46: Abnormal Blood Gas

I. PROBLEM

An abnormal blood gas value for a neonate is reported by the laboratory.

II. IMMEDIATE QUESTIONS

1. What component of the blood gas is abnormal? Accepted normal values for an arterial blood gas on room air are pH 7.35–7.45 (pH varies with age, a pH >7.30 is generally acceptable), Paco₂, 35–45 mm Hg (slightly higher accepted if the blood pH remains normal), and Pao₂ 50–95 mm Hg (depends on gestational age). (See Table 8–1) 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. General blood gas concepts are as follows:

   1. pH is proportional to HCO₃ (base excess)

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

      2. Metabolic alkalosis. Abnormal ↑ HCO₃ with ↑ pH.

   2. pH is inversely proportional to Pco₂

      1. Respiratory acidosis. Abnormal ↑ Pco₂with ↓ pH.

      2. Respiratory alkalosis. Abnormal ↓ Pco₂ with ↑ pH.

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 on the basis of one 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 (heelstick) 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.

   2. Venous blood samples. Give a lower pH value, significantly lower Po₂, and a higher Pco₂ than arterial samples. It is good for HCO₃ estimation,

   3. Capillary (heelstick) samples. Give a satisfactory assessment of the infant's pH and Pco₂ but do not give an accurate Pao₂. Capillary samples give a similar or lower pH value (not as low as venous pH), similar or slightly higher Pco₂, and lower Po₂ than arterial samples; capillary blood gas measurements are not reliable in an infant who is hypotensive or in shock.

4. Is the infant on ventilatory support? Management of abnormal blood gas levels is approached differently in an intubated infant than in a patient breathing room air.

III. DIFFERENTIAL DIAGNOSIS

1. Metabolic acidosis (pH <7.30–7.35 with a normal to low CO₂). After birth it is normal ...