Match each clinical presentation with its corresponding primary acid-base disorder.

A previously healthy patient complains of profuse watery stools for several days. Arterial blood gas reveals a pH of $7.28$ and PaCO $_2$ of $32\text{ mm Hg}$ , while the basic metabolic panel shows an anion gap of $10\text{ mEq/L}$ .Normal anion gap metabolic acidosis
During an acute asthma attack, a highly anxious patient presents with a respiratory rate of $32\text{/min}$ . Laboratory analysis demonstrates a pH of $7.52$ , PaCO $_2$ of $26\text{ mm Hg}$ , and a bicarbonate level of $22\text{ mEq/L}$ .Acute respiratory alkalosis
Following $48$ hours of relentless emesis due to suspected food poisoning, a patient's laboratory workup indicates a pH of $7.50$ , a PaCO $_2$ of $46\text{ mm Hg}$ , and a serum bicarbonate of $34\text{ mEq/L}$ .Metabolic alkalosis
In a patient admitted to the intensive care unit with profound hypotension and suspected sepsis, blood tests show a pH of $7.22$ , PaCO $_2$ of $30\text{ mm Hg}$ , and an anion gap of $24\text{ mEq/L}$ .High anion gap metabolic acidosis

Answer

The clinical scenarios correctly match their respective primary acid-base disturbances based on the primary driver of the pH change: bicarbonate loss in diarrhea (normal gap acidosis), CO2 blow-off in asthma (respiratory alkalosis), acid loss in vomiting (metabolic alkalosis), and lactic acid buildup in sepsis (high gap acidosis).

Each presentation classically alters the acid-base balance through a specific physiologic mechanism. Diarrhea causes direct bicarbonate loss (normal AG metabolic acidosis). Hyperventilation from an acute asthma attack blows off CO2 (respiratory alkalosis). Severe vomiting removes gastric acid and volume (metabolic alkalosis). Poor perfusion from septic shock generates systemic lactate (high AG metabolic acidosis).

Step-by-Step Solution

Assess the pH in each scenario to determine if the primary disorder is an acidosis (pH

< 7.35

) or an alkalosis (pH

> 7.45

Identifies the fundamental directional disturbance of the acid-base derangement.

Provides the baseline needed to categorize the primary process before evaluating compensation.

Evaluate the PaCO

_2

and serum bicarbonate levels to identify if the primary etiology is respiratory or metabolic.

Differentiates metabolic conditions (driven by bicarbonate) from respiratory conditions (driven by carbon dioxide).

Determines which physiological system is responsible for the abnormal pH.

For patients presenting with metabolic acidosis, evaluate the calculated anion gap using the formula:

Na^+ - (Cl^- + HCO_3^-)

Categorizes the acidosis as either a normal or high anion gap metabolic acidosis.

Narrows down the differential diagnosis significantly, such as distinguishing diarrhea (normal gap) from lactic acidosis (high gap).

Key Concept

Basic interpretation of primary acid-base disorders requires assessing the pH, identifying the primary alteration in PaCO2 or bicarbonate, and calculating the anion gap in all cases of metabolic acidosis.

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