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ventilatory failure. Table 5.9 provides an overview summary of acute ventilatory failure superimposed on chronic ventilatory failure and acute alveolar hyperventilation superimposed on chronic ventilatory failure, in relationship to typical baseline ABG values of a patient with chronic ventilatory failure.

TABLE 5.8

Acute Ventilatory Failure Superimposed on Chronic Ventilatory Failure (Acute Hypoventilation on Compensated Respiratory Acidosis)

TABLE 5.9

Overview Examples of Acute Changes in Chronic Ventilatory Failure

Metabolic Acid-Base Abnormalities

Metabolic acid-base disturbances are subdivided into the following two categories: metabolic acidosis and metabolic alkalosis (see Box 5.1). An overview of the metabolic acid-base disturbances are presented in the following section.

Metabolic Alkalosis

The presence of other bases not related to either a decreased PaCO2 level or renal compensation also can be identified by using the PCO2/HCO3–/pH nomogram illustrated in Fig. 5.1. The presence of metabolic alkalosis is verified when the calculated HCO3– and pH readings are both higher than expected for a particular PaCO2 level in terms of the absolute PCO2/HCO3–/pH relationship. For example, according to the normal blood buffer line, an HCO3– reading of 35 mEq/L and a pH level of 7.54 would both be higher than expected in a patient who has a PaCO2 level of 40 mm Hg (see Fig. 5.1). This

extremely common condition is known as metabolic alkalosis. Table 5.10 provides an example of metabolic alkalosis. Box 5.4 provides common causes of metabolic alkalosis.

TABLE 5.10

Metabolic Alkalosis

Box 5.4

Common Causes of Metabolic Acid-Base Abnormalities

Metabolic Acidosis

•Lactic acidosis (most common)

•Ketoacidosis (most commonly associated with diabetes mellitus)

•Salicylate intoxication (aspirin overdose)

•Renal failure

•Chronic diarrhea

Metabolic Alkalosis

•Hypokalemia

•Hypochloremia

•Gastric suctioning

•Vomiting

•Excessive administration of corticosteroids

•Excessive administration of sodium bicarbonate

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acids must be identified for the patient to be appropriately treated. For example, metabolic acidosis caused by lactic acids requires oxygen therapy to reverse the accumulation of the lactic acids. Metabolic acidosis caused by ketone acids requires insulin therapy to help facilitate the movement of glucose into the cells and normalize metabolism.

It is interesting to note that metabolic acidosis caused by an excessive loss of HCO3– (e.g., from renal disease or severe diarrhea) does not cause an increase in the anion gap. This is because as the HCO3– level decreases, the Cl− level usually increases to maintain electroneutrality. In short, for every HCO3– ion lost, a Cl− anion takes its place (i.e., the law of electroneutrality). This action maintains a normal anion gap. Metabolic acidosis caused by decreased HCO3– with an

increased Cl− is commonly called hyperchloremic metabolic acidosis.

Thus when metabolic acidosis is accompanied by an increased anion gap, the most likely cause of the acidosis is the accumulation of fixed acids. When metabolic acidosis is seen with a normal anion gap, the most likely cause of the acidosis is an excessive loss of HCO3– (e.g., caused by renal failure or severe diarrhea).

Errors Associated With Arterial Blood Gas Measurements

Because an ABG error can occur before, during, or after the analysis of the sample, the respiratory therapist must always be on alert for ABG results that do not fit the patient's current clinical condition. Accurate and efficient ABG samplings, along with the correct analysis and interpretation of ABG values, are no simple matters, as anyone who has performed these procedures can testify. Fast and precisely accurate ABG results are often life-critical!

In general, the types of ABG errors can be classified as (1) preanalytic errors, (2) analytic errors, (3) postanalytic errors, and (4) interpretation errors. Preanalytic errors include errors that occur either before or after the sample analysis—for example, improper sample or data handling. Analytic errors include errors that occur during the actual analysis of the ABG sample—for example, blood gas machine malfunctions and poor individual technique. Postanalytic errors include the recording of the ABG results after analysis—for example, incorrect patient name or FIO2 setting. Interpretation errors are

the incorrect classification of any ABG results (see Box 5.1). Table 5.13 provides an overview of common errors of ABG measurements.

