A 65-year-old woman is receiving mechanical ventilation in the intensive care unit for acu… — USMLE Step 2 CK

A $65$ -year-old woman is receiving mechanical ventilation in the intensive care unit for acute respiratory failure secondary to community-acquired pneumonia. She was intubated $2$ days ago and is currently sedated and paralyzed with a continuous cisatracurium infusion. She is receiving targeted antibiotic therapy with intravenous ceftriaxone and azithromycin. Current vital signs show a temperature of $38.2^\circ$ C ( $100.8^\circ$ F), blood pressure of $92/64$ mmHg on a norepinephrine infusion, and a pulse of $112$ /min. Auscultation reveals coarse crackles in the right lower lung field.

She is on assist-control volume ventilation with the following settings:

Setting	Value
Tidal volume	$420$ mL ( $6$ mL/kg ideal body weight)
Respiratory rate	$14$ /min
FiO $_2$	$0.50$
PEEP	$8$ cm H $_2$ O

Arterial blood gas analysis reveals:

Parameter	Value
pH	$7.22$
PaCO $_2$	$40$ mmHg
PaO $_2$	$78$ mmHg
HCO $_3^-$	$16$ mEq/L

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

Increase the respiratory rate on the ventilatorAnswer
B
Decrease the respiratory rate on the ventilator
C
Administer intravenous furosemide
D
Add intravenous vancomycin to the current regimen
E
Administer a continuous infusion of sodium bicarbonate

Answer

The most appropriate next step in management is to increase the respiratory rate on the ventilator.

The correct action is to increase the respiratory rate. This patient has a primary metabolic acidosis with an HCO

_3^-

16

mEq/L. According to Winter's formula (Expected PaCO

_2 = 1.5 \times

HCO

_3^- + 8

), her expected PaCO

_2

should be approximately

32

mmHg. Her actual PaCO

_2

40

mmHg, indicating a superimposed respiratory acidosis due to inadequate alveolar ventilation. Since she is chemically paralyzed, she is entirely dependent on the ventilator for her minute ventilation and cannot mount a spontaneous respiratory compensation. Therefore, the clinician must increase her minute ventilation (most safely achieved by increasing the respiratory rate rather than tidal volume, to maintain lung-protective ventilation) to correct the severe acidemia.

Step-by-Step Solution

Identify the primary acid-base disorder.

The arterial blood gas shows acidemia (pH

7.22

) with a low bicarbonate (

16

mEq/L), confirming a primary metabolic acidosis.

Determining the primary disorder is the foundational step in evaluating an arterial blood gas.

Calculate the expected respiratory compensation using Winter's formula.

Expected PaCO

_2

= (

1.5 \times 16) + 8 = 32 \pm 2

mmHg. The acceptable range is

30

34

mmHg.

This calculation determines if the patient's respiratory response is appropriate for the degree of metabolic acidosis.

Compare the actual PaCO

_2

to the expected PaCO

_2

The patient's actual PaCO

_2

40

mmHg, which is significantly higher than the expected maximum of

34

mmHg. This indicates a concomitant respiratory acidosis.

A higher than expected PaCO

_2

in the setting of metabolic acidosis means ventilation is inadequate.

Determine the appropriate intervention for the patient's clinical state.

Because the patient is paralyzed and receiving assist-control ventilation, she cannot spontaneously increase her minute ventilation. The clinician must increase the respiratory rate on the ventilator to blow off excess carbon dioxide.

Mechanical adjustments are required to achieve necessary physiologic compensation in a paralyzed patient.

Key Concept

Patients with severe metabolic acidosis require a compensatory increase in minute ventilation; if they are pharmacologically paralyzed on mechanical ventilation, the clinician must proactively increase the respiratory rate to prevent a superimposed respiratory acidosis.

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