Chapter 40

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

2. Immediate questions

1. What component of the blood gas is abnormal? Accepted normal values for an arterial blood gas sample are pH, 7.35–7.45 (pH varies with age, a pH >7.30 is generally acceptable), PaCO2, 35–45 mm Hg (slightly higher values accepted if the blood pH remains normal), and PaO2 between 55 and 65 mm Hg on room air. Blood gas measures pH, PCO2, and oxygen (O2), and all the other components (base deficit and bicarbonate) are calculated based on the three levels measured. If one of the components is low (eg, falsely low carbon dioxide [CO2]), this falsely elevates the base deficit.

1. Is this blood gas value very different from the patient's previous blood gas determination? This is a key question. If the patient has had metabolic acidosis on the last five 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.

1. How was the sample collected? Blood gas measurements can be reported on arterial, venous, or capillary blood samples. Arterial blood samples are the best indicator of pH, PaCO2, and PaO2. Venous blood samples give a lower pH value and a higher PCO2 than arterial samples. Capillary samples give a fair assessment of the infant's pH and PCO2 but do not give an accurate PaO2. Capillary samples give a lower pH value (not as low as venous pH) and a slightly higher PCO2 than arterial samples. An accurate capillary blood gas measurement cannot be obtained on an infant who is hypotensive or in shock.

1. 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.

3. Differential diagnosis

1. Metabolic acidosis is defined as a pH <7.30–7.35 with a normal CO2 value and a base deficit >5.

1. Common causes

1. Sepsis.

1. Necrotizing enterocolitis (NEC).

1. Hypothermia or cold stress.

1. Asphyxia.

1. Periventricular-intraventricular hemorrhage.

1. Patent ductus arteriosus (PDA).

1. Shock.

1. Factitious acidosis (excessive heparin in the syringe). Air contamination can give a large base deficit.

1. Drugs (eg, acetazolamide), benzyl alcohol in doxapram, and topical carbonic anhydrase inhibitors (eg, dorzolamide) have been reported to cause metabolic acidosis.

1. Parenteral nutrition.

1. Less common causes

1. Renal tubular acidosis is a defect in the reabsorption of bicarbonate or the secretion of hydrogen ion and can present in three forms: proximal, distal, or mixed.

1. Inborn errors of metabolism. See Table 93–1 for those diseases that present with metabolic acidosis.

1. Maternal use of salicylates and maternal acidosis.

1. Renal failure and renal bicarbonate losses.

1. Congenital lactic acidosis.

1. Gastrointestinal losses such as frequent loose stools and short bowel syndrome.

1. Metabolic alkalosis is defined as a pH value >7.45 ...

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