TABLE 5.13

Common Errors of Arterial Blood Gas Measurements

Misidentification of patient

Inappropriately transported sample

Analytic Errors Include

Poor quality assurance (QA) and quality control (QC) programs*

Malfunctioning PO2 and PCO2 electrodes

Out-of-date reagents (cleaning, rinse, and calibration solutions)

Postanalytic Errors Include

Incorrect patient name/location/demographics on report Typographic and transcription errors (e.g., 74 instead of 47 mm Hg) Incorrect FIO2 or ventilator setting on patient chart

Failure to wait at least 20 minutes after an FIO2 or ventilator setting change before sampling

Incorrect sampling time recorded

Failure to notify appropriate personnel of critical results (e.g., impending ventilatory failure and/or acute ventilatory failure)

Slow turnaround time for results to get back to the patient's bedside to be interpreted by the medical staff

Interpretation Errors Include

The incorrect interpretation of any of the acid-base disturbances discussed in this chapter (see Box 5.1) Interpretation errors can result in serious harm and/or death to the patient (e.g., failure to correctly identify

impending ventilatory failure, or acute ventilatory failure, or to identify a mixed acid-base disorders)

*NOTE: Two quick internal checks of blood gas accuracy entail (1) calculating the Henderson-Hasselbalch equation to determine if the measured arterial blood gas (ABG) values correlate and (2) calculating the alveolar-arterial oxygen gradient for the same purpose. The Henderson-Hasselbalch equation ensures that the pH, PCO2,

and HCO3– determinations are at least internally consistent. The alveolar-arterial oxygen gradient ensures that the FIO2, PaO2, and PaCO2 calculations are at least

reasonable. For a complete review of these two equations, see Des Jardins, T. (2019). Cardiopulmonary anatomy and physiology: essentials of respiratory care (7th ed.). Clifton Park, NY: Delmar/Cengage Learning.

The “take home message” from this brief section is to be aware of the common sources of ABG errors and be ready to repeat the test if the ABG data do not correlate with the clinical situation. Unexpected or questionable ABG results should always be thoroughly investigated. Failure to do so is unacceptable! Noninvasive “reality checks” on ABG results include observation of the patient's sensorium and vital signs, presence or absence of cyanosis, and the timely readings from pulse oximeters and transcutaneous PO2 and PCO2 electrodes.

Self-Assessment Questions

1.During acute alveolar hyperventilation, which of the following occurs?

1.HCO3– decreases.

2.PaCO2 increases.

3.HCO3– increases.

4.PaCO2 decreases.

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a.2 only

b.3 only

c.1 and 4 only

d.2, 3, and 4 only

2.When lactic acidosis is present, which of the following will occur? 1. pH will likely be lower than expected for a particular PaCO2.

2.HCO3– will likely be higher than expected for a particular PaCO2.

3.pH will likely be higher than expected for a particular PaCO2.

4.HCO3– will likely be lower than expected for a particular PaCO2.

a.2 only

b.3 only

c.2 and 3 only

d.1 and 4 only

3.What is the clinical interpretation of the following ABG values (in addition to hypoxemia)? pH: 7.17

PaCO2: 77 mm Hg

HCO3–: 27 mEq/L

PaO2: 54 mm Hg

a.Acute alveolar hyperventilation superimposed on chronic ventilatory failure

b.Acute ventilatory failure

c.Acute alveolar hyperventilation

d.Acute ventilatory failure superimposed on chronic ventilatory failure

4.A 74-year-old man with a long history of emphysema and chronic bronchitis enters the emergency department in respiratory distress. His respiratory rate is 34 breaths per minute and labored. His heart rate is 115 beats per minute, and his blood pressure is 170/120. What is the clinical interpretation of the following ABG values (in addition to hypoxemia)?

pH: 7.51

PaCO2: 68 mm Hg HCO3–: 52 mEq/L PaO2: 49 mm Hg

a.Acute alveolar hyperventilation superimposed on chronic ventilatory failure

b.Acute ventilatory failure

c.Acute alveolar hyperventilation

d.Acute ventilatory failure superimposed on chronic ventilatory failure

5.Which of the following is classified as metabolic acidosis?

a.pH 7.23; PaCO2 63; HCO3– 26; PaO2 52

b.pH 7.16; PaCO2 38; HCO3– 13; PaO2 86

c.pH 7.56; PaCO2 27; HCO3– 23; PaO2 101

d.pH 7.64; PaCO2 49; HCO3– 51; PaO2 91

6.Which of the following cause metabolic acidosis?

1.Hypokalemia

2.Renal failure

3.Excessive administration of sodium bicarbonate

4.Hypochloremia

a.1 only

b.2 only

c.1 and 4 only

d.2 and 3 only

7.Using the general rule of thumb for the PCO2/HCO3–/pH relationship, if the PaCO2 suddenly increased to 90 mm Hg in a patient who normally has a pH of 7.40, a PaCO2 of 40 mm Hg, and an HCO3– of 24 mEq/L, the pH will decrease to approximately what level?

a.7.15

b.7.10

c.7.05

d.7.00

8.Which of the following is classified as metabolic alkalosis?

a.pH 7.23; PaCO2 63; HCO3– 26; PaO2 52

b.pH 7.16; PaCO2 38; HCO3– 13; PaO2 86

c.pH 7.56; PaCO2 27; HCO3– 23; PaO2 101

d.pH 7.64; PaCO2 44; HCO3– 46; PaO2 91

9.Lactic acidosis develops from which of the following?

