Synonym/Acronym
CO2 combining power, CO2, TCO 2.
Rationale
To assess the effect of total carbon dioxide levels on respiratory and metabolic acid-base balance.
Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction.
Normal Findings
Method: Colorimetry, enzyme assay, or PCO 2 electrode.
| Carbon Dioxide | Conventional and SI Units |
|---|
| Plasma or serum (venous) | | | Infant–2 yr | 13–29 mEq/L or mmol/L | | 2 yr–older adult | 23–29 mEq/L or mmol/L | | Whole blood (venous) | | | Infant–2 yr | 18–28 mEq/L or mmol/L | | 2 yr–older adult | 22–26 mEq/L or mmol/L |
|
Consideration may be given to verification of critical findings before action is taken. Policies vary among facilities and may include requesting immediate recollection and retesting by the laboratory or retesting using a rapid point-of-care testing instrument at the bedside, if available.
Observe the patient for signs and symptoms of excessive or insufficient CO2 levels, and report these findings to the HCP. If the patient has been vomiting for several days and is breathing shallowly, or if the patient has had gastric suctioning and is breathing shallowly, this may indicate elevated CO2 levels. Decreased CO2 levels are evidenced by deep, vigorous breathing and flushed skin.
Study type: Blood collected in a gold-, red-, red/gray-, green-top [lithium or sodium heparin] tube or heparinized syringe; related body system: Respiratory and Urinary systems.
Serum or plasma carbon dioxide (CO2) measurement is usually done as part of an electrolyte panel. Total CO2 (TCO 2) is an important component of the body’s buffering capability, and measurements are used mainly in the evaluation of acid-base balance. It is important to understand the differences between TCO 2 (CO2 content) and CO2 gas (PCO 2). Total CO2 reflects the majority of CO2 in the body, mainly in the form of bicarbonate (HCO3 -); is present as a base; and is regulated by the kidneys. CO2 gas contributes little to the TCO 2 level, is acidic, and is regulated by the lungs (see study titled “Blood Gases”).
CO2 provides the basis for the principal buffering system of the extracellular fluid system, which is the bicarbonate–carbonic acid buffer system. CO2 circulates in the body either bound to protein or physically dissolved. Constituents in the blood that contribute to TCO 2 levels are bicarbonate, carbamino compounds, and carbonic acid (carbonic acid includes undissociated carbonic acid and dissolved CO2). Bicarbonate is the second-largest group of anions in the extracellular fluid (chloride is the largest). TCO 2 levels closely reflect bicarbonate levels in the blood, because 90% to 95% of CO2 circulates as HCO3 -.
Factors That May Alter the Results of the Study
- Drugs and other substances that may cause an increase in TCO 2 levels include acetylsalicylic acid, aldosterone, barbiturates, bicarbonate, carbenicillin, corticosteroids, dexamethasone, ethacrynic acid, and laxatives (chronic misuse).
- Drugs and other substances that may cause a decrease in TCO 2 levels include acetazolamide, acetylsalicylic acid (initially), amiloride, ammonium chloride, fluorides, furosemide, hydrochlothiazide, mannitol, metformin, methicillin, metolazone, nitrofurantoin, paraldehyde, spironolactone, tetracycline, triamterene, and xylitol.
- Prompt and proper specimen processing, storage, and analysis are important to achieve accurate results. The specimen should be stored under anaerobic conditions after collection to prevent the diffusion of CO2 gas from the specimen. Falsely decreased values result from uncovered specimens. It is estimated that CO2 diffuses from the sample at the rate of 6 mmol/hr.
Increased In
Interpretation requires clinical information and evaluation of other electrolytes.
