Acid-Base Analysis

This chapter describes how to identify acid-base disorders using the pH, PCO₂ and bicarbonate (HCO₃) concentration in blood. Included are: (a) simple rules for the identification of primary, secondary, and mixed acid-base disorders, (b) formulas for determining the expected acid-base changes for each of the primary acid-base disorders, and (c) a description of the “anion gap” and how it is used.

I. Acid-Base Balance

According to traditional concepts of acid-base physiology, the hydrogen ion (H⁺) concentration in extracellular fluid is determined by the balance between the partial pressure of carbon dioxide (PCO₂) and the bicarbonate (HCO₃) concentration (1):

(k is a proportionality constant). This means that all acid-base disorders are defined by two variables: PCO₂ and HCO₃. This is shown in Table 23.1.

A. Types of Acid-Base Disorders

A respiratory acid-base disorder is a change in [H⁺] that is a
direct result of a change in PCO₂. According to Equation 23.1, an increase in PCO₂ will increase the [H⁺] and produce a respiratory acidosis, while a decrease in PCO₂ will decrease the [H⁺] and produce a respiratory alkalosis.
A metabolic acid-base disorder is a change in [H⁺] that is a direct result of a change in HCO₃. Equation 23.1 predicts that an increase in HCO₃ will decrease the [H⁺] and produce a metabolic alkalosis, while a decrease in HCO₃ will increase the [H⁺] and produce a metabolic acidosis.
Acid base disorders can be primary (the principal disturbance) or secondary (an additional disturbance).

Table 23.1 Acid-Base Disorders and Compensatory Responses

ΔH⁺ = ΔPCO₂ / ΔHCO₃
Acid-Base	Primary	Compensatory
Disorder	Change	Response
Respiratory Acidosis	(↑PCO₂)	(↑HCO₃)
Respiratory Alkalosis	(↓PCO₂)	(↓HCO₃)
Metabolic Acidosis	(↓HCO₃)	(↓PCO₂)
Metabolic Alkalosis	(↑HCO₃)	(↑PCO₂)

B. Compensatory Responses

Compensatory responses are designed to limit the change in H⁺ concentration produced by the primary acid-base disorder. This is accomplished by changing the secondary variable in the same direction as the primary variable (e.g., a primary increase in PCO₂ is accompanied by a compensatory increase in HCO₃), as shown in Table 23.1.
Compensatory responses do not completely correct the change in [H⁺] produced by the primary acid-base disorder (2).
The specific features of compensatory responses are described next. The equations that describe these responses are shown in Table 23.2.

C. Responses to Primary Metabolic Disorders

The response to a metabolic acid-base disorder involves a change in minute ventilation that is mediated by peripheral chemoreceptors in the carotid body, located at the carotid bifurcation in the neck.

1. Response to Metabolic Acidosis

The compensatory response to metabolic acidosis is an increase in minute ventilation (tidal volume and respiratory rate) and a subsequent decrease in arterial PCO₂ (PaCO₂). This response appears in 30–120 minutes, and can take 12 to 24 hours to complete (2). The magnitude of the
response is defined by the equation below (2).

Using a normal PaCO₂ of 40 mm Hg and a normal HCO₃ of 24 mEq/L, the above equation can be rewritten as follows:

EXAMPLE: For a primary metabolic acidosis with a plasma HCO₃ of 14 mEq/L, the ΔHCO₃ is 24 − 14 = 10 mEq/L, the ΔPaCO₂ is 1.2 * 10 = 12 mm Hg, and the expected PaCO₂ is 40 − 12 = 28 mm Hg. If the measured PaCO₂ is >28 mm Hg, there is a secondary respiratory acidosis, and if the measured PaCO₂ is <28 mm Hg, there is a secondary respiratory alkalosis.

2. Response to Metabolic Alkalosis

The compensatory response to metabolic alkalosis is a decrease in minute ventilation and a subsequent increase in PaCO₂. This response is not as vigorous as the response to metabolic acidosis (because the baseline activity of peripheral chemoreceptors is low, so they are easier to stimulate than inhibit). The magnitude of the response is defined by the equation below (2).

Using a normal PaCO₂ of 40 mm Hg and a normal HCO₃ of 24 mEq/L, the above equation can be rewritten as follows:

EXAMPLE: For a metabolic alkalosis with a plasma HCO₃ of 40 mEq/L, the ΔHCO₃ is 40 − 24 = 16 mEq/L, the ΔPaCO₂ is 0.7 * 16 = 11 mm Hg, and the expected PaCO₂ is 40 + 11 = 51 mm Hg.

Table 23.2 Equations for the Expected Response to Primary Acid-Base Disorders

Primary Disorder	Compensatory Response
Metabolic Acidosis	ΔPaCO₂ = 1.2 × Δ HCO₃ Expected PaCO₂ = 40 − [1.2 × (24 − HCO₃)]
Metabolic Alkalosis	Δ PaCO₂ = 0.7 × Δ HCO₃ Expected PaCO₂ = 40 + [0.7 × (HCO₃ − 24)]
Acute Respiratory Acidosis	Δ HCO₃ = 0.1 × Δ PaCO₂ Expected HCO₃ = 24 + [0.1 × (PaCO₂ − 40)]
Acute Respiratory Alkalosis	Δ HCO₃ = 0.2 × Δ PaCO₂ Expected HCO₃ = 24 − [0.2 × (40 − PaCO₂)]
Chronic Respiratory Acidosis	Δ HCO₃ = 0.4 × Δ PaCO₂ Expected HCO₃ = 24 + [0.4 × (PaCO₂ − 40)]
Chronic Respiratory Alkalosis	Δ HCO₃ = 0.4 × Δ PaCO₂ Expected HCO₃ = 24 − [0.4 × (40 − PaCO₂)]
From Reference 2.