What is the characteristic blood pH and PCO2 (partial pressure of carbon dioxide) level in uncompensated metabolic alkalosis?

Uncompensated Metabolic Alkalosis: pH and PCO2 Characteristics

In uncompensated metabolic alkalosis, pH will be elevated without an elevation of PCO2 (option B is correct).

Understanding Acid-Base Disorders and Compensation

Metabolic alkalosis is characterized by a primary increase in serum bicarbonate (HCO3-) concentration, which leads to an elevation in blood pH above 7.45 1. In a normal physiological response to metabolic alkalosis, the body attempts to compensate through respiratory mechanisms:

• Compensated state: The body responds to metabolic alkalosis by hypoventilation, which increases PCO2 levels to partially offset the alkalemia 2
• Uncompensated state: When this compensatory mechanism is absent or inadequate, the PCO2 remains normal while pH remains elevated 1

Pathophysiology of Uncompensated Metabolic Alkalosis

In uncompensated metabolic alkalosis:

• The primary disturbance is an increase in serum bicarbonate concentration
• The respiratory system has not yet responded with hypoventilation
• The PCO2 remains within normal range (35-45 mmHg)
• The pH remains elevated (>7.45) due to the lack of respiratory compensation 3

This differs from compensated metabolic alkalosis, where:

• The respiratory system responds with hypoventilation
• PCO2 rises as a compensatory mechanism
• The pH moves closer to normal range due to this compensation 2

Distinguishing Between Options

Let's analyze each option:

• A: pH elevated with elevated PCO2 - This describes compensated metabolic alkalosis, not uncompensated
• B: pH elevated without elevated PCO2 - This correctly describes uncompensated metabolic alkalosis
• C: pH normal with normal PCO2 - This describes a normal acid-base status, not alkalosis
• D: pH elevated with decreased PCO2 - This describes a respiratory alkalosis or mixed disorder
• E: pH normal with elevated PCO2 - This describes compensated respiratory acidosis, not metabolic alkalosis

Clinical Implications

Understanding the difference between compensated and uncompensated metabolic alkalosis is crucial for:

1. Determining the acuity of the condition (uncompensated states are typically more acute)
2. Identifying potential complications (severe metabolic alkalosis with pH ≥7.55 is associated with increased mortality) 3
3. Guiding appropriate management strategies

Common Causes of Metabolic Alkalosis

Metabolic alkalosis typically results from:

• Gastrointestinal hydrogen and chloride loss (vomiting)
• Renal causes (diuretic use, hypokalemia)
• Excess alkali intake
• Aldosterone or cortisol excess 1, 2

For metabolic alkalosis to persist, both generation and maintenance factors must be present, with the kidneys failing to excrete excess bicarbonate appropriately 3.