Ch04-9781975113285

Useful Formulae

4

Thomas M. De Fer

INTRODUCTION

• The most important formula for the intern year:

(discharges + transfers) (admissions + cross cover) 2

Sleep (h) =

× number of interns

• Many of these formulae can be found on applications for your electronic devices or on Web sites. A-a O 2 GRADIENT

− Pa O

A-a gradient = PA O 2

2

2  × 713) −  (PaCO 2

PAO

 = (FiO

/0.8)

2

( 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.

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