Chapter 51: Abnormal Blood Gas

I. PROBLEM

An abnormal blood gas value for a neonate is reported by the laboratory. Blood gas measures pH, PCO₂, and oxygen (O₂), and all the other components (base excess, bicarbonate concentration, and oxygen saturation) are calculated based on the 3 levels measured. Accepted normal values for an arterial blood gas on room air are pH 7.35 to 7.45 (pH varies with age; a pH >7.30 is generally acceptable), PaCO₂ 35 to 45 mm Hg (slightly higher accepted if the blood pH remains normal), and PaO₂ 50 to 95 mm Hg (based on gestational age). (See Table 9–1, for normal range of arterial blood gas values for term and preterm infants.)

II. IMMEDIATE QUESTIONS

1. What component of the blood gas is abnormal?

   1. What is the pH? The pH determines the presence of acidemia or alkalemia. Is it acidic (pH <7.35) or alkalotic (pH >7.45)?

      1. pH is proportional to HCO₃ (base excess)

         1. Metabolic acidosis. Abnormal ↓ in HCO₃ ↓ pH.

         2. Metabolic alkalosis. Abnormal ↑ HCO₃ ↑ pH.

      2. pH is inversely proportional to PCO₂

         1. Respiratory acidosis. Abnormal ↑ PCO₂ ↓ pH.

         2. Respiratory alkalosis. Abnormal ↓ PCO₂ ↑ pH.

   2. What is the PCO₂? Is it increased or decreased? PCO₂ reflects alveolar ventilation and is mediated by the respiratory system, and the only way to remove it is through the lungs. If it is abnormal, it suggests a respiratory disorder.

   3. What is the PaO₂? Is it increased or decreased? Hypoxia (inadequate oxygenation) or hyperoxia (excess supply of oxygen) can occur.

   4. What is the HCO₃? Is it increased or decreased? Bicarbonate is a weak base that is mediated by the renal system as part of the acid-base homeostasis. It is retained or excreted by the kidney to regulate acid-base balance. If bicarbonate is abnormal, it is a metabolic disorder.

2. Is this blood gas value very different from the patient’s previous blood gas determination? If the patient has had metabolic acidosis on the last 5 blood gas measurements and now has metabolic alkalosis, it might be best to repeat the blood gas measurements before initiating treatment. Do not treat the infant based on 1 abnormal gas value, especially if the infant’s clinical status has not changed.

3. How was the sample collected? Blood gas measurements can be reported on arterial, venous, or capillary (heel stick) blood samples.

   1. Arterial blood samples. Best indicator of pH, PaCO₂, and PaO₂. The gold standard of obtaining a blood gas is to obtain one from an indwelling arterial catheter (peripheral or umbilical). Blood gases by intermittent arterial punctures may not accurately reflect the infant’s respiratory status. A sudden decrease in the PaCO₂ and PaO₂ can occur during the puncture. Crying can decrease the PaCO₂, HCO₃, and oxygen saturation. Arterial blood gas reference values per ...