Equations

Airway resistance equation

R = (PIP - Pplat) / Flow (L/s)

Airway resistance normal values

spontaneous = 0.6-2.4 cm H2O/L/s

intubated = >5 cm H2O/L/s

Static compliance equation

Cs = (corrected Vt) / (Pplat - PEEP)

Static compliance normal values

60-100 mL/cm H2O

Dynamic compliance equation

Cdyn = (corrected Vt) / (PIP - PEEP)

Dynamic compliance normal values

50-100 mL/cm H2O

Alveolar oxygen tension equation when FiO2 < 60% or PaCO2 > 60 mm Hg

PAO2 = (PB - PH2O)(FiO2) - (PaCO2/0.8)

Alveolar oxygen tension equation when FiO2 ≥ 60% or PaCO2 < 60 mm Hg

PAO2 = (PB - PH2O)(FiO2) - PaCO2

Alveolar-arterial tension gradient equation

P(A-a)O2 = PAO2 - PaO2

Alveolar-arterial tension gradient normal values

room air = < 4 mm Hg per every 10 years of age

100% O2 = 2% shunt per every 50 mm Hg difference

Henderson-Hasselbach equation

pH = 6.1 + log [HCO3- / (PaCO2 × 0.03)]

Estimated FiO2 based on L of flow equation

FiO2 = 21 + (L x 4%)

Minute ventilation equation

VE = Vt x f

Minute ventilation normal values

spontaneous = 5-8 L/min

intubated = depends on set variables

Alveolar minute ventilation equation

VA = (Vt - VD) x f

Estimated anatomical dead space equation

VD = kg of BW x (1 mL/kg)

Male IBW equation

IBW = 50 + 2.3 (height - 60)

Female IBW equation

IBW = 45.5 + 2.3 (height - 60)

RSBI equation

RSBI = f / Vt (in L)

RSBI normal values

< 100 breaths/min/L

Mean arterial pressure equation

MAP = [systolic BP + (2 x diastolic BP)] / 3

Mean arterial pressure normal values

> 60 mm Hg

Dead space to tidal volume ratio equation

VD/VT = (PaCO2 - PECO2) / (PaCO2)

Dead space to tidal volume ratio normal values

spontaneous = 20-40%

intubated = 40-60%

Cardiac output equation

CO = (130 x BSA) / (CaO2 - CvO2)

Cardiac output normal values

4-8 L/min

Cardiac index equation

CI = CO / BSA

Cardiac index normal values

2.5-3.5 L/min/m²

Gaseous O2 tank duration

Duration = (PSI x tank factor) / Flow

Liquid O2 tank duration

Duration = (344 x liquid weight in lbs) / Flow

Arterial oxygen content equation

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

Arterial oxygen content normal values

16-20 vol%

Mixed venous oxygen content equation

CvO2 = (1.34 x Hb x SvO2) + (PvO2 × 0.003)

Mixed venous oxygen content normal values

12-15 vol%

Mean airway pressure during constant pressure ventilation equation

Paw = [(f x I-time) / 60] x (PIP - PEEP) + PEEP

Mean airway pressure during constant flow ventilation equation

Paw = 0.5[(f x I-time) / 60] x (PIP - PEEP) + PEEP

Mean airway pressure normal values

<30 cm H2O

Systemic vascular resistance equation

SVR = (MAP - RA) x (80 / CO)

Systemic vascular resistance normal values

800-1500 dyne x sec/cm^5

Pulmonary vascular resistance equation

PVR = (PA - PCWP) x (80 / CO)

Pulmonary vascular resistance normal values

50-150 dyne x sec/cm^5

Stroke volume equation

SV = CO x HR

Stroke volume normal values

40-80 mL

Stroke volume index equation

SVI = SV / BSA

Stroke volume index normal values

33-47 mL/m²

Desired FiO2 equation

Desired FiO2 = (desired PaO2 x known FiO2) / known PaO2

Desired frequency equation

Desired f = (known PaCO2 x known f) / desired PaCO2

Desired tidal volume equation

Desired Vt = (known Vt x known PaCO2) / desired PaCO2

Cerebral perfusion pressure equation

CPP = MAP - ICP

Cerebral perfusion pressure normal values

70 - 80 mm Hg

Intracranial pressure normal values

8-12 mm Hg (technically up to 20 mm Hg)