Arterial Blood Gas Analysis

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The pH is a measurement of the acidity or alkalinity of the blood.
It is inversely proportional to the number of hydrogen ions (H+) in the blood.
The more H+ present, the lower the pH will be.
Likewise, the fewer H+ present, the higher the pH will be.
The pH of a solution is measured on a scale of 1 (very acidic) to 14 (very alkalotic).
A liquid with a pH of 7, such as water, is neutral (neither acidic nor alkalotic).

Blood Acidity

The normal blood pH range is 7.35 to 7.45.
In order for normal metabolism to take place, the body must maintain this narrow range at all times.
When the pH is below 7.35, the blood is said to be acidic.
Changes in body system functions that occur in an acidic state include a decrease in the force of cardiac contractions, a decrease in the vascular response to catecholamines, and a diminished response to the effects and actions of certain medications.

Blood Alkalinity

When the pH is above 7.45, the blood is said to be alkalotic.
An alkalotic state interferes with tissue oxygenation and normal neurological and muscular functions.

Significant changes in the blood pH above 7.8 or below 6.8 will interfere with cellular functioning, and if uncorrected, will lead to death.
So how is the body able to self-regulate acid-base balance in order to maintain pH within the normal range?
It is accomplished using buffer mechanisms between the respiratory and renal systems.
Let’s examine each system separately.

The Respiratory Buffer Response

A normal by-product of cellular metabolism is carbon dioxide (CO2).
CO2 is carried in the blood to the lungs, where excess CO2 combines with water (H2O) to form carbonic acid (H2CO3).
The blood pH will change according to the level of carbonic acid present.
This triggers the lungs to either increase or decrease the rate and depth of ventilation until the appropriate amount of CO2 has been re-established.
Activation of the lungs to compensate for an imbalance starts to occur within 1 to 3 minutes.

The Renal Buffer Response

In an effort to maintain the pH of the blood within its normal range, the kidneys excrete or retain bicarbonate (HCO3-).
As the blood pH decreases, the kidneys will compensate by retaining HCO3- and as the pH rises, the kidneys excrete HCO3- through the urine.
Although the kidneys provide an excellent means of regulating acid-base balance, the system may take hours to days to correct the imbalance.

Respiratory Acidosis

Respiratory acidosis is defined as a pH less than 7.35 with a PaCO2 greater than 45 mm Hg.

Acidosis is caused by an accumulation of CO2 which combines with water in the body to produce carbonic acid, thus, lowering the pH of the blood. Any condition that results in hypoventilation can cause respiratory acidosis.

These conditions include:

CNS depression related to head injury, medications such as narcotics, sedatives, or anesthesia
Impaired respiratory muscle function related to spinal cord injury, neuromuscular diseases, or neuromuscular blocking drugs
Pulmonary disorders such as atelectasis, pneumonia, pneumothorax, pulmonary edema, or bronchial obstruction
Massive pulmonary embolus
Hypoventilation due to pain, chest wall injury/deformity, or abdominal distension.

The signs and symptoms of respiratory acidosis are centered within the pulmonary, nervous, and cardiovascular systems.

Pulmonary symptoms include: dyspnea, respiratory distress, and shallow respirations.
Nervous system manifestations include: headache, restlessness, and confusion.
If CO2 levels become very high, drowsiness and unresponsiveness may occur.
Cardiovascular symptoms include palpitation and arrhythmias. Increasing ventilation will correct respiratory acidosis.

Respiratory Alkalosis

Respiratory alkalosis is defined as a pH greater than 7.45 with a PaCO2 less than 35 mm Hg.

Any condition that causes hyperventilation can result in respiratory alkalosis.

These conditions include:

Psychological responses, such as anxiety or fear, Pain.
Increased metabolic demands, such as fever, sepsis, pregnancy, or thyrotoxicosis.
Medications, such as respiratory stimulants.
Central nervous system lesions.

Signs and symptoms of respiratory alkalosis are largely associated with the nervous and cardiovascular systems. Nervous system alterations include light-headedness, numbness and tingling, confusion, inability to concentrate, and blurred vision.

