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Introduction ⬇

Total Carbon Dioxide Content (TCO2)

In normal blood plasma, more than 95% of the total CO2 content (TCO2) is contributed by bicarbonate ion (HCO3−), which is regulated by the kidneys. The other 5% is contributed by the dissolved CO2 gas and by carbonic acid (H2CO3). Dissolved CO2 gas, which is regulated by the lungs, therefore contributes little to the TCO2, and the TCO2 gives little information about the lungs.

The HCO3− in the extracellular spaces exists first as CO2 and then as H2CO3; later, much of it is changed to sodium bicarbonate (NaHCO3) by the buffers in the plasma and erythrocytes. This test is a general measure of the alkalinity or acidity of venous, arterial, or capillary blood. It measures the CO2 contributions from dissolved CO2 gas, total H2CO3, HCO3−, and carbaminohemoglobin (CO2HHb).

Procedure ⬆ ⬇

  1. Collect a venous or arterial blood sample of 5 mL in a heparinized syringe. (See Chapter 2 for venous and arterial blood collection.)

  2. Label the sample with patient’s name, identification number, date, time, mode of O2 therapy, and flow rate.

  3. Place the sample on ice and transfer it to the blood gas laboratory.

  4. Use the following formula:

Clinical Implications ⬆ ⬇

  1. Increased TCO2 occurs in:

    1. Severe vomiting

    2. Emphysema

    3. Aldosteronism

    4. Use of mercurial diuretics

  2. Decreased TCO2 occurs in:

    1. Severe diarrhea

    2. Starvation

    3. Acute kidney injury

    4. Salicylate toxicity

    5. Diabetic acidosis

    6. Use of chlorothiazide diuretics

Table 14.4 presents the changes in pH, HCO3−, and PaCO2 that occur in various ventilatory disturbances and acid–base imbalances.

Clinical Alert

  1. A double use of the term CO2 is one of the main reasons why understanding of acid–base problems may be difficult. Use the terms CO2 content and CO2 gas to avoid confusion. Remember the following:

    1. CO2content (i.e., TCO2) is mainly bicarbonate and a base. It is a solution and is regulated by the kidneys.

    2. CO2gas is mainly acid. It is regulated by the lungs.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain the purpose, benefits, and risks of arterial blood sampling. Assess the patient’s ability to comply with the procedure.

  2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed.

  2. Assess, monitor, and intervene appropriately for acid–base imbalances.

  3. Monitor and intervene for bleeding at the puncture site and for respiratory or ventilatory disturbances.

  4. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆ ⬇

A number of drugs can either increase or decrease TCO2.

Reference Values ⬆

Normal

  • 23–30 mEq/L or 23–30 mmol/L

Clinical Alert

Critical ValueCo2 content: 6.0 mEq/L (6.0 mmol/L