Indications for Arterial Blood Gas Sampling

ABGs are reported to be the most frequently performed test in the ICU.⁴ There are, however, no published guidelines and few clinical studies that provide guidance as to the indications for ABG sampling.⁶ It is likely that many ABGs are performed unnecessarily. Muakkassa and coworkers studied the relationship between the presence of an arterial line and ABG sampling.⁷ These authors demonstrated that patients with an arterial line had more ABGs drawn than those who did not, regardless of the value of the PaO₂, PaCO₂, the Acute Physiology and Chronic Health Evaluation (APACHE) II score, or the use of a ventilator. In that study, multivariate analysis demonstrated that the presence of an arterial line was the most powerful predictor of the number of ABGs drawn per patient independent of all other measures of the patient’s clinical status. Roberts and Ostryznuik demonstrated that with use of a protocol they were able to reduce the number of ABGs by 44%, with no negative effects on patient outcomes.⁴

The ubiquitous use of pulse oximetry in the ICU has made the need for frequent ABG sampling to monitor arterial oxygenation unnecessary. Furthermore (as discussed later), venous blood gas analysis can be used to estimate arterial pH and bicarbonate (HCO₃⁻) but not arterial carbon dioxide tension (PaCO₂). Previously, ABGs were drawn after every ventilator change and with each step of the weaning process; such an approach is no longer recommended.

The indications for ABG analysis should be guided by clinical circumstances. However, as a “general rule” all patients should have an ABG performed on admission to the ICU and/or following (10-15 minutes) endotracheal intubation. Patients with respiratory failure should have an ABG performed at least every 24 to 48 hours. Patients with type II respiratory failure (see definitions later in this chapter) will require more frequent ABG sampling than those with type I respiratory failure. Furthermore, patients with complex acid-base disorders and patients undergoing permissive hypoventilation will require more frequent ABG sampling.

Arterial Blood Gas Sampling

ABG specimens may be obtained from an indwelling arterial catheter or by direct arterial puncture using a heparinized 1- to 5-mL syringe. Indwelling arterial catheters should generally not be placed for the sole purpose of ABG sampling, as they are associated with rare but serious complications. Arterial puncture is usually performed at the radial site. When a radial pulse is not palpable, the brachial or femoral arteries are suitable alternatives. Serious complications from arterial puncture are uncommon; the most common include pain and hematoma formation at the puncture site. Laceration of the artery (with bleeding), thrombosis, and aneurismal formation are rare but serious complications.^8,⁹

ABG analysis is typically performed on whole blood. The partial pressure of oxygen (PaO_2,), partial pressure of carbon dioxide (PaCO₂), and pH are directly measured with standard electrodes and digital analyzers; oxygen saturation is calculated from standard O₂ dissociation curves and may be directly measured with a co-oximeter. The bicarbonate (HCO₃⁻) concentration is calculated using the Henderson-Hasselbalch equation:

where pK_A is the negative logarithm of the dissociation constant of carbonic acid. The base excess is defined as the quantity of strong acid required to titrate blood to pH 7.40 with a PaCO₂ of 40 mm Hg at 37°C. In practice, acid is not titrated as suggested but calculated using a variety of established formulae or normograms. The base excess thus “removes” the respiratory element of acid-base disturbance and identifies the metabolic contribution to interpret with pH and [H⁺]. The standard bicarbonate is broadly similar and is the calculated [HCO₃⁻] at a PaCO₂ of 40 mm Hg. Although the base excess and standard bicarbonate allow for a metabolic acidosis to be diagnosed, they provide few clues as to the pathophysiology or underlying diagnosis.

As with any diagnostic test, it is important that the specimen for an ABG measurement be collected and processed correctly and that quality assurance methods exist to ensure the accuracy of the measurements. Aside from interlaboratory variation, errors in calibration and electrode contamination with protein or other fluids may alter results. Heparin is usually added to the blood to prevent coagulation, and dilution with older liquid solutions previously caused spuriously low PaCO₂. Sample preparation is important because air bubbles falsely elevate PaO₂.

To avoid errors in blood gas interpretation, the following points must be considered before obtaining the sample:

1 Steady state: Blood sampling must be done during steady state following the initiation or change in oxygen therapy or changes in ventilatory parameters in patients on mechanical ventilation. In most ICU patients, a steady state is reached between 3 and 10 minutes and in about 20 to 30 minutes in patients with chronic airway obstruction.¹⁰

2 Anticoagulants: Excess of heparin may affect the pH. Only 0.05 mL is required to anticoagulate 1 mL of blood. The dead space volume of a standard 5-mL syringe with 1-inch, 22-gauge needle is 0.02 mL; filling the syringe dead space with heparin provides sufficient volume to anticoagulate a 5-mL blood sample. Today, calibrated volumes of dry (sodium or lithium) heparin are used in ABG kits, minimizing this problem.

