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