A 64-year-old man is evaluated in the intensive care unit for increasing respiratory effor… — USMLE Step 2 CK

A 64-year-old man is evaluated in the intensive care unit for increasing respiratory effort. He was admitted $48$ hours ago for severe left lower lobe community-acquired pneumonia and sepsis. Blood cultures obtained at admission have since grown *Streptococcus pneumoniae* sensitive to ceftriaxone, and he is currently receiving targeted therapy with intravenous ceftriaxone.

Over the last few hours, he has become increasingly lethargic and is using accessory muscles of respiration. Vital signs are:
- Temperature: $38.1^\circ\text{C}$ ( $100.6^\circ\text{F}$ )
- Blood pressure: $95/60\text{ mm Hg}$
- Pulse: $118\text{/min}$
- Respirations: $34\text{/min}$
- Pulse oximetry: $90\%$ on a high-flow nasal cannula.

Laboratory and arterial blood gas analysis reveals:

Test	Value	Reference Range
pH	$7.17$	$7.35-7.45$
PaCO2	$39\text{ mm Hg}$	$35-45\text{ mm Hg}$
PaO2	$62\text{ mm Hg}$	$75-105\text{ mm Hg}$
HCO3	$14\text{ mEq/L}$	$22-28\text{ mEq/L}$
TSH	$0.4\text{ }\mu\text{U/mL}$	$0.5-5.0\text{ }\mu\text{U/mL}$
Free T4	$0.8\text{ ng/dL}$	$0.9-1.7\text{ ng/dL}$

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

Endotracheal intubation and mechanical ventilationAnswer
B
Administration of intravenous sodium bicarbonate
C
Broadening antibiotic coverage to include intravenous vancomycin and piperacillin-tazobactam
D
Administration of intravenous furosemide
E
Administration of intravenous levothyroxine

Answer

The most appropriate next step is endotracheal intubation and mechanical ventilation.

The correct intervention is endotracheal intubation and mechanical ventilation. This patient is exhibiting signs of impending respiratory failure due to muscle exhaustion. Although his PaCO2 is

39\text{ mm Hg}

(which falls within the normal laboratory reference range of

35-45\text{ mm Hg}

), Winter's formula (

1.5 \times 14 + 8 = 29\text{ mm Hg}

) reveals that his expected PaCO2 should be much lower to adequately compensate for his severe metabolic acidosis. The inappropriately high PaCO2, combined with lethargy, tachypnea, and accessory muscle use, confirms a mixed primary metabolic and respiratory acidosis secondary to respiratory muscle fatigue. Immediate definitive airway management is required.

Step-by-Step Solution

Identify the primary acid-base disturbance.

The patient has severe acidemia (

pH = 7.17

) with a low bicarbonate (

14\text{ mEq/L}

), indicating a primary metabolic acidosis (likely lactic acidosis from sepsis).

Establishing the primary disturbance is required to determine the expected physiological compensation.

Calculate expected respiratory compensation using Winter's formula.

Expected PaCO2

= (1.5 \times 14) + 8 = 29 \pm 2\text{ mm Hg}

(Range:

27-31\text{ mm Hg}

Winter's formula determines if the respiratory system is adequately compensating for the metabolic acidosis.

Compare actual PaCO2 to expected PaCO2.

The actual PaCO2 is

39\text{ mm Hg}

, which is significantly higher than the expected range of

27-31\text{ mm Hg}

An actual PaCO2 higher than expected defines a concomitant primary respiratory acidosis.

Interpret the clinical context of the ABG findings.

A "normal" PaCO2 (

39\text{ mm Hg}

) in a severely tachypneic patient (

34\text{ breaths/min}

) with lethargy indicates diaphragmatic fatigue and impending hypercapnic respiratory arrest.

Clinical signs of fatigue combined with a pseudonormal PaCO2 require immediate intervention.

Select the appropriate clinical intervention.

Endotracheal intubation and mechanical ventilation are indicated to secure the airway and perform the work of breathing.

Non-invasive or pharmacological measures are insufficient for a lethargic patient in acute respiratory failure.

Key Concept

In patients with severe metabolic acidosis, a 'normal' or elevated PaCO2 indicates a superimposed respiratory acidosis due to respiratory muscle fatigue, necessitating immediate mechanical ventilation.

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