Redraw ABG and BMP simultaneously
Primary Respiratory Alkalosis (see Step 2)?
Primary Respiratory Acidosis (see Step 2)?
History and physical examination to determine
cause
History and physical examination to determine cause (Step 5) and treat appropriately (Step 6)
pH decrease more than expected or HCO3− increase less
than expected?
pH increase more than expected or HCO3− decrease less
than expected?
Are the data correct (see Step 1)?
Is the disorder acute or chronic
(see Step 3)?
Is the disorder acute or chronic
(see Step 3)?
ACUTE ACUTE
Possible mixed acid-base disorder present based on
inappropriate pH change and clinical history (see
Step 4)?
Possible mixed acid-base disorder present based on
inappropriate pH change and clinical history (see
Step 4)?
Secondary metabolic acidosis
Secondary metabolic acidosis Secondary
metabolic alkalosis
Secondary metabolic alkalosis CHRONIC
Yes
No
No
Yes
s e Y s
e
Y No No
Yes
ABG, arterial blood gas; BMP, basic metabolic panel.
Acid–Base Disorders rRespiratory Acid–Base Disorders 2 2 5
TABLE 26.2 Causes of Respiratory Alkalosis Normal P(A-a)O2Gradient
Mechanical hyperventilation Drugs
Central nervous system Salicylates
Psychogenic hyperventilation Progesterone
Fever Catecholamines
Pain Hypoxia
Encephalitis/meningitis High altitude
Tumors Severe anemia
Pregnancy Endotoxinemia
Hyperthyroidism Cirrhosis
Elevated P(A-a)O2gradienta
V/Q mismatch Shunt
aThe differential diagnosis of respiratory alkalosis with an elevated alveolar-arterial oxygen gradient (P(A-a)O2) is the same as hypoxia with an elevated P(A-a)O2listed in Chapter 7, Algorithm 7.2.
P[A-a]O2, alveolar-arterial oxygen gradient; V/Q, ventilation/perfusion.
ventilated patients, minute ventilation should be decreased by decreasing the res- piratory rate and/or tidal volume. In psychogenic hyperventilation, reassurance and anxiolytics can be used. In patients at high altitudes, acetazolamide can be used to induce a metabolic acidosis to compensate for the respiratory alkalosis.
Step 1—Obtain an ABG (see section on arterial cannulation) and ensure the [HCO3−] is accurate using the Henderson Equation.a
[HCO3−=24×PaCO2/[H+] H1concentration and corresponding pH
pH [H+mEq/L]
7.10 79
7.20 63
7.30 50
7.40 40
7.50 32
7.60 25
Step 2—Determine the underlying abnormality, that is, a primary respiratory acidosis or alkalosis (metabolic acid–base disorders are discussed separately).
1. A primary respiratory acid–base disorder is present if the PaCO2is altered and the change in pH is in the opposite direction.
aConversely, obtain ABG and basic metabolic panel simultaneously and compare [HCO3−] in the ABG and basic metabolic panel to ensure accuracy. They should agree within 2 mmol/L.
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2 2 6 A C I D – B A S E D I S O R D E R S
a. A respiratory acidosis is present if the PaCO2 is>45 mm Hg and the pH is decreased.
b. A respiratory alkalosis is present if the PaCO2 is<35 mm Hg and the pH is increased.
Step 3—Examine the pH, PaCO2, and serum HCO3−to determine if the respiratory acid–base disorder is acute or chronic.
1. Each acute change in the PaCO2of 10 mm Hg will change the pH 0.08 (or 0.008 for each 1 mm Hg change) in the opposite direction.
2. Each partially metabolically compensated change in the PaCO2of 10 mm Hg will change the pH 0.03 to 0.08 in the opposite direction.
3. Each fully compensated change in the PaCO2of 10 mm Hg will change the pH approximately 0.03,but not back to normal.
4. Table 26.1 shows the expected metabolic compensation for acute and chronic respiratory acid–base disorders, which occurs through changes in serum HCO3−. Deviations in the serum HCO3−that are significantly less than or greater than expected suggest a superimposed metabolic acid–base disorder.
Step 4—Determine if a mixed respiratory and metabolic acid–base disorder is present.
1. A superimposed metabolic acid–base disorder is present if the change in pH ismore than 0.08 in theoppositedirection of each 10 mm Hg change in the PaCO2. a. A metabolic acidosis is superimposed on a respiratory acidosis if the decrease
in pH is greater than expected based on the PaCO2. For example, pH 7.1 and PaCO2 60 mm HG. The expected pH for an acute increase in the PaCO2 to 60 mm Hg is 7.24. Thus, a superimposed metabolic acidosis is present.
b. A metabolic alkalosis is superimposed on a respiratory alkalosis if the increase in pH is greater than expected based on the PaCO2. For example, pH 7.55 and PaCO2 30 mm Hg. The expected pH for an acute decrease in the PaCO2 to 30 mm Hg is 7.48. Thus, a superimposed metabolic alkalosis is present.
2. A combined acidosis and alkalosis is present if the PaCO2is increased or decreased and the pH is normal (∼7.4).
a. As stated, compensation back to normal does not occur. Therefore, a normal pH with an altered PaCO2, regardless of metabolic compensation, supports a superimposed metabolic disorder.
Step 5—Correlate the above steps with the patients’ clinical history and physical exam to arrive at the correct diagnosis.
For example, consider an ABG with pH 7.20, PaCO280, and plasma HCO3−
33 mEq/L. The primary disorder is a respiratory acidosis, evidenced by the decrease in pH and increase in PaCO2 (Step 2). The expected plasma HCO3− would be 28 mEq/L if this were an acute acidosis, or 36 mEq/L if it were chronic (Step 3). If this were an acute change in the PaCO2, the pH would be 7.08 (0.08 change in pH for each 10 mm Hg change in PaCO2). Thus, this cannot be a simple acute respiratory aci- dosis (Step 4). However, there are multiple valid explanations for these ABG findings.
One possibility is that it represents a partially compensated respiratory acidosis. Con- versely, this could represent an acute respiratory acidosis with a superimposed metabolic alkalosis (explaining the higher than normal serum HCO3−). A final possibility is that it represents an acute respiratory acidosis on a compensated chronic respiratory
Acid–Base Disorders rRespiratory Acid–Base Disorders 2 2 7
acidosis. Thus, the patient’s clinical history and physical exam must be incorporated to determine the correct interpretation. If the patient had severe, compensated COPD complicated by acute cardiogenic pulmonary edema, an acute respiratory acidosis on a chronic compensated respiratory acidosis would seem likely.
Step 6—Once the etiology has been determined, treat appropriately (see end of open- ing text).
S U G G E S T E D R E A D I N G S
Epstein SK, Singh N. Respiratory acidosis.Respir Care.2001;46:366–383.
Review of the pathophysiology, evaluation, and causes of respiratory acidosis.
Foster GT, Vazin ND, Sassoon CS. Respiratory alkalosis.Respir Care.2001;46:384–391.
Review of the pathophysiology, evaluation, and causes of respiratory alkalosis.
Kaufman D, Kitching AJ, Kellum JA. Acid-base balance. In: Hall JB, Schmidt GA, Wood LD, eds. Principles of critical care. 3rd ed online. New York: McGraw-Hill, 2002:1202–1208.
Comprehensive textbook review of acid-base disorders.
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