3 Delay in processing of the sample: Because blood is a living tissue, O₂ is being consumed and CO₂ is produced in the blood sample. Red blood cell glycolysis may generate lactic acid and change pH. Significant increases in PaCO₂ and decreases in pH occur when samples are stored at room temperature for more than 20 minutes. In circumstances when a delay in excess of 20 minutes is anticipated, the sample should be placed in ice; iced samples can be processed up to 2 hours after collection without affecting the blood gas values.

5 Collection equipment and technique: Increased dead space in the syringe lowers PaCO₂ content. Needle size rarely causes variability; however, a smaller (25 g) needle is recommended. In patients with an indwelling arterial line, a discard volume of at least twice the dead space (priming volume from sample port to catheter tip) is required to prevent sample dilution.¹¹

6 Hypotension: Severe hypotension may require forceful aspiration, and results—particularly for PaO₂—may be falsely low.

7 Hyperventilation: Hyperventilation resulting from anxiety and/or pain may acutely alter results from baseline values.

8 Leukocytosis: Leukocytosis accelerates the decline of PaO₂ and pH and elevation of PaCO₂ within a stored sample. This PaO₂ decrease is more pronounced at higher PaO₂ levels, is attributable to cellular oxygen consumption, and may be attenuated when samples are stored at colder temperatures.

9 Hypothermia: Blood gas values are temperature dependent, and if blood samples are warmed to 37°C before analysis (as is common in most laboratories), PO₂ and PCO₂ will be overestimated and pH underestimated in hypothermic patients. The following correction formulas can be used:

• Subtract 5 mm Hg PO₂ per 1°C that the patient’s temperature is less than 37°C.

• Subtract 2 mm Hg PCO₂ per 1°C that the patient’s temperature is less than 37°C.

• Add 0.012 pH units per 1°C that the patient’s temperature is less than 37°C.

Arterial Blood Gas Analysis

An ABG provides a rapid and accurate assessment of oxygenation, ventilation, and acid-base status. These three processes are closely interrelated with each other, and an alteration in one process will affect the other two. However, for the sake of simplicity and ease of understanding, each will be discussed separately.

Alveolar Ventilation

The arterial CO₂ content as reflected by arterial CO₂ tension (PaCO₂) at any given moment depends on the quantity of CO₂ produced and its excretion through alveolar ventilation (VA) and can be expressed by the equation, PaCO₂ ∼ CO₂/VA. The alveolar ventilation is that portion of total ventilation that participates in gas exchange with pulmonary blood. If it is assumed that CO₂ production is constant, then CO₂ homeostasis can be simplified to 1/VA ∼ PaCO₂. Thus PaCO₂ becomes very useful for the assessment of alveolar ventilation. High PaCO₂ (>45 mm Hg) indicates alveolar hypoventilation, and low PaCO₂ (<35 mm Hg) implies alveolar hyperventilation.

Oxygenation

The ultimate aim of the cardiorespiratory system is to provide adequate delivery of oxygen to the tissues. This is largely dependent upon cardiac output, hemoglobin (Hb) concentration, and Hb saturation. The PaO₂ is a measure of the oxygen tension in plasma; while the dissolved fraction makes a negligible contribution to oxygen delivery (<2%), it is a major factor affecting Hb saturation. In turn, the PaO₂ is dependent on the concentration of oxygen in the inspired air (FIO₂), oxygen exchange in the lung (V/Q mismatching), and the venous oxygen saturation (SmvO₂). The PaO₂ must always be interpreted in conjunction with the FIO₂ and age.

Relation Between PaO₂ and FIO₂

The PaO₂ alone provides little information regarding the efficiency of oxygen loading into the pulmonary capillary blood. The PaO₂ is determined largely by the FIO₂ and the degree of intrapulmonary shunting (Figure 45-1). The PaO₂ must therefore always be interpreted in conjunction with the FIO₂. The PaO₂ alone does not quantitate the degree of intrapulmonary shunt, which is required for assessing the severity of the underlying lung disease and in guiding the approach to oxygen therapy and respiratory support. There are various formulas for calculating the intrapulmonary shunt, including the classic “shunt equation,” which is the gold standard but requires mixed venous sampling through a pulmonary artery catheter, and the alveolar-arterial oxygen gradient equation (Table 45-1). Clinically the PaO₂-to-FIO₂ ratio (PaO₂/FIO₂) is most commonly used to quantitate the degree of ventilation/perfusion mismatching (V/Q). Since the normal PaO₂ in an adult breathing room air with an FIO₂ of 0.21 is 80 to 100 mm Hg, the normal value for PaO₂/FIO₂ is between 400 and 500 mm Hg. A PaO₂/FIO₂ ratio of less than 200 most often indicates a shunt of greater than 20%. A notable limitation of the PaO₂/FIO₂ is that it does not take into account changes in PaCO₂ at a low FIO₂, which tends to have a considerable effect on the ratio.

Figure 45-1 The effect on PaO₂ of increasing FIO₂ according to shunt fraction.

TABLE 45-1 Formulas for Evaluating Patients in Respiratory Failure

Age

The normal arterial oxygen tension decreases with age (see Table 45-1). The normal PaO₂ at sea level and breathing room air is approximately 85 to 90 mm Hg at the age of 60 and 80 to 85 mm Hg at the age of 80 years.

