Respiratory Therapy Formulas and Normal Values

The following are the most common respiratory therapy formulas and normal values used by respiratory therapists.

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

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

18. Nasal intubation = 26-29cm

19. PEEP therapy = greater than 6-8 CWP

20. Humidity should be set at 37 degrees Celcius.

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

22.  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.

23.  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 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%

24.  Temperature Conversion: Celcus = Fahrenheit minus 32/ 9       Fahrenheit = (Celcus*9)/32

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