1. Inadequate tissue oxygenation

2.Renal failure

3.An inadequate insulin level

4.Anaerobic metabolism

5.An inadequate glucose level

a.1 only

b.2 only

c.1 and 4 only

d.3 and 5 only

10.Metabolic alkalosis can develop from which of the following?

1.Hyperchloremia

2.Hypokalemia

3.Hypochloremia

4.Hyperkalemia

a.4 only

b.1 and 3 only

c.1 and 4 only

d.2 and 3 only

11.During acute alveolar hypoventilation, the blood:

1.HCO3– increases

2.pH decreases

3.PCO2 increases

4.HCO3– decreases

a.2 only

b.4 only

c.2 and 3 only

d.1, 2, and 3 only

12.During acute alveolar hyperventilation, the blood:

1.PCO2 increases

2.HCO3– increases

3.HCO3– decreases

4.pH increases

a.2 only

b.4 only

c.1 and 3 only

d.3 and 4 only

13.In chronic hypoventilation, kidney compensation has likely occurred when the:

1.HCO3– is higher than expected for a particular PaCO2

2.pH is lower than expected for a particular PaCO2

3.HCO3– is lower than expected for a particular PaCO2

4.pH is higher than expected for a particular PaCO2

a.1 only

b.2 only

c.1 and 4 only

d.3 and 4 only

14.Which of the following represents acute alveolar hyperventilation?

a.pH 7.56; PaCO2 51; HCO3– 44

b.pH 7.45; PaCO2 37; HCO3– 25

c.pH 7.53; PaCO2 46; HCO3– 29

d.pH 7.58; PaCO2 26; HCO3– 21

15.Which of the following represents compensated metabolic alkalosis?

a.pH 7.55; PaCO2 21; HCO3– 19

b.pH 7.52; PaCO2 45; HCO3– 29

c.pH 7.45; PaCO2 26; HCO3– 18

d.pH 7.45; PaCO2 61; HCO3– 41

1For a complete review of the role of the relationship in acid-base balance, see Des Jardins, T. (2019). Cardiopulmonary anatomy and physiology: essentials of respiratory care (7th ed.). Clifton Park, NY: Delmar/Cengage Learning.

2The isobars on the PCO2/HCO3–/pH nomogram illustrate the pH changes that develop in the blood as a result of metabolic changes (i.e., HCO3– changes) or a combination of metabolic and respiratory (CO2) changes.

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Mixed Venous Blood

Mixed Venous Oxygen Saturation () Moderate Hypoxemia

Nanostraws

Oxygen Consumption (VO2)

Oxygen Content of Arterial Blood (CaO2)

Oxygen Content of Mixed Venous Blood () Oxygen Content of Pulmonary Capillary Blood (CcO2) Oxygen Extraction Ratio (O2ER)

Oxyhemoglobin Dissociation Curve

Oxyhemoglobin Equilibrium Curve PaO2/FIO2 Ratio

PaO2/PAO2 Ratio Polycythemia

Pulmonary Capillary Blood (CcO2)

Pulmonary Shunt Fraction (QS/QT) Severe Hypoxemia Thermodilution

Total Oxygen Delivery (DO2)

Oxygen transport between the lungs and the metabolizing cells is a function of the blood itself and the cardiovascular system (blood vessels and heart). Oxygen is carried in the blood in two ways: as dissolved oxygen in the blood plasma and oxygen bound to the hemoglobin (Hb). Most oxygen is carried to the tissue cells bound to hemoglobin. The following pages provide the essential knowledge cornerstones needed to effectively and safely assess the patient's oxygenation status.

Oxygen Transport Review

Oxygen Dissolved in the Blood Plasma

A small amount of oxygen that diffuses from the alveoli to the pulmonary capillary blood remains in the dissolved form. The term dissolved means that the gas molecule (in this case oxygen) maintains its exact molecular structure and freely moves throughout the plasma of the blood in its normal gaseous state. Clinically, it is the dissolved oxygen that is measured to assess the patient's partial pressure of oxygen (PO2).

At normal body temperature, about 0.003 mL of oxygen will dissolve in each 100 mL of blood for every 1 mm Hg of PO2.

Therefore in the normal individual with an arterial PaO2 of 100 mm Hg, only about 0.3 mL of oxygen exists in the dissolved

form in every 100 mL of plasma (0.003 × 100 mm Hg = 0.3 mL). Clinically, this is written as 0.3 mL/dL.1 Relative to the total oxygen transport, only a small amount of oxygen is carried to the tissue cells in the form of dissolved oxygen.

