Approach to acid-base disorders: Clinical sciences

An acid-base disorder is any process that causes the arterial pH to move outside its normal range of 7.35 to 7.45. When the arterial pH decreases below this range, the process is called acidosis. On the other hand, if the pH increases above this range, then it’s referred to as alkalosis. Now, depending on the cause, acid-base disorders can be classified as either respiratory or metabolic. Respiratory acid-base disorders are due to an increase or decrease in the removal of carbon dioxide from the body through the lungs. On the other hand, metabolic acid-base disorders result from the loss or accumulation of acids or bicarbonate.

Now, if a patient presents with a chief concern suggesting an acid-base disorder, you should first perform an ABCDE assessment to determine if your patient is unstable or stable. If your patient is unstable, stabilize their airway, breathing, and circulation, which may require endotracheal intubation with mechanical ventilation. Next, obtain IV access and put your patient on continuous vital sign monitoring, including blood pressure, heart rate, and pulse oximetry. Finally, if needed, don’t forget to provide supplemental oxygen!

Now, let’s go back to the ABCDE assessment and discuss stable patients. In this case, first, obtain a focused history and physical examination, and order labs, including a comprehensive metabolic panel or CMP, and an arterial blood gas or ABG. The history will vary greatly depending on the type of acid-base disorder and the underlying cause. Your patient might report fatigue, nausea, and vomiting, or recent medication changes, including diuretics or salicylates. The physical exam might reveal an abnormal respiratory rate, whereas the CMP might show electrolyte abnormalities depending on the type of the disorder, mainly involving bicarbonate, potassium, and chloride.

Next, assess the arterial pH from the ABG results. If the pH is under 7.35, you can diagnose acidosis. Your next step is to determine whether the acidosis is either respiratory or metabolic, which involves assessing both the pCO2 from the ABG and serum bicarbonate level from the CMP.

If the pCO2 is increased above its normal range, which is 35 to 45 millimeters of mercury, then that's respiratory acidosis. This means there’s a process causing reduced carbon dioxide elimination through the lungs, such as a decreased rate or depth of breathing, or an impaired alveolar gas exchange.

Here’s a clinical pearl! Once you’ve diagnosed respiratory acidosis, evaluate for metabolic compensation by checking the serum bicarbonate level. Uncompensated respiratory acidosis will have a low pH and elevated pCO2, but with a normal bicarbonate level. On the other hand, if there is metabolic compensation, the bicarbonate level will be elevated, because the kidneys are retaining it to restore normal pH.

Now that we’re done with respiratory acidosis, let’s go back and take a look at bicarbonate! Normally, serum bicarbonate levels are between 22 and 27 milliequivalents per liter. If the bicarbonate level is decreased in addition to having a low pH, diagnose metabolic acidosis.

Okay, here are some clinical pearls to keep in mind! After diagnosing metabolic acidosis, check the pCO2 to see if there is respiratory compensation. If the pCO2 is normal, in the setting of a low pH and low bicarbonate levels, your patient is experiencing uncompensated metabolic acidosis. If there’s respiratory compensation, more CO2 will be expelled through the lungs by an increased respiratory rate or depth, so the pCO2 will be low.

However, if you notice a high pCO2 in the setting of a low pH and low bicarbonate level, your patient is experiencing mixed respiratory and metabolic acidosis. For example, this might occur in a patient with diabetic ketoacidosis, a type of metabolic acidosis, which can be associated with cerebral edema and subsequent central respiratory depression, hypoventilation, and respiratory acidosis. As you might expect, when two or more processes causing acidosis occur simultaneously, the pH is lower than you would expect from a single metabolic disturbance.

Now, let’s take a look at individuals with a pH between 7.35 and 7.45. In this case, your patient has a normal arterial blood pH. This could mean there’s no acid-base disorder, or it could mean there’s a mixed acid-base disorder, where two or more acid-base disorders coexist and offset each other. These patients may present with normal or only slightly abnormal pH in the direction of the predominant, or more severe acid-base disorder.