A 68-year-old woman with a history of heart failure with preserved ejection fraction (HFpE… — USMLE Step 2 CK

A $68$ -year-old woman with a history of heart failure with preserved ejection fraction (HFpEF) and hypertension is brought to the emergency department by her daughter due to $4$ days of profound, watery diarrhea and generalized weakness. She reports having up to $10$ to $12$ bowel movements a day since returning from a cruise.

Vital signs are as follows:
- Temperature: $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ )
- Heart rate: $122/\text{min}$
- Blood pressure: $82/50$ mmHg
- Respiratory rate: $26/\text{min}$
- Oxygen saturation: $96\%$ on room air

Physical examination reveals a lethargic patient. Her mucous membranes are extremely dry, and capillary refill time is $4$ seconds. Jugular venous pressure is undetectable. Lungs are clear to auscultation bilaterally. Cardiac examination reveals regular tachycardia with no murmurs, rubs, or gallops. The abdomen is soft, non-distended, and has hyperactive bowel sounds with mild, diffuse tenderness to deep palpation. There is no guarding or rebound tenderness.

Laboratory results are as follows:

Laboratory Test	Result	Reference Range
Sodium	$137$ mEq/L	$136-145$ mEq/L
Potassium	$2.6$ mEq/L	$3.5-5.0$ mEq/L
Chloride	$115$ mEq/L	$98-106$ mEq/L
Bicarbonate	$12$ mEq/L	$22-28$ mEq/L
Blood Urea Nitrogen (BUN)	$54$ mg/dL	$8-20$ mg/dL
Creatinine	$2.2$ mg/dL	$0.6-1.2$ mg/dL
Lactic acid	$1.8$ mmol/L	$0.5-2.2$ mmol/L

Arterial blood gas (on room air):
- pH: $7.25$
- $pCO_2$ : $26$ mmHg
- $pO_2$ : $92$ mmHg

Which of the following is the most appropriate next step in management?

Intravenous administration of isotonic saline and potassium chlorideAnswer
B
Endotracheal intubation and mechanical ventilation
C
Intravenous administration of furosemide and non-invasive positive pressure ventilation
D
Intravenous administration of broad-spectrum antibiotics
E
Abdominal radiograph and nasogastric tube decompression

Answer

Intravenous administration of isotonic saline and potassium chloride

This patient is presenting with severe hypovolemic shock, prerenal acute kidney injury, severe hypokalemia, and a normal anion gap metabolic acidosis (NAGMA), all resulting from profound gastrointestinal fluid and bicarbonate losses (likely traveler's diarrhea or norovirus). The most urgent and appropriate next step is aggressive fluid resuscitation with intravenous isotonic saline to restore intravascular volume and organ perfusion, along with potassium chloride to correct the life-threatening hypokalemia. Her normal lactic acid level and clear lungs further support that her shock is purely hypovolemic rather than septic or cardiogenic.

Step-by-Step Solution

Interpret the clinical presentation and vital signs.

The patient presents with severe hypotension (

82/50

mmHg), tachycardia, and dry mucous membranes in the setting of profound diarrhea, indicating severe hypovolemic shock.

Establishing the patient's hemodynamic status is the first priority to guide immediate life-saving interventions.

Analyze the basic metabolic panel and calculate the anion gap.

Anion Gap =

Na^+ - (Cl^- + HCO_3^-) = 137 - (115 + 12) = 10

. The patient has a normal anion gap metabolic acidosis (NAGMA), severe hypokalemia (

2.6

mEq/L), and prerenal acute kidney injury (BUN:Cr ratio

> 20

Identifying the nature of the metabolic derangement confirms the etiology (bicarbonate loss from diarrhea) and highlights electrolyte deficits that require immediate replacement.

Assess respiratory compensation using Winter's formula.

Expected

pCO_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2

. The patient's actual

pCO_2

26

mmHg, indicating perfectly compensated metabolic acidosis.

This calculation confirms that the patient's tachypnea is an appropriate physiologic response and not a sign of impending primary respiratory failure.

Determine the most appropriate next step in management.

The immediate administration of intravenous isotonic saline and potassium chloride.

Aggressive volume resuscitation and electrolyte correction are the mainstays of therapy for hypovolemic shock secondary to severe acute gastroenteritis.

Key Concept

Management of severe hypovolemic shock, severe hypokalemia, and normal anion gap metabolic acidosis secondary to acute gastroenteritis.

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