Wednesday, July 14, 2010

Respiratory Therapy Formulas

The following are some respiratory therapy formulas that you learned in RT school, have probably since forgotten, but could still find useful from time to time:

Respiratory Therapy Formulas and normal values:

1. Ideal Body Weight (IBW):


  • a. Female: 100 lb for 1st 5ft + 5lbs ea additional inch
  • b. Male: 106 lb for 1st 5 ft + 6lbs ea additional inch
2. Static Compliance: (VT/Static pressure – PEEP)


  • a. Normal = 60-100
  • b. less than 60 = lungs becoming less compliant
  • c. greater than 25 is acceptable
  • d. less than 25 is unacceptable
3. Desired FiO2 = Desired PaO2 + Known FiO2 divided by known PaO2

(Normal PaO2 on 21% or room air = 105)

4. Desired Ve= Known Ve*Known PaCO2 divided by desired PaCO2

5. Desired Vt = (Known PaCO2 x Known Vt)/Desired PaCO2

6. Desired f = (Known PaCO2 x Known f)/Desired PaCO2

7. RAW: PIP–Plateau/ Flow, or PIP–plateau

8. French size sx catheter = ETT size * 3/2

9. PAO2: (713 *Fio2 – PaCO2)/0.8or 0.1 if 100% O2

10. A-a gradient (ratio or A-ADO2): PAO2 – PaO2


  • a. Normal on RA = 10-40 or on 100% = 25 – 70
  • b. Increased 66-300 = acute lung injury
  • c. greater than 300 = severe shunting, ARDS (unacceptable)
11. To determine cause of hypoxia, refer to the A-a gradient:


  • a. If normal, hypoxia caused by hypoventilation,consider drug overdose, neuromuscular disorder.
  • b. If abnormal & SpO2 improves with increased FiO2. Consider PE, pneumothorax, asthma,emphysema, pneumonia, bronchitis, heartfailure, congenital heart disease, aging.
  • c. If abnormal & refractory hypoxemia occurs, hypoxia caused by shunting problem considerpneumonia, atelectasis, pulmonary edema or ARDS.
12. Shunt % = A-a gradient/20


  • normal=20%
  • if greater than 20 an increase in PEEP is indicated
13. a-A ratio: PaO2/PAO2


  • a. Normal = 80% (74% elderly)
  • b. 60% = V/Q imbalance
  • c. 15% = shunting
14. P/F Ratio: PaO2/FiO2


  • a. Normal = 300 – 500
  • b. Acute lung injury = 200 – 300
  • c. less than 200 = ARDS (shunt)
15. Expected PaO2 = FiO2 x5

Even though normal PaO2 is 105 on room air, a PaO2 of 200 on 100% FiO2 is not necessarily good. It should be 500. Therefore you know patient still not oxygenating effectively.

16. Actual PaO2/ Expected PaO2 = % of patient expected PaO2:
  • a. Should be recorded daily
  • b. Shows if patient is oxygenating better
  • c. Better indicator than simply looking at actual PaO2 and FiO2
  • d. Normal = zero (patient requiring no supplememtal oxygen)

Examples of % expected PaO2: (Despite lower PaO2, patient still oxygenating better)

  • e. January 1 PaO2 40 on 100% FiO2 = 80%
  • f. January 5 PaO2 60 on 40% FiO2 = 30%
  • g. January 6 PaO2 55 on 50% FiO2 = 20%

Another example of % expected PaO2 (PaO2 look good, but is patient really oxygenating?)

  • h. January 1 PaO2 200 on 100% FiO2 = 40%
  • i. January 5 PaO2 100 on 100% = 20%
  • j. January 6 PaO2 100 on 90% = 22%
  • k. January 10 PaO2 55 on 80% = 13%

17. e-cylinder time remaining=0.30(PSI) / LPM

18. Oral intubation = 21-25cm @ lip.

19. Nasal intubation = 26-29cm

20. PEEP therapy = greater than6-8 CWP

21. Humidity should be set at 37 degrees Celcius.

22. Suction:Adult=100-120,Child=80-100,Infant=60-80

23. Patient WOB (available on newer microprocessor ventilators)

  • a. Less than 0.8 = normal
  • b. Measures effectiveness of rise time and sensitivity.
  • c. Measured in spontaneous mode.

(Post updated Janurary 21, 2011)

Note: There are other RT Formulas, yet these are the ones I have used on occasion while working. If you find another formula you find worthwhile, please email me with the formula and a note about when you would use it, and I will post it for others to use.

3 comments:

Chris G said...

For 13. a/A Ratio, write means to write PaO2/PAO2, not PaO2-PAO2. Using division will result in a %.

Anonymous said...

For number three, should be "x" times, not "+" plus.

Anonymous said...

Chris G: For number 13 the only difference is when you subtract Pa02-PA02 the answer will be in mmHg..EX. 100PA02-85Pa02= 15mmHg