Arterial Blood gas analysis (ABG), is a test that measures the amount of oxygen (O2) and carbon dioxide ( CO2) in the blood, as well as the acidity (pH) of the blood.
It is an essential part of diagnosing and managing a patient’s oxygenation status and acid-base balance. The usefulness of this diagnostic tool is dependent on being able to correctly interpret the results.
Purpose of Arterial Blood Gas Analysis
- Evaluates how effectively the lungs are delivering oxygen to the blood and how efficiently they are eliminating carbon dioxide from it.
- It indicates how well the lungs and kidneys are interacting to maintain normal blood pH (acid-base balance).
- To assess respiratory disease and other conditions that may affect the lungs, and to manage patients receiving oxygen therapy (respiratory therapy.
- To determine the pH of the blood and the partial pressures of carbon dioxide (PaCO2) and oxygen (PaO2) within it.
- To assess the effectiveness of gaseous exchange and ventilation, be it spontaneous or mechanical.
- It allows patients' metabolic status to be assessed, giving an indication of how they are coping with their illness.
If the pH becomes deranged, normal cell metabolism is affected.
For you to understand Arterial Blood Gas analysis you will need to have knowledge about the acid-base balance, buffer systems and acid-base balance disorders which we shall look at briefly before we proceed.
Overview of Acid-Base Balance and Its Disorders
The pH is a measurement of the acidity or alkalinity of the blood.
The more H+ present, the lower the pH will be, the fewer H+ present, the higher the pH will be.
The pH of a solution is measured on a scale from 1 (very acidic) to 14 (very alkalotic).
A liquid with a pH of 7, such as water, is neutral (neither acidic nor alkalotic).
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,
- A diminished response to the effects and actions of certain medications
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 functioning.
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.
The body self-regulate acid-base balance in order to maintain pH within the normal range through the use of delicate buffer mechanisms between the respiratory and renal systems.
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
The carbonic acid present triggers the lungs to either increase or decreases 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 from hours to days to correct the imbalance.
When the respiratory system and renal systems are working together, they are able to keep the blood pH balanced by maintaining 1 part acid to 20 parts
Acid-Base Disorders
Respiratory Acidosis:
It 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
Causes of respiratory acidosis
Any condition that results in hypoventilation can cause respiratory acidosis. These conditions include:
- Central nervous system depression related to head injury
- Central nervous system depression related to 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
Pulmonary symptoms include dyspnea, respiratory distress, and/or shallow respirations.
Nervous system manifestations include headache, restlessness, and confusion.
If CO2 levels become extremely high, drowsiness and unresponsiveness may be noted.
Cardiovascular symptoms include tachycardia and dysrhythmias.
Increasing ventilation will correct respiratory acidosis.
The method for achieving this will vary with the cause of hypoventilation.
Causes that can be treated rapidly include pain pneumothorax and CNS depression related to medications. If the cause cannot be readily resolved, the patient may require mechanical ventilation while treatment is rendered.
Although patients with hypoventilation often require supplemental oxygen, it is important to remember that oxygen alone will not correct the problem
Respiratory Alkalosis
Respiratory alkalosis is defined as a pH greater than 7.45 with a PaCO2 less than 35 mm Hg.
Causes of respiratory acidosis
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
Nervous system alterations include light-headedness, numbness and tingling, confusion, inability to concentrate, and blurred vision.
Cardiac symptoms include dysrhythmias and palpitations.
Additionally, the patient may experience dry mouth, diaphoresis, and tetanic spasms of the arms and legs.
Treatment of respiratory alkalosis centers on resolving the underlying problem.
Patients presenting with respiratory alkalosis have dramatically increased work of breathing and must be monitored closely for respiratory muscle fatigue.
When the 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 of less than 7.35.
Causes of metabolic acidosis
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 the base.
Causes of increased acids include:
- Renal failure
- Diabetic ketoacidosis
- Anaerobic metabolism
- Starvation
- Salicylate intoxication
Signs and symptoms
Nervous system manifestations include headache, confusion, and restlessness progressing to lethargy, then stupor or coma.
Cardiac dysrhythmias are common and Kussmaul respirations occur in an effort to compensate for the pH by blowing off more CO2.
Warm, flushed skin, as well as nausea and vomiting, are commonly noted.
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.
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.
Causes of metabolic alkalosis
Either an excess of base or a loss of acid within the body can cause metabolic alkalosis.
Excess base occurs from the 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.
Signs and symptoms
Neurologic symptoms include dizziness, lethargy, disorientation, seizures, and coma.
Musculoskeletal symptoms include weakness, muscle twitching, muscle cramps, and tetany.
The patient 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 (Diamox™), but the resolution of the imbalance will be slow.
Information provided by an Arterial Blood Gas Analysis
PaCO2
This is the partial pressure of carbon dioxide dissolved within the arterial blood. It is used to assess the effectiveness of ventilation.
A high PaCO2 (respiratory acidosis) indicates underventilation.
A low PaCO2 (respiratory alkalosis) indicates hyper- or overventilation.
