Up until now, much of our testing has been on a Nellcor Puritan Bennett 840 Ventilator. 

The goal is to make sure whatever we build works with as many ventilators as possible. Having access to these devices is not easy and we're lucky to have support from medical professionals to provide access and their working knowledge of these systems. 

The LTV1200 is  more of a portable unit, compared to what you would normally see in use in an ICU. As such, it has less features, or patient protections that an ICU ventilator would have. We tested it with the intention of confirming the ventilator splitting circuits would still work. 

One thing we learned was the tidal volume measurements were showing very large on my meters. We also noticed that air was always flowing in the breathing circuits. We think this is how the LTV1200 senses any occlusions. 

Looking at the serial plotter, we see that the flow rate is not zero between breaths and tidal volume is accumulating when it shouldn't.

Zooming in on the flow, there is a constant minimum flow that drops down to 2.4SLM, but not sharply. I'm likely not seeing a brief spike that goes to zero (or negative) for me to trigger my flow measurement to stop. 

NOTE: I just realized I had my calibration smoothing code still in play, when I was trying to get smooth flowrate readings off my blower set to fixed rates. It's sampling at 1khz, averaging over 1000 points and plotting every only plotting every 50ms. This is accounting for the slow decay in the flow and needs to be removed for measuring flow waveforms.

In our meter placement, we're measuring flow rate on the expiratory branch, just after the patient wye. The ventilator controls the flow and pressure in the breathing circuit. So to truly measure the tidal volume, we need to do one of the following

• Move the flow meter between the wye and the patient, then it will see bidirectional flow and be a true indication of the flow. We are hoping not to do this, to avoid an extra cable running to the patient and concern of patient proximity to the meter (meter rubbing on face, meter getting disconnected)
• Set the flow trigger to be a bit higher than 2.4SLM, which will result in less accurate tidal volume readings.
• Add a pressure sensor, then trigger flow measurements when the pressure increases above minimum PEEP for most accurate readings.