Actual blood gases from a patient that I took care of two nights ago.
The first thing I look at is the Base Excess (BE). BE is a calculated value:
BE (meq/l) = HCO3 (observed) + 10 [pH (observed) -7.40] – 24
If it is negative there is acidosis. If it is positive then there is alkalosis (rarely if ever seen in a trauma patient). This patient arrived with a BE of -13, severe acidosis. In trauma, there are usually two contributors to this kind of BE, lactic acidosis and hyperchloremia. In the first ABG (far left) I would have expected the BE to be closer to -7. One unit owing to chloride 107 (upper limit of normal is 106) and 6 units owing to the lactate of 6. I can not explain the -13 unless the chloride is a lab error. The middle left and the middle right ABGs are more what I expect. The -12 (middle left ABG) is due to chloride 116 (ten units higher than normal) and the lactate of 3. The middle right ABG was after two amps of bicarb were given. This drove the chloride down to 108 (two units above normal) plus the lactate of 1.8 = BE -3.
If there are other unmeasured acids in the patient, this will be reflected in an elevated anion gap.
The other parameter to look at is the pCO2. Try to keep it around 40 by changing the ventilator tidal volume or frequency as necessary.
The pH doesn’t tell me much as it is the result of what is going on with ventilation AND metabolically. By itself, it does not point to the cause of the derangement the way that looking at the BE, Cl, and lactic acid do.
I pay little attention to the bicarbonate. It moves inversely to the Cl and the lactate. Surgeons used to follow the bicarbonate in a trauma reasoning that its decline was due to a rising lactate owing to poor perfusion and their response was to give more and more blood, which was diluted with normal saline, which in turn caused hyperchloremia and more bicarbonate excretion by the kidney. A viscous cycle. It is best to transfuse on the basis of the HGB instead of the bicarbonate.
A rising lactate is indicative of hypoperfusion and shock. This is treated by increasing perfusion (blood transfusion most often, inotropes rarely).
Hyperchloremia is treated with sodium bicarbonate. It can be prevented by diluting blood with Plasmalyte instead of with normal saline or giving sodium bicarbonate if normal saline is used to dilute blood.
CORRECTION OF ACIDOSIS with sodium bicarbonate:
0.3 X BE X Kg