- Acute intermittent porphyria (related to severe vomiting associated with acute attacks)
- Airway obstruction (related to impaired elimination from weak breathing responses)
- Asthmatic shock (related to impaired elimination from abnormal breathing responses)
- Brain tumor (related to abnormal blood circulation)
- Bronchitis (chronic) (related to impaired elimination from weak breathing responses)
- Cardiac disorders(related to lack of blood circulation)
- Chronic obstructive pulmonary disease (COPD) (related to impaired elimination from weak breathing responses)
- Depression of respiratory center (related to impaired elimination from weak breathing responses)
- Electrolyte disturbance (severe) (response to maintain acid-base balance)
- Hypothyroidism (related to impaired elimination from weak breathing responses)
- Hypoventilation(related to impaired elimination from weak breathing responses)
- Metabolic alkalosis (various causes; excessive vomiting)
- Myopathy (related to impaired ventilation)
- Pneumonia(related to impaired elimination from weak breathing responses)
- Poliomyelitis(related to impaired elimination from weak breathing responses)
- Respiratory acidosis (related to impaired elimination)
- Tuberculosis (pulmonary) (related to impaired elimination from weak breathing responses)
Decreased In
Interpretation requires clinical information and evaluation of other electrolytes
- Acute kidney injury(response to buildup of ketoacids)
- Anxiety(related to hyperventilation; too much CO2 is exhaled)
- Dehydration(response to metabolic acidosis that develops)
- Diabetic ketoacidosis(response to buildup of ketoacids)
- Diarrhea (severe) (acidosis related to loss of base ions such as HCO3; most of CO2 content is in this form)
- High fever (response to neutralize acidosis present during fever)
- Metabolic acidosis (response to neutralize acidosis)
- Respiratory alkalosis (hyperventilation; too much CO2 is exhaled)
- Salicylate intoxication (response to neutralize related metabolic acidosis)
- Starvation(CO2 buffer system used to neutralize buildup of ketoacids)
Before the Study: Planning and Implementation
Teaching the Patient What to Expect
- Discuss how this test can assist in measuring the amount of carbon dioxide in the body.
- Explain that a blood sample is needed for the test.
Safety Considerations
- Confusion caused by acid-base imbalances and increased fall and injury risk.
- Timely interventions may include medication evaluation; fall and injury prevention through appropriate use of postural support, bed alarm, or the appropriate use of restraints; pharmacological interventions; and accurate intake and output to assess fluid status.
After the Study: Implementation & Evaluation Potential Nursing Actions
Avoiding Complications
- Timely and appropriate interventions are necessary to reverse the effects of acid-base imbalances.
Treatment Considerations
- Identify the underlying cause of the metabolic disturbance.
- Assess for a history of respiratory disease.
- Monitor and trend vital signs: heart rate, blood pressure, pulse rate, and respirations.
- Assess, monitor, and trend both cardiac and peripheral vascular status.
- Administer ordered sodium bicarbonate and evaluate effectiveness.
- Monitor and trend arterial blood gas.
- Administer oxygen as ordered and monitor saturation.
- Encourage slow deep breathing to slow respiratory rate.
- Provide emotional support to reduce anxiety and enhance coping skills.
- Treat the medical condition and correlate confusion with the need to correct altered electrolytes.
Nutritional Considerations
- Abnormal CO2 values may be associated with diseases of the respiratory system.
- Malnutrition is commonly seen in patients with severe respiratory disease related to fatigue, lack of appetite, and gastrointestinal distress.
- Research estimates the daily caloric intake required for respiration of patients with chronic obstructive pulmonary disease is 10 times higher than that of healthy individuals.
- Emphasize the importance of adequate intake of vitamins A and C to prevent pulmonary infection and to decrease the extent of lung tissue damage.
- Stress the importance of following the prescribed diet as a therapeutic intervention.
Clinical Judgement
- Consider how respiratory disease impacts the ability to breathe and how to decrease anxiety associated with fears of suffocation.
Follow-Up and Desired Outcomes
- Agrees to follow the therapeutic regime recommended by the HCP.
- Agrees to consult with a registered dietician to address dietary concerns.