Arterial Blood Gases
Test Description
Arterial blood gases (ABGs) are performed when information is needed regarding
the acid-base status of the patient. The acid-base balance of the body is controlled
via three mechanisms: the buffering system, the respiratory system, and the renal
system.
The buffering system assists with maintaining acid-base balance through the
retention or loss of hydrogen ions (H+). There are also minor buffers in the blood in
the form of phosphates and proteins.
The effect of the respiratory system occurs through the carbonic acid bicarbonate
buffer system. In order for the pH of the blood to be within the normal
range, these two substances need to be in a 20:1 ratio—20 parts bicarbonate for
every 1 part carbonic acid (H2CO3). The breakdown of carbonic acid forms carbon
dioxide and water, thus the carbonic acid level can be measured indirectly with the
PCO2 level. The PCO2 level is controlled by the lungs. The lungs are able to respond
relatively quickly to changes in the acid-base balance of the body through the
amount of CO2 retained. The more CO2 retained, the more carbonic acid in the body,
which leads to a state known as acidosis. When less CO2 is retained, the result is
less carbonic acid in the body, or alkalosis.
Although the lungs are capable of making rapid changes in the acid-base balance
of the body, they are only approximately 80% efficient since they must continue
to be the site of oxygen exchange. In order to bring the body into a normal
acid-base balance once again, another mechanism must be involved. This mechanism
involves the kidneys. The kidneys make changes in the acid-base balance at
a slower rate than the lungs, taking several days for their effect to be fully noted.
The kidneys regulate the pH of the blood through the excretion or retention of H+
ions, bicarbonate (HCO3
–), sodium, potassium, and chloride.
Unlike the lungs, the kidneys are 100% efficient; that is, they will continue to
work on the acid-base problem until either the pH of the blood returns to the normal
range, a state known as full compensation, or the condition worsens. If all of
the compensatory mechanisms (buffering system, lungs, and kidneys) are unsuccessful
in bringing the acid-base imbalance under control, the problem progresses.
Severe acidotic states lead to coma and death, due to depression of the central
nervous system. Alkalotic states stimulate the central nervous system, leading to
irritability, tetany, and possibly death. Acidotic states are generally considered more
life-threatening than alkalotic states.
THE EVIDENCE FOR PRACTICE
In patients with known chronic obstructive pulmonary disease (COPD), it is recommended
that arterial blood gases be obtained with O2 saturation of less than 88%, positive history of
hypercapnia, questionable accuracy of oximetry, somnolence, or other evidence of impending
respiratory failure (e.g., respiratory rate greater than 40 breaths per minute).
pH
The pH of the blood is the negative logarithm of the H+ ion concentration in the blood. For
example, if the H+ ion concentration is 1 × 10−7, the pH is 7; if the H+ ion concentration is 1 ×
10−6, the pH is 6. Thus, the pH of 6 has a higher H+ ion concentration and is more acidic. A blood
pH of normal range is needed for many of the chemical reactions in the body to take place. The
normal range of pH for arterial blood is 7.35 to 7.45. Blood pH less than 7.35 is considered
acidemia or acidosis. Blood pH greater than 7.45 is considered alkalemia or alkalosis.
Normal Values 7.35–7.45
Contributing Factors to Abnormal Values
• Drugs which may increase the blood pH (more alkaline): sodium bicarbonate.
PARTIAL PRESSURE OF CARBON DIOXIDE (PCO2, PACO2 )
The partial pressure of carbon dioxide in the arterial blood, designated as PaCO2, is the
amount of pressure exerted by CO2 dissolved in the blood. It is measured in millimeters of
mercury (mm Hg) or torr (1 torr = 1 mm Hg). The normal range for PaCO2 is 35 to 45 torr,
but lower values are normal at higher altitudes where the atmospheric pressure is decreased.
When the lungs retain CO2, the level of CO2 in the blood increases. This is known as
hypercarbia or hypercapnia, which is an acidotic state. This problem is exhibited in clinical
signs and symptoms of headache, dizziness, and decreasing levels of consciousness. When
the lungs expire more CO2 than normal, the level of CO2 in the blood decreases, an alkalotic
state known as hypocarbia or hypocapnia. This results in the patient complaining of tingling
of the fingers, muscle twitching, lightheadedness, and dizziness.
Normal Values 35–45 torr (mm Hg)
Contributing Factors to Abnormal Values
• Failure to expel all air from the syringe will result in a falsely low PaCO2 value.
• Drugs which may increase PaCO2: aldosterone, ethacrynic acid, hydrocortisone,
metolazone, prednisone, sodium bicarbonate, thiazides.
• Drugs which may decrease PaCO2: acetazolamide, dimercaprol, methicillin, nitrofurantoin,
tetracycline, triamterene.
