Ventilator Sharing Using Volume-controlled Ventilation during the COVID-19 Pandemic

To the Editor:

In a recent article, Beitler and colleagues described their methodology and lessons learned from ventilator sharing during the acute shortage caused by the coronavirus disease (COVID-19) pandemic (1). We applaud their efforts during unprecedented circumstances, as we similarly assessed the safety and feasibility of ventilator sharing at a time of near depletion. In their assessment, each patient was matched with identical ventilator settings before sharing pressure-control ventilation. In our assessment, we used the Vent Multiplexor device to modulate flow in a volume-control mode and permit individual adjustments of Vt to two patients.

At baseline, patient A had a Vt of 350 ml (5.5 ml/kg predicted body weight), driving pressure of 14 cm H₂O with positive end-expiratory pressure (PEEP) of 14 cm H₂O, and pH 7.36 with Pa_CO2 56 mm Hg; patient B had a Vt of 450 ml (6.8 ml/kg predicted body weight), driving pressure of 12 cm H₂O with PEEP of 10 cm H₂O, and pH 7.42 with Pa_CO2 54 mm Hg. Each had different static lung compliances (A = 25 ml/cm H₂O; B = 37.5 ml/cm H₂O). Both had a respiratory rate of 20 breaths/min and required vasopressor support and neuromuscular blockade for the assessment; neither had underlying lung disease. Before the assessment, consent was obtained from both patients’ families. The Vent Multiplexor was assembled within the circuit described in Figure 1. In the assessment, the device was adjusted to deliver different flow ratios to patients, with vitals, end-tidal carbon dioxide, plateau pressures, and arterial blood gases monitored over a 2-hour period.

Figure 1.

Circuit diagram showing the inspiratory and expiratory segments of 2 patients on the Vent Multiplexor, including placement of one-way valves, filters, and monometers. C = clamp; HMEF = heat moisture exchanger filter; M = manometer; V = one-way valve; VM = Vent Multiplexor.

During the assessment, both were initially placed on volume assist control mode with total Vt of 800 ml split at a 1:1 ratio through the device and PEEP at 12 cm H₂O, with increased end-tidal carbon dioxide noted for each patient. The device was adjusted to deliver unequal Vts at a ratio of 6:5 (Patient A:B), noting plateau pressure of 32 cm H₂O for patient A compared with 28 cm H₂O for patient B. In response, the device was adjusted to deliver a flow ratio of 5:6 resulting in plateau pressures of 30 cm H₂O each, calculated Vt of 373 ml for patient A and 447 ml for patient B. At the conclusion of the 2-hour assessment, patient A had Pa_CO2 57 mm Hg with pH 7.38, whereas patient B had Pa_CO2 63 mm Hg with pH 7.36.

Our assessment demonstrated that the Vent Multiplexor allowed for successful coventilation, using individual and adjustable Vts to maintain standard-of-practice lung-protective ventilation, in two patients with COVID-19. This is distinct from pressure-controlled ventilation used by Beitler and colleagues, and the ability to deliver individualized volumes would permit the correction of respiratory alkaloses and acidosis without the addition or removal of dead space, respectively, to the circuit, as was required in their study. Beitler and colleagues matched patients by exact ventilator requirements, and a recent in vitro study by Tonetti and colleagues proposes matching patients by compliance (2). Matching compliance is both inherently challenging and potentially harmful, as it varies over time. We demonstrate that exact matching of compliance and Vt is not required with this device. Individualized pressure monitoring is available to inform flow adjustments and mitigate the risk of barotrauma, which is not modifiable in uncontrolled vent splitting.

Ventiltor sharing has been previously explored in laboratory and animal models to assess feasibility (3, 4), and noninvasive coventilation has been demonstrated in healthy volunteers (5). The COVID-19 pandemic required an assessment of feasibility and safety of coventilation in diseased lungs, which brings up several issues, both physiologic and ethical, as detailed in a recent consensus statement (6). This device addresses some of these issues by allowing low-Vt ventilation and adjustable settings for each patient. Both our assessment and the parameters set by Beitler and colleagues allow ventilator alarms to detect circuit disconnections when sum measurements fell beyond the set limit. Individual pulse oximetry and capnography alert for life-threatening disconnections. We agree with the authors that this approach is not intended for clinically deteriorating patients, but the Vent Multiplexor device allows for additional control of coventilation in settings of severe ventilator shortages. Critical care physicians would be able to support patients, while mitigating possible harm, until additional ventilators are obtained.