Wednesday, July 14, 2010

Respiratory Therapy Formulas

Listed here are the most commonly used respiratory therapy formulas.

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

This formula is nice for determining how well a patient is oxygenating.  A normal PaO2 on room air is 104 (normal range is 80-100).  Is a patient with a PO2 of 200 on 100% NRB oxygenating well? 100 FiO2 times 5 equals 500 expected PaO2.  The patient should have a PaO2 of 500 on an NRB. This means this patient is not oxygenating well.

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 = 1 (patient requiring no supplememtal oxygen)
    • 21% FiO2 in room air * 5 = 105
    • 105/105 = 1  (Remember, this is an estimate.  Actual PaO2 is usually listed as 104)
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 looks 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=0.28(PSI) / LPM  (or click here for calculator)

18. Oral intubation for adult usually = 21-25cm @ lip.

19. Nasal intubation for adult usually = 26-29cm

20. PEEP therapy = greater than 6-8 CWP  (Dr. Tobin disputes this claim)

21. Humidity should be set at 37 degrees Celsius.

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.
Please note:  some of the values are rounded off for ease of calculating.  Some values or normal ranges may vary slightly based on the source.  

Post originally published on 7/4/2010 on respiratorytherapytcave.blogspot.com and edited for accuracy by Rick Frea

4 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

Jessica4282 said...

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

An e-cylinder is 0.28, not 0.30