BICARBONATE (HCO3–)
As discussed previously, bicarbonate works with carbonic acid to help regulate the pH of
the blood. There are two ways in which bicarbonate may be measured. The first is through
direct measurement of the bicarbonate level. The second is an indirect measurement using
the values for total CO2 content and PaCO2 in the following formula:
HCO3− = Total CO2 − (0.03 × PaCO2).
When the bicarbonate level is less than 22, the value is considered acidotic;
greater than 26, alkalotic.
Normal Values :22–26 mEq/L (22–26 mmol/L)
Contributing Factors to Abnormal Values
• Drugs which may increase bicarbonate: alkaline salts, diuretics.
• Drugs which may decrease bicarbonate: acid salts.
BASE EXCESS/DEFICIT
Determination of the base excess/deficit provides information about the total buffer
anions (bicarbonate, hemoglobin, phosphates, and plasma proteins) and whether changes
in acid-base balance are respiratory or nonrespiratory (metabolic) in nature. Values below
−2 mEq/L indicate a base deficit, which correlates to a decrease in bicarbonate level.
Values greater than +2 mEq/L indicate a base excess. This information assists in the planning
of appropriate treatment for the patient.
Normal Values
ANALYSIS OF ARTERIAL BLOOD GASES
Following these steps should simplify the analysis of ABG results:
1. Determine whether the pH is acidotic (<7.35) or alkalotic (>7.45). (Note: If the pH is
normal and the PCO2 and HCO3– are abnormal .)
2. Determine whether the PCO2 is acidotic (>45) or alkalotic (<35).
3. Determine whether the HCO3− is acidotic (<22) or alkalotic (>26).
4. Compare the above 3 values and find the two, which “match” in terms of acidity/alkalinity
to determine the underlying acid-base imbalance.This step is summarized in Table A-1
5. If the third value (the one, which did not “match” in acidity/alkalinity) is: Normal: the
imbalance is uncompensated; Abnormal: The imbalance is partially compensated. For
example, if the pH and PCO2 are both acidotic and the HCO3− is alkalotic, the analysis
is a “partially compensated respiratory acidosis.”
6. If the pH is normal, but the PCO A 2 and HCO3
− are abnormal, the imbalance is considered fully
compensated. To determine the underlying, or initial imbalance, look at the base excess,
shown as BE on a laboratory report. If the base excess is normal when the pH is normal, the
underlying problem was metabolic. The HCO3
− value, which is considered the metabolic component,
is then referred to in order to determine whether the problem was acidotic or alkalotic.
The above steps are summarized in Table A-2: ABG Analysis
Possible Meanings of Abnormal Values
Respiratory Acidosis: (increased PCO2 due to hypoventilation)
Anesthesia/drugs
Asthma
Cardiac arrest
Chronic bronchitis
Congestive heart failure
Emphysema
Head trauma
Neuromuscular depression
Obesity
Pickwickian syndrome
Pneumonia
Pulmonary edema
Respiratory failure
Respiratory Alkalosis: (decreased PCO2 due to hyperventilation)
Adult cystic fibrosis
Anemia
Anxiety
Carbon monoxide poisoning
Cerebral hemorrhage
Fever
Heart failure
Hypoxia
Improperly set ventilator
Myocardial infarction
Pain
Pregnancy (third trimester)
Pulmonary emboli
Metabolic Acidosis: (decreased HCO3− due to either excess acid production or loss of
bicarbonate)
Cardiac arrest (lactic acidosis)
Diabetic ketoacidosis
Diarrhea
Renal failure
Renal tubular acidosis
Starvation (ketoacidosis)
Metabolic Alkalosis: (increased HCO3− due to either excessive intake of bicarbonate
or lactate, or increased loss of chloride, hydrogen, and potassium ions)
DiureticsHypochloremia
Hypokalemia
Ingestion of sodium bicarbonate, antacids
Nasogastric suctioning
Sodium bicarbonate infusion
Vomiting
PARTIAL PRESSURE OF OXYGEN (PAO2, PO2)
The partial pressure of oxygen in the arterial blood, designated as PaO2, is the amount of pressure
exerted by O2 dissolved in the blood. It is measured in millimeters of mercury (mm Hg)
or torr (1 torr = 1 mm Hg). The PaO2 is used to measure how effective the lungs are in oxygenating
the blood. When the value is below normal, the patient is said to be hypoxic. The
PaO2 is directly influenced by the amount of oxygen inhaled; thus, it can also be used to
assess the effectiveness of oxygen therapy.
