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Arterial Blood Gases
The Test

How is it used?
When is it ordered?
What does the test result mean?
Is there anything else I should know?

The 6 Easy Steps to ABG Analysis:
1. Is the pH normal?
2. Is the CO2 normal?
3. Is the HCO3 normal?
4. Match the CO2 or the HCO3 with the pH
5. Does the CO2 or the HCO3 go the opposite direction of the pH?
6. Are the pO2 and the O2 saturation normal?

Step 1: Analyze the pH The first step in analyzing ABGs is to look at the pH. Normal blood pH is 7.4, plus or minus 0.05, forming the range 7.35 to 7.45. If blood pH falls below 7.35 it is acidic. If blood pH rises above 7.45, it is alkalotic. If it falls into the normal range, label what side of 7.4 it falls on. Lower than 7.4 is normal/acidic, higher than 7.4 is normal/alkalotic. Label it.

Step2: Analyze the CO2 The second step is to examine the pCO2. Normal pCO2 levels are 35-­‐45mmHg. Below 35 is alkalotic, above 45 is acidic. Label it.

Step 3: Analyze the HCO3 The third step is to look at the HCO3 level. A normal HCO3 level is 22-­‐26 mEq/L. If the HCO3 is below 22, the patient is acidotic. If the HCO3 is above 26, the patient is alkalotic. Label it.

Step 4: Match the CO2 or the HCO3 with the pH Next match either the pCO2 or the HCO3 with the pH to determine the acid-­‐base disorder. For example, if the pH is acidotic, and the CO2 is acidotic, then the acid-­‐base disturbance is being caused by the respiratory system. Therefore, we call it a respiratory acidosis. However, if the pH is alkalotic and the HCO3 is alkalotic, the acid-­‐ base disturbance is being caused by the metabolic (or renal) system. Therefore, it will be a metabolic alkalosis.

Step 5: Does the CO2 or HCO3 go the opposite direction of the pH? Fifth, does either the CO2 or HCO3 go in the opposite direction of the pH? If so, there is compensation by that system. For example, the pH is acidotic, the CO2 is acidotic, and the HCO3 is alkalotic. The CO2 matches the pH making the primary acid-­‐base disorder respiratory acidosis. The HCO3 is opposite of the pH and would be evidence of compensation from the metabolic system.

Step 6: Analyze the pO2 and the O2 saturation. Finally, evaluate the PaO2 and O2 sat. If they are below normal there is evidence of hypoxemia.
Normal Values (At sea level): Range:
pH 7.35-­‐7.45
pCO2 35-­‐45 mmHg
pO2 80-­‐100 mmHg
O2 Saturation 95-­‐100%
HCO3-­‐ 22-­‐26 mEq/L
Base Excess + or -­‐ 2
NOW LET’S PUT THE 6 STEPS INTO ACTION WITH AN EXAMPLE:
pH 7.27 acidotic
CO2 53 acidotic
pO2 50 low
O2 sat. 79% low
HCO3 24 normal
Step 1: The pH is less than 7.35, therefore is acidotic.
Step 2: The CO2 is greater than 45, and is therefore acidotic.
Step 3: The HCO3 is normal.
Step 4: The CO2 matches the pH, because they are both acidotic. Therefore the imbalance is respiratory acidosis. It is acidotic because the pH is acidotic, it is respiratory because the CO2 matches the pH.
Step 5: The HCO3 is normal, therefore there is no compensation. If the HCO3 is alkalotic (opposite direction) then compensation would be present.
Step 6: Lastly, the PaO2 and O2 sat are low indicating hypoxemia.
The full diagnosis for this blood gas is:
Uncompensated respiratory acidosis with hypoxemia.
This patient has an acute respiratory disorder.
HERE’S ANOTHER EXAMPLE:
pH 7.52 alkalotic
CO2 29 alkalotic
pO2 100 normal
O2 sat. 98% normal
HCO3 23 normal
Step 1: The pH is greater than 7.45, therefore is alkalotic.
Step 2: The CO2 is less than 35, and is therefore alkalotic.
Step 3: The HCO3 is normal.
Step 4: The CO2 matches the pH, because they are both alkalotic. Therefore the imbalance is respiratory alkalosis. It is alkalotic because the pH is alkalotic; it is respiratory because the CO2 matches the pH.
Step 5: The HCO3 is normal, therefore there is no compensation. If the HCO3 is acidotic (opposite direction) then compensation would be present. Step 6: Lastly, the PaO2 and O2 sat are normal indicating normal oxygenation.
The full diagnosis for this blood gas is:
Uncompensated respiratory alkalosis. This patient is probably hyperventilating.