Cardiac symptoms include arrhythmias and palpitations. Additionally, the patient may experience dry mouth, diaphoresis, and tetanic spasms of the arms and legs.
Patients presenting with respiratory alkalosis have increased work of breathing and must be monitored for respiratory muscle fatigue.
When respiratory muscles become exhausted, acute respiratory failure may ensue.

Metabolic Acidosis

Metabolic acidosis is defined as a bicarbonate level of less than 22 mEq/L with a pH less than 7.35.

Metabolic acidosis is caused by either a deficit of base in the bloodstream or an excess of acids, other than CO2.

Diarrhea and intestinal fistulas may cause decreased levels of base.

Causes of increased acids include:

Renal failure
Diabetic ketoacidosis
Anaerobic metabolism
Starvation
Salicylate intoxication

Symptoms of metabolic acidosis center around the CNS, CVS, pulmonary, and GI systems.

CNS features include headache, confusion, and restlessness progressing to lethargy, then stupor or coma.
Cardiac arrhythmias are common & Kussmaul breathing occur in an effort to compensate for the low pH by blowing off more CO2.
Warm, flushed skin, as well as nausea and vomiting are commonly noted.

As with most acid-base imbalances, the treatment of metabolic acidosis is dependent upon the cause. The presence of metabolic acidosis should spur a search for hypoxic tissue somewhere in the body.

Hypoxemia can lead to anaerobic metabolism system-wide, but hypoxia of any tissue bed will produce metabolic acids as a result of anaerobic metabolism even if the PaO2 is normal.

The only appropriate way to treat this source of acidosis is to restore tissue perfusion to the hypoxic tissues.

Warning:

Current research has shown that the use of sodium bicarbonate is indicated only for known bicarbonate-responsive acidosis, such as that seen with renal failure.
Routine use of sodium bicarbonate to treat metabolic acidosis results in subsequent metabolic alkalosis with hypernatremia and should be avoided.

Metabolic Alkalosis

Metabolic alkalosis is defined as a bicarbonate level greater than 26 mEq/liter with a pH greater than 7.45.

Either an excess of base or a loss of acid within the body can cause metabolic alkalosis.

Excess base occurs from ingestion of antacids, excess use of bicarbonate, or use of lactate in dialysis.

Loss of acids can occur secondary to protracted vomiting, gastric suction, hypochloremia, excess administration of diuretics, or high levels of aldosterone.

Symptoms of metabolic alkalosis are mainly neurological and musculoskeletal.

Neurologic symptoms include dizziness, lethargy, disorientation, seizures, and coma.
Musculoskeletal symptoms include weakness, muscle twitching, muscle cramps, and tetany.
Patients may also experience nausea, vomiting, and respiratory depression.

Metabolic alkalosis is one of the most difficult acid-base imbalances to treat.

Bicarbonate excretion through the kidneys can be stimulated with drugs such as acetazolamide, but resolution of the imbalance will be slow. In severe cases, IV administration of acids may be used.

Arterial blood gas measurement is a blood test that is performed to determine the concentration of oxygen, carbon dioxide, and bicarbonate, as well as the pH, in the blood.
Its main use is in pulmonology, as many lung diseases feature poor gas exchange, but it is also used in nephrology & Acute Medicine.
As its name implies, the sample is taken from an artery, not from venipuncture.

Obtaining and Processing the Sample

Arterial blood is taken from any easily accessible artery (typically radial, brachial, or femoral) or out of an arterial line. The syringe is prepacked and contains a small amount of heparin to prevent coagulation.
Once the sample is obtained, care should be taken to eliminate visible gas bubbles, as these bubbles can dissolve into the sample and cause inaccurate results. The sealed syringe is taken to a blood gas analyzer.
If the sample cannot be immediately analyzed, it should be chilled in an ice bath to slow metabolic processes that may also cause inaccuracy.
The machine aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen and carbon dioxide.
The bicarbonate concentration is calculated. Some blood gas analyzers can also measure glucose, lactate, hemoglobins, dys-hemoglobins, oxygen saturation, bilirubin, and electrolytes (sodium, potassium, calcium, and chloride).
The results are usually available for interpretation within five minutes.

