A critical care team evaluates a 64-year-old woman in the surgical intensive care unit. Sh… — USMLE Step 2 CK

A critical care team evaluates a $64$ -year-old woman in the surgical intensive care unit. She was admitted $6$ days ago following an exploratory laparotomy for a perforated duodenal ulcer. Her past medical history is notable for hypertension and mild chronic obstructive pulmonary disease. She remains intubated and mechanically ventilated. Over the past $72$ hours, she has been afebrile and hemodynamically stable without vasopressor support. A nasogastric tube set to low continuous wall suction has drained approximately $1.5\text{ L}$ of greenish fluid daily.

The respiratory therapist initiates a spontaneous breathing trial (SBT) using pressure support ventilation. Thirty minutes into the trial, the patient exhibits a weak respiratory effort with a tidal volume of $250\text{ mL}$ . Capnography shows a gradually rising end-tidal carbon dioxide, and the SBT is aborted.

Current laboratory evaluation reveals:

Laboratory Test	Result
Serum sodium	$138\text{ mEq/L}$
Serum potassium	$2.6\text{ mEq/L}$
Serum chloride	$86\text{ mEq/L}$
Serum bicarbonate	$38\text{ mEq/L}$
Blood urea nitrogen	$48\text{ mg/dL}$
Serum creatinine	$1.2\text{ mg/dL}$

Arterial blood gas (drawn while breathing comfortably on pressure support ventilation just prior to the SBT):
* pH: $7.52$
* PaCO $_2$ : $49\text{ mm Hg}$
* PaO $_2$ : $85\text{ mm Hg}$

Urine chloride is $8\text{ mEq/L}$ .

Which of the following is the most appropriate management to improve her likelihood of successful extubation?

A
Administration of intravenous acetazolamide
Administration of intravenous normal saline and potassium chlorideAnswer
C
Increase the set respiratory rate on the ventilator
D
Administration of intravenous methylprednisolone and inhaled albuterol
E
Initiation of intravenous vancomycin and cefepime

Answer

The most appropriate next step is the administration of intravenous normal saline and potassium chloride to correct the contraction alkalosis.

The patient has a saline-responsive metabolic alkalosis (contraction alkalosis) caused by massive gastric acid loss from nasogastric suctioning. This is confirmed by the high pH (7.52), elevated bicarbonate (38 mEq/L), prerenal azotemia, hypokalemia, and low urine chloride (8 mEq/L). The body compensates for metabolic alkalosis by decreasing the respiratory rate to retain CO2 and lower the pH back toward normal. Therefore, the patient's hypercapnia (PaCO2 49 mm Hg) is an appropriate physiological response. This compensatory hypoventilation suppresses the intrinsic respiratory drive, causing the patient to take shallow breaths and fail the spontaneous breathing trial. To successfully wean this patient, the underlying metabolic alkalosis must be corrected. Intravenous normal saline restores intravascular volume, which shuts off the renin-angiotensin-aldosterone system, while potassium chloride repletes the severe potassium deficit. This allows the kidneys to excrete the excess bicarbonate, normalizing the pH and restoring a normal respiratory drive.

Step-by-Step Solution

Analyze the arterial blood gas and basic metabolic panel.

pH is 7.52 (alkalemia) and HCO3- is 38 mEq/L, confirming a primary metabolic alkalosis. BUN to Creatinine ratio is 40:1, indicating significant volume depletion (prerenal state).

Establishing the primary acid-base disorder and volume status is essential for diagnosing the cause of weaning failure.

Calculate the expected respiratory compensation for the metabolic alkalosis.

Using the formula: Expected PaCO2 = 40 + [0.7 * (Measured HCO3 - 24)]. Expected PaCO2 = 40 + [0.7 * (38 - 24)] = 40 + 9.8 = 49.8 mm Hg. The patient's PaCO2 of 49 mm Hg represents perfect physiological compensation.

This confirms that the hypercapnia is not a primary respiratory acidosis (e.g., from COPD) but an appropriate reflex to lower the pH.

Identify the etiology of the metabolic alkalosis and the reason for the failed spontaneous breathing trial (SBT).

High nasogastric suction output causes loss of HCl and volume contraction (saline-responsive metabolic alkalosis, confirmed by urine Cl < 20 mEq/L). The compensatory hypoventilation to retain CO2 suppresses her intrinsic respiratory drive, causing her to fail the SBT.

Understanding the mechanism allows for targeted therapy to reverse the physiological block to extubation.

Select the appropriate targeted treatment.

Administer intravenous 0.9% NaCl and KCl to restore intravascular volume, replete chloride, and reverse the secondary hyperaldosteronism.

Correcting the metabolic alkalosis will remove the drive for compensatory hypoventilation, allowing the patient to breathe adequately off the ventilator.

Key Concept

Severe metabolic alkalosis causes compensatory hypoventilation, which can manifest as failure to wean from mechanical ventilation. Treatment of saline-responsive (contraction) alkalosis requires volume expansion with isotonic saline and potassium repletion.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic