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Useful Formulae
Thomas M. De Fer
4
INTRODUCTION
•
The most important formula for the intern year:
Sleep (h) =
(discharges + transfers)
× number of interns
(admissions + cross cover)
2
•
Many of these formulae can be found on applications for your
electronic devices or on Web sites.
A-a O
2
GRADIENT
A-a gradient = PA
O
2
−
Pa
O
2
PAO
2
= (FiO
2
× 713)
−
(PaCO
2
/0.8)
(
all units in mm Hg
)
•
Estimate for upper limit of normal in room air (in mm Hg) by
age (years) = (age/4) + 4.
•
Causes of increased A-a gradient: V/Q mismatch, intrapulmonary
right-to-left shunt, intracardiac right-to-left shunt, impaired diffu-
sion (room air only)
ANION GAP (SERUM)
AG = [Na
+
]
−
([Cl
−
] + [HCO
3
–
])
(
all in mmol/L
)
•
Normal = 8 to 12 mmol/L
•
See Chapter 15, Acid-Base Disorders, for differential diagnosis.