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

The correct answer is B: pH will be elevated without an elevation of PCO2. In uncompensated metabolic alkalosis, there is a primary increase in serum bicarbonate leading to elevated pH, but without the compensatory respiratory response that would increase PCO2 1, 2.

Understanding Metabolic Alkalosis

Metabolic alkalosis is characterized by:

• Primary elevation of serum bicarbonate (HCO3-)
• Elevated arterial pH (>7.45)
• In compensated states, a secondary increase in PCO2 due to adaptive hypoventilation 1

However, in the uncompensated state:

• The pH is elevated due to the primary increase in bicarbonate
• The respiratory system has not yet responded with hypoventilation
• Therefore, PCO2 remains within normal range 2

Pathophysiology of Metabolic Alkalosis

The development of metabolic alkalosis involves two key processes:

1. Generation: Either loss of fixed acid (via gastrointestinal tract or kidneys) or accumulation of bicarbonate in the extracellular fluid 1
2. Maintenance: Factors that impair the kidney's ability to eliminate excess bicarbonate 1

Common causes include:

• Vomiting (loss of gastric acid)
• Diuretic therapy (especially chloruretic diuretics)
• Aldosterone excess
• Excessive alkali ingestion 1, 3

Compensation Mechanism

In a fully compensated metabolic alkalosis:

• The respiratory system responds by decreasing alveolar ventilation
• This leads to CO2 retention and increased PCO2
• The increased PCO2 helps offset the alkalosis by forming carbonic acid 2

However, in uncompensated metabolic alkalosis:

• This respiratory compensation has not yet occurred or is inadequate
• PCO2 remains normal while pH is elevated 2

Distinguishing Between Options

Let's analyze each option:

• A: pH will be elevated with an elevation of PCO2 - This describes compensated metabolic alkalosis, not uncompensated
• B: pH will be elevated without an elevation of PCO2 - This correctly describes uncompensated metabolic alkalosis
• C: pH will be normal with a normal PCO2 - This describes a normal acid-base status
• D: pH will be elevated with a decrease in PCO2 - This describes respiratory alkalosis or mixed respiratory and metabolic alkalosis
• E: pH will be normal with an elevation of PCO2 - This describes compensated respiratory acidosis

The key distinction is that in uncompensated metabolic alkalosis, the respiratory system has not yet responded with hypoventilation to increase PCO2 and attempt to normalize the pH 1, 2.