Pediatric Respiratory Alkalosis Workup

Updated: Dec 22, 2019
  • Author: Mary C Mancini, MD, PhD, MMM; Chief Editor: Michael R Bye, MD  more...
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Workup

Laboratory Studies

A simple step-wise approach proves useful for further workup in patients with respiratory alkalosis, such as the following:

  • Step 1: Prove the presence of respiratory alkalosis by an ABG. A PCO2 less than 35 indicates alveolar hyperventilation. A pH greater than 7.4 is highly suggestive of alkalosis. When both are found, respiratory alkalosis is likely.

  • Step 2: Assess the chronicity of hyperventilation. Reference range HCO3- with a pH greater than 7.45 suggests acute hyperventilation, whereas low HCO3- with a pH of 7.4-7.45 suggests a chronic partially compensated process.

  • Step 3: An arterial-alveolar oxygen gradient within the reference range and a pH greater than 7.4 is consistent with hyperventilation secondary to direct CNS stimulation, with normal lung function.

  • Step 4: Arterial pH less than 7.4 is usually observed with alveolar hyperventilation as compensation for metabolic acidosis (overcompensation for metabolic acidosis is very rare).

  • Step 5: Respiratory alkalosis is likely with hypoxemia with alveolar hyperventilation. However, determining if the alkalosis is caused by the hypoxia or if the hypoxia and the alkalosis are caused by the underlying pulmonary disease is difficult.

Measurement of arterial pH, HCO3-, and PCO2 are crucial. Transcutaneous or end-tidal PCO2 may be used in place of arterial PCO2; however, transcutaneous PCO2 requires normal skin perfusion, and end-tidal pCO2 is useful only in the presence of normal lung function and when no other acid-base disturbance is suspected. Furthermore, the noninvasive tests do not measure the pH.

A detailed history and careful physical examination should indicate an underlying disorder.

Standard nomograms (see image below) help diagnose simple acid-base disorders, despite the following limitations:

  • They describe acid-base status in children with a steady-state condition. Hence, nomograms are not helpful for patients with rapidly changing status.

  • Nomograms lose precision at extremes.

  • Values falling in respiratory alkalosis may overlap with other mixed disorders and ultimately require clinical judgment.

Acid-base nomogram shows confidence bands for simp Acid-base nomogram shows confidence bands for simple acid-base disturbances. Conversion factor is 1 torr = 0.13 kPa.

Hyperventilation syndrome is often considered a diagnosis of exclusion. Physicians must consider other causes before making the diagnosis. However, in the typical patient with a normal alveolar-arterial oxygen gradient with an acute stress, the diagnosis can be made with confidence.

Drug screening may be helpful.

Next:

Imaging Studies

Chest radiography may be indicated.

Ventilation/perfusion imaging, helical chest CT imaging, or CT angiography may be performed if pulmonary embolism is suspected.

CT imaging or MRI of the brain may be indicated if CNS pathology is suspected.

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