The normal range for a healthy person is 4.7-6.0 kPa or 35-45 mmHg although in chronic pulmonary diseases it may be considerably higher and still normal for that patient. (CPOD).
PaO2
This is the partial pressure of oxygen dissolved within the arterial blood and will determine oxygen binding to hemoglobin (SaO2).
It is of vital importance but is not used in determining patients' acid-base status and normally low readings indicate hypoxemia.
The normal range -9.3-13.3 kPa or 80-100 mmHg.
SaO2
Oxygen saturation measures how much of the hemoglobin (Hb) in the red blood cells are carrying oxygen (O2). Although similar to SpO2 (measured by a pulse oximeter), it is more accurate.
The normal levels are 97% and above, although levels above 90% are often acceptable in critically ill patients.
pH
The pH measures hydrogen ions (H+) in blood.
The pH of blood usually between 7.35 to 7.45.
A pH of less than 7.0 is called acid and a pH greater than 7.0 is called basic (alkaline). So blood is slightly basic.
HCO3 (Bicarbonate)
Bicarbonate is a chemical (buffer) that keeps the pH of blood from becoming too acidic or too basic & indicates whether a metabolic problem is present (such as ketoacidosis).
A low HCO3- indicates metabolic acidosis, a high HCO3- indicates metabolic alkalosis.
HCO3- levels can also become abnormal when the kidneys are working to compensate for a respiratory issue so as to normalize the blood pH.
Normal range - 22–26 mmol/l.
Base Excess (BE)
The base excess is used for the assessment of the metabolic component of acid-base disorders and indicates whether the patient has metabolic acidosis or metabolic alkalosis.
A negative base excess indicates that the patient has metabolic acidosis (primary or secondary to respiratory alkalosis).
A positive base excess indicates that the patient has metabolic alkalosis (primary or secondary to respiratory acidosis).
Normal range - -2 to +2 mmol/l.
When the Arterial Blood Gas Analysis test is ordered?
Arterial blood gas analysis tests are ordered there symptoms of an O2/CO2 or pH imbalance, such as difficulty breathing or shortness of breath.
Also if there are known respiratory, metabolic, or kidney disease.
Those experiencing respiratory distress to evaluate oxygenation and acid/base balance.
Patients who are “on oxygen” (have supplemental oxygen) may have their blood gases measured at intervals to monitor the effectiveness of treatment.
patients with head or neck trauma, injuries that may affect breathing.
Patients undergoing prolonged anesthesia – particularly for cardiac bypass surgery or brain surgery – may have their blood gases monitored during and for a period after the procedure.
Extraction and analysis
Blood is most commonly drawn from the radial artery because it is easily accessible, can be compressed to control bleeding, and has less risk for occlusion.
The femoral artery (or less often, the brachial artery) is also used, especially during emergency situations or with children.
Blood can also be taken from an arterial catheter already placed in one of these arteries.
The syringe is pre-packaged and contains a small amount of heparin, to prevent coagulation or needs to be heparinized.
Eliminate visible gas bubbles from the sample, 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 is chilled in an ice bath in a glass syringe to slow metabolic processes which can cause inaccuracy.
Samples drawn in plastic syringes should not be iced and should always be analyzed within 30 minutes.
The machine used for analysis aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen and carbon dioxide.
The bicarbonate concentration is also calculated.
These results are usually available for interpretation within five minutes.
Steps to an Arterial Blood Gas Analysis Interpretation
The arterial blood gas is used to evaluate both acid-base balance and 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
- Evaluate arterial blood gas results.
- Normal values are:
- pH 7.35 - 7.45
- PCO2 35 - 45mmHg
- PO2 80 - 100mmHg
- SO2 95 - 100%
- Hco3 22 - 26mmols
- BE -2 - +2 mmol
Step 1
Look at PH it can be: low, high or normal
pH > more 7.45 (alkalosis)
pH < less 7.35 (acidosis)
pH 7.35 7.45 (normal)
Step 2
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 in respiratory problems, 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 PaCO2 values. If pH and PaCO2 are indeed moving in opposite directions, then the problem is primarily respiratory in nature.
Step 3
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.
Determine the primary cause of the disturbances
It is done by evaluating the Paco2 and HCO3 in relation to pH
PH > more 7.4 (Alkalosis)
a) If the PCO2 is less than 40 mmHg the primary disturbance is respiratory alkalosis.
b) If the PCO2 is more than 45 mmHg the primary disturbance is respiratory acidosis.
c) If the HCO3 is less than 22mmols the primary disturbance is metabolic acidosis.
d) If the HCO3 is more than 26 mmols the primary disturbance is metabolic alkalosis.
Compensation
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 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 order to look for evidence of partial compensation, review the following three steps:
1. Assess the pH.
This step remains the same and allows us to determine if an acidotic or alkalotic state exists.
2. 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.
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.
The Lungs 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 partial respiratory compensation.
Partially compensated states
Assess HCO3.
If the pH and HCO3 move in the same direction it indicates 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.
Fully compensated states
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.
