A 71-year-old woman is evaluated in the intensive care unit on the second day of her admis… — USMLE Step 2 CK

A 71-year-old woman is evaluated in the intensive care unit on the second day of her admission for community-acquired pneumonia that progressed to septic shock. She was intubated and placed on mechanical ventilation upon arrival. The patient is currently receiving an intravenous infusion of norepinephrine and is deeply sedated, triggering no spontaneous breaths.

Blood and sputum cultures obtained at admission have grown *Streptococcus pneumoniae* susceptible to penicillin and ceftriaxone. Her current empiric antimicrobial regimen consists of intravenous piperacillin-tazobactam and vancomycin. Her temperature is 38.4°C (101.1°F), blood pressure is 92/58 mm Hg, and heart rate is 112/min.

Mechanical ventilator settings (assist-control volume-cycled mode) are as follows:

Parameter	Setting
Respiratory rate	$12$ /min
Tidal volume	$450$ mL (Ideal body weight: $60$ kg)
Fraction of inspired oxygen ( $FiO_2$ )	$0.40$
Positive end-expiratory pressure (PEEP)	$5$ cm $H_2O$

Laboratory and arterial blood gas values are obtained:

Test	Value
Sodium	$138$ mEq/L
Potassium	$4.8$ mEq/L
Chloride	$102$ mEq/L
Bicarbonate	$14$ mEq/L
Blood urea nitrogen	$38$ mg/dL
Creatinine	$1.9$ mg/dL
Lactic acid	$4.2$ mmol/L
$pH$	$7.23$
$PaCO_2$	$37$ mm Hg
$PaO_2$	$84$ mm Hg

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

Increase the set respiratory rateAnswer
B
Administer intravenous sodium bicarbonate
C
Maintain the current mechanical ventilator settings
D
Administer intravenous furosemide
E
Transition to intravenous meropenem

Answer

Increase the set respiratory rate

This patient has severe sepsis resulting in a high anion gap metabolic acidosis (

AG = 138 - [102 + 14] = 22

). In metabolic acidosis, normal physiologic compensation involves hyperventilation to decrease

PaCO_2

. Expected

PaCO_2

can be calculated using Winter's formula:

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

. For this patient, the expected

PaCO_2

(1.5 \times 14) + 8 = 29 \pm 2

mm Hg. Because the actual

PaCO_2

37

mm Hg, the patient has a concomitant respiratory acidosis. Since she is deeply sedated and solely dependent on the ventilator, her minute ventilation is fixed at the clinician's settings. A respiratory rate of

12

/min is inadequate to provide the necessary compensatory hyperventilation. The most appropriate next step is to increase her minute ventilation by increasing the set respiratory rate on the ventilator, which will blow off

CO_2

and improve her profound acidemia (

pH

7.23

Step-by-Step Solution

Calculate the anion gap to determine the primary acid-base disorder.

Anion Gap =

138 - (102 + 14) = 22

. Normal is 8-12, indicating a primary high anion gap metabolic acidosis (due to lactic acidosis and acute kidney injury).

Identifying the primary disturbance is required before evaluating for respiratory compensation.

Apply Winter's formula to calculate the expected

PaCO_2

for the given bicarbonate level.

Expected

PaCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2

mm Hg (range

27

31

mm Hg).

This determines whether the patient's respiratory system is appropriately compensating for the metabolic acidosis.

Compare the actual

PaCO_2

to the expected

PaCO_2

The actual

PaCO_2

(

37

mm Hg) is significantly higher than the expected range (

27

31

mm Hg).

An actual

PaCO_2

higher than expected indicates a superimposed primary respiratory acidosis.

Determine the appropriate clinical intervention for a deeply sedated, mechanically ventilated patient with respiratory acidosis.

The set respiratory rate on the ventilator should be increased.

Because the patient is heavily sedated and not triggering spontaneous breaths, her minute ventilation is fixed. To blow off excess

CO_2

and correct the respiratory acidosis, the clinician must increase the minute ventilation, most safely achieved by increasing the respiratory rate.

Key Concept

In patients with severe metabolic acidosis who are mechanically ventilated and deeply sedated, the clinician must manually adjust ventilator settings (e.g., increase respiratory rate) to match the required physiologic respiratory compensation, as the patient cannot spontaneously increase their minute ventilation.

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