The PaO₂ is primarily used for assessment of oxygenation status, since PaO₂ accurately assesses arterial oxygenation from 30 to 200 mm Hg, whereas SaO₂ is normally a reliable predictor of PaO₂ only in the range of 30 to 60 mm Hg. However, oxygen saturation as measured by pulse oximetry (SpO₂) or by ABG analysis (SaO₂) is a better indicator of arterial oxygen content than PaO₂, since approximately 98% of oxygen is carried in blood combined with Hb. Hypoxemia is defined as a PaO₂ of less than 80 mm Hg at sea level in an adult patient breathing room air; the concomitant decrease in cell/tissue oxygen tension is known as hypoxia (or tissue hypoxia). The degree of hypoxia in patients with hypoxemia depends on the severity of the hypoxemia and the ability of the cardiovascular system to compensate. Hypoxia is unlikely in mild hypoxemia (PaO₂ = 60-79 mm Hg). Moderate hypoxemia (PaO₂ = 45-59 mm Hg) may be associated with hypoxia in patients with anemia or cardiovascular dysfunction. Hypoxia is almost always (but with a few exceptions) associated with severe hypoxemia (PaO₂ <45 mm Hg). However, it must be recognized that the human body has an extraordinary capacity to adapt to hypoxemia. Indeed, patients with cyanotic heart disease do not have evidence of tissue hypoxia at rest. Most remarkably, at the top of Mount Everest (29,028 ft; 253 torr) and without supplemental oxygen, experienced mountain climbers have been reported to have a mean PaO₂ of between 24 and 28 mm Hg in the absence of tissue hypoxia.^12,¹³

Acute respiratory failure occurs when the pulmonary system is no longer able to meet the metabolic demands of the body. Respiratory failure is classically divided into two types:

• Type I, hypoxemic respiratory failure: PaO₂ ≤ 60 mm Hg when breathing room air (sea level).

• Type II, hypercapnic respiratory failure: PaCO₂ ≥ 50 mm Hg.

Acid-Base Balance

The normal diet generates volatile acid (CO₂), primarily from carbohydrate metabolism, and nonvolatile acid (hydrogen ion, H⁺) from protein metabolism. The aim of the body’s homeostatic system is to maintain pH within a narrow range, and pH homeostasis is accomplished through the interaction of the lungs, kidneys, and blood buffers. Alveolar ventilation allows for excretion of CO₂. The kidneys must reclaim filtered bicarbonate (HCO₃⁻), because any urinary loss leads to gain of H⁺. In addition, the kidney must excrete the daily acid load generated from dietary protein intake. Less than half of this acid load is excreted as titratable acids (i.e., phosphoric and sulfuric acids); the remaining acid load is excreted as ammonium. The blood pH is determined by the occurrence of these physiologic processes and by the buffer systems present in the body.

The history of assessing the acid-base equilibrium and associated disorders is intertwined with the evolution of the definition of an acid. In the 1950s, clinical chemists combined the Henderson-Hasselbalch equation and the Brønsted-Lowry definition of an acid to produce the current bicarbonate ion–centered approach to metabolic acid-base disorders.¹⁴ Stewart repackaged pre-1950 ideas of acid-base in the late 1970s, including the Van Slyke definition of an acid.¹⁵ Stewart also used laws of physical chemistry to produce a new acid-base approach.¹⁴ This approach, using the strong ion difference (SID) and the concentration of weak acids (particularly albumin), pushes bicarbonate into a minor role as an acid-base indicator rather than as an important mechanism:

The SID is not identical to anion gap (AG) and contains [lactate], although it does share a number of parameters, and the trends will often be close. The normal SID has not been well established, but the quoted range is 40 to 42 mEq/L.

As the SID approaches zero, anions “accumulate” and acidity increases. This approach provides a physicochemical model for “hyperchloremic acidosis” following 0.9% saline administration,²¹ and the systemic alkalosis of hypoalbuminemia (regarded as a weak acid).

Most clinicians use the bicarbonate ion–centered approach for the diagnosis and management of acid-base disorders; this approach is easier to understand and more practical. Furthermore, there are no clinical data to suggest that the Steward approach has any advantages over the classic (bicarbonate) approach.¹⁶ The Henderson-Hasselbalch equation describes the fixed interrelationship between PaCO₂, pH, and HCO₃⁻ being described as pH = pK_c log HCO₃⁻/dissCO₂. If all the constants are removed, the equation can be simplified to pH = HCO₃⁻/PaCO₂ (∼Kidney/Lung). The HCO₃⁻ is controlled mainly by the kidney and blood buffers. The lungs control the level of PaCO₂ by regulating the level of volatile acid, carbonic acid, in the blood. Buffer systems can act within a fraction of a second to prevent excessive change in pH. The respiratory system takes about 1 to 15 minutes and kidneys many minutes to days to readjust H⁺ ion concentration.