Hematology

Question:

A 2-day-old neonate becomes lethargic and uninterested in breast-feeding. Physical examination reveals tachypnea with a normal heart rate and breath sounds. Initial blood chemistry values include normal glucose, sodium, potassium, chloride, and bicarbonate (HCO3–) levels; initial blood gas values reveal a pH of 7.53, partial pressure of oxygen (PO2) of 103 mmHg, and partial pressure of carbon dioxide (PCO2) of 27 mmHg. Which of the following is the most appropriate treatment?

Correct Answer: E.  Decrease the respiratory rate to treat respiratory alkalosis

Tachypnea in term infants may result from brain injuries or metabolic diseases that irritate the respiratory center. The increased respiratory rate removes (“blows off”) carbon dioxide from the lung alveoli and lowers blood CO2, forcing a shift in the indicated equilibrium toward the left:
CO2 + H2O H2CO3 H+ + HCO3-
Carbonic acid (H2CO3) can be ignored because negligible amounts are present at physiologic pH, leaving the equilibrium:
CO2 + H2O H+ + HCO3-
The leftward shift to replenish exhaled CO2 decreases the hydrogen ion (H+) concentration and increases the pH (-log10[H+]) to produce alkalosis (blood pH above the physiologic norm of 7.4). This respiratory alkalosis is best treated by diminishing the respiratory rate to elevate the blood [CO2], force the above equilibrium to the right, elevate the [H+], and decrease the pH. The newborn does not have acidosis, defined as a blood pH below 7.4, either from excess blood acids (metabolic acidosis) or from increased [CO2] (respiratory acidosis). The baby also does not have metabolic alkalosis, caused by loss of hydrogen ion from the kidney (e.g., with defective tubular filtration) or stomach (e.g., with severe vomiting).