HERE IS ANOTHER
EXAMPLE:
pH 7.18 acidotic
CO2 44 normal
pO2 92 normal
O2 sat. 95% normal
HCO3 16 acidotic
Step 1: The pH is less than 7.35, therefore is acidotic. Step 2: The CO2 is normal. Step 3: The HCO3 is less than 22, and is therefore acidotic. Step 4: The HCO3 matches the pH, because they are both acidotic. Therefore the imbalance is metabolic acidosis. It is acidotic because the pH is acidotic, it is metabolic because the HCO3 matches the pH. Step 5: The CO2 is normal, therefore there is no compensation. If the CO2 is alkalotic (opposite direction) then compensation would be present. Step 6: Lastly, the PaO2 and O2 sat are normal indicating normal oxygenation. The full diagnosis for this blood gas is: Uncompensated metabolic acidosis. This patient probably has an acute metabolic disorder such as DKA.

10 LET’S TRY ANOTHER:
pH 7.60 alkalotic
CO2 37 normal
pO2 92 normal
O2 sat. 98% normal
HCO3 35 alkalotic
Step 1: The pH is greater than 7.45, therefore is alkalotic.
Step 2: The CO2 is normal.
Step 3: The HCO3 is greater than 26, and therefore is alkalotic.
Step 4: The HCO3 matches the pH, because they are both alkalotic. Therefore the imbalance is metabolic alkalosis. It is alkalotic because the pH is alkalotic, it is metabolic because the HCO3 matches the pH.
Step 5: The CO2 is normal, therefore there is no compensation. If the CO2 is acidotic (opposite direction) then compensation would be present.
Step 6: Lastly, the PaO2 and O2 sat are normal. The full diagnosis for this blood gas is: Uncompensated metabolic alkalosis. This patient probably is losing stomach acid from vomiting or NG tube drainage.

11 ONE LAST EXAMPLE:
pH 7.30 acidotic
CO2 30 alkalotic
pO2 68 low
O2 sat. 92% low
HCO3 14 acidotic
Step 1: The pH is less than 7.35, therefore is acidotic.
Step 2: The CO2 is less than 35, and is therefore alkalotic.
Step 3: The HCO3 is less than 22, and therefore is acidotic.
Step 4: The HCO3 matches the pH, because they are both acidotic. Therefore the imbalance is a metabolic acidosis. It is acidotic because the pH is acidotic, it is metabolic because the HCO3 matches the pH.
Step 5: The CO2 is alkalotic and goes the opposite direction of the pH, so there is compensation. Because the pH is not in the normal range the compensation is called partial.
Step 6: Lastly, the PaO2 and O2 sat are low indicating hypoxemia. The full diagnosis for this blood gas is: Partially-­‐compensated metabolic acidosis with hypoxemia. There are a number of conditions that can cause metabolic acidosis: renal failure, diarrhea, poisonings, diabetic ketoacidosis, and shock, to name a few. This patient is probably in shock, because his metabolic acidosis associated with poor oxygenation.

How is it used?

Blood gas measurements are used to evaluate your oxygenation and acid/base status. They are typically ordered if you have worsening symptoms of an acid/base imbalance, difficulty breathing, or shortness of breath. Blood gases may be ordered along with other tests, such as electrolytes to determine if an electrolyte imbalance is present, glucose to evaluate blood sugar concentrations, and BUN and creatinine tests to evaluate kidney function.

If you are on continuing supplemental oxygen therapy, blood gases may be used to monitor the effectiveness of that treatment.

When is it ordered?

Blood gas tests are ordered when you have symptoms of an oxygen/carbon dioxide or pH imbalance, such as difficulty breathing, shortness of breath, nausea or vomiting.

Blood gas measurements may be ordered when you are known to have a respiratory, metabolic, or kidney disease and are experiencing respiratory distress.

