Sunday, September 14, 2014

Calculating Tank Time

As a respiratory therapist, it may become necessary for you to know the amount of time an oxygen tank or oxygen cylinder is expected to last when running at a given liter flow.  This can be determined by calculating tank time.  The following is an example of a scenario in which it would be necessary for you to calculate tank time:

Photo taken by Janelle Gardiner, September 15, 2014.
You are a respiratory therapist on the night shift at a 300-bed, level two trauma center.  You are asked by the charge nurse of the cardiovascular-thoracic unit to arrange for home oxygen for a patient who is being discharged in the morning.  The patient is a truck-driver from Boise, Idaho, who suffered a myocardial infarction on his route and subsequently underwent Coronary Artery Bypass Graft at your hospital.  The patient will be traveling home via his 18-wheeler.  It will take approximately eight hours for the patient to travel from your location to Boise, including adequate time for rest stops and meals.  The patient will be discharged on a nasal cannula at 4 LPM.  The nurse would like to know how many E cylinders the patient will need to ensure the patient has enough oxygen to get home.  The patient needs to know how long to expect one full E cylinder to last.  This will allow the patient to plan to change tanks at appropriate time intervals.

This tank time formula will be helpful in your calculations.


The following steps are provided to guide you in obtaining the information necessary to provide both the nurse and the patient with the information needed.
  1. Identify the tank factor for the size cylinder being used.  To simplify, use a tank factor of 0.28 for an E cylinder, and 3.14 for an H cylinder.  For a more complete listing of tank factors, please refer to Table 37-4 in Egan's Fundamentals of Respiratory Care (Kacmarek, Stoller, & Heuer, 2013).
  2. Note the liter flow for the particular scenario.
  3. Assume a full cylinder contains 2200 psig.
  4. Insert know figures into the formula.  Tank factor * psig in cylinder.
  5. Divide the result of step 4 by the liter flow as noted in step 2.  This will give you the number of minutes the cylinder will last at the specified liter flow and pressure (psi) remaining in the cylinder.
  6. Divide the result of step 5 by 60 to determine the number of hours and minutes one cylinder will last.
Photo taken by Janelle Gardiner
Contribute to the discussion by posting information needed by the nurse and the patient in the comments section below.  If you are the first to respond, provide a scenario of your own for the next person to use to calculate tank time.

Beyond Tank Time
For the patient to be completely prepared for discharge, they will need to know other things in addition to time intervals at which to change tanks.  Comment on what other patient education you could provide at the time of discharge.  What arrangements need to made in Boise?  Are there other ancillary services that need to be involved in discharge planning?


Recommendations for teaching tank time calculations:
  • Provide the students with some relevant scenarios similar to the one provided above.  The scenarios can be for in-hospital transport, inter-hospital transport, or discharge for home use.  For optimal learning, the student should be able to find clinical relevance in the scenarios provided.
  • Discuss with students the format for finding a similar scenario on the National Board for Respiratory Care (NBRC) Certified Respiratory Therapist exam.  Help the student understand that all questions will be asked in a multiple choice format.  The students must be able to perform these calculations without the use of a calculator, as the NBRC does not allow the student to use a calculator on the exam.  Therefore, you may suggest they can use a tank factor of three for an H cylinder, and 0.3 for an E cylinder to simply the calculations.  Historically, multiple choice responses are far enough apart from each other that these types of estimations can be made to arrive at the correct answer.  
References:


Kacmerek, R. M., Stoller, J. K., & Heuer, A. J. (Eds.). (2013).  Egan’s fundamentals of respiratory care (10th ed.).  St. Louis, Missouri: Elsevier.

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