Acid–base disorders are caused by disturbances in hydrogen ion (H+) homeostasis, which is ordinarily maintained by extracellular buffering, renal regulation of hydrogen ion and bicarbonate excretion, and ventilatory regulation of carbon dioxide (CO2) elimination.
Buffering refers to the ability of a solution to resist change in pH after the addition of a strong acid or base. The body’s principal extracellular buffer system is the carbonic acid/bicarbonate (H2CO3/HCO3−) system.
Most of the body’s acid production is in the form of CO2 and is produced from catabolism of carbohydrates, proteins, and lipids.
There are four primary types of acid–base disturbances, which can occur independently or together as a compensatory response.
Metabolic acid–base disorders are caused by changes in plasma bicarbonate concentration (HCO3−). Metabolic acidosis is characterized by decreased HCO3−, and metabolic alkalosis is characterized by increased HCO3−.
Respiratory acid–base disorders are caused by altered alveolar ventilation, producing changes in arterial carbon dioxide tension (Paco2). Respiratory acidosis is characterized by increased Paco2, whereas respiratory alkalosis is characterized by decreased Paco2.
Blood gases (Table 72–1), serum electrolytes, medical history, and clinical condition are the primary tools for determining the cause of acid–base disorders and for designing therapy.
Arterial blood gases (ABGs) are measured to determine oxygenation and acid–base status (Fig. 72–1). Low pH values (<7.35) indicate acidemia, whereas high values (>7.45) indicate alkalemia. The Paco2 value helps determine whether there is a primary respiratory abnormality, whereas the HCO3− concentration helps determine whether there is a primary metabolic abnormality. Steps in acid–base diagnosis and interpretation are described in Tables 72–2 and 72–3.
TABLE 72–1Normal Blood Gas Values |Favorite Table|Download (.pdf) TABLE 72–1 Normal Blood Gas Values
| ||Arterial Blood ||Mixed Venous Blood |
|pH ||7.40 (7.35–7.45) ||7.38 (7.33–7.43) |
|Po2 ||80–100 mm Hg (10.6–13.3 kPa) ||35–40 mm Hg (4.7–5.3 kPa) |
|Sao2 ||95% (0.95) ||70–75% (0.70–0.75) |
|Pco2 ||35–45 mm Hg (4.7–6.0 kPa) ||45–51 mm Hg (6.0–6.8 kPa) |
|HCO3– ||22–26 mEq/L (22–26 mmol/L) ||24–28 mEq/L (24–28 mmol/L) |
Analysis of arterial blood gases (ABGs).
(HCO3−, bicarbonate; Pco2, partial pressure of carbon dioxide.)
TABLE 72–2Steps in Acid–Base Diagnosis
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