Vol. 15 •Issue 10 • Page 24
ABGs as Easy as 123
Caregivers Can Become Expert Analysts Fast
By Michael J. Dickmann, MMS, RRT
Interpreting ABGs is not as difficult a task as it may seem. Because the basics follow a logical pattern, a caregiver can quickly become an expert at analysis quite fast–well almost. First of all, it is important to know normal values and what they mean.1 Here are the basics:
• pH–normal range is 7.35-7.45. An expression of hydrogen ion concentration, pH is a reflection of acidity or alkalinity of the blood.
• PaCO2 – normal range is 35Ð45 mm Hg. The PaCO2 is the most useful blood gas measurement because it directly reflects the adequacy of alveolar ventilation.
• HCO3– – normal range is 22Ð26 mEq/L. This is a measurement of the primary base in arterial blood.
• PaO2 – normal range is 80Ð100 mm Hg. This is the partial pressure of oxygen in arterial blood.
• O2 Sat – normal range greater than 95 percent. This reflects the amount of hemoglobin that has combined with oxygen.
Once you know the values, you need to look at what they mean through interpretation. This process follows a normal four-step process.
STEP 1: Check the pH value (7.35 to 7.45 in the normal range) with 7.40 being true normal.
• If it is less than 7.40, this may be an acidotic state.
• If it is greater than 7.40, this may be an alkalotic state.
STEP 2: Check the PaCO2 value (35 to 45 mm Hg is the normal range) which indicates the adequacy of ventilation.
• If it is less than 35 mm Hg, it indicates alkalosis.
• If it is greater than 45 mm Hg, it indicates acidosis.
• Question: Does it move with or opposite the pH? If it moves opposite the pH, this indicates a respiratory imbalance. If it moves in the same direction as the pH, the respiratory system is compensating for a metabolic imbalance. (See Compensation Made Simple).
STEP 3: Check the HCO3 value (22 to 26 mEq/L is the normal range) which is the metabolic indicator.
• If it is less than 22 mEq/L, it shows acidosis.
• If it is greater than 26 mEq/L, it shows alkalosis.
• Question: Does it move with or opposite the pH? If it moves in the same direction as the pH, this indicates a metabolic imbalance. If it moves opposite the pH, the metabolic system is compensating for a respiratory imbalance. (See Compensation Made Simple below).
One of the easiest ways I have found to put this all together is to think of a seesaw and an elevator. Use the seesaw to determine whether there is a respiratory problem.
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.
OXYGENATION STATUS
Above all else, caregivers cannot neglect the oxygenation status of their patients. Most of the time this is one of the first things examined. While an adequate PaO2 is important to the patient, it has nothing to do with the acid-base status of the patient.
STEP 4: Check the PaO2 value (80 to 100 mm Hg is the normal range). If out of normal, patient may have hypoxemia which can be classified as follows:2
• Mild hypoxemia is a PaO2 < 80 mm Hg.
• Moderate hypoxemia is a PaO2 < 60 mm Hg.
• Severe hypoxemia is a PaO2 < 40 mm Hg.
For every year over 60 years, subtract 1 mm Hg for limits of mild and moderate hypoxemia. At any age, a PaO2 less than 40 mm Hg indicates severe hypoxemia.
Once you know the basics, try your hand at a case study. Remember, in respiratory acidosis and respiratory alkalosis, PaCO2 is causing the abnormal pH and HCO3- is the compensating factor. In metabolic acidosis and alkalosis, HCO3- is causing the abnormal pH and PaCO2 is the compensating factor.
Step 1: Evaluate the pH. Is it less than 7.40? If so, this may be an indication of acidosis. Is it greater than 7.40? If so, this may be an indication of alkalosis.
Step 2: Evaluate the PaCO2. If it is abnormal, does it seesaw with the pH? If yes, your patient has a respiratory imbalance. If no, the lungs are compensating for a metabolic imbalance.
Step 3: Evaluate the HCO3-. If it is abnormal, does it take the same elevator as the pH? If yes, your patient has a metabolic imbalance. If no, the kidneys are compensating for a respiratory imbalance.
Step 4: Evaluate the PaO2. A value greater than 80 mm Hg is normal. Any value less than 80 mm Hg indicates some degree of hypoxemia. (See Oxygenation Status).
EXAMPLE: ABG results show a pH of 7.68, PaCO2 of 24 mm Hg, HCO3- of 18 mEq/L, PaO2 of 73 mm Hg.
Step 1: Evaluate the pH. This value is well above 7.40 indicating alkalosis.
Step 2: Evaluate the PaCO2. It is below normal and seesaws with the pH, indicating respiratory alkalosis.
Step 3: Evaluate the HCO3-. It is also below normal, but the pH did not take the same elevator. Therefore, the metabolic imbalance is compensating for the respiratory alkalosis.
Step 4: Evaluate the PaO2. Slightly below normal indicating mild hypoxemia.
Interpretation: partially compensated respiratory alkalosis with mild hypoxemia.
