Equations:

Air/O2 entrainment ratio:

  • 24 = 25:1

  • 28 = 10:1

  • 30 = 8:1

  • 35 = 5:1

  • 40 = 3:1

  • 45 = 2:1

  • 50 = 1.7:1

  • 60 = 1:1

Depending on O2 percentages, if the percentage is less than 40% use 21%, if the percentage is 40% or higher, use 20%.

Equation: 100 - X (FiO2) / X (FiO2) - 20 or 21

Example: 100 - 40 / 40 - 20 = 60/20 = 3:1

Example: 100 - 24 / 24 - 20 = 76/3 = 25:1


Minute ventilation & alveolar ventilation:

VE: RR x VT

Normal value: 5-6 L/min in a healthy adult, should be kept below 10 L/min

Description: The total volume of air that is inhaled and exhaled by the lungs within a minute.

VA: VT - VD (VD = IBW/lb) x RR

Normal value: 4.5 L/min

Description: The volume of fresh air that reaches the alveoli for gas exchange per minute, it excludes the air trapped in the respiratory dead space. It is vital because it determines how well O2 enters the blood and CO2 leaves the body.


Formulas from VE:

VT: VE x RR

Normal value: 5-8 mL/kg in a healthy adult, 6-8 mL/kg in a ventilated patient

Description: The normal amount of air that moves in and out of the lungs within each normal breath

RR: VE/VT

Normal value: 12-20 breaths/min

Description: The number of breaths and individual takes per minute


IBW for male and female:

Male: 106 + 6 (height - 60)

Female: 105 + 5 (height - 60)

  • This is for lbs, to convert from lbs to kg, divide by 2.2


Oxygenation formulas:

PAO2: (PB - 47) FiO2 - (PaCO2 × 1.25)

Description: Estimates the partial pressure of O2 in the alveoli.

P(A-a)O2: PAO2 - PaO2

Normal values:

  • On 100% O2; 25-65 mmHg

  • On RA; 10-25 mmHg

Description: Estimates the difference between partial pressure in the alveoli, and the O2 in the arterial blood. It is typically used in order to find the cause of low blood O2 levels.

P(a/A): PaO2/PAO2

Normal values:

  • 0.74-0.90 = Normal

  • < or equal to 0.60 = Mismatch

  • < or equal to 0.15 = Shunt

Description: Estimates the percentage of alveolar O2 that successfully reaches the arterial blood

P/F ratio:

Normal values:

350 - 450 = Normal

300 - 200 = Mild ARDS

200 - 100 = Moderate ARDS

< 100 = Severe ARDS

Description: Estimates whether ARDS is present in an individual


Shunt formulas:

CcO2: (1.34 x Hb) + (PAO2 × 0.003)

Normal value: 20 vol%

Description: Total content of O2 in the pulmonary capillary blood

CaO2: (1.34 x Hb x SaO2) + (PaO2 × 0.003)

Normal value: 16-20 vol%

Description: Total content of O2 in the arterial blood

CvO2: (1.35 x Hb x SvO2) + (PvO2 × 0.003)

Normal value: 12-15 vol%

Description: Total content of O2 in the mixed venous blood

QS/QT: CcO2 - CaO2 / CcO2 - CvO2

Normal values:

  • < 10% = Normal

  • 10 - 20% = Mild

  • 20 - 30% = Moderate

  • > 30% = Severe


Compliance & RAW:

C = V/P

Cstat: VT / PPlat - PEEP

Normal values:

  • 60-100 mL/cmH2O in non-ventilated patient

  • 40-60 mL/cmH2O in ventilated patient

Description: No gas flow is present

Cdyn: VT / PIP - PEEP

Description: Gas flow is present

  • Cdyn will always be lower than or equal to Cstat, because Cdyn includes the extra pulmonary pressure in order to overcome RAW

RAW: (PIP - PPlat) / Flow x 60

Normal values:

  • 0.6-2.6 mL/cmH2O/L/Sec in non-ventilated patient

  • 5-7 mL/cmH2O/L/Sec in ventilated patient

Description: Opposition to airflow caused by friction as air moves through the respiratory tract during breathing


Changing PaCO2:

Desired VE: Known VE x Known PaCO2 / Desired PaCO2

Desired VT: Known VT x Known PaCO2 / Desired PaCO2

Desired RR: Known RR x Known PaCO2 / Desired PaCO2


Changing FiO2:

Desired FiO2: Known FiO2 x Desired FiO2 / Known PaO2


Dead space calculation:

VD/VT: PaCO2 - PETCO2 / PaCO2


Other:

Pack years: Number of packs/day x number of years smoked

Estimating HR from an ECG tracing: HR = 300 / number of large boxes between R waves or HR = 1500 / number of small boxes between R waves

Inspiratory flow: VT (L) / TI in seconds