YOU'RE WORKING in the ED when Jay Patel, 68, arrives complaining of pain in his left arm after falling from a ladder. Mr. Patel has a history of chronic obstructive pulmonary disease (COPD) and appears to be in respiratory distress.

The physical exam reveals an underweight man with a body mass index of 18. He has a barrel chest (increased anteroposterior diameter of the thorax), and he's using accessory muscles of respiration and pursed-lip breathing. His breath sounds are clear but diminished in all lung fields. His vital signs are temperature, 98.8[degrees] F; heart rate, 128 beats/minute; respirations, 28 breaths/minute; BP, 166/98 mm Hg. His SpO₂ is 92% on room air.

After obtaining a left upper extremity X-ray, the ED physician diagnoses a displaced ulnar fracture requiring surgical repair. Meanwhile, you've obtained Mr. Patel's arterial blood gas (ABG) values: pH, 7.43; PaCO₂, 52 mm Hg; PaO₂, 70 mm Hg; HCO₃^-, 34 mEq/L; and SaO₂, 91%. What do these values tell you about Mr. Patel's clinical status?

This article describes a step-by-step approach to interpreting ABG results and discusses how these results affect nursing interventions and medical treatments. As a refresher, let's review each of the values measured by ABG analysis.

Getting down to basics

An ABG analysis has five basic components:

* pH measures the blood's acidity or alkalinity.

* PaCO₂ measures the partial pressure of carbon dioxide (CO₂) dissolved in arterial blood.

* PaO₂ measures the partial pressure of oxygen dissolved in arterial blood.

* HCO₃^- measures the concentration of bicarbonate ions.

* SaO₂ measures the percentage of hemoglobin saturated with oxygen.

SaO2 and PaO2: An oxygenation review. About 97% of oxygen in the blood is carried by hemoglobin in the form of oxyhemoglobin (HbO₂) in red blood cells (RBCs). This is measured as SaO₂. Normal HbO₂ saturation should be greater than 95%; if the value drops to 90% or less, immediately assess the patient and administer supplemental oxygen. The remaining 3% of oxygen is dissolved in blood and measured as PaO₂.

The SaO₂ is related to the patient's PaO₂ level: As oxygen is dissolving into blood, it's also combining with hemoglobin. When PaO₂ is high, hemoglobin quickly takes up oxygen molecules. An SaO₂ of 100% indicates that hemoglobin is completely saturated.

The relationship between PaO₂ and SaO₂ is shown by the S-shaped HbO₂ dissociation curve. Changes in certain parameters that occur in the body will cause the curve to shift to the left or right. (See Why does the HbO2 curve shift?)

* A shift to the left indicates hemoglobin's increased affinity for oxygen (inhibiting oxygen release to the cells). This can be caused by increased pH, decreased body temperature, or decreased PaCO₂.

PaCO2: A respiratory parameter. The PaCO₂ measures the partial pressure that dissolved CO₂ exerts in the plasma. It's directly related to the amount of CO₂ being produced by the cells.

HCO3-: Metabolic parameter. The bicarbonate ion (HCO₃^-) is the acid-base component regulated by the kidneys. Acting as one of the body's buffer systems, the kidneys retain or excrete the alkalotic bicarbonate ion as needed. You can use the HCO₃^- value to determine whether the source of an acid-base disturbance is respiratory or metabolic. Generally speaking, an HCO₃^- level below 22 mEq/L indicates metabolic acidosis; above 26 mEq/L indicates metabolic alkalosis.

An uncompensated status indicates that one of the body systems (respiratory or renal) has made no attempt to compensate for the changing pH. (Often this is just a matter of time because uncompensated acid-base disturbances either resolve fairly quickly or trigger a compensatory response.) A partially compensated status indicates that the opposing body system is attempting to compensate but hasn't changed enough to bring the pH back to normal. In this case, the opposing body system value will be outside its normal range in the direction opposite to the problem.

A fully compensated status occurs when pH is within normal limits and values for the respiratory and metabolic components are outside their normal ranges but in opposite directions.

Conversely, as CO₂ decreases, fewer hydrogen ions are generated and pH increases, resulting in respiratory alkalosis. Increases in HCO₃^- in the blood take hydrogen ions out of circulation, resulting in metabolic alkalosis; decreases in HCO₃^- leave more hydrogen ions in circulation, resulting in metabolic acidosis.

You can take a systematic approach to apply these principles to your practice. Suppose your patient's ABG results are as follows: pH, 7.52; PaCO₂, 30 mm Hg; HCO₃^-, 24 mEq/L; PaO₂, 89 mm Hg; and SaO₂, 96%. You can see immediately that the pH is elevated, the PaCO₂ is low, and the remaining values are within normal limits. How do you assess what these values tell you about the patient's condition? Follow these steps:

Step 1: Examine the PaO₂ and the SaO₂ levels to determine whether hypoxemia exists and intervene if necessary. In our example, both of these values are within normal limits, so the patient isn't hypoxemic. Continue to monitor the oxygenation status, and label the oxygenation status normal.

Step 2: Examine the pH and determine whether it indicates or tends toward acidosis or alkalosis. Note that a pH between 7.35 and 7.39 is considered normal but a little acidic; a pH between 7.41 and 7.45 is considered normal but a little alkalotic. In the example, the pH of 7.52 is definitely high, so label it alkalosis.