Normal Values
75–100 torr (mm Hg)—on room, or ambient air
Elderly: value decreases with age
Possible Meanings of Abnormal Values
Increased Decreased
High doses of oxygen Anemia
Polycythemia Atelectasis
Cardiac decompensation
Emphysema
Hypoventilation
Insufficient atmospheric oxygen
Pneumonia
Pulmonary edema
Pulmonary embolism
Contributing Factors to Abnormal Values
• Falsely-elevated PaO2 levels may occur due to failure to expel all air from the
syringe when drawing the arterial blood sample.
OXYGEN CONTENT (O2, O2CT)
The amount of oxygen, which the blood can contain is based upon the amount of oxygen
carried by the hemoglobin and upon the amount of oxygen contained in the plasma. One
gram of hemoglobin can carry up to 1.34 mL of oxygen. In addition, up to 0.3 mL of oxygen
can be carried in 100 mL of blood plasma.
The oxygen content is a measurement of the actual amount of oxygen being carried in
the blood. This value is determined through the following formula:
O Content =SaO %/100%*Hgb* 1.34 (PaO*0.003)
Normal Values
Arterial: 15–22 mL/100 mL of blood (15–22%)
Venous: 11–16 mL/100 mL of blood (11–16%)
Possible Meanings of Abnormal Values
Decreased
Asthma
Chronic bronchitis
Emphysema
Flail chest
Hypoventilation
Kyphoscoliosis
Neuromuscular impairment
Obesity
Postoperative respiratory complications
OXYGEN SATURATION (SAO2, SO2, O2 SAT)
The oxygen saturation value is a comparison of the actual amount of oxygen carried by
the hemoglobin compared to the amount of oxygen which the hemoglobin is capable
of carrying. Thus, if the hemoglobin is carrying the amount of oxygen it is capable of
carrying, the oxygen saturation is approximately 100%. Oxygen saturation may be measured
with arterial blood gases or through pulse oximetry, a noninvasive procedure.
Normal Values
95–100%
Possible Meanings of Abnormal Values
Increased Decreased
Adequate oxygen therapy Carbon monoxide poisoning
Hypoxia
Contributing Factors to Abnormal Values
• The oxygen saturation is affected by the partial pressure of oxygen in the blood, by
the body temperature, by the pH of the blood, and by the structure of the hemoglobin.
Interventions/Implications
Pretest
• Explain to the patient the purpose of the test, noting that the puncture is momentarily
painful. (Note: Some institutions allow for anesthetizing the area with 1% Xylocaine.)
• Perform the Allen’s test to assess for adequate collateral circulation in the ulnar artery.
This collateral circulation is important should the radial artery become obstructed by a
thrombus following the arterial puncture.
• To perform the Allen’s test, apply pressure to both the radial and ulnar pulses of one
of the patient’s wrists until the pulses are obliterated. The hand will blanch (pale)
due to lack of circulation to the hand. Release the pressure on the ulnar artery. If the
hand returns to normal color immediately, the test is considered positive, and the
arterial puncture may be done in that wrist. If the hand remains pale, the ulnar circulation
is inadequate. The test is considered negative and the other arm should be
tested for adequacy. Should both arms be found to inappropriate sites, the use of the
femoral artery may need to be explored.
• No fasting is required prior to the test.
Procedure
• The area over the radial artery in the wrist is anesthetized with 1% Xylocaine, if allowed
per institutional policy.
• An airtight syringe containing 0.2 mL of heparin is used to draw a 3 to 5 mL arterial
blood sample. (Note: Venous blood can be used if arterial blood is unaccessible; however,
venous is useful only for evaluating pH, PaCO2, and base excess.)
• All air bubbles are expelled from the syringe. The syringe is capped to prevent loss of
gases from the sample.
• The syringe is labeled, placed in ice, and taken to the laboratory immediately for
analysis.
• Gloves are worn throughout the procedure.
Posttest
• Apply continuous pressure on the radial site for at least 5 minutes, 10 minutes if the
femoral site was used.
• Apply dressing, periodically assessing for continued bleeding, especially if the patient A
has bleeding problems or is receiving anticoagulant therapy.
• Assess the extremity for signs of circulatory impairment: changes in the color, movement,
temperature, sensation, and, if femoral site, pulses distal to the puncture site.
• Note on the laboratory slip whether the patient was breathing ambient (room) air or, if
receiving supplemental oxygen, the oxygen flow.
• Report abnormal findings to the primary care provider.
Clincal alert
• Possible complications: Circulatory impairment due to arterial occlusion, nerve
damage during arterial puncture.
CONTRAINDICATION!
• No palpable pulse in the extremity
• Negative Allen’s test in the extremity
• Any skin infection in the area of the proposed arterial puncture
• Any arteriovenous (AV) shunt in area of proposed arterial puncture
• Severe coagulopathies
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