Acid-base status

Published on 13/02/2015 by admin

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Last modified 13/02/2015

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Acid-base status

Ian MacVeigh, MD

Recognizing, diagnosing, and treating acid-base disorders are absolutely essential skills for all anesthesia providers. Having a clear understanding of the terminology and physiology related to acid-base disorders and employing standard criteria for the assessment and diagnosis of these often-puzzling derangements results in quicker recognition and more effective treatment of acid-base disorders.

Terminology

Normal values of pH, defined as the negative logarithm of the hydrogen ion concentration [H⁺] (expressed in extracellular fluids in nanoequivalents per liter), range from 7.35 to 7.45. Changes in pH are inversely related to changes in [H⁺]: a 20% increase in [H⁺] decreases the pH by 0.1; conversely, a 20% decrease in [H⁺] increases the pH by a 0.1 (Table 47-1).

Table 47-1

pH for Given Hydrogen Ion Concentrations

pH	[H⁺] (nEq/L)
7.0	100
7.1	80
7.2	64
7.3	50
7.4	40
7.5	30
7.6	24
7.7	20

The terms acidemia and alkalemia refer to the pH of blood. Acidosis, on the other hand, refers to the process that either adds acid or removes alkali from body fluids; conversely, alkalosis is the process that either adds alkali or removes acid from body fluids. Compensation refers to the body’s homeostatic mechanisms that generate or eliminate [H⁺] to normalize pH in response to a pathologic acid-base disturbance. Base excess, an assessment of the metabolic component of an acid-base disturbance, quantifies the amount of acid that must be added to a blood sample to return the pH of the sample to 7.40 if the patient’s PaCO₂ were 40 mm Hg. A positive base excess value indicates that the patient has a metabolic alkalosis (acid would have to be added to the blood to reach a normal pH); a negative value indicates that the patient has a metabolic acidosis and alkali would have to be added to normalize the pH. Blood gas results are, by convention, reported as pH, PaCO₂, PaO₂, HCO₃⁻, and base excess; the latter two are calculated. The HCO₃⁻ is derived using the Henderson-Hasselbalch equation, which can be expressed as either of the following:

< ?xml:namespace prefix = "mml" />pH=pKa+PaCO2HCO3–

[H+]=24×PaCO2HCO3–

Tight control of the pH requires a fairly constant PaCO₂/HCO₃⁻ ratio, which allows one to check the validity of an arterial blood gas (ABG) sample (Table 47-2).

Table 47-2

Examples of the Use of Henderson-Hasselbalch Equation to Calculate H⁺ Concentration with a Known HCO₃⁻ and PaCO₂

PaCO₂	HCO₃⁻	[H⁺] (nEq/L) will be	Calculation [H⁺]	pH
40	24	40	24 × 40/24 = 40	7.4
60	24	60	24 × 60/24 = 60	7.2
20	24	20	24 × 20/24 = 20	7.7
40	16	60	24 × 40/16 = 60	7.2
60	16	90	24 × 60/16 = 90	7.05
20	16	30	24 × 20/16 = 30	7.5

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