OxVent: Design and evaluation of a rapidly-manufactured Covid-19 ventilator

Abstract

Background: The manufacturing of any standard mechanical ventilator cannot rapidly be upscaled to several thousand units per week, largely due to supply chain limitations. The aim of this study was to design, verify and perform a pre-clinical evaluation of a mechanical ventilator based on components not required for standard ventilators, and that met the specifications provided by the Medicines and Healthcare Products Regulatory Agency (MHRA) for rapidly-manufactured ventilator systems (RMVS).

Methods: The design utilises closed-loop negative feedback control, with real-time monitoring and alarms. Using a standard test lung, we determined the difference between delivered and target tidal volume (VT) at respiratory rates between 20 and 29 breaths per minute, and the ventilator's ability to deliver consistent VT during continuous operation for >14 days (RMVS specification). Additionally, four anaesthetised domestic pigs (3 male-1 female) were studied before and after lung injury to provide evidence of the ventilator's functionality, and ability to support spontaneous breathing.

Findings: Continuous operation lasted 23 days, when the greatest difference between delivered and target VT was 10% at inspiratory flow rates >825 mL/s. In the pre-clinical evaluation, the VT difference was -1 (-90 to 88) mL [mean (LoA)], and positive end-expiratory pressure (PEEP) difference was -2 (-8 to 4) cmH₂O. VT delivery being triggered by pressures below PEEP demonstrated spontaneous ventilation support.

Interpretation: The mechanical ventilator presented meets the MHRA therapy standards for RMVS and, being based on largely available components, can be manufactured at scale.

Funding: Work supported by Wellcome/EPSRC Centre for Medical Engineering,King's Together Fund and Oxford University.

Keywords: Biomedical engineering; Covid-19; Critical care; Respiration (artificial).

Figure 1

Overview of ‘bag in bottle’ principle. During inhalation (a), the resuscitator bag is compressed within a sealed pressure vessel (‘bottle’). During exhalation (b), patient air exits through the PEEP valve; the bottle exhausts through a separate solenoid. Note: heat and moisture exchange (HME) filters are not shown for clarity.

Figure 2

System block diagram for the closed-loop negative feedback proportional, integral, derivative (PID) algorithm used within the system. The control algorithm adjusts the solenoid valve current based on the error between the integrated tidal volume (VT) delivered and the set target volume; this VT error is calculated on a breath-by-breath basis.

Figure 3

The OxVent system, with all required supply and patient connections. The device comprises two main assemblies: the electronics/control enclosure (white panel, top), and ventilator box (‘bag in bottle’, bottom). The system has inlets for electrical power, compressed air at 4 bar, and oxygen (the concentration of which is set via rotameter). Flow measurement is achieved via a spirometry kit placed close to the patient airway.

Figure 4

Limits of VT delivery in different flow rates configurations. In each case, the worst-case result amongst three test devices is shown, measured as the difference between delivery and set value. a) VT delivery error versus set inspiratory flow rate showed greater under-delivery for flows >825 mL/s. b) Re-plotting the same data as a function of set VT shows that high VTs could be attained with flow rates under 825 mL/s, whereas even lower VTs were unattainable with flows at or above 825 mL/s (for example, 40 mL under-delivery at VT = 450 mL and flow >825 mL/s).

Figure 5

Variation in supply pressure, both decreasing (left) and increasing (right). Violin plots over consecutive cycles are shown, and error bounds of ± 10% are denoted with red dotted lines. Different colours illustrate the different supply pressure conditions tested. For decreasing pressures, mean VT delivery was within 10% of target at and above 3.5 bar (delivery at 3.5 bar was right on the lower bound). For increasing pressures, mean VT delivery was robust at all pressures up to and including 6 bar, though inter-breath variability exceeded the bounds at higher pressures.

Figure 6

Long-term ventilation under high-flow rates (VT = 600 mL, RR = 27/min, I:E = 1:2). Violin plots over consecutive cycles are shown, and error bounds of ±10% are denoted with red dotted lines. VT delivery was within bounds for 23 days (surpassing the two-week RMVS requirement); the test was terminated at 24 days due to mechanical failure of the resuscitator bag after approximately 860 k cycles.

Figure 7

Flow and pressure characteristics. To facilitate the comparison between conditions, time is provided as percentage of inspiration/expiration due to different inspiratory/expiratory times used during the protocol. Thin lines represent each experiment, while thick lines show values grouped by PEEP [points and error bars represent mean (SD)]. Different colours represent different levels of PEEP in cmH₂O: PEEP 5 [red; n = 35], PEEP 10 [green; n = 32], PEEP 20 [blue; n = 17].

Figure 8

Example 30 s periods of ventilation showing volume-controlled mandatory ventilation (VCV) in the left panel and assisted spontaneous ventilation (ASV) in the right panel [n = 1]. ASV was tested after lung injury, when pulmonary compliance was reduced, hence leading to greater peak inspiratory pressures.