This value varies in proportion with changes in PaCO₂, in contrast with the standard bicarbonate which will not change with changes in PaCO₂, only with changes in acid or base content of the blood sample. Note in the examples below, all calculated on the same sample of blood, how the actual bicarbonate reflects the PaCO₂ changes, while the standard bicarbonate and base excess remain constant.

Care must be taken to use the standard not the actual bicarbonate whenever you attempt to draw conclusions about a baby’s acid–base status from the ‘bicarbonate’.

Base excess

Base excess (BE) is the difference between the ‘buffer base’ of the measured sample and ‘normal buffer base’. BE approximates the amount of acid or base needed to titrate 1 litre of blood to pH 7.4. ‘Acidosis’ and ‘alkalosis’, as we use the terms physiologically, are relative to a pH of 7.4. Blood with pH 7.4 and PaCO₂ 40 mmHg at 100% O₂ saturation has a base excess of zero (0.0). A negative BE (actually a base deficit) indicates metabolic acidosis; and a positive BE indicates a metabolic alkalosis. The BE, therefore, indicates the metabolic contribution to any measured acidosis or alkalosis.

For example, in the first example in Table 5.1 above, with a pH of 7.125, there is only a small metabolic acid contribution to this significant acidosis (BE −5.6). The major contributor to the acidosis is the high PaCO₂ of 80 mmHg (i.e. respiratory acidosis). A contrasting pH of 7.125 with a PaCO₂ of 40 mmHg has no respiratory contribution, and a major metabolic contribution with a BE of −16. A spontaneously breathing newborn baby with this degree of metabolic acidosis would more likely have a PaCO₂ of around 28 mmHg, created by over-breathing in response to the low pH — thus raising the pH to 7.19, a partial ‘respiratory compensation’ for a metabolic acidosis.

Table 5.1 Example calculated values for actual bicarbonate (HCO₃⁻)

Note that the BE also can have a ‘standard’ and an ‘actual’ value, with different connotations from those of bicarbonate. The standard base excess (SBE) applies to the BE of the extracellular fluid, while the actual base excess (ABE) applies to that of the blood only.

Alveolar–arterial difference in partial pressure of oxygen, pO₂(A–a)_e

This is derived from the alveolar gas equation. A large pO₂(A–a) value indicates that there is a lot more oxygen in the alveolus than in the arteries, and the patient has a serious problem getting oxygen into the blood from the alveoli. There is a major ventilation–perfusion (V/Q) mismatch and/or a major gas diffusion problem.

Arterial/alveolar ratio of partial pressures of oxygen, pO₂(a/A)_e (a/A ratio)

This is simply another way of expressing the difference between alveolar (A) and arterial (a) pO₂, by expressing one as a fraction of the other. Here it is printed as a percentage, but may also often be quoted as a decimal fraction (e.g. 0.337 equates to 33.7%).

In clinical studies examining the effectiveness of Exosurf (a synthetic surfactant), an a/A ratio of ≤0.22 was the most commonly used index of severity of disease. An a/A ratio of 0.22 approximates having a PaO₂ of 50 mmHg in 50% oxygen, with a reasonably normal PaCO₂.

Partial pressures of oxygen at 50% saturation, p50(act)_c and p50(st)_c

The pO₂ values at 50% saturation for your actual sample (act) and your sample without fetal haemoglobin (st) tell you whether the patient’s oxygen–haemoglobin dissociation curve is ‘left-shifted’.

Blood oximetry values

These are measured values. The haemoglobin (Hb) is measured, and the oxygen saturation is assessed using the absorbance at multiple wavelengths of light. The cooximeter in the blood gas machine needs to make a correction to its measured values because of its use of multiple wavelengths of light. The machine uses its calculated fetal haemoglobin (HbF) concentration for this correction.

The pulse oximeters we use to monitor babies do not need correction of their values for HbF concentration (hence their usefulness), because they measure at only two wavelengths (660 and 940 nm) where the differences in absorption between fetal and adult haemoglobin of the same saturation are insignificant.

Many preterm infants have a HbF concentration in excess of 90% at birth. Term infants have about 80%. The cHbF drops quickly with transfusion, which is always with adult blood.

Alkalosis

In neonatal practice two causes of metabolic alkalosis are seen:

1. chronic diuretic administration, with hypochloraemia; and

2. compensation for chronic respiratory acidosis.

In the latter, the patient does not actually become ‘alkalaemic’. An example would be pH 7.35, PaCO₂ 80 mmHg — the BE would then be +14.5.

Acidosis

In neonatal practice, many instances of respiratory and metabolic acidosis are seen. In treating these, the best principle is to treat respiratory acidosis with respiratory treatments, and treat metabolic acidosis with metabolic management (e.g. administration of alkali, volume expansion for better perfusion). Many times it is appropriate to accept a respiratory acidosis, particularly in an infant with respiratory disease who is spontaneously breathing. Treatment of respiratory acidosis in a ventilated infant will depend on the underlying lung pathology. The principle is to increase alveolar ventilation. See section on conventional mechanical ventilation, page 37.

Treatment of metabolic acidosis with NaHCO₃ is most efficient in the absence of a concomitant respiratory acidosis, since the reaction

HCO₃⁻ + H⁺ ↔ H₂CO₃ ↔ H₂O + CO₂

must occur for the bicarbonate to ‘mop up’ hydrogen ions. If CO₂ cannot be removed there is little point in administering bicarbonate, e.g. if the PaCO₂ is >60 mmHg.

• When bicarbonate is given, it should be given slowly, at a rate not exceeding 1 mmol/min (i.e. 1 mL/min of 8.4% NaHCO₃).

• No dose should be given over less than 5 minutes.

• Be sure that all bicarbonate has cleared the line before sampling for further blood gas analysis if giving through a UAC (umbilical arterial catheter).

• Never give bicarbonate in the same line as calcium — it makes concrete!

A ‘half-correction’ dose of sodium bicarbonate NaHCO₃ to give for metabolic acidaemia can be calculated from: