ABG BASICS What are the normal value for ABGs? • pH: 7.35 to 7.45 • CO2: 35 to 45 mm Hg • HCO3 -: 24 to 28 mEq/L Remember that the CO2 and HCO3-determine arterial pH. However, ABG reports also include two additional values that evaluate the patient's oxygen status. • O2: 80 to 100 mm Hg • SaO2: 95 to 100% How do I know if an abnormal pH is due to the lungs (respiratory) or to the kidneys (metabolic)? • Check the pH value. Where is it in comparison to normal alkaline or acid? • Check the C02 value. High CO2 means more of the gas is available for conversion to carbonic acid. Low CO2 indicates less carbon dioxide is available for conversion to carbonic acid. The CO2 represents the lungs' side of the balance, respiratory control. • Check the HCO3 - value. A high value means a large amount of bicarbonate build-up, while a low value indicates bicarbonate loss. The HCO 3 - represents the kidneys' side of the balance, metabolic control. • Determine which value (CO2 or HCO3 -) can create the patient's pH. • Here are four examples of laboratory values to illustrate each of the main pH imbalances.

Example 1: pH = 7.31 CO2 = 50 mm Hg HCO3-= 24 mEq/L

Analysis: pH is less than 7.35, so the condition is acidosis. CO2 is above 45 mm Hg. HCO3- is normal.

Conclusion: This is an example of respiratory acidosis. (Note: A pH of 7.31 could not be caused by a bicarbonate value that is WNL.)

Example 2: pH = 7.31 CO2 = 44 mm Hg HCO3-= 20 mEq/L Analysis: pH is less than 7.35, so the condition is acidosis.

CO2 is normal. HCO3 - is less than 24 mEq/L.

Conclusion: This is an example of metabolic acidosis. (Note: This is the same pH as the first example, but it is due to too little bicarbonate).

Example 3: pH = 7.48 CO2 = 33 mm Hg HCO3 -= 24 mEq/L Analysis: pH is more than 7.45, so the condition is alkalosis.

CO2 is below 35 mm Hg. HCO3- is normal.

Conclusion: This is an example of respiratory alkalosis. (Note: A pH of 7.45 could not be caused by a bicarbonate value that is WNL.)

Example 4 pH = 7.48 CO2 = 43 mm Hg HCO3 -= 33 mEq/L Analysis: pH is more than 7.45, so the condition is alkalosis.

CO2 is normal. HCO3 - is above 28mEq/L.

Conclusion: This is an example of metabolic alkalosis. (Note: This is the same pH as the third example, but it is due to too much bicarbonate.)

Can I tell by looking at an, ABG report if the body is attempting to get back into balance? Yes. But sometimes it is tricky. What you are asking about is called compensation. As one

system goes out oh balance, the other system kicks in to restore and maintain the all important ratio of I part carbonic acid to 20 parts bicarbonate.

When you look back at the four examples, you see imbalances without evidence that the body is attempting to regain the ratio. These are examples of uncompensated situations. The following is an example of a compensated situation. Example: pH = 7.4 CO2 = 60 mm Hg HCO3 - = 37 mEq/L Note that the carbon dioxide and bicarbonate values are both abnormal, but the pH is WNL. In this case, the kidneys are retaining bicarbonate to counter the lungs’ retention of carbon dioxide, thus, keeping the 1 to 20 ratio and a pH that is WNL. r

Compensation can be full or partial. In full compensation, the pH is WNL but both carbon dioxide and bicarbonate are abnormal. In partial compensation, the pH is still abnormal as well as the carbon dioxide and/or the bicarbonate values Remember, compensation takes time. Acid-base imbalances with a respiratory etiology are generally corrected quicker than imbalances with a metabolic etiology. What is base excess that I see on ABG reports? Base excess (BE) is a value that indicates the sum of the body's buffer anions—that is negative ions. Bicarbonate accounts for only about half of the body's anions. (Total buffer anions are in the range of 45 to 50 mEq/L, while total bicarbonate range is 24 to 28 mEq/L.) Therefore, base excess gives a better view of the metabolic side of the acid-base balance. Normal values for base excess are -3 to +3 mEq/L. Positive values, those above +3 mEq/L, indicate base excess. Negative values, those below -3 mEq/L, indicate base deficit. The following are two ABG examples that include BE values. Example: pH = 7.16 CO2 = 57 mm Hg HCO3- = 25 mEq/L BE=+1 Both the bicarbonate and BE values are WNL. This is a case of respiratory acidosis with no evidence that compensation has begun. Example: pH =7.18 CO2 = 41 mm Hg HCO3- = 14 mEq/L BE = -4 The respiratory value (carbon dioxide) is WNL, while the bicarbonate is below normal. Note that the base excess is actually a deficit (-4). This value indicates that there is a true deficit of bicarbonate. This example is typical of one that might be seen in an individual with diabetic ketoacidosis. So this represents metabolic acidosis where, as yet, there is no compensation by the lungs. Can you determine the 1:20 ratio from the ABG values? Yes. But you do not need to. The pH tells you about the CO2 and HCO3- balance. Once the pH is beyond the norm in either direction the 1:20 ratio has been lost. Although carbonic acid is seldom measured, its normal blood value is 3% of the carbon dioxide value, or 1.05 to 1.35 mEq/L. Remember that normal bicarbonate is 22 to 26 mEq/L. Using these normal ranges, the ratio of carbonic acid to bicarbonate can be calculated.

Note that in this calculation we are using carbonic acid, not carbon dioxide. You cannot take the carbon dioxide value and divide it into the bicarbonate value to determine the ratio. So, although calculating the ratio is possible, it is not practical to do so.

When should I be alert for possible acid-base imbalance? When the pH has a metabolic etiology, the pH values and the bicarbonate value go together; that

is, high pH plus high HCO3 - in metabolic alkalosis and low pH plus low HCO3 - in metabolic acidosis. When the imbalance has a respiratory etiology, the pH value and the carbon dioxide values are opposite; that is, low pH and high CO2 in respiratory acidosis and high pH and low CO2, in respiratory alkalosis.

Fall 2003 LPN 142