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 – separating the skin and the plate. This is seen from the outside as visible chest rise.
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.
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.
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.
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 – giving 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.
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)
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)
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.