NIV devices working as an invasive ventilator
To operate as an invasive ICU ventilator the NIV device must have the following features:
- To allow pressure control and/or S/T modes.
- The NIV device must reach an inspiratory positive airway pressure (IPAP) ≥ 30 cmH2O and an expiratory positive airway pressure (EPAP) ≥ 10 cmH2
- An obligated gas leak port to washout CO2 from the single limb NIV circuit.
- An external O2 source supply. We have administered O2 through a port placed between the single-limb circuit and the NIV device.
- A heat and moisture exchange (HME) antibacterial/antiviral filter must be placed between the endotracheal tube (ETT) and the NIV circuit. The HME prevents SARS-CoV-2 virus dispersion and keeps humidity of inhaled gas.
Three NIV circuit configurations were proposed to implement ventilation in intubated patients (Figure 1) . These are circuit configurations described for such altered standard of care that must be only used in the context of COVID-19 pandemic. The first option is the standard circuit configuration that has a leak at the HME filter, nearest to the patient. The second one is a modified Bain system ideated to perform general anesthesia, which has the advantage to keep heat and humidity in the breathing gases . The circuit has double coaxial tubes one inside the other. The outer tube is connected to the NIV device and delivers inspired gases to the patient while the inner tube transports expired gases to the ambient (leak closer to the NIV equipment). The third option has a non-rebreathing valve like Ruben or Duckbill (similar to those found in Ambu® resuscitation bags) , which delivers gases to the patient and then eliminates gases to the ambient throughout a PEEP valve. This atypical circuit option has the theoretical advantage to deliver more O2 and to decrease CO2 re-breathing.
The analysis was done in the Simulation Center of the Buenos Aires Association of Anesthesia, Analgesia and Reanimation. Data was collected by the ASL 5000 Breathing Simulator (IngMar Medical, Pittsburgh, USA), which was connected to the NIV device (Stellar 150, ResMed Inc., Sydney, Australia) by a cuffed nº 8 endotracheal tube. This device can be used for noninvasive and invasive ventilation according to manufacturer’s specifications. Respiratory mechanics and CO2-O2 signals were obtained with sensors placed at the airways opening (S5 device, GE Healthcare/Datex-Ohmeda, Helsinki, Finland). The O2 was measured by paramagnetic sensor with an accuracy < 2% of reading, rise time < 260 milliseconds and measurement range between 0 to 100% [9,10]. Sensors were calibrated before protocol as described by the manufacturer. Data was recorded by the software Datex Collect (GE Healthcare/Datex-Ohmeda, Helsinki, Finland) in a laptop and was analyzed off-line.
We simulated two kinds of patients based on lung mechanics. One patient with healthy lungs, with a respiratory compliance of 50 mL/cmH2O and airways resistance of 5 cmH2O/L/s, while ventilated with an inspiratory positive airways pressure (IPAP) of 22 cmH2O, an expiratory positive airways pressure (EPAP) of 8 cmH2O, a respiratory rate of 15 bpm and an inspiratory time of 0.9 seconds. The other simulated patient with compromised lungs, a respiratory compliance of 30 mL/cmH2O, an airways resistance of 12 cmH2O/L/s while ventilated with an IPAP of 28 cmH2O, an EPAP of 14 cmH2O, a respiratory rate of 25 bpm and an inspiratory time of 0.9 seconds.
In each simulated patient and for all three NIV circuit configurations (Figure 1) we have tested with an increased O2 supply, from 1 to 10 L/min in incremental steps of 1 L/min, to a final maximum flow of 15 L/min. Each step of O2 flow was maintained by 3 minutes separated by a washout period of another 3 minutes to reach a stable condition. Simulation was repeated three times in different days by different operators. Data is presented as mean ± SD.
Data of anesthetized patients
Preliminary data recorded in three American Society of Anesthesiologists Classification 1 patients undergoing laparoscopic procedures was used to illustrate the results of simulations. In these patients, the ventilatory settings and the recording system (S5 device with Datex Collect software both, GE Healthcare/Datex-Ohmeda, Helsinki, Finland) were similar than the simulations for healthy lungs.