A 26-year-old woman is brought to the emergency department after being found unresponsive.… — USMLE Step 2 CK

A $26$ -year-old woman is brought to the emergency department after being found unresponsive. She has a history of type 1 diabetes mellitus. On presentation, she is lethargic and oriented only to person. Temperature is $36.8^\circ\text{C}$ ( $98.2^\circ\text{F}$ ), blood pressure is $90/60\text{ mm Hg}$ , heart rate is $135/\text{min}$ , and respirations are $40/\text{min}$ and deep.

Initial laboratory and arterial blood gas (on room air) results show:

Parameter	Value
Sodium	$138\text{ mEq/L}$
Potassium	$5.2\text{ mEq/L}$
Bicarbonate	$6\text{ mEq/L}$
Glucose	$850\text{ mg/dL}$
pH	$7.17$
PaCO2	$17\text{ mm Hg}$
PaO2	$105\text{ mm Hg}$

The patient is started on aggressive intravenous fluid resuscitation and an insulin infusion. One hour later, her respiratory effort diminishes significantly, and she becomes completely unresponsive (Glasgow Coma Scale score of 6). She undergoes rapid sequence intubation with etomidate and rocuronium.

Following intubation, she is placed on volume-assist control mechanical ventilation with the following settings:
- Tidal volume: $450\text{ mL}$ ( $6\text{ mL/kg}$ ideal body weight)
- Respiratory rate: $14/\text{min}$
- FiO2: $100\%$
- PEEP: $5\text{ cm H}_2\text{O}$

Twenty minutes later, the ventilator alarms for low blood pressure. The patient's blood pressure has dropped precipitously to $62/40\text{ mm Hg}$ , and her heart rate is $148/\text{min}$ . A repeat arterial blood gas reveals:

Parameter	Value
pH	$6.84$
PaCO2	$36\text{ mm Hg}$
PaO2	$320\text{ mm Hg}$
Bicarbonate	$6\text{ mEq/L}$

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

Increase the set respiratory rateAnswer
B
Administer a bolus of intravenous sodium bicarbonate
C
Initiate an intravenous infusion of norepinephrine
D
Decrease the set tidal volume to $300\text{ mL}$
E
Administer broad-spectrum intravenous antibiotics

Answer

The correct answer is to increase the set respiratory rate.

The correct answer is to increase the set respiratory rate. This patient with severe diabetic ketoacidosis presented with a profound metabolic acidosis (bicarbonate

6\text{ mEq/L}

). Prior to intubation, she was maintaining an appropriate compensatory respiratory alkalosis (Kussmaul respirations) with a PaCO2 of

17\text{ mm Hg}

, keeping her pH at a viable

7.17

. Following intubation and paralysis, she became completely dependent on the ventilator. The standard ventilator settings (respiratory rate of

14/\text{min}

, tidal volume of

450\text{ mL}

) provided a typical minute ventilation of approximately

6.3\text{ L/min}

. While appropriate for most patients, this is grossly inadequate for someone requiring massive compensatory hyperventilation. Consequently, her PaCO2 rapidly rose from

17\text{ mm Hg}

36\text{ mm Hg}

. Although

36\text{ mm Hg}

is within the conventional normal range, for this patient, it represents an acute, severe superimposed respiratory acidosis, causing her pH to plummet to

6.84

. Severe acidemia (pH

< 7.1

) causes profound negative inotropy and systemic vasodilation (acidemic shock). The most critical, immediate life-saving step is to increase her minute ventilation by increasing the set respiratory rate to match her pre-intubation compensatory state, thereby lowering the PaCO2 and restoring her pH.

Step-by-Step Solution

Analyze the patient's pre-intubation acid-base status and compensatory mechanism.

The patient presented with severe metabolic acidosis (bicarbonate

6\text{ mEq/L}

) but maintained a viable pH of

7.17

through massive compensatory hyperventilation, driving her PaCO2 down to

17\text{ mm Hg}

Understanding the baseline physiology is essential to identifying the cause of deterioration after mechanical ventilation is initiated.

Compare the post-intubation arterial blood gas to the pre-intubation values.

Following intubation, the PaCO2 rapidly doubled from

17\text{ mm Hg}

36\text{ mm Hg}

, causing the pH to plummet from

7.17

6.84

. This represents an acute, iatrogenic respiratory acidosis superimposed on the underlying metabolic acidosis.

The standard ventilator settings provided a 'normal' minute ventilation, which is grossly inadequate for a patient requiring massive hyperventilation to compensate for DKA.

Correlate the acid-base derangement with the sudden hemodynamic collapse.

The profound drop in pH to

6.84

directly caused myocardial depression and systemic vasodilation, resulting in acute hypotensive shock.

Severe acidemia (pH

< 7.1

) significantly impairs cardiovascular function and diminishes the efficacy of endogenous catecholamines.

Determine the most appropriate immediate intervention to reverse the shock.

The minute ventilation must be drastically increased by raising the set respiratory rate on the ventilator.

Increasing the respiratory rate will blow off the retained CO2, returning the PaCO2 to the patient's compensatory target (

\sim17\text{ mm Hg}

), thereby correcting the life-threatening acidemia.

Key Concept

Mechanical ventilation in patients with severe metabolic acidosis requires setting a high minute ventilation (via a high respiratory rate) to match their pre-intubation compensatory respiratory alkalosis; providing 'normal' ventilation causes rapid, life-threatening acidemic shock.

Estimated Time:2m 0s

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