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pH Sleuth: The Case of Acid-Base Imbalance

Acid–base imbalances can cause even the most astute nurse to shudder with fear at the thought of interpreting these numbers. RN.com has simplified this process, providing a simple framework for you to use. These guidelines will give you the confidence and knowledge you need to detect and correct acid-base disorders!

Back to basics: Remember that the body’s acid-base balance depends on delicately balanced chemical reactions that require the pH of blood to remain slightly alkaline within a normal range of 7.35-7.45. For normal cell function and metabolism, the blood’s pH must stay within this narrow range. A pH greater than 7.45 creates alkaline conditions and a pH lower than 7.35 is too acidic for the body. The cause of acid-base imbalances is either respiratory or metabolic in origin.

Facts to follow: Hydrogen ions (H+) increase acidity. The more hydrogen ions (H+) that are present in the blood, the lower the pH will be. Conversely, the lower the H+ in the blood, the higher the pH of the blood will be. In addition, the partial pressure of arterial carbon dioxide (Paco2) affects pH balance. When Paco2 falls, pH rises and when Paco2 rises, pH falls (Fournier, M. 2009).

Note the numbers: Memorize the following important normal values for adults:
pH: 7.35-7.45
Paco2: 35 – 45 mmHg
HCO3-: 22-26mEq/L

Most important information: When the acid-base balance in the body is disturbed, compensatory measures come into play to restore balance. The lungs (Paco2) compensate for metabolic imbalances and the kidneys (H+) compensate for respiratory imbalances (Grogono, A. MD, 2008). In respiratory acid-base imbalances, Ph and Paco2 move in opposite directions. Bicarbonate (HCO3-) remains normal until compensation occurs. In metabolic acid-base disturbances, pH and HCO3 move in the same direction (Fournier, M. 2009). Paco2 remains normal until compensation occurs. If the pH returns to normal, the response is called complete compensation. If the pH remains abnormal despite the compensatory effort, the response is called partial compensation.

Putting it all together: Based on Michelle Fornier’s (2009) four step analysis, RN.com offers you a simple six-step guide to perform an arterial blood gas (ABG) analysis. This analysis will allow you to assess the status of your patient’s acid-base balance and monitor the response to treatment.

Step One: Record or list the patient’s values for pH, Paco2 and HCO3-.
Step Two: Compare each of the above three values from the patient’s to normal standard values. If a result shows excessive acid (pH low or Paco2 high), write an “A” next to it. If a result shows excessive base (pH high or HCO3- high), write a “B” next to it. If a result shows a normal reading, write an “N” next to it.
Step Three: Look at the pH value: If it is low, make a notation that the patient has an acidosis. If the pH is high, note that the patient has an alkalosis.
Step Four: If you have written the same letter more than once, circle that letter.
Step Five: If you have circled pH and Paco2, your patient has a respiratory disorder. If you circled pH and HCO3-, your patient has a metabolic disorder. If you circled all three values, your patient has a combined respiratory and metabolic acid-base disturbance!
Step Six: To check for compensation, look at the result you did NOT circle. If this value has moved away from the normal value in the OPPOSITE direction of the values circled, compensation is occurring. If the value remains within normal range, no compensation has occurred. Once compensation is complete and effective, the pH will be within normal limits.

This article was adapted from Michelle Fournier’s article entitled: “Perfecting your Acid-Base Balancing Act,” published in American Nurse Today (January 2009) Volume 4, Number 1, p17-22.

Additional Reference: Grogono, Alan MD (2008). Acid Base Balance. Retrieved June 25, 2010 from: http://www.uam.es/departamentos/medicina/anesnet/gtoa/acido-base/ab_physiol.html