Arterial blood gas (ABG) analysis provides measurements of pH and the partial pressures of oxygen and carbon dioxide in arterial blood. These values can be used to assess the blood's acid‒base balance and the status of pulmonary gas exchange. ABG analysis is often used to help assess patients' oxygenation as well as help manage their respiratory and metabolic (renal) acid‒base balance and their electrolyte status. (The patient's clinical condition has to be taken into consideration concurrently.)
The pH (power of hydrogen) scale is designed to reflect changes in the hydrogen ion (H+) concentration in a simplified way. (An acid is a substance that releases hydrogen ion when dissolved in solution, while a base accepts it.) When the H+ concentration increases, the pH value decreases. Comparatively speaking, a blood pH value of 7.2 is more acidic than a pH value of 7.3. The pH of human blood must be maintained within a narrow range, 7.35 to 7.45. When blood pH is below the normal range (pH < 7.35), a problem of acidemia (abnormal acidity of the blood) is present. When blood pH value exceeds 7.45, there is a problem of alkalemia (abnormal alkalinity of the blood).
Partial pressure describes the pressure of a specific gas as part of the total gas mixture. PaO2 is a measure of the partial pressure exerted by oxygen when dissolved in the arterial blood. PaCO2 is the arterial (partial) pressure of carbon dioxide. CO2 is produced by cell metabolism when O2 is consumed for energy; it is excreted by the lungs. (Other acids are excreted in the urine by the kidneys.)
Hypoxemia refers to a state in which the arterial blood oxygen is reduced. Hypoxia refers to a state in which tissues receive an oxygen supply inadequate to maintain normal aerobic metabolism. An increased pulse can indicate the heart's attempt to compensate for such a reduced oxygen supply to the body tissues.
The oxygen saturation level indicates the percentage of hemoglobin saturated with O2. In most cases, it can be assessed adequately with a pulse oximeter, by applying the probe over a pulsating vascular bed, such as the fingertips or earlobes. A decrease in the oxygen saturation level may be an early indication of respiratory compromise. Unlike ABG analysis, however, pulse oximetry does not provide information on the partial pressure of carbon dioxide.
The lungs are responsible for CO2 removal, with the effectiveness of this process being determined by alveolar ventilation. Increased PaCO2 (hypercapnia) and respiratory acidosis usually result from ineffective breathing (hypoventilation). The initial assessment findings in such a case may include a compensatory increase in the rate and depth of respiration, as well as