When you are "on oxygen" (ventilation), you may have your blood gases measured at intervals to monitor the effectiveness of treatment.

Blood gases may also be ordered when you have head or neck trauma, injuries that may affect breathing. When you are undergoing prolonged anesthesia – particularly for cardiac bypass surgery or brain surgery – you may have your blood gases monitored during and for a period after the procedure.

Checking the blood gases from the umbilical cord of newborns may uncover respiratory problems as well as determine the baby's acid/base status. Testing is usually only done if a newborn's condition indicates that he or she may be having difficulty breathing.

What does the test result mean?

Abnormal results of any of the blood gas components may mean that:

  • you are not getting enough oxygen
  • you are not getting rid of enough carbon dioxide
  • there is a problem with kidney function

The results of the PO2 component of the tests for blood gases relates to how much oxygen you are able to breathe in and the amount of oxygen in your blood. Low levels may mean you are not getting enough oxygen while results that are within normal range usually mean your oxygen intake is sufficient.

The results of the other components of the tests for blood gases are interrelated and the results must be considered together. Certain combinations of results, if abnormal, may indicate a condition that is causing acidosis or alkalosis:

  • Respiratory acidosis is characterized by a lower pH and an increased PCO2 and is due to respiratory depression – not enough oxygen in and carbon dioxide out. This can be caused by many things, including pneumonia, chronic obstructive pulmonary disease (COPD), and over-sedation from narcotics.
  • Respiratory alkalosis, characterized by a raised pH and a decreased PCO2, is due to over ventilation caused by hyperventilating, pain, emotional distress, or certain lung diseases that interfere with oxygen exchange.
  • Metabolic acidosis is characterized by a lower pH and decreased HCO3-; the blood is too acidic on a metabolic/kidney level. Causes include diabetes, shock, and renal failure.
  • Metabolic alkalosis is characterized by an elevated pH and increased HCO3- and is seen in hypokalemia, chronic vomiting (losing acid from the stomach), and sodium bicarbonate overdose.

Combinations of results that may be seen in certain conditions are summarized below:

pH result Bicarbonate result PCO2 result Condition Common causes
Less than 7.4 Low Low Metabolic acidosis Kidney failure, shock, diabetic ketoacidosis
Greater than 7.4 High High Metabolic alkalosis Chronic vomiting, low blood potassium
Less than 7.4 High High Respiratory acidosis Lung diseases such as pneumonia, COPD
Greater than 7.4 Low Low Respiratory alkalosis Hyperventilation, pain, anxiety

If left untreated, these conditions can create an imbalance that can eventually be life-threatening. Your doctor will provide the necessary medical intervention for you to regain your body's normal balance, but the underlying cause of the imbalance must also be addressed.

Is there anything else I should know?

Arterial blood sample collection is usually more painful than regular venipuncture. You will experience moderate discomfort, and a compress is required for some time to prevent any bleeding from the site.

Sometimes mixed venous blood taken from a central line is used in particular situations, such as in cardiac catheterization labs and by transplant services. Careful interpretation of the results is required. Peripheral venous blood, such as that taken from a vein in the arm, is of no use for oxygen status.

Common Questions

1.  Can this test be done in a doctor's office?

Blood gas measurements, performed by trained personnel, are usually done in a hospital, emergency room, surgical center, ambulance, or large laboratory setting since the analysis should be done immediately after sample collection and specialized equipment is required. Most doctors do not have such capabilities in their offices.

2.  I've had pneumonia before and currently have asthma. Why has my doctor never ordered this test on me?

Most cases of pneumonia or asthma can be diagnosed by symptoms and monitored by listening to your chest sounds or by examining the results of spirometry tests or chest x-rays. Most of the time, asthma will respond to your usual medications and pneumonia to rest and possibly antibiotics. Blood gas tests may be necessary if you have severe or acute breathing problems or prolonged, chronic ones. In these cases, blood gas tests are usually done in an emergency room or hospital setting.

