**Question:** Lab values: pH 7.42, paCO2- 25, HCO3- 18. Interpret-

A. Metabolic acidosis
B. Respiratory alkalosis
C. Normal pH
D. Respiratory acidosis

**Correct Answer:** D. Respiratory acidosis

**Core Concept:**

In medical interpretation of laboratory values, understanding the relationship between pH, partial pressure of carbon dioxide (paCO2), and bicarbonate (HCO3-) levels is essential. pH (potential of hydrogen) is a measure of acidity or alkalinity of a solution, with values closer to 7 being neutral (7.0), values below 7 are acidic, and values above 7 are alkaline. PaCO2 represents the partial pressure of carbon dioxide in arterial blood, while HCO3- is a measure of total bicarbonate ions in plasma.

**Why the Correct Answer is Right:**

In this question, we have pH 7.42, paCO2 of 25 mmHg, and HCO3- of 18 mmol/L. The normal pH range is 7.35-7.45, and the normal paCO2 range is 35-45 mmHg. A pH of 7.42 is within the normal range, indicating a stable acid-base balance. A paCO2 of 25 mmHg is outside the normal range, specifically lower than the lower limit (35 mmHg). This is indicative of elevated carbon dioxide levels in the blood, which is characteristic of respiratory acidosis.

**Why Each Wrong Option is Incorrect:**

A. Metabolic acidosis (paCO2 is usually elevated in this condition)
B. Respiratory alkalosis (paCO2 is usually decreased in this condition)
C. Normal pH
D. Respiratory acidosis

**Core Concept**:

Metabolic acidosis typically results from elevated lactate, ketone bodies, or urea levels, leading to a decrease in blood pH. Respiratory alkalosis occurs when there is a decrease in CO2 levels, causing an increase in blood pH.

**Why Each Wrong Option is Incorrect:**

A. Metabolic acidosis: Metabolic acidosis is characterized by increased lactate, ketone bodies, or urea levels, all leading to a decrease in blood pH, which is not observed in this question.
B. Respiratory alkalosis: Respiratory alkalosis results from decreased CO2 levels, leading to increased blood pH, which contradicts the given lab values.
C. Normal pH: The pH of 7.42 indicates an abnormal lab value, not a normal one.
D. Respiratory acidosis: A paCO2 of 25 mmHg is elevated, which is the opposite of the expected decrease in CO2 levels seen in respiratory acidosis.

**Clinical Pearls:**

Respiratory acidosis is a condition where there is an increase in carbon dioxide (CO2) levels, leading to a decrease in pH. This can be seen in conditions like chronic