A 71-year-old woman is evaluated on the medical ward due to acute-onset confusion and prog… — USMLE Step 2 CK

A 71-year-old woman is evaluated on the medical ward due to acute-onset confusion and progressively worsening shortness of breath. She was admitted 3 days ago for a severe exacerbation of chronic obstructive pulmonary disease (COPD). She has a history of ischemic cardiomyopathy (ejection fraction 40%). Sputum cultures collected on admission have just returned positive for *Moraxella catarrhalis*, which is sensitive to amoxicillin. She is currently receiving intravenous ceftriaxone, nebulized bronchodilators, and systemic glucocorticoids.

Current vital signs include a temperature of $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure of $138/85\text{ mm Hg}$ , pulse of $108/\text{min}$ , and respiratory rate of $26/\text{min}$ . On physical examination, the patient is somnolent but arousable to verbal stimuli. Lung auscultation reveals prolonged expiration with bilateral wheezing. The jugular venous pressure is $7\text{ cm H}_2\text{O}$ (normal $<8\text{ cm H}_2\text{O}$ ). Heart sounds are normal. There is no peripheral edema.

Laboratory studies:

Test	Result
Leukocyte count	$9,200/\text{mm}^3$
Serum sodium	$138\text{ mEq/L}$
Serum potassium	$4.5\text{ mEq/L}$
Serum chloride	$96\text{ mEq/L}$
Serum bicarbonate	$14\text{ mEq/L}$
Blood urea nitrogen	$42\text{ mg/dL}$
Serum creatinine	$1.8\text{ mg/dL}$ (baseline $1.1\text{ mg/dL}$ )

Arterial blood gas (room air):

Test	Result
pH	$7.24$
$PaCO_2$	$34\text{ mm Hg}$
$PaO_2$	$58\text{ mm Hg}$

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

Endotracheal intubation and mechanical ventilationCevap
B
Escalation of antibiotics to intravenous piperacillin-tazobactam
C
Administration of intravenous furosemide
D
Administration of a continuous sodium bicarbonate infusion
E
Continuation of current management and repeat arterial blood gas in 2 hours

Cevap

Endotracheal intubation and mechanical ventilation

The correct answer is endotracheal intubation and mechanical ventilation. This patient presents with a high anion gap metabolic acidosis (

pH\ 7.24

HCO_3^-\ 14\text{ mEq/L}

) likely due to uremia or lactic acidosis from tissue hypoperfusion. To determine if her respiratory compensation is adequate, Winter's formula is applied:

Expected\ PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2

. For a bicarbonate of 14, the expected

PaCO_2

29 \pm 2\text{ mm Hg}

. Her actual

PaCO_2

34\text{ mm Hg}

, which is inappropriately high. This indicates a superimposed primary respiratory acidosis due to respiratory muscle fatigue, typical in prolonged, severe COPD exacerbations. Combined with her new-onset somnolence, she is in impending respiratory failure and cannot adequately protect her airway. Because altered mental status is a direct contraindication to non-invasive positive pressure ventilation (NIPPV), invasive mechanical ventilation is the only appropriate life-saving step.

Adım Adım Çözüm

Analyze the arterial blood gas (ABG) to determine the primary acid-base disorder.

The pH is 7.24 (acidemia) and serum bicarbonate is 14 mEq/L (low), indicating a primary metabolic acidosis.

Establishing the primary disturbance is required before assessing the adequacy of physiological compensation.

Calculate the expected respiratory compensation using Winter's formula.

Expected PaCO2 = (1.5 × 14) + 8 = 29 ± 2 mm Hg.

Winter's formula determines the expected PaCO2 if the lungs are adequately compensating for the metabolic acidosis.

Compare the expected PaCO2 with the actual measured PaCO2.

The actual PaCO2 is 34 mm Hg, which is higher than the expected range (27-31 mm Hg), indicating a superimposed primary respiratory acidosis.

An actual PaCO2 higher than the expected PaCO2 reveals that the respiratory system is failing to compensate, defining a mixed acid-base disorder.

Synthesize the acid-base findings with the clinical presentation.

The patient has a mixed metabolic and respiratory acidosis accompanied by acute-onset somnolence.

The combination of failed respiratory compensation and altered mental status indicates profound respiratory muscle fatigue and impending respiratory failure.

Select the most appropriate airway management strategy.

Endotracheal intubation and mechanical ventilation are selected.

Non-invasive positive pressure ventilation (NIPPV) is contraindicated in patients with altered mental status who cannot protect their airway; therefore, invasive mechanical ventilation is required.

Anahtar Kavram

Identifying a mixed acid-base disorder using Winter's formula and recognizing indications for invasive mechanical ventilation in a patient with respiratory muscle fatigue and altered mental status.

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