Author Archives: csprick

Resus Anne compression sensor

The compression/ventilation sensor in the Resus Anne / QCPR manikins from Laerdal have been the subject of previous posts where I looked at the implications of the linear optical encoder on ventilation volume measurements.

Another issue that we have encountered several times is that one of the wires that connects to this module fatigues over time and eventually breaks internally (the white one).¬† The pattern when this happens is typically a gradually decreasing measured depth of compressions until they don’t measure at all.

overview

The wire connects to the sensor module with a small IDC (insulation displacement connector) from TE/AMP (173977-8) which are $0.75 USD each from Digikey (A98616-ND) or $0.95 AUD each (in a pack of 10) from RS Components (680-1304).

connector

Unfortunately, the tool to insert the wire is ~$1300 USD ūüė¶¬† 58074-1 Hand crimper without die set – Digikey Part A2031-ND¬† $105.77 USD + 58372-1 Tool head assem 2mm terminator
Digikey part A99129-ND  $1,191.20 USD

crimp-tool

So naturally I made my own punch tool.  Not as fancy, but should get the job done.

tool-tip

The tip is a piece of stainless steel out of the scrap bin.  1.15 mm thick, ~5mm wide

The tip is cut/filed down to 3mm wide by 3.25mm high with two slits in the end.¬† These slits were cut with a jewellers saw using a 3/0 blade (0.24mm = 0.095″wide) and about 1 mm deep (because I got tired of cutting) about 0.75mm from each edge of the tip and just over 1mm apart.

This drawing was taken off the official connector drawing.  Ignore the tolerances.  Close enough seems to work fine.

drawing

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Spherical camera

I did a little experiment looking at camera spacing, so I made a single side.  This allowed experiments with lens spacing via the mounting bolt through slots in the grey plastic plate.  The ledge provides the registration for alignment.

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All the pieces are coming together. ¬†I whipped up a frame with proper angles and a bit more rigid than cardboard out of some PVC sheet. ¬†I had a great time making jigs and cutting everything out in my shop at home. ¬†Mostly table saw with a crosscut sled and some precisely angled blocks and a router attachment for my Dremel. ¬†All glued together with PVC cement. ¬†The slots on the top allow access to attach and tighten the nuts on the back of the camera mounting slots. ¬†There’s a hold in the base under the top camera mounting hole.

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The camera on top is located such that the lens is at the centre of the pentagon. ¬†We’ll see what sort of gaps that leaves in the ceiling. ¬†The whole thing sits on an old IV stand

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3D Printer!

I looked around a bunch and finally settled on the Prusa i3 Mk2 (beta) from Aus3D. ¬†One of the primary things that influenced me in this choice was that Aus3D is local in Adelaide. ¬†When I first inquired about a printer, I was tossing up between the Aus3D and the original from Prusa Research. ¬†The price was about the same (once you factor in currency conversion and shipping). ¬†Chris from Aus3D responded to my email suggesting that he was in the final stages of upgrading his kit to Mk2 with a bunch of improvements. ¬†He said I could have a beta version. ¬†I missed out on a few features that came out with the official release, but I’m happy with the printer.

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Some of the features and design choices I particularly liked was the use of proper lead screws, thick acrylic frame, infrared z-probe with auto bed leveling, and a E3D v6 lite hot end which I upgraded to a full E3d-v6.  I also really like that it had pre-drilled holes for a Raspberry Pi to run OctoPrint.  All of the steppers had connectors, not just wires and the limit switches were on small PCBs with proper connectors and mounting.  All of this added up to some good engineering choices and a local supplier as a bonus.

Quite a bit of a learning curve here too, and it took me some time to get consistent prints.  The design of the GoPro holders also evolved (left to right) as I learned what size feature I could reproduce and became familiar with the material properties.

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The camera is held in by a small protrusion in the back of the holder which mates with the slot on the end of the camera. ¬†Then it’s just a matter of an elastic band to keep it from tipping out. ¬†The tolerances are tight enough that its a snug fit anyway.

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So, lots of printing later…

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There were a good 8 prototypes that didn’t get used. ¬†Near the end I was coming to the end of the spool of blue ColorFabb (a PLA/PHA blend) that I got with the printer. ¬†I picked up some green PLA from Bilby3D (another Australian company). ¬†I had been printing on kapton tape at 60C, but the new green filament wasn’t sticking at all. ¬†A switch to blue painter’s tape and a bed temp of 45C per the tech support folks at Bilby and we’re back in business. ¬†I used 2 of the green holders to mark ‘front’ on the spherical camera.

3D360 second try

Inspired by eleVRant’s website, I created a cardboard & duct tape version of a horizontal rig.

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This seemed to work ok, but was a little inconsistent with angles ¬†and stitching. ¬†Instead of spending another $1000 USD on another holder from 360Heros, I decided to get (build) a 3D printer of my own. ¬†This would also cost about $1k, but then I’ll have a printer to use for other things. ¬†All at the expense of time…

3D360 first try

Apparently, other people thought this was a great idea too, so I got some funding from CEdICT at Flinders University to purchase some equipment to try this experiment. ¬† I got 12 Hero4 Silver GoPros (and a remote), a 3DH3Pro12 holder from 360Heros.com¬†and some software (PTGUI, Video-Stitch and 360CamMan). ¬†I also got a couple 7-port USB3 hubs (for charging) and a couple microSD card readers. ¬†The Hero4 doesn’t show up as a disk when connected via USB, so you can’t use 360CamMan to pull the videos off. ¬†Have to either use the GoPro software or pull the SD cards and use the card readers. ¬†I may also need some other software (Premiere Pro and a plugin called quicks3D), but I haven’t jumped on this yet. ¬†So far, ffmpeg is serving my video rotation and combining needs. ¬†I’ll post details in a later post once I’ve figured it out.

All set to go! ¬†Wow, that’s a steep learning curve. ¬†Talk about millions of settings all interacting. Hmmm, it’s not turning out like I anticipated. ¬†I discovered this web page:¬†http://elevr.com/elevrant-360-stereo-consumer-cameras/¬†which also references this page:¬†http://elevr.com/stereo-polygons/¬†which implies that maybe this rig has some flaws.

3D360 video?

Probably back in 2014, I attended an ‘Emerging Technologies Brainstorming’ session hosted by CEDICT at Flinders University. ¬†One of the things we talked about was use of the Oculus Rift. ¬†I had just ordered a DK2 a week or so prior – just because it was cool and not that expensive. ¬†I was looking for something ‘official’ to do with it.

The idea was to create a procedures video library for health professional students (medicine, nursing, paramedics, etc). ¬†This was largely inspired by the Moveo Foundation in France¬†where they filmed a procedure (in stereo) from the surgeon’s PoV. ¬†The idea was that when assisting as a student, you are sometimes given a job – e.g. hold a retractor. ¬†You know that you have to do this job well and will get yelled at if you don’t, so you concentrate on doing a good job. ¬†The result of this is that you miss out on the details of the procedure, and you certainly don’t have a view from the surgeon’s perspective. ¬†Having a video from the surgeon’s perspective allows anyone to see what is done and review it at leisure. ¬†A commentary by the surgeon, even after the fact could be added for further instructional value.

We thought that there might be many procedures that could be filmed in this way for students to view when learning how to do them.  The question is, how much effort is needed to produce these videos?  Options range from a single camera up to a spherical stereo video (11 or 12 GoPro cameras) best viewed with a VR rig (like the Oculus Rift).  Obviously the more cameras, the more complex and time consuming the post-production.

  • Single camera
  • Stereo pair¬†(2)
  • Spherical (5 or 6)
  • Spherical stereo (11 or 12)

 

Qantum Ventilation

A question was raised recently regarding ventilation with Laerdal QCPR manikins.  Basically, the question was about required volumes (which is a physiological discussion) and how the manikins represent these volumes, as chest rise and as reported via software. These manikins use a lung bag placed between a plate and the chest skin to represent the lungs.  As this bag is inflated, it lifts the chest skin Рseparating the skin and the plate.  This is seen from the outside as visible chest rise.

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There is a clever little valve in the breathing circuit that allows flow only from the white side (input) to the branch (lung) and then only from the branch to the grey side (exhaust). ¬†This is designed to direct gasses away from the rescurer to reduce potential cross-contamination between rescurers. ¬†I don’t think this valve impacts on the volumes.

IMG_0744directional valve

Note that the Resus Anne Simulator with the intubation airway doesn’t use this valve, and exhausts gas back through the airway. ¬†This manikin also has a check valve on the oesophagus to simulate the pressure needed to insufflate the stomach.

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There is a sensor that measures this inflation by measuring the distance that the centre of the chest skin has risen above (separated from) the plate. The sensor uses a pattern of lines and a photodetector to sense displacement.

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This implies that there are discreet values of displacement and thus reported volumes.  There are approximately 11 lines per centimeter, or 22 transitions per centimeter Рgiving a resolution of just under 0.5mm.  Which seems to be reported as about 14mL.  There is some inherent error in this apparatus.  The sticker with the lines is not always straight, and the sliding plastic piece that the sticker is on, can rotate slightly in the housing. The sensor that detects the displacement reports similar resolutions.

We measured the flow into a manikin using a Wright Respirometer Type P.M. Wright Respirometer

and compared those values with that reported by the Wireless Skillreporter software from Laerdal.  In a new QCPR Anne with unmodified lungs, the reported value is slightly (25%) higher than the measured flow.  (18% @ 400 mL to 15%  @ 700 mL)

QCPR measures vs reported volumes

Some years ago, we decreased the volume of the lung bag in one of our Resus Anne Simulators in an attempt to increase the amount of chest rise for a given ventilation volume.  This manikin was measured in the same way and the results are shown in the second graph.  In this case, the volumes reported by the software are slightly smaller than measured. (19% @ 400 mL to 7.5% @ 700 mL)

IMG_20150504_111122RASim volume measurements

Conclusions:

Neither lung matches the measured volumes exactly.  Shrinking the lung does move the curve in the right direction.  The volume used here (190mm lung width) was too small although it did give good chest rise.

I think it is possible to adjust the lung size to match the measured ventilation volumes and this will still give good chest rise.

An interesting side note is that the volumes reported from the software appear to be quantised with a volume of about 14ml.  This is not surprising given the nature of the sensor used to measure lung inflation.

The question of what volumes should be used is a physiological discussion. I will address some of the literature surrounding this topic in another post.