Components of the ABG

pH: Measurement of acidity or alkalinity, based on the hydrogen (H+) ions present. The normal range is 7.35 to 7.45.
PaO2: The partial pressure of oxygen that is dissolved in arterial blood. The normal range is 80 to 100 mm Hg.
SaO2: The arterial oxygen saturation. The normal range is 95% to 100%.
PaCO2: The amount of carbon dioxide dissolved in arterial blood. The normal range is 35 to 45 mm Hg.
HCO3: The calculated value of the amount of bicarbonate in the bloodstream. The normal range is 22 to 26 mEq/liter.
B.E.: The base excess indicates the amount of excess or insufficient level of bicarbonate in the system. The normal range is -2 to +2 mEq/liter. (A negative base excess indicates a base deficit in the blood.)

Steps to an Arterial Blood Gas Interpretation

The arterial blood gas is used to evaluate both acid-base balance and tissue oxygenation, each representing separate conditions.
Acid-base evaluation requires a focus on three of the reported components: pH, PaCO2, and HCO3.
This process involves three steps.

Step One

Assess the pH to determine if the blood is within normal range, alkalotic, or acidotic.
If it is above 7.45, the blood is alkalotic. If it is below 7.35, the blood is acidotic.

Step Two

If the blood is alkalotic or acidotic, we now need to determine if it is caused primarily by a respiratory or metabolic problem.
To do this, assess the PaCO2 level.
Remember that with a respiratory problem, as the pH decreases below 7.35, the PaCO2 should rise. If the pH rises above 7.45, the PaCO2 should fall. Compare the pH and the PaCO2 values. If pH and PaCO2 are indeed moving in opposite directions, then the problem is primarily respiratory in nature.

Step Three

Finally, assess the HCO3 value.
Recall that with a metabolic problem, normally as the pH increases, the HCO3 should also increase.
Likewise, as the pH decreases, so should the HCO3.
Compare the two values. If they are moving in the same direction, then the problem is primarily metabolic in nature.

The following chart summarizes the relationships between pH, PaCO2, and HCO3:

	pH	PaCO2	HCO3
Respiratory Acidosis	↓	↑	normal
Respiratory Alkalosis	↑	↓	normal
Metabolic Acidosis	↓	normal	↓
Metabolic Alkalosis	↑	normal	↑

Examples

Example 1

John Josphen (JJ) is a 45-year-old female admitted to the ER with a severe asthma attack. She has been experiencing increasing shortness of breath since admission three hours ago. Her arterial blood gas result is as follows:

Clinical Laboratory

PATIENT: John, Josphen
DATE: 6/4/08 10:43am
pH: 7.22
PaCO2: 55
HCO3-: 25

Follow the Steps:

Assess the pH. It is low (normal 7.35-7.45); therefore, we have acidosis.
Assess the PaCO2. It is high (normal 35-45) and in the opposite direction of the pH.
Assess the HCO3. It has remained within the normal range (22-26).

The following chart summarizes the relationships between pH, PaCO2, and HCO3:

	pH	PaCO2	HCO3
Respiratory Acidosis	↓	↑	Normal

Refer to the chart. Acidosis is present (decreased pH) with the PaCO2 being increased, reflecting a primary respiratory problem. For this patient, we need to improve the ventilation status by providing oxygen therapy, mechanical ventilation, pulmonary toilet, or by administering bronchodilators.

Example 2

John Doe is a 55-year-old male admitted to the ER with a recurring bowel obstruction. He has been experiencing intractable vomiting for the last several hours despite the use of anti-emetics. Here is his arterial blood gas result:

PATIENT: DOE, JOHN

DATE: 3/6/08 08:30

pH: 7.50
PaCO2: 42
HCO3-: 33

Follow the Three Steps Again:

Assess the pH. It is high (normal 7.35-7.45), therefore, indicating alkalosis.
Assess the PaCO2. It is within the normal range (normal 35-45).
Assess the HCO3. It is high (normal 22-26) and moving in the same direction as the pH.

