Correction of Acidosis

Respiratory acidosis is corrected by controlling ventilation. Metabolic acidosis is corrected by infusing sodium bicarbonate. However, bicarbonate should not be administered unless the physician is reasonably sure that the neonate's intravascular volume is appropriate.

Several potentially serious problems are associated with administering sodium bicarbonate:

1. Sodium bicarbonate is hypertonic, about 1800 mOsm/L. If a large volume of sodium bicarbonate is administered rapidly (>1 mEq/kg/min), the intravascular volume may expand rapidly and cause intracranial hemorrhage.
2. The complete reaction of hydrogen ions with 50 mEq of bicarbonate generates approximately 1250 mL of CO₂ . If ventilation is adequate, the CO₂ is quickly exhaled, and the PaCO₂ rises 1 to 3 mm Hg. If ventilation is inadequate, as it is in asphyxiated neonates, the PaCO₂ increases significantly, and because the CO₂ freely diffuses into cells, it may lead to cardiac arrest. Because it dilates cerebral vessels, PaCO₂ increases cerebral blood flow and may cause intracranial hemorrhage. To prevent the rise in PaCO₂ , ventilation should be controlled. Sodium bicarbonate should not be infused to correct metabolic acidosis unless ventilation is adequate.
3. Administering bicarbonate also may induce hypotension ( Fig. 59-8 ). This occurs because acidotic, hypovolemic neonates have intense peripheral vasoconstriction, which preserves their arterial blood pressure. Correcting the acidosis reduces the PVR and induces hypotension because the neonate's blood volume is inadequate to fill the expanded vascular space.
4. Sodium bicarbonate also may interfere with myocardial function, especially if the acidosis for which it was given is ongoing.^{[92] [93]} This is rare clinically in neonates.
5. Until recently, it was believed that the CO₂ produced by the reaction of bicarbonate with hydrogen ions decreased the intracellular pH of brain. However, this is not the case.^[94]

Figure 59-8 The effects of sodium bicarbonate on aortic blood pressure (PAO), heart rate, and hematocrit level. Notice that hypotension occurred after administration of sodium bicarbonate. The hematocrit level decreased as fluid was "pulled" into the intravascular space to compensate for the hypovolemia that was present since birth. Raising the pH decreased the peripheral vasoconstriction produced by the preexisting acidosis. Giving albumin increased the aortic pressure to normal. On the basis of the final hematocrit level, the initial blood volume was approximately 30% less than predicted. (Adapted from Phibbs RH: Problems of neonatal intensive care units. In Lucey JF [ed]: Report on the 59th Ross Conference on Pediatric Research, 1969, Columbus, OH.)

Trishydroxymethylaminomethane (THAM) is an alternative to sodium bicarbonate. Rather than produce CO₂ , THAM binds it in addition to binding the hydrogen ion of fixed acids. This feature of THAM is very beneficial to neonates who already have an elevated PaCO₂ .

Who Requires Alkali Therapy?

Despite the potential problems associated with bicarbonate administration, there are situations in which it is useful. If the Apgar score is 2 or less at 2 minutes or is 5 or less at 5 minutes despite tactile stimulation and controlled ventilation with oxygen, the neonate should be given 2 mEq/kg of sodium bicarbonate while the lungs are being ventilated. Bicarbonate should not be infused into a catheter whose tip rests in the liver, because the hypertonic solution may cause hepatic necrosis. Ventilation should be controlled as the drug is infused. Blood gases and pH should be measured. If the pH is below 7.00, the PaCO₂ is below 35 mm Hg, and the blood volume is adequate, one fourth of the base deficit should be corrected with sodium bicarbonate. If the pH is above 7.10, ventilation of the lungs should be continued, and arterial pH and blood gases should be measured again in 5 minutes. If the pH is 7.15 or higher on the repeat measurement, ventilation of the lungs should continue, and bicarbonate therapy should be delayed. If the repeat blood gas determination shows a decrease or no change

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Figure 59-9 The effects of the rapid infusion of sodium bicarbonate on PaO₂ and pH when ventilation was held constant. The PaO₂ increased when the pH increased above 7.10 to 7.20. (Adapted from Gregory GA: Resuscitation of the newborn. Anesthesiology 43:225, 1975.)

Metabolic acidosis occurs when tissue perfusion is poor. At birth, underperfusion is usually caused by hypovolemia or heart failure (i.e., congenital heart disease, including congenital bradycardia and severe acidosis). Heart failure usually occurs when the pH is below 7.00. Raising the pH to 7.15 or higher usually improves cardiac output. As a result of this increase, liver perfusion increases and metabolic acids are metabolized. If heart failure is caused by cardiac disease (e.g., congenital bradycardia from arrhythmias, erythroblastosis, congenital cardiac anomalies), the cardiac output should be increased

TABLE 59-3 -- Relationships of skin color, capillary refill time, pulse volume, and extremity temperature to hypovolemia

Amount of Volume Depletion (%)	Skin Color	Capillary Refill Time (sec)	Posterior Tibial Pulse Volume	Skin Temperature
None	Pink	<2	Full	Warm
5	Pale	3–4	Moderately full	Cold from midcalf and midforearm out
10	Gray	4–5	Markedly diminished or absent	Cold midthigh and upper arm out
15	Mottled	>5	Markedly diminished or absent	Entire extremity cold

with a continuous infusion of isoproterenol (starting with 0.05 µg/kg/min and increasing the dose as necessary), or a transvenous pacemaker should be inserted. In these patients, cardiac output is usually best when the heart rate is raised to 160 to 190 beats/min. Hypoglycemia may also cause heart failure, but hypoglycemia is uncommon in asphyxiated neonates. If it is low, the blood glucose concentration should be increased to normal (50 to 90 mg/dL) by infusing 5 mL/kg of 10% dextrose in water over 3 to 5 minutes. This infusion should be followed with a continuous infusion of enough glucose to maintain a normal glucose concentration. Blood glucose concentration should be measured 5 minutes later to determine if the glucose concentration is normal. If the acidosis is caused by hypovolemia, as it usually is, the blood volume should be expanded before bicarbonate is administered.