Common respiratory equations

Here are some commonly used respiratory equations and a simple calculator.

Alveolar gas equation

PAO2 = (FiO2 × (Patm - PH2O) - (PaCO2 / RQ)

FiO2 Patm PH2O PaCO2 RQ

PaO2/FiO2 Ratio (P/F Ratio)

P/F Ratio = (PaO2 / FiO2)

PaO2 FiO2

Minute Ventilation (VE)

VE = (RR × VT)

RR VT

Alveolar Minute Ventilation(VA)

VA = Respiratory Rate x (Tidal Volume – Deadspace)

RR VT Deadspace

Ideal Body Weight

Females: IBW = (105 + 5 × (height in inches - 60))

Males: IBW = (105 + 6 × (height in inches - 60))

Male Female Height

Rapid Shallow Breathing Index (RSBI)

RSBI = Rate / Tidal Volume

Rate Tidal Volume

FiO2 Estimation for Nasal Cannula

FiO2 = 20 + (4 x Liter Flow)

Liter Flow

Cardiac Index (CI)

CI = Cardiac Output / Body Surface Area

Cardiac Output Body Surface Area

Cardiac Output (QT)

QT = Heart Rate x Stroke Volume

Heart Rate Stroke Volume

Cardiac Output (CO) Fick’s Method

CO = (O2 Consumption / CaO2 – CvO2)

O2 Consumption CaO2 CvO2

Mean Arterial Pressure (MAP)

MAP = (Systolic BP + (2 x Diastolic BP)) / 3

Systolic BP Diastolic BP

Stroke Volume (SV)

SV = Cardiac Output / Heart Rate

Cardiac Output Heart Rate

Maximum Heart Rate (HRmax)

HRmax = 220 – Age

Age

Systemic Vascular Resistance (SVR)

SVR = (MAP – CVP) x (80 / Cardiac Output)

MAP CVP Cardiac Output

Pulmonary Vascular Resistance (PVR)

PVR = (MPAP – PCWP) x (80 / Cardiac Output)

MPAP PCWP Cardiac Output

Static Compliance (Cst)

Cst = Tidal Volume / (Plateau Pressure – PEEP)

Tidal Volume Plateau Pressure PEEP

Dynamic Compliance (Cdyn)

Cdyn = Tidal Volume / (Peak Pressure – PEEP)

Tidal Volume Peak Pressure PEEP

Simple Calculator

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John Slama RRT