Arterial blood gas analysis

1

What information does an arterial blood gas provide?

Arterial blood gas (ABG) machines provide a direct measurement of partial pressure of oxygen in arterial blood (PaO ₂ ), partial pressure of carbon dioxide in arterial blood (PaCO ₂ ), pH by using electrodes that measure changes in voltage, current, and resistance. It uses this data to calculate bicarbonate ion ( ${\mathrm{HCO}}_3^{-}$ ), base excess, and oxygen saturation. ABG machines may also measure Na, K, iCa, glucose, and lactate.

• Oxygenation (PaO ₂ ). The PaO ₂ is the amount of oxygen dissolved in blood and provides information on the efficiency of oxygenation.

• Ventilation (PaCO ₂ ). The adequacy of ventilation is inversely proportional to the PaCO ₂ .

• Acid-base status (pH, ${\mathrm{HCO}}_3^{-}$ , and base excess). A pH greater than 7.45 indicates alkalemia, and a pH less than 7.35 indicates acidemia. The base excess measures the metabolic component of the acid-base disturbance.

2

What is a CO-oximeter and what information does it provide?

A CO-oximeter is a device that measures hemoglobin absorbance of electromagnetic waves of varying wavelength. This can be used to measure total hemoglobin (tHb), oxyhemoglobin (O ₂ Hb), deoxyhemoglobin (HHb), methemoglobin (MetHb), and carboxyhemoglobin (COHb). A CO-oximeter is similar to a pulse oximeter, except a pulse oximeter only measures two wavelengths, which correspond to deoxyhemoglobin and oxyhemoglobin. However, a CO-oximeter can measure hundreds of wavelengths, which can be used to accurately measure the various molecular configurations of hemoglobin (e.g., COHb). Although some arterial blood gas machines include a CO-oximeter, many ABG machines do not have this functionality.

3

What are the normal ABG values in a healthy patient breathing room air at sea level?

See Table 7.1 .

4

How is the regulation of acid-base balance traditionally described?

Acid-base balance is traditionally described using the Henderson-Hasselbalch equation, which states that changes in ${\mathrm{HCO}}_3^{-}$ and PaCO ₂ determine pH by the following relationship:

To prevent a change in pH, any increase or decrease in the PaCO ₂ should be accompanied by a compensatory increase or decrease in the ${\mathrm{HCO}}_3^{-}$ , and vice versa. The importance of other physiological noncarbonic acid buffers was later recognized and partly integrated into the base deficit and corrected anion gap (AG), both of which aid in interpreting complex acid-base disorders.

5

What is meant by pH?

pH stands for “Power of the Hydrogen ion” and represents the negative logarithm of the hydrogen ion (H ⁺ ) concentration in the extracellular fluid. As with any “p” designation (signifying a negative logarithm), when the entities being measured get larger, the pH, pKa, and so on, get smaller. Normally the [H ⁺ ] in extracellular fluid is 40 × 10 ⁻⁹ mol/L, a very small number. By taking the negative log of this value, we obtain a pH of 7.4, a much simpler way to describe [H ⁺ ]. Note, that because we are using a logarithmic scale, small changes in the pH represent large changes in the [H ⁺ ] of the extracellular fluid. For example, a pH of 7.2 corresponds to a [H ⁺ ] equal to 60 × 10 ⁻⁹ mol/L, an increase of 50%!

6

Why is the pH of the extracellular fluid important?

The pH of the extracellular fluid is important because hydrogen ions react highly with cellular proteins, altering their function. Avoiding acidemia and alkalemia by tightly regulating hydrogen ions is essential for normal cellular function. Deviations from the normal pH of 7.4 suggest that some physiological processes are in disorder and causes need to be determined and treated.

7

What are the major consequences of acidemia?

Severe acidemia is defined as a blood pH lower than 7.20 and is associated with the following major effects:

• Impairment of cardiac contractility and cardiac output

• Impaired responsiveness to catecholamines

• Increased susceptibility to dysrhythmias

• Arteriolar vasodilation resulting in hypotension

• Vasoconstriction of the pulmonary vasculature and subsequent increased pulmonary vascular resistance

• Centralization of blood volume, eventually leading to pulmonary edema and dyspnea

• Hyperventilation (a compensatory response)

• Confusion, obtundation, and coma

• Insulin resistance

• Inhibition of glycolysis and adenosine triphosphate synthesis

• Coagulopathy

• Hyperkalemia (occurs primarily with metabolic acidosis but not respiratory acidosis)

8

What are the major consequences of alkalemia?

Severe alkalemia is defined as blood pH greater than 7.60 and is associated with the following major effects:

• Increased cardiac contractility until pH greater than 7.7, when a decrease is seen

• Refractory ventricular dysrhythmias

• Coronary artery vasoconstriction

• Hypoventilation (which can lead to hypercapnia and hypoxemia in spontaneously ventilating patients). In patients who are being mechanically ventilated, weaning may be made more difficult as a result of hypoventilation

• Cerebral vasoconstriction

• Neurological manifestations, such as lethargy, delirium, stupor, tetany, and seizures

• Hypokalemia, hypocalcemia, hypomagnesemia, and hypophosphatemia

• Stimulation of anaerobic glycolysis and lactate production