The following chart summarizes the relationships between pH, PaCO2, and HCO3:

	pH	PaCO2	HCO3
Metabolic Alkalosis	↑	Normal	↑

Again, look at the chart. Alkalosis is present (increased pH) with the HCO3 increased, reflecting a primary metabolic problem. Treatment of this patient might include the administration of I.V. fluids and measures to reduce the excess base.

COMPENSATION

Thus far, we have looked at simple ABG values without any evidence of compensation occurring.
Now, see what happens when an acid-base imbalance exists over a period of time. When a patient develops an acid-base imbalance, the body attempts to compensate.
Remember that the lungs and the kidneys are the primary buffer response systems in the body.
The body tries to overcome either a respiratory or metabolic dysfunction in an attempt to return the pH into the normal range.
A patient’s ABG can be uncompensated, partially compensated, or fully compensated.
When an acid-base disorder is either uncompensated or partially compensated, the pH remains outside the normal range.
In fully compensated states, the pH has returned to within the normal range, although the other values may still be abnormal.
Be aware that neither system has the ability to overcompensate.
In our first two examples, the patients were uncompensated. In both cases, the pH was outside of the normal range, the primary source of the acid-base imbalance was readily identified, but the compensatory buffering system values remained in the normal range.
Now let’s look at arterial blood gas results when there is evidence of partial compensation.

In order to look for evidence of partial compensation, review the following three steps:

Assess the pH. This step remains the same and allows us to determine if an acidotic or alkalotic state exists.
Assess the PaCO2. In an uncompensated state, we have already seen that the pH and PaCO2 move in opposite directions when indicating that the primary problem is respiratory.
But what if the pH and PaCO2 are moving in the same direction? That is not what we would expect to see happen.
- We would then conclude that the primary problem was metabolic. In this case, the decreasing PaCO2 indicates that the lungs, acting as a buffer response, are attempting to correct the pH back into its normal range by decreasing the PaCO2 ("blowing off the excess CO2").
- If evidence of compensation is present, but the pH has not yet been corrected to within its normal range, this would be described as a metabolic disorder with a partial respiratory compensation.
Assess the HCO3.
- In our earlier uncompensated examples, the pH and HCO3 move in the same direction, indicating that the primary problem was metabolic.
- But what if our results show the pH and HCO3 moving in opposite directions? That is not what we would expect to see.
- We would conclude that the primary acid-base disorder is respiratory, and that the kidneys, again acting as a buffer response system, are compensating by retaining HCO3, ultimately attempting to return the pH back towards the normal range.

The following tables demonstrate the relationships between the pH, PaCO2, and HCO3 in partially and fully compensated states.

Fully Compensated States
	pH	PaCO2	HCO3-
Respiratory Acidosis	normal, but <7.40	↑	↑
Respiratory Alkalosis	normal, but >7.40	↓	↓
Metabolic Acidosis	normal, but <7.40	↓	↓
Metabolic Alkalosis	normal, but >7.40	↑	↑

Partially Compensated States
	pH	PaCO2	HCO3-
Respiratory Acidosis	↓	↑	↑
Respiratory Alkalosis	↑	↓	↓
Metabolic Acidosis	↓	↓	↓
Metabolic Alkalosis	↑	↑	↑

Notice that the only difference between partially and fully compensated states is whether or not the pH has returned to within the normal range. In compensated acid-base disorders, the pH will frequently fall either on the low or high side of neutral (7.40).

Making note of where the pH falls within the normal range is helpful in determining if the original acid-base disorder was acidosis or alkalosis.

