To the Editor:
The Oxygen cylinder bears a label showing how many liters it contains when full. The pressure on the pressure gauge will show, usually in gross divisions from full to empty, how full the cylinder is. But there is no indication of how much volume is in the cylinder at a particular moment. If this were known, we could calculate how long the cylinder content would last at a particular flow rate. But as this calculation is usually required when patients are transferred, it becomes just another thing to remember. A precalculation would save time and would guarantee accuracy.
I advocate the use of a table for oxygen cylinders showing how long they will last using a particular flow rate. An example of the proposed table is shown (Table 1) and can be fixed to the cylinders and be readily checked by the user. This can save time if the person bringing the cylinder does not know how to calculate how long the cylinder content will last but can check with the clinician to determine how much flow will be required and for how long. Thus the clinician will not be presented with a cylinder that has to be replaced because it will not last the journey.
The calculations on this table are based on the concept that the volume content of the cylinder is proportional to the pressure displayed on the gauge. This concept is derived from the famous formula PV/T = constant, where P is pressure in the cylinder, V is the volume of the cylinder, and T is the temperature. As volume is a constant and temperature is usually constant in the hospital environment, the pressure gauge can provide an accurate reading for volume as well as for pressure. This is an appropriate application for oxygen, as its critical temperature is −119°C, and it is kept in a gaseous form in the cylinder. These calculations are rounded to the nearest lesser round values (e.g., 63.75 min rounded to 63 min). Although it may be tempting to make the estimated duration intentionally less by some value (e.g., 5 min) as a safety margin, this may cause some to overestimate this safety margin on the assumption that it may be forgotten in time, thus reintroducing the guessing factor in estimating the cylinder’s volume.
It is important to note that at other institutions, the E cylinder contains 679 liters, or have new BOC oxygen E cylinders that contain only 643 liters, which will overestimate the content by 6%, therefore carrying the risk of earlier unexpected running out of oxygen.
Thus, the reader should be aware that the table is based on cylinders with a volume of 680 liters, and departments may have to recalculate the numbers based on the volume of oxygen in the E cylinders at their hospital.
So I suggest that the best procedure is to look at the label on the cylinder and check how long it will last at the selected flow rate.
Y. M. Shouman, MD