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Tuesday, October 21, 2014

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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