Multiplace Delivery System
Monthly Hyperbaric Safety Notice: October 2005
Calibrating the multiplace ventilator
In
this notice a method of accurately measuring ventilator volumes at various pressures
is described.
Safety Issue
Unless you are you using a volume controlled, typically air driven, ventilator you need some way to measure your expired tidal volumes. The built in LED display or the externally mounted volumeter likely is giving you erroneous information and you may be delivering inadequate tidal volumes.
A commonly used multiplace ventilator is the Sieman’s Servo 900c with the AC circuitry separated to the outside of the chamber. It measures the expired volumes by air blown over a membrane. When a volumeter aka respirometer measures volumes it is counting the revolutions turned by the turbines in the exhaust tunnel. Both of these measurement techniques are affected by air mass and hence by the ambient pressure inside the chamber.
Method
Use a 3-liter manual calibration syringe, a Y connector, a one-way valve, the volumeter and a short length of corrugated hose. Assemble as illustrated.
Completely fill the syringe through the one-way valve on one side of the Y valve. With a steady pressure completely empty the syringe. Record the volume on the volumeter. Do this at least a dozen times at each depth, record the results and calculate the mean. Do this at the surface and each of your common treatment pressures.
Enter on a table IE.
| ATA | Vol Delivered | Vol Measured | Correction Factor (Vol Delivered/ Vol Measured ) |
| 1 | 3.00 | 3.28 | 1.1 |
| 2.4 | 3.00 | 3.34 | 1.12 |
| 3 | 3.00 | 3.51 | 1.17 |
| 6 | 3.00 | 3.60 | 1.20 |
Clinical implications
In order to accurately delivered the ordered tidal volume multiply the ordered volume by your correction factor for the appropriate chamber pressure.
IE: for ordered TV of 750 cc at 3 ATA.
750 X 1.17 = 878 cc expired volume by volumeter.
Discussion with your pulmonary/ critical care physicians will establish their usual range of preferences for tidal volumes I.E. 600 –1000 cc.
The calibrated volumeter can also be used to calibrate the expired volume readout on the ventilator itself. Use a test lung and allow the ventilators to warm up first.
Reference
Kindwall, E.P. Hyperbaric Medicine Procedures, 8 th edition, 69-71.
Contributing Author: Claude Wreford-Brown, ACHRN
Claude
is the clinical manager of the hyperbaric medicine department at Virginia Mason
Medical Center in Seattle, Washington, USA. Claude is active within the Baromedical
Nurses Association, and currently serves as its president. He contributed chapters
to the Baromedical Nursing textbook and is an Undersea and Hyperbaric Medical
Society facility accreditation surveyor. Claude has attended both the Hyperbaric
Safety Director training course and the National Oceanic and Atmospheric Administrations
Diving Medical Officer Course.
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