effort was recently accepted for publication in the journal [*Critical Care*](https://ccforum.biomedcentral.com/).
More information is provided via the video below.
Associate Clinical Professor, Department of Anaesthesiology, UCSD
MADLab PhD student
Mechanical and Aerospace Engineering, UCSD
MADLab PhD student
Mechanical and Aerospace Engineering, UCSD
MADLab, Center for Medical Devices, Department of Mechanical and Aerospace Engineering, Jacobs School of Engineering, UCSD
School of Medicine, UCSD
Post-doc, Department of Anaesthesiology, UCSD
Lonnie (MD,PhD) completed her MD from the University of Copenhagen, Denmark in 2007 and has worked in Emergency Medicine and Intensive Care. Dr. Petersen received her PhD in Gravitational Physiology and Space Medicine in 2016. Currently an assistant Professor the University of California, San Diego and supported by NASA, DoD, and the Novo Nordic Foundation as well as being a Sapera Aude Fellow (National Research Council). Her research is rooted in cardiovascular, cerebral and exercise physiology always with an integrative physiology approach.
Casper “Johan" (MD) graduated from University of Copenhagen in 2008 and has worked in Cardiology and Emergency Medicine. He has focused on research in renal physiology and sympathetic reflexes as well as physiological fluid shifts in regard to space and aviation medicine and countermeasure development for long term space travel. Dr. Petersen is supported by NASA , Kratos and ONR and currently holds a position as Assistant Project Scientist at University of California, San Diego.
James (BSAero, MSME, PhD) leads the Medically Advanced Devices Laboratory in the Center for Medical Devices at the University of California-San Diego. He is a professor in both the Department of Mechanical and Aerospace Engineering, Jacobs School of Engineering, and the Department of Surgery, School of Medicine. He has over 270 peer-reviewed research publications, including 140 journal papers and eight book chapters, and 29 patents in process or granted, completed 34 postgraduate students and supervised 20 postdoctoral staff, and been awarded over $25 million in competitive grant-based research funding over his career. He is a fellow of the IEEE.
Work(s) (the “Work”) by:
COVID-19 Acute Ventilation Rapid Response Taskforce (AVERT) Medically Advanced Devices Laboratory
Department of Mechanical and Aerospace Engineering
Jacobs School of Engineering and the School of Medicine University of California, San Diego
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