## INTRODUCTION

The assessment of an emergency patient's acid–base status must begin with a clinical suspicion that an underlying acid–base disorder exists. That an acid–base disorder might exist in a patient presenting obtunded, hypotensive, hypoperfused, or obviously in extremis is rarely surprising. Patients with more subtle presentations or chronic, well-compensated acid–base disorders often elude clinicians in today's busy and overtasked emergency departments (EDs). One must remain diligent for clinical signs, astutely reviewing basic electrolyte panels, and remain open to the possibility that patients may be or become more ill than they first appear. Knowing when to investigate for the possibility of an acid–base disorder or evaluate for complex mixed acid–base disorders requires astute clinical acumen. Unfortunately, many emergency providers today lack the ability to perform mixed acid–base assessments with facility, and many mixed or complex disorders, no doubt, go undiagnosed or undertreated.

In this chapter, we hope to review the measures of acid–base status routinely available to emergency medicine critical care practitioners, their utility, as well as their liabilities. Using these measures, this chapter will provide a rational guide to the interpretation and initial management of a patient's acid–base status.

## THE HENDERSON–HASSELBALCH EQUATION

The Henderson–Hasselbalch equation in its original form is of limited clinical utility, and is given as follows:

$pH = pk + log[HCO3−][H2CO3]$

The Kassirer–Bleich equation is obtained by inserting known constants into the Henderson–Hasselbalch equation, then taking the antilog of each side.1 The resultant equation is much more conceptually useful in understanding clinical acid–base interactions:

$[H+] = 24 × Pco2[HCO3−]$

The Kassirer–Bleich equation makes clear the interactions between the PCO2, the bicarbonate concentration, and the free hydrogen ion concentration. If any two of these values are known, the other can be calculated.

## MEASURES OF ACID–BASE STATUS

### Serum Bicarbonate

The serum bicarbonate concentration is often one of the first pieces of measured laboratory data available for clinical assessment of acid–base status. Regardless of how it is labeled when reported, this value is actually a measured total CO2 concentration.2 The total CO2 concentration is a combination of bicarbonate, carbonic acid, and dissolved carbon dioxide. The amount of dissolved carbon dioxide can be calculated if the measured PCO2 is known by multiplying the PCO2 by the solubility coefficient of CO2 in the blood, 0.03. Hence:

$“Bicarbonate concentration”=total CO2=[HCO3−]+[H2CO3]+(0.03)(Pco2)$

Most of the time, the relative ...

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