Therefore, venous blood gas and ABG values should not be used interchangeably; arterial blood sampling remains the preferred method for assessing a patient's acid-base and respiratory status. Interpretation of acid-base imbalances and compensation begins with an understanding of the principles of the regulatory mechanisms that control the body's acidity and alkalinity-the pH balance-which is necessary for homeostasis.
Two body systems, the respiratory system and the renal system, are primarily responsible for controlling blood pH levels. With a basic knowledge of these mechanisms, clinicians can determine the presence of respiratory and renal acid-base imbalances in a variety of critical conditions including head injury, drug overdose, stroke, diabetic ketoacidosis, and pulmonary edema. Yet it can be more difficult to interpret combinations of respiratory and metabolic imbalances and understand how the body compensates for such complex imbalances.
In this article, we focus on basic ABG analysis and interpretation, discuss the combinations of imbalances that may occur, and review the compensatory mechanisms that arise as a result.
We also present a three-step method of ABG analysis, with examples of various values. The respiratory system maintains normal blood pH levels through the regulation of carbon dioxide, an acid that exits the body through exhalation. When carbon dioxide levels increase in the body, the respiratory center is stimulated to initiate breathing.
Hyperventilation causes a decrease in carbon dioxide and leads to alkalosis-an increased pH level. Hypoventilation causes an increase in carbon dioxide and leads to acidosis-a decreased pH level.
The respiratory system can regulate changes in blood pH levels within minutes, simply by increasing or decreasing the respiratory rate. Bicarbonate is a base that acts as a buffer in the blood. Increased bicarbonate levels and decreased hydrogen ions in the blood lead to alkalosis and a rise in pH level. Decreased bicarbonate levels and increased hydrogen ions in the blood cause acidosis and a decrease in pH level. The renal system takes several hours or even days to adjust these changes in blood pH levels.
The PaO 2 level reveals the patient's oxygenation status; carbon dioxide is considered an acid and bicarbonate a base. Some institutions may use slightly different reference ranges, but variations are not clinically significant. Abnormalities in the PaCO 2 level indicate a respiratory problem, and abnormalities in the HCO 3 level indicate a metabolic problem. Louis: Mosby-Year Book; Clinical Application of Blood Gases.
Louis: Mosby -Year Book; Wilkins RL. Interpretation of blood gases. Clinical Assessment in Respiratory Care. Michael J. With both pH and PaCO2 being within the normal range, the seesaw is balanced.
If the pH goes up and the PaCO2 goes down, this is respiratory alkalosis. If the pH goes down and the PaCO2 goes up, this is respiratory acidosis. The elevator model is used to determine whether there is a metabolic problem.
With both pH and HCO3- being within the normal range, the elevator is in neutral. If both the pH and the HCO3- go up, this is metabolic alkalosis. If both the pH and the HCO3- go down, this is metabolic acidosis. This value is well above 7.
Slightly below normal indicating mild hypoxemia. Interpretation: partially compensated respiratory alkalosis with mild hypoxemia. This value is well below 7. Example: The pH is 7. The patient is partially compensated when he has an imbalance and some compensation occurs.
The patient would be fully compensated if he had an imbalance with a normal pH. For additional information on this or related content, please email contact advanceweb. We are working as fast as we can to fix this issue. If you are an author and would like your content updated sooner, please email us with the name of the article, and your updated author bio. If you have new content that you would like to contribute please call or email us with your topic suggestions.
Thanks as always for being part of our community. Close Menu. The next value is the carbon dioxide level, and this will tell you if the problem is respiratory in origin, as CO 2 is regulated by the lungs Berman et al. The normal range for P a CO 2 is 35 to 45 mmHg. Finally, bicarbonate ions, or HCO 3 - , will tell you if the problem is related to metabolic changes in your patient and refers to the renal system Berman et al.
Put simply, when the numbers in an ABG result fall outside of these ranges, you can then determine what type of problem the patient is experiencing. The lower the number, the more acidotic the patient is. For instance, a pH of 3 is severely acidotic and requires emergency intervention.
Alkalosis is the opposite. The higher the pH, the more base is in the blood sample, which can disrupt the normal functioning of the body. If the cause is respiratory in nature, the P a CO 2 will be out of the normal range, whereas for metabolic problems the HCO 3 - will be abnormal. We can investigate this by looking at the opposing component of the problem. If the other level or component is within normal ranges, then the problem is non-compensated or uncompensated.
Ultimately, the body is yet to fix the problem or has been unable to fix the problem. On the flip side, if the pH was not normal but the HCO3 was normal, it would be uncompensated.
Our answer is: metabolic acidosis, fully compensated by the means of respiratory alkalosis. Again pull from your memory bank regarding normal values for ABGs as we did in the previous problem and set-up the problem using the Tic Tac Toe method:.
There is a tic tac present: there is a three in a row with base, pH, HCO3 as seen in the picture above. We can see that this is a metabolic problem due to alkalosis.
But now we must determine if this is uncompensated, partially, or fully compensated? The pH is abnormal and this rules out full compensation, but is it uncompensated or partial? To answer this you must look at the system that will attempt to correct the metabolic alkalosis, which would be the respiratory system, hence the PaCO2. It is normal or abnormal? It is abnormal!
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