Oxygen Bound to Hemoglobin

In the healthy individual, over 98% of the oxygen that diffuses into the pulmonary capillary blood chemically combines with hemoglobin. Clinically, the weight measurement of hemoglobin, in reference to 100 mL of blood, is known as the grams per deciliter (g/dL).2 The normal hemoglobin value for men is 14 to 16 g/dL. The normal hemoglobin value for women is 12 to 15 g/dL. The normal hemoglobin value for infants is 14 to 20 g/dL.

Each gram of hemoglobin is capable of carrying about 1.34 mL of oxygen. Therefore if the hemoglobin level is 12 g/dL and the hemoglobin is fully saturated with oxygen (i.e., carrying all the oxygen that is physically possible), about 16.08 O2

mL/dL will be bound to the hemoglobin:

However, because of normal physiologic shunts (e.g., Thebesian venous drainage and bronchial venous drainage), the actual normal hemoglobin saturation is only about 97% (see chapter 11, Pathophysiologic mechanisms of Hyoxemic Respiratory Failure). Therefore the final amount of arterial oxygen shown in the previous calculation must be adjusted by 97% as follows:

Total Oxygen Content of Blood

To calculate the total amount of oxygen in each 100 mL of blood, the dissolved oxygen and the oxygen bound to the hemoglobin must be added together. The following case example summarizes the mathematics required to determine the total oxygen content of the patient's blood.

Case Example

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FIGURE 6.1 Oxyhemoglobin dissociation curve at a normal pH.

The steep portion of the oxyhemoglobin dissociation curve falls between 10 and 60 mm Hg, and the upper flat portion falls between 70 and 100 mm Hg. The steep part of the curve demonstrates that oxygen quickly combines with hemoglobin as the PO2 increases or, the converse, quickly breaks away (or dissociates) from the hemoglobin as the PO2 decreases.5 It is also interesting to note that very little additional oxygen combines with hemoglobin between a PO2 of 60 and 100 mm Hg. In fact, a PO2 increase from 60 to 100 mm Hg increases the total saturation of hemoglobin by only 7% (from 90% to 97% saturated) (see Fig. 6.1).

Oxygenation Indices

A number of oxygen transport measurements are available to assess the oxygenation status of the critically ill patient. Results from these studies can provide important information to adjust therapeutic interventions. The oxygen transport studies can be divided into the oxygen tension–based indices and the oxygen saturation–based and content-based indices.6

Oxygen Tension–Based Indices

Arterial Oxygen Tension (PaO2)

The PaO2 has withstood the test of time as a good indicator of the patient's oxygenation status. In general, an appropriate PaO2 on an inspired low oxygen concentration almost always indicates good tissue oxygenation. The PaO2, however, can be misleading in a number of clinical situations. For example, the PaO2 may give a “falsely normal” impression of true tissue

oxygenation when the patient has (1) a low hemoglobin concentration, (2) a decreased cardiac output or reduced blood flow to specific organs (e.g., the heart), (3) peripheral shunting, or (4) been exposed to carbon monoxide. In all of these cases, the PaO2 may be at an appropriate level (normal PaO2) but the actual oxygen content available for tissue metabolism

—the oxygen bound to the hemoglobin—is inadequate.

Alveolar-Arterial Oxygen Tension Difference (P[A-a]O2)

The alveolar-arterial oxygen tension difference (P[A-a]O2) is the oxygen tension difference between the alveoli and arterial blood. The P(A-a)O2 is also known as the alveolar-arterial oxygen tension gradient. The information required for the P(A-a)O2 is obtained from (1) the patient's calculated alveolar oxygen tension (PAO2), which is derived from the ideal alveolar gas equation to determine PAO2 and (2) the patient's PaO2, which is obtained from an arterial blood gas

analysis.

The ideal alveolar gas equation is written as follows:

where PB is the barometric pressure, PAO2 is the partial pressure of oxygen within the alveoli, PH2O is the partial pressure of water vapor in the alveoli (which is 47 mm Hg), FIO2 is the fractional concentration of inspired oxygen, PaCO2

is the partial pressure of arterial carbon dioxide, and RQ is the respiratory quotient. The RQ is the ratio of carbon dioxide production (VCO2) divided by oxygen consumption (VO2). Under normal circumstances, about 250 mL of oxygen per

minute is consumed by the tissue cells and about 200 mL per minute of carbon dioxide is excreted into the lung. Thus the RQ is normally about 0.8 but can range from 0.7 to 1.0. Clinically, a value of 0.8 is generally used for the RQ.

For example, if the patient is receiving an FIO2 of 0.30 on a day when the barometric pressure is 750 mm Hg, and if the patient's PaCO2 is 70 mm Hg and PaO2 is 60 mm Hg, the P(A-a)O2 can be calculated as follows:

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