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Contents

Symbols 

Chapter:
Nomenclature
Author(s):

Harriet S. Meyer

Symbols

Symbols and their subgrouping into main symbols and modifiers are consistent with approved nomenclature formulated circa 1980 by the Commission of Respiratory Physiology (International Union of Physiological Sciences) and the Publications Committee of the American Physiological Society.2,3 The following groupings of pulmonary-respiratory symbols are adapted from Fishman.2

Main symbols are typically capital letters set on the line and are the first elements of an expression. The same letter may stand for one entity in respiratory mechanics and another in gas exchange (eg, P stands for pressure in respiratory mechanics and partial pressure in gas exchange). The following are examples (note dots above some letters to indicate flow):

C

compliance, concentration

D

diffusing capacity

F

fractional concentration in a dry gas

P

pressure, partial pressure

Q

volume of blood

Q˙

perfusion (volume of blood per unit time or blood flow)

R

resistance, gas (respiratory) exchange ratio

S

saturation

sG

specific conductance

V

volume of gas

v˙

ventilation (volume per unit time)

Modifiers are set as small capitals (not subscript):

a

alveolar

b

barometric

ds

dead space

e

expired, expiratory

et

end-tidal

i

inspired, inspiratory

l

lung

t

tidal

Lowercase-letter modifiers (which are not subscript) follow small-capital modifiers, if both appear; note bar in last term:

a

arterial

aw

airway

b

blood

c

capillary

c′

pulmonary end-capillary

i

ideal

max

maximum

p

pulse oximetry

v

venous

v¯

mixed venous

Gas abbreviations are usually the last element of the symbol, given as small capitals:

co

carbon monoxide

co2

carbon dioxide

n2

nitrogen

o2

oxygen

(Note: At other times, when gas abbreviations are used on their own, large capitals are used, eg, carbon monoxide [CO].)

The main symbols and modifiers are combined in various ways to derive terms; common examples are the following:

Term

Expansion

Typical Units of Measure2,4-8

Pco2

partial pressure of carbon dioxide

mm Hg or kPa

Paco2

partial pressure of carbon dioxide, arterial

mm Hg or kPa

Po2

partial pressure of oxygen

mm Hg or kPa

Pao2

partial pressure of oxygen, arterial

mm Hg or kPa

(Note: The above 4 terms may be given without expansion at first mention; see also 14.11, Abbreviations, Clinical, Technical, and Other Common Terms, and 18.0, Units of Measure.)

Term

Expansion

Typical Units of Measure2,4-8

Pao2

partial pressure of oxygen, alveolar

mm Hg or kPa

Pv¯O2

partial pressure of oxygen, mixed venous

mm Hg or kPa

Pb

barometric pressure

mm Hg or kPa

Pao2 - Pao2

alveolar-arterial difference (or gradient) in partial pressure of oxygen (preferred to AaDo2)

mm Hg or kPa

Cao2

oxygen concentration (or content), arterial

mL/dL or mmol/L

Cc′o2

oxygen concentration (or content), pulmonary end-capillary

mL/dL

Cl

lung compliance

L/cm H2O or L/mm Hg or L/kPa

Dlco

diffusing capacity of lung for carbon monoxide

mL·min−1·mm Hg−1

Fen2

fractional concentration of nitrogen in expired gas

fraction

Fio2

fraction of inspired oxygen

fraction

Pemax

maximum expiratory pressure

cm H2O or mm Hg

Pimax

maximum inspiratory pressure

cm H2O or mm Hg

Raw

airway resistance

cm H2O·L−1 ·s−1 or kPa ·L−1 ·s−1

Sao2

arterial oxygen saturation

%

sGaw

specific airway conductance

L · s−1 · cm H2O−1 or L · s−1 · kPa−1

Spo2

oxygen saturation as measured by pulse oximetry

%

Vds

volume of dead space

mL or L

V˙E

expired volume per unit time

L/min

V˙O2

oxygen consumption

mL/min or L/min or mmol/min

V˙o2max

maximum oxygen consumption

mL/min or L/min or mmol/min

V˙/Q˙

ventilation perfusion ratio (also V˙A/Q˙)

ratio

Vt

tidal volume

mL or L

*Note: Sometimes quantities are given per unit body weight, eg, Vt in liters per kilogram.

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