Example 3

John Doe is admitted to the hospital. He is a kidney dialysis patient who has missed his last two appointments at the dialysis center. His arterial blood gas values are reported as follows:

PATIENT: John Doe
DATE: 7/11/07 22:20
pH: 7.32
PaCO2: 32
HCO3-: 18

Follow the Three Steps:

Assess the pH. It is low (normal 7.35-7.45); therefore, we have acidosis.
Assess the PaCO2. It is low. Normally, we would expect the pH and PaCO2 to move in opposite directions, but this is not the case. Because the pH and PaCO2 are moving in the same direction, it indicates that the acid-base disorder is primarily metabolic. In this case, the lungs, acting as the primary acid-base buffer, are now attempting to compensate by “blowing off excessive CO2”, and therefore increasing the pH.
Assess the HCO3-. It is low (normal 22-26). We would expect the pH and the HCO3- to move in the same direction, confirming that the primary problem is metabolic.

What is your interpretation? Because there is evidence of compensation (pH and PaCO2 moving in the same direction) and because the pH remains below the normal range, we would interpret this ABG result as a partially compensated metabolic acidosis.

	pH	PaCO2	HCO3-
Metabolic Acidosis	↓	↓	↓

Example 4

Jane Doe is a patient with chronic COPD being admitted for surgery. Her admission lab work reveals an ABG with the following values:

Clinical Laboratory
PATIENT: DOE, JANE
DATE: 2/16/08 17:30
pH: 7.35
PaCO2: 48
HCO3-: 28

Follow the Three Steps:

Assess the pH. It is within the normal range, but on the low side of neutral (<7.40).
Assess the PaCO2. It is high (normal 35-45). We would expect the pH and PaCO2 to move in opposite directions if the primary problem is respiratory.
Assess the HCO3-. It is also high (22-26). Normally, the pH and HCO3- should move in the same direction. Because they are moving in opposite directions, it confirms that the primary acid-base disorder is respiratory and that the kidneys are attempting to compensate by retaining HCO3.

Because the pH has returned into the low normal range, we would interpret this ABG as a fully compensated respiratory acidosis.

	pH	PaCO2	HCO3-
Respiratory Acidosis	normal, but <7.40	↑	↑

Example 5

Johnson Stone is a trauma patient with an altered mental status. His initial ABG result is as follows:

PATIENT: JOHNSON STONE
DATE: 7/12/02 07:30
pH: 7.33
PaCO2: 62
HCO3: 35

Follow the Three Steps:

Assess the pH. It is low (normal 7.35-7.45). This indicates that an acidosis exists.
Assess the PaCO2. It is high (normal 35-45). The pH and PaCO2 are moving in opposite directions, as we would expect if the problem were primarily respiratory in nature.
Assess the HCO3. It is high (normal 22-26). Normally, the pH and HCO3 should move in the same direction. Because they are moving in opposite directions, it also confirms that the primary acid-base disorder is respiratory in nature. In this case, the kidneys are attempting to compensate by retaining HCO3 in the blood in an order to return the pH back towards its normal range. Because there is evidence of compensation occurring (pH and HCO3 moving in opposite directions), and seeing that the pH has not yet been restored to its normal range, we would interpret this ABG result as a partially compensated respiratory acidosis.

Example 6

Jane MAC is a 54-year-old female admitted for an ileus. She had been experiencing nausea and vomiting. An NG tube has been in place for the last 24 hours. Here are the last ABG results:

PATIENT: JANEMAC
DATE: 3/19/02 07:30
pH: 7.43
PaCO2: 48
HCO3: 36

Follow the Three Steps:

Assess the pH. It is normal, but on the high side of neutral (>7.40).
Assess the PaCO2. It is high (normal 35-45). Normally, we would expect the pH and PaCO2 to move in opposite directions. In this case, they are moving in the same direction indicating that the primary acid-base disorder is metabolic in nature. In this case, the lungs, acting as the primary acid-base buffer system, are retaining CO2 (hypoventilation) in order to help lower the pH back towards its normal range.
Assess the HCO3. It is high (normal 22-26). Because it is moving in the same direction, as we would expect, it confirms the primary acid-base disorder is metabolic in nature.

pH	PaCO2	HCO3-
Metabolic Alkalosis	Normal, but >7.40	↑	↑

What is your interpretation? Because there is evidence of compensation occurring (pH and PaCO2 moving in the same direction) and because the pH has effectively been returned to within its normal range, we would call this fully compensated metabolic alkalosis.

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