EXAMPLE: ABG results show a pH of 7.10, PaCO2 of 25 mm Hg, HCO3- of 8 mEq/L, PaO2 of 40 mm Hg.
Step 1: Evaluate the pH. This value is well below 7.40, indicating acidosis.
Step 2: Evaluate the PaCO2. It is below normal (alkalotic) and did not seesaw with the pH. The imbalance is compensating for a metabolic problem.
Step 3: Evaluate the HCO3-. It is well below normal, taking the same elevator as the pH indicating metabolic acidosis.
Step 4: Evaluate the PaO2. It is well below normal, indicating severe hypoxemia. Interpretation: partially compensated metabolic acidosis with severe hypoxemia.
COMPENSATION MADE SIMPLE
As we already know, the body does its best to keep the system in balance. When there is an imbalance in the acid-base system, the body responds by activating compensatory mechanisms that minimize pH changes. If the imbalance is either respiratory acidosis or alkalosis, renal compensation takes place (increase or decrease in HCO3-). To bring the pH back to a normal range may take several days. If the imbalance is either metabolic acidosis or alkalosis, respiratory compensation takes place (increase or decrease in PaCO2). To bring the pH back to the normal range may take several hours. Patients are uncompensated when they have an imbalance, but the compensating mechanism remains normal.
Example: The pH is 7.16, PaCO2 is 65 mm Hg, HCO3- is 24 mEq/l.
This patient has respiratory acidosis (seesaw: pH down, PaCO2 up). The HCO3- is within normal limits, so the interpretation is uncompensated respiratory acidosis.
The patient is partially compensated when he has an imbalance and some compensation occurs.
Example: The pH is 7.16, PaCO2 is 65 mm Hg, HCO3- is 30 mEq/l.
This patient has respiratory acidosis
(seesaw: pH down, PaCO2 up). The HCO3- is elevated in an attempt to bring the pH back within normal limits. The interpretation is partially compensated respiratory acidosis.
The patient would be fully compensated if he had an imbalance with a normal pH.
Example: The pH is 7.37, PaCO2 is 65 mm Hg, HCO3- is 35 mEq/l.
This patient has respiratory acidosis (seesaw: pH down, PaCO2 up). The HCO3- is elevated, bringing the pH back within limits. The interpretation is fully compensated respiratory acidosis.
Let’s try one more.
The patient is uncompensated when he has an imbalance, but the compensating mechanism remains normal.
Example: The pH is 7.58, PaCO2 is 40 mm Hg, HCO3- is 40 mEq/l.
This patient has metabolic alkalosis (elevator: pH up, HCO3- up). The PaCO2 is within normal limits, so the interpretation is uncompensated metabolic alkalosis.
The patient is partially compensated when he has an imbalance and some compensation occurs.
Example: The pH is 7.58, PaCO2 is 50 mm Hg, HCO3- is 40 mEq/l.
This patient has metabolic alkalosis (elevator: pH up, HCO3- up). The PaCO2 is increased in an attempt to bring the pH back within normal limits. The interpretation is partially compensated metabolic alkalosis.
The patient would be fully compensated if he had an imbalance with a normal pH.
Example: The pH is 7.43, PaCO2 is 55 mm Hg, HCO3- is 40 mEq/l.
This patient has metabolic alkalosis (elevator: pH up, HCO3- up). The PaCO2 is increased, bringing the pH back within normal limits. The interpretation is fully compensated metabolic alkalosis.
ACID BASE DISORDERS
• Respiratory acidosis results from hypoventilation that is manifested by the accumulation of CO2 in the blood. This decreases blood pH. Examples of specific causes are acute upper airway obstruction, massive pulmonary edema, drug overdose, neuromuscular disease and head trauma.
• Respiratory alkalosis results from hyperventilation that is manifested by excess elimination of CO2 from the blood. This increases blood pH. Examples of specific causes are anxiety, fever, stimulant drugs and patients on mechanical ventilation.
• Metabolic acidosis results when an increased load of H+ ions is present or when a decreased ability to excrete acids exists. Examples of specific causes are diarrhea, renal disease, ketoacidosis, lactic acidosis and ingestion of certain toxins.
• Metabolic alkalosis results from elevation of serum bicarbonate. Examples of specific causes are hypokalemia, nasogastric suction, vomiting, diuretic therapy and excessive administration of sodium bicarbonate.3
REFERENCES 1. Beachey W. Acid-base balance. In: Scanlon C. Egan’s Fundamentals of Respiratory Care. 7th ed. St. Louis: Mosby-Year Book; 1999.
2. Shapiro BA, Peruzzi WT, Templin R. Clinical Application of Blood Gases. 5th ed. St. Louis: Mosby -Year Book; 1994.
3. Wilkins RL. Interpretation of blood gases. In: Wilkins RL, Krider SJ, Sheldon, RL. Clinical Assessment in Respiratory Care. 4th ed. St. Louis: Mosby-Year Book; 2000.
Michael J. Dickmann is an assistant professor in the Respiratory Care Technology Program at St. Philip’s College, San Antonio, Texas, and a recent graduate of Nova Southeastern University.