3.  Is there any other way to measure my oxygen levels?

A pulse oximeter is a noninvasive (no needlestick to obtain a blood sample is required) way of continuously monitoring O2 saturation only. A small clip-like device (called a sensor) is attached to the end of the finger or earlobe. The sensor reads light that is transmitted through the skin. Pulse oximeters are useful for monitoring trends in O2 saturation, but their accuracy can be affected by the presence of abnormal forms of hemoglobin, like carboxyhemoglobin (see below), low blood pressure due to poor perfusion, and very low levels of hemoglobin due to severe anemia.

4.  Why does my lab report also list carbyoxyhemoglobin? What is it?

If your blood gases were measured using an instrument known as a co-oximeter, then your lab report may also list results for carboxyhemoglobin and other altered forms of hemoglobin. A co-oximeter is a blood gas analyzer that can measure concentrations of these hemoglobin derivates in addition to the usual blood gas measurements. A co-oximeter is not always used, so these values are not reported for all blood gas tests.

Carboxyhemoglobin is an altered form or derivative of hemoglobin that forms when carbon monoxide binds to hemoglobin. Levels of carboxyhemoglobin are often elevated with carbon monoxide poisoning, and a co-oximeter is used to measure carboxyhemoglobin levels and to monitor oxygen therapy. Hemoglobin binds to carbon monoxide about 210 times more strongly than to oxygen. This significantly decreases hemoglobin's ability to carry oxygen through the body, which can lead to a serious, life-threatening condition.

Other hemoglobin derivates include sulfhemoglobin (or sulfmethemoglobin) and methemoglobin, which may result from the ingestion of certain medicines or exposure to chemicals. These altered forms of hemoglobin, like carboxyhemoglobin, cannot function properly in carrying oxygen and are commonly measured by a co-oximeter.


pH
CO2
HCO3

Normal Arterial Blood Gas Values

pH 7.35-7.45
PaCO2 35-45 mm Hg
PaO2  80-95 mm Hg
HCO3  22-26 mEq/L
O2 Saturation 95-99%
BE  +/- 1

Four-Step Guide to ABG Analysis

  1. Is the pH normal, acidotic or alkalotic?
  2. Are the pCO2 or HCO3 abnormal?  Which one appears to influence the pH?
  3. If both the pCO2 and HCO3 are abnormal, the one which deviates most from the norm is most likely causing an abnormal pH. 
  4. Check the pO2.  Is the patient hypoxic?

I used Swearingen's handbook (1990) to base the results of this calculator.  The book makes the distinction between acute and chronic disorders based on symptoms from identical ABGs.  This calculator only differentiates between acute (pH abnormal) and compensated (pH normal).  Compensation can be seen when both the PCO2 and HCO3 rise or fall together to maintain a normal pH.  Part compensation occurs when the PCO2 and HCO3 rise or fall together but the pH remains abnormal.  This indicates a compensatory mechanism attempted to restore a normal pH.  I have not put exact limits into the calculator.  For example, it will perceive respiratory acidosis as any pH < 7.35 and any CO2 > 45 (i.e. a pH of 1 and CO2 of 1000).  These results do not naturally occur.

pH PaCO2 HCO3
Respiratory Acidosis
Acute < 7.35 > 45 Normal
Partly Compensated < 7.35 > 45 > 26
Compensated Normal > 45 > 26
Respiratory Alkalosis
Acute > 7.45 < 35 Normal
Partly Compensated > 7.45 < 35 < 22
Compensated Normal < 35 < 22
Metabolic Acidosis
Acute < 7.35 Normal < 22
Partly Compensated < 7.35 < 35 < 22
Compensated Normal < 35 < 22
Metabolic Alkalosis
Acute > 7.45 Normal > 26
Partly Compensated > 7.45 > 45 > 26
Compensated Normal > 45 > 26

Mixed Disorders

It's possible to have more than one disorder influencing blood gas values.  For example ABG's with an alkalemic pH may exhibit respiratory acidosis and metabolic alkalosis.  These disorders are termed complex acid-base or mixed disorders.

*This table is able to classify most clinical blood gas values but not all.  In cases where blood gas values do not fall into any of the above classifications, an answer "unable to determine" will appear when using the interpreter.  For example a pH of 7.428, pCO2 43.6, and a HCO3 of 29.1 do not match any of the classifications (I found these results in someone's chart).  While the pH and pCO2 are normal, the HCO3 is abnormally high.