2007 OPEN FORUM Abstracts
THE EFFECT OF A HYPERBARIC ENVIRONMENT ON THE SERVO 900CâS DELIVERED TIDAL VOLUME
E. Lopez1, R. Hase1
Background: We wondered what actually happened to delivered tidal volumes during a hyperbaric dive, because of the effects of Boyle's Law in an environment where ambient pressures may vary as much as 600%.
Methods: We use the Siemens Servo 900C for mechanical ventilation needs in the hyperbaric chamber, configured with the pneumatic system placed inside the chamber while the control section remains outside; the two are connected via through-hull cabling. We set the machine to deliver 800cc VT at a Volume Control rate of 10. The inspiratory time percent was set to 33, with no pause time percent. Peak pressure cycle/alarm was set to 100 cm H2O. We collected and recorded exhaled title volume readings from the Servo's multiparameter display, recording 10 breaths at each chamber depth, beginning at 0 feet salt water (FSW) depth (sea level ambient) and progressed to depths of 16, 33, 60, 66, 99, 132, and 165 FSW. At each depth, ten breaths’ exhaled volume data were recorded following pressure stabilization at each level. The data were then entered on a Microsoft Excel spreadsheet with average exhaled volumes calculated at each depth. These volumes were then corrected using our measured volume correction nomogram for the Servo 900C, which gave the true delivered tidal volume at each depth. We plotted these on a graph and saw that the relationship best fit a straight line relationship, so we used the Excel spreadsheet’s LINEST function to calculate a best-fit regression line for the data.
Results: Our first observation was that the volumes displayed on the 900C’s digital display did not decrease substantially as the pressure increased through the test range. In fact, there was only an approximate 12% difference in the exhaled volumes comparing surface (0 FSW) data with maximum depth (165 FSW) data. However, once the displayed volumes were corrected using our exhaled volume correction nomogram, the delivered volumes demonstrated a greater than 55% decrease between surface and maximum depth readings. Our study demonstrated that while the Servo 900C’s flow transducers read increasingly high over increasing dive depth, the delivered volumes become simultaneously increasingly smaller. These two phenomena together mask the effect of an increasingly hyperbaric environment on delivered tidal volumes with this ventilator, and demonstrate the vital necessity of measuring the effect of hyperbaric pressure on any mechanical ventilator used in hyperbaric medicine.