Step 3: Examine the PaCO₂ and determine whether it indicates acidosis or alkalosis. In this example, the PaCO₂ is low, so the respiratory component indicates alkalosis. Label the PaCO₂alkalosis.

Step 4: Examine the HCO₃^- and determine whether it indicates acidosis or alkalosis. In the example, this metabolic component is in the normal range so label the HCO₃^-normal.

Step 5: Identify the origin of the acid-base disturbance as respiratory or metabolic. Circle the acidosis or alkalosis that matches the label given to the pH. In this case, PaCO₂ (the respiratory component) matches the pH (alkalosis), indicating respiratory alkalosis.

Step 6: Now determine whether the patient is in compensation. Is the pH within normal limits? Are both PaCO₂ and HCO₃^- abnormal in opposition to each other, with one alkalotic and the other acidic? If you answered yes to both questions, the patient is fully compensated.

If pH isn't within normal limits, look at the value that didn't match the pH. This is the HCO₃^- in our example. It's within normal limits, so the patient is uncompensated. If this value had been outside the normal limits on the acidic side, the patient would be partially compensated because the pH was outside normal limits on the alkalotic side. If the HCO₃^- had been outside the normal limits on the alkalotic side, the patient could have a combined respiratory and metabolic alkalosis.

The distinction between partial and full compensation depends on the pH. If pH is within normal range due to the "balance" between PaCO₂ and HCO₃^-, this is labeled as fully compensated. If pH is outside the normal range and both PaCO₂ and HCO₃^- are also outside normal range but in opposition to each other (one alkalotic and the other acidic), the body is trying to compensate but hasn't yet succeeded and we have a partially compensated acid-base status.

For more practice, see Test your ABG interpretation skills.

A case in point: Mr. Patel

Let's return to Mr. Patel, the patient we met at the beginning of this article. His ABG results from the ED were pH, 7.43; PaCO₂, 52 mm Hg; PaO₂, 70 mm Hg; HCO₃^-, 34 mEq/L; and SaO₂, 91%.

As always, your interpretation of ABGs must take the patient's clinical status into account. Mr. Patel is most likely tachypneic and tachycardic due to his pain and anxiety. Knowing that he has a history of COPD, you suspect he has chronic respiratory acidosis or an elevated PaCO₂ and an elevated HCO₃^-, which would result in a compensated state. His pH would be in the normal range but leaning a little toward an acidosis if he were stable and relatively healthy.

However, this isn't the case now: His pH is leaning into the alkalotic range. Using the original step-by-step approach, you interpret his condition as compensated metabolic alkalosis with mild hypoxemia. His pH is "alkalotic" with a high (or alkalotic) HCO₃^-, and a high (or "acidic") PaCO₂.

Now the tough part...based on his HCO₃^- of 34 mEq/L and our useful tip earlier (5 mEq/L increase in HCO₃^- for every 10 mm Hg increase in PaCO₂ for chronic respiratory acidosis), you'd expect a PaCO₂ of 60 mm Hg. Responding to pain, his respiratory rate has increased and he's "blowing off" CO₂. As the PaCO₂ value moves down to 52 mm Hg, the pH moves to a higher level than he'd normally have. The correct interpretation of this patient's ABG profile is chronic respiratory acidosis complicated by alveolar hyperventilation due to pain.

The next day, a second ABG shows the following results: pH, 7.38; PaCO₂, 61 mm Hg; PaO₂, 82 mm Hg; HCO₃^-, 34 mEq/L; and SaO₂, 93%. This is what Mr. Patel's usual ABG results should be: a fully compensated respiratory acidosis with corrected hypoxemia when receiving oxygen therapy. He feels fairly comfortable given the circumstances and is back to his usual baseline status, but he may need supplemental oxygen for home use if he remains hypoxemic on room air or has documented drops in oxygenation during activity.

Practice makes perfect

With practice and careful thought, you can improve your skill and accuracy at ABG interpretation. By adding your knowledge of your patient's clinical status to what's happening with his oxygenation, ventilation, and acid-base balance, you can intervene correctly and deliver better patient care.

Why does the HbO

The HbO₂ curve shifts to the left or right when certain factors change-notably dissolved CO₂ in the blood (PaCO₂), body temperature, pH, or 2,3-bisphosphoglycerate (BPG, also called diphosphoglycerate, a substance in RBCs). The HbO₂ curve shifts occur naturally in the body due to the relative values of PaCO₂. When blood enters the pulmonary capillary system and reaches the alveoli, PaCO₂ diffuses out of the blood into the alveoli. This creates a relatively low level of PaCO₂ in the blood, which shifts the curve to the left and increases the affinity of hemoglobin for oxygen. Hemoglobin molecules quickly take up the oxygen as it diffuses out of the alveoli.

The other end of the oxygen transport system is in the capillary bed at the tissue level throughout the body. Here, the opposite circumstances occur. PaCO₂ is being generated by cellular metabolism and moves out of the cells into the blood. This creates a relatively high level of PaCO₂ in the blood, which shifts the curve to the right and decreases the affinity of hemoglobin for oxygen. Hemoglobin molecules quickly release the oxygen being carried so it can diffuse into cells to replenish their supply.

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