Base Deficit

What is Base Deficit?
For parents with a child suffering from a birth injury there are a bewildering number of laboratory tests and imaging which healthcare providers use to manage the care of a newborn baby.  If a baby has suffered a possible brain injury due to oxygen deprivation during labour and delivery, it is likely that cooling will be recommended by the treating doctors.  Cooling is discussed in more detail in the blog “Therapeutic Hypothermia for the Newborn with HIE”.  The determination of the base deficit is one laboratory test in particular that is important in determining if there was oxygen deprivation during labour and delivery sufficient to cause a brain injury.  This test involves an analysis of the blood drawn from the vein and artery of the umbilical cord shortly after delivery.

Process
The laboratory will analyze these blood samples and measure two important components:
  (1) the relative acidity of the blood (measured by the pH level); and 
  (2) the level of carbon dioxide in the blood (referred to as pCO2).

When these two measurements are known it is then possible to calculate the base deficit.  Base deficit is referred to as a positive number, whereas the mirror image is referred to as base excess which is a negative number.

The importance of base deficit requires an understanding of respiratory and metabolic acidosis. 
 
Respiratory Acidosis
During labor, and specifically during contractions, there is a reduction in the blood flow through the umbilical cord to the fetus which often results in a reduction of the blood flow circulation in the fetus, including the fetal brain.  The decrease in blood flow results in a decrease of oxygen delivery to and disposal of carbon dioxide from the fetal brain cells.  This results in an increase of carbon dioxide in the blood which in turn results in an increase of the blood acidity.  The increase of carbon dioxide increases the acidity in the fetal blood which, in turn, results in a decrease in the pH.  This is referred to as respiratory acidosis.  The medical consensus is that a drop in pH due to respiratory acidosis is not indicative of a decrease in oxygenation to the fetus sufficient to cause a brain injury.

Metabolic Acidosis
In some situations the oxygen supply to the brain is diminished to such an extent that the fetus compensates by chemically producing oxygen.  This process does not result in an increase in the carbon dioxide level, but does result in increased acidosis and thus a decrease in the pH level.  A fetus has the ability to tolerate, to a point, the metabolic increase in blood acidity.  A fetus has a bicarbonate buffer which helps to protect from the increase in acidity.  In simplistic terms, the base deficit measures the depletion of the buffer caused by this metabolic acidosis.  The importance of this is that the medical consensus is that some degree of metabolic acidosis is required for the fetus to suffer a brain injury due to oxygen deprivation. 

Base Deficit Calculation
Thus, there are two processes that result in a decrease in the pH.  One is through an increase in the carbon dioxide, referred to as respiratory acidosis, and the other is through the chemical production of oxygen which does not change the level of carbon dioxide.  A formula was developed to assess the drop in the pH level and how much was due to an increase in the carbon dioxide.  The drop in the pH that was not due to the increase in the carbon dioxide must therefore be caused by metabolic acidosis.  This results in a change to the base deficit.  The larger the base deficit, the more significant the metabolic acidosis is. 

Evidence of Brain Injury
The normal base deficit reading is zero or slightly above.  If a fetus experiences metabolic acidosis, the bicarbonate is used up, creating a deficit (which is the positive number) or an excess (which is the negative number).  The higher the level of metabolic acidosis, the more likely that the fetus has suffered a neurological injury due to oxygen deprivation at or near the time of delivery.  Various guidelines have suggested that a base deficit of +12 or more is strong evidence that any injury suffered by the newborn was due to oxygen deprivation at birth. However, it is certainly possible to have neurological injuries at lower base deficits and, conversely, no neurological injury when the base deficit exceeds +12. 

Conclusion 
One of the most important factors that neonatologists consider in whether to cool a newborn baby is whether there is evidence that the baby suffered a brain injury due to oxygen deprivation during labour and delivery.  The base deficit results provide very important evidence on that very point.  The base deficit (excess) is an important consideration for healthcare providers in determining whether cooling is appropriate and for birth injury lawyers in relation to being able to prove that any brain injury suffered was due to oxygen deprivation at birth.

Written by Joe Miller, QC