Age-specific normal acid–base values in children

Acid–base values change slightly from the moment of birth until adulthood (Table 9.1). The first hour after birth is characterized by a mixed (predominantly respiratory) acidosis which steadily normalizes in the normal neonate. Preterm neonates may have a lower HCO₃ and lower PaCO₂ for the first month to 6 weeks of life.

Acid–base diagram

Figure 9.1 can be used to elucidate the nature of an acid–base disturbance by plotting pH against PCO₂ or HCO₃.

Fig 9.1 An example of an acid–base chart.

Universal acid–base formula

There is also a ‘rule-of-thumb’ formula that may be used to rapidly assess acid–base status:

Where Δ = change, ≈ = approximately equal to, PaCO₂ = arterial partial pressure of carbon dioxide (mmHg), HCO₃ = serum bicarbonate (mmol/L) and K⁺ = serum potassium (mmol/L).

This predicts that a change in pH of 0.1 may be caused by a change in PaCO₂ of 12 mmHg or a change in HCO₃ of 6 mmol/L. Similarly, changes in PaCO₂ or HCO₃ of these amounts will be required to compensate for a change in pH. Potassium will also rise by 0.3 to 0.5 mmol/L for each 0.1 fall in pH and vice versa.

There are more specific equations to predict the expected values for compensation in acid–base derangement (Box 9.1).

Anion gap

Where AG = anion gap (mmol/L), Na⁺ = serum sodium (mmol/L), HCO₃⁻ = serum bicarbonate (mmol/L) and Cl⁻ = serum chloride (mmol/L).

The upper limit of normal for the anion gap is 12 mmol/L. Some formulas include K⁺ on the cationic side of the equation, in which case the upper limit of normal is slightly higher (16 mmol/L).

Sodium deficit in hyponatraemia

Where NS = volume (mL) of 0.9% normal saline to administer to correct Na⁺ deficit, wt = weight (kg), desired Na⁺ = the minimum level required (normally 130 mmol/L), and rate of administration = (mL/hr) based on a maximum infusion of 0.6 mmol/kg/hr.

Parkland formula

Where fluid requirement (mL) = the total fluid volume to infuse (over and above normal maintenance fluids): half in the first 8 hours from the time of the burn and the second half in the subsequent 16 hours; wt = weight in kg and %BSA = burn surface area as a percentage of total body surface area (see Fig. 1.7).

Acute lung injury ratio

Where PaO₂ = partial pressure of oxygen (mmHg) and F_iO₂ = fraction of inspired oxygen (in decimal form, e.g. 0.4).

This is also known as the P/F ratio. A value of <300 indicates the presence of acute lung injury and a value of <200 the presence of acute respiratory distress syndrome (ARDS).

Oxygen index

Where F_iO₂ = fraction of inspired oxygen (in percentage form, e.g. 40%), MAP = mean airway pressure (mmHg) and PaO₂ = partial pressure of oxygen (mmHg).

An oxygen index >40 suggests the need for extracorporeal membrane oxygenation techniques in appropriately selected neonates.

Estimated minute ventilation requirements

This formula allows for the estimation of ventilation requirements of the critically ill child (based on metabolic factors) to maintain a normal PaCO₂:

Where MV = minute ventilation (L/min), wt = weight (kg) and RR = ventilator respiratory rate.

This formula allows for the determination of the estimated required ventilation rate for children with a tidal volume of 6 mL/kg, based on expected metabolic requirements.

Predicted peak expiratory flow rate

Where PEFR = peak expiratory flow rate (L/min) and ht = height (cm).

This formula approximates the 50th centile for peak expiratory flow rate in children taller than 100 cm. More precise values should be obtained from tables or electronic applications as soon as time permits (Table 9.2).

Endotracheal tube size and length

Where ETT = endotracheal tube internal diameter (mm) and Age = age in years. For infants less than a year use a 3.5 mm ETT; for children between 1 and 2 years of age a 4 mm ETT; after 2 years use the formula above.

For infants less than a year use 3.0 mm ETT; for children between 1 and 2 years of age a 3.5 mm ETT; after 2 years use the formula above.

Minimum systolic blood pressure