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Difficulty: MediumPulmonary and Critical Care

A 5555-year-old man with a history of type 1 diabetes mellitus and asthma is brought to the emergency department due to worsening shortness of breath, fever, and a productive cough over the past 33 days. His temperature is 38.8∘C38.8^\circ\text{C} (101.8∘F101.8^\circ\text{F}), blood pressure is 105/65105/65 mm Hg, pulse is 118118/min, and respirations are 3232/min. Pulse oximetry shows 88%88\% on room air. Physical examination reveals the use of accessory muscles of respiration, diffuse expiratory wheezes, and crackles over the right lower lung field. Laboratory studies reveal a blood glucose of 450450 mg/dL and strongly positive serum ketones. Arterial blood gas analysis on room air shows:

VariableValue
pH7.227.22
PaCO2PaCO_23838 mm Hg
PaO2PaO_25858 mm Hg
HCO3−HCO_3^-1212 mEq/L

Which of the following best describes this patient's current acid-base disturbance?

  1. Primary metabolic acidosis with concurrent respiratory acidosisAnswer
  2. B
    Primary metabolic acidosis with appropriate respiratory compensation
  3. C
    Primary metabolic acidosis with concurrent respiratory alkalosis
  4. D
    Primary respiratory acidosis with concurrent metabolic acidosis
  5. E
    Primary respiratory alkalosis with appropriate metabolic compensation

Answer

Primary metabolic acidosis with concurrent respiratory acidosis
The correct answer accurately identifies that this patient has two distinct acid-base disturbances. The primary disorder is a metabolic acidosis, driven by his diabetic ketoacidosis (evidenced by blood glucose of 450450 mg/dL, positive ketones, and a low HCO3−HCO_3^- of 1212 mEq/L resulting in a pH of 7.227.22). When a primary metabolic acidosis is present, the expected compensatory response is hyperventilation to decrease the PaCO2PaCO_2. The adequacy of this compensation is calculated using Winter's formula: Expected PaCO2=(1.5×[HCO3−])+8±2PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2. In this patient, the expected PaCO2=(1.5×12)+8=26±2PaCO_2 = (1.5 \times 12) + 8 = 26 \pm 2 mm Hg (range 2424-2828 mm Hg). The patient's actual PaCO2PaCO_2 is 3838 mm Hg. Because the actual PaCO2PaCO_2 is greater than the expected PaCO2PaCO_2, the patient is relatively retaining CO2CO_2. This indicates a superimposed (concurrent) respiratory acidosis. Clinically, this patient with an asthma exacerbation and pneumonia is tachypneic (3232/min) but is experiencing respiratory muscle fatigue, leading to impending respiratory failure and an inability to adequately blow off CO2CO_2.

Step-by-Step Solution

1
Identify the primary acid-base disorder based on pH and HCO3−HCO_3^-.
The pH is 7.227.22 (acidemia) and the HCO3−HCO_3^- is 1212 mEq/L (low), which indicates a primary metabolic acidosis.
Establishing the primary disturbance is the first step in diagnosing complex acid-base disorders.
2
Calculate the expected respiratory compensation using Winter's formula.
Expected PaCO2=(1.5×[HCO3−])+8±2PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2. For this patient: (1.5×12)+8=26±2(1.5 \times 12) + 8 = 26 \pm 2 mm Hg. The expected range is 2424-2828 mm Hg.
Winter's formula determines if the respiratory response (change in PaCO2PaCO_2) is appropriate for the degree of metabolic acidosis.
3
Compare the actual PaCO2PaCO_2 to the expected PaCO2PaCO_2 to identify any superimposed disorders.
The patient's actual PaCO2PaCO_2 is 3838 mm Hg, which is significantly greater than the expected range of 2424-2828 mm Hg. This relative retention of CO2CO_2 indicates a concurrent respiratory acidosis.
A PaCO2PaCO_2 higher than expected in the setting of metabolic acidosis means the patient is failing to adequately hyperventilate, representing an additional, superimposed respiratory acidosis.

Key Concept

In a patient with primary metabolic acidosis, an actual PaCO2 that is higher than the expected PaCO2 calculated by Winter's formula indicates a superimposed respiratory acidosis, often serving as a sign of impending respiratory failure.
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Topics

Pulmonary and Critical Care

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