Three essential questions to answer in a critically ill patient with acid–base disorder:
- What disorder does the patient have?
- How severe is the disorder?
- What is the underlying etiology?
Basic Terminology and Normal Values
|Alkalemia||Arterial pH >7.45|
|Acidemia||Arterial pH <7.35|
|Alkalosis||Abnormal process or condition that lowers arterial pH|
|Acidosis||Abnormal process or condition that raises arterial pH|
|PaCO2||40 mm Hg (35–45)|
|PaO2||100 mm Hg|
|HCO3||24 mEq/L (22–26)|
|Anion gap (Na − [Cl + HCO3])||8–12|
- Impairment of cardiac contractility
- Arteriolar dilatation
- Attenuation of cardiovascular responsiveness to catecholamines
- Increased sensitivity and decreased threshold for arrhythmias
- Increased pulmonary vascular resistance
- Arteriolar constriction
- Reduction in coronary blood flow
- Decreased threshold for arrhythmias
- Decreased strength of respiratory muscles and promotion of muscle fatigue
- Respiratory failure
- Hypercapnia and hypoxemia
- Inhibition of anaerobic glycolysis
- Insulin resistance
- Reduction in ATP synthesis
- Stimulation of anaerobic glycolysis
- Decreased ionized calcium
- Inhibition of metabolism and cell volume
- Regulation, altered mental status
- Reduction in cerebral blood flow
- Altered mental status
Knowing the clinical scenario is important for a correct interpretation of acid–base abnormalities.
There are two main approaches.
Five steps (see Figure 207-1):
Determine arterial pH (acidemia or alkalemia)
Identify the primary acid–base abnormality (metabolic/respiratory)
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