Blood gases are helpful to determine the adequacy of respiratory function (oxygenation and ventilation) as well as the baby's acid-base balance.
Blood gases can be taken from the following sites:
Arterial sites - either a peripheral arterial stab or an indwelling arterial line, arterial stabs may be taken from the radial artery (provided there is also a palpable ulnar pulse) or from the brachial artery, although this is in close proximity to the median nerve. Arterial specimens are required to assess pO2. It is always important to note the FiO2 (percentage inspired oxygen) when interpreting blood gases.
Venous sites - (from an intravenous cannula) more accurate from umbilical vein catheter (UVC) or central venous catheter (CVC) than peripheral IV.
Capillary sites - (heel prick) specimens are the least useful, particularly if the baby has decreased perfusion or is cold.
The pH is a negative logarithm of hydrogen ion concentration [H+], normal range 7.35-7.45. Thus a decrease in pH from 7.0 to 6.0 represents a ten-fold increase in [H+].
- pH > 7.45 is an alkalosis.
- pH < 7.35 is an acidosis.
While a pH range of 7.35-7.45 reflects physiologically normal values, the 'clinical' range that is targeted for care may differ (for example, a pH range of 7.25-7.35 may be chosen as a means of targeting the amount of ventilatory support provided).
The pH is proportional to HCO3 (or base excess), therefore:
- An abnormal rise in HCO3 (or base excess) increases the pH (metabolic alkalosis).
- An abnormal fall in HCO3 (or base excess) decreases the pH (metabolic acidosis).
The pH is inversely proportional to pCO2, therefore:
- An abnormal increase in pCO2 decreases the pH (respiratory acidosis).
- An abnormal decrease in pCO2 increases the pH (respiratory alkalosis).
- Many organic acids are produced during normal metabolism. Sometimes they can accumulate in the blood (such as lactic acid).
- The hydrogen ion (H+) may be 'mopped up' by buffers including bicarbonate (HCO3), haemoglobin and phosphate. Bicarbonate is unique because it can be converted to CO2, which can be blown off by the lungs (provided the baby is not in respiratory failure). The following bi-directional equation demonstrates this.
Respiratory acidosis (pCO2 >= 50 mmHg, pH < 7.35)
Respiratory acidosis occurs when the pCO2 is abnormally high.
Causes of respiratory acidosis
- inadequate alveolar ventilation
- depression of the breathing centre in the brain
- upper airway obstruction
- stiffness of the chest wall
- a significant ventilation/perfusion imbalance.
Chronic respiratory acidosis
If the respiratory acidosis is chronic, the body will respond by trying to excrete acid and retain bicarbonate in the urine resulting in a compensatory rise in serum bicarbonate. This will lead to a compensated respiratory acidosis with an elevated base excess.
Treatment of respiratory acidosis
The treatment of a respiratory acidosis is to address the underlying cause and to consider the need for commencing respiratory support or increasing mechanical ventilation. The latter is achieved by either increasing the tidal volume (increasing PIP or decreasing PEEP), or increasing the set tidal volume if the baby is ventilated using a targeted ventilation mode such as 'volume guarantee', or by increasing the respiratory rate. Commencing continuous positive airway pressure (CPAP) or high flow may also improve ventilation and reduce pCO2, but generally only if the lungs are underexpanded.
Respiratory alkalosis (pCO2 < 35 mmHg, pH > 7.45)
Respiratory alkalosis occurs when the pCO2 is abnormally low.
Causes of respiratory alkalosis
- excessive mechanical ventilation or abnormal control of ventilation (such as during hypoxic-ischaemic encephalopathy).
Treatment of respiratory alkalosis
The treatment of a respiratory alkalosis is to wean the mechanical ventilation by first reducing PIP or tidal volume, then respiratory rate.
Metabolic acidosis (HCO3< 18 mmol/L or B.E. < minus 4.0 mEq/L, pH < 7.35)
Metabolic acidosis may occur where there is a rise in free H+ ions and/or a loss of base (HCO3). In this case the anion gap is increased.
A spontaneously breathing baby may compensate for a primary (intracellular or extracellular) metabolic acidosis by over ventilating, although the pH will never become alkalotic (as the baby will never over compensate).
Causes of metabolic acidosis
Causes include the following:
- Lactic acidosis secondary to tissue hypoxia (for example, hypotension, sepsis) or the inability to excrete/buffer accumulated organic acids (for example, protein loading and renal immaturity).
- Excessive loss of HCO3 in the urine or gut. In this case the anion gap is normal. This is a common cause of metabolic acidosis, particularly in the extremely premature infant.
- Metabolic acidosis is rarely due to an inborn error of metabolism.
Treatment of metabolic acidosis
The treatment of a metabolic acidosis is to treat the underlying cause, consider:
- Volume expansion (for example, 10 mL/kg of normal saline) if the baby is thought to be hypovolaemic or to administer NaHCO3 if the metabolic acidosis is severe or refractory (for example, bicarbonate wasting).
- Bicarbonate should not be given if the pCO2 is elevated as the pH will not change (according to the above formula, a metabolic acidosis is merely being replaced by a respiratory acidosis).
Metabolic alkalosis (HCO3 > 25 mmol/L or B.E. > plus 4.0 mEq/L, pH > 7.45)
Metabolic alkalosis occurs where the plasma HCO3 or base excess is abnormally high or there is a loss of metabolic acids.
Causes of metabolic alkalosis
- hypochloraemia (the level of bicarbonate and chloride in plasma are reciprocally related), which may be due to diuretic therapy or upper gastrointestinal obstruction (such as pyloric stenosis) or persistent vomiting
- too much bicarbonate.
Treatment of metabolic alkalosis
The treatment of a metabolic alkalosis is to treat the underlying cause (for example, chloride replacement).
Base excess refers to the amount of base that would have to be added to one litre of the baby's blood at 40 mmHg pCO2 to return the pH to normal. Issues to note:
- This is one way to look at the metabolic component.
- It is a calculated value and will be erroneous if the pCO2 is not normal (for every 10 mmHg rise in pCO2 the base excess will be reduced by 1 mEq/L).
- In these circumstances, the 'metabolic' component of the blood gas should be assessed using the plasma HCO3 level (not the base excess measurement).
Any one of the above four scenarios can occur in isolation, with or without compensation. These are classified as simple acid-base disorders.
When a combination of simple acid-base disturbances occurs, the baby has a mixed acid-base disorder. When there is a mixed disorder, it is sometimes difficult to know which is primary and which is the compensatory component. In such circumstances a helpful principle is that normal physiological processes never over-compensate. The pH can be relatively normal in the following situations:
- respiratory acidosis with metabolic compensation
- metabolic acidosis with respiratory compensation
- metabolic alkalosis with respiratory compensation
- respiratory alkalosis with metabolic compensation.
Respiratory alkalosis with metabolic compensation is extremely unusual in neonates.
The blood gas machine
Details about the machine:
- The blood gas machine measures pH, pCO2 and pO2 and may measure glucose and lactate.
- It calculates HCO3, base excess and oxygen saturation.
- Measurements that are inaccurate, including Hb and oxygen saturation, should not be used to decide therapy (although newer machines contain co-oximetry and are very accurate).
Areas of uncertainty in clinical practice
The main controversy relates to the use of bicarbonate (HCO3) for the treatment of a metabolic acidosis. Please note:
- There is no evidence that the correction of an acute metabolic acidosis improves survival or long-term neurodevelopmental outcome.
- The extra pCO2 produced (in the above equation) can cross cell membranes and paradoxically worsen the intracellular acidosis (as it combines with intracellular water, and with the equation in reverse, produces excess H+, which can't cross back out).
- Sodium bicarbonate is hyperosmolar and, if given rapidly, particularly to premature babies, may cause intraventricular haemorrhage.
- The exact reference ranges for pH, pCO2, HCO3 and base excess will vary from unit to unit.
The above ranges are given for practical demonstration only.
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First published: May 2016
Last web update: 2018
Review by: May 2019
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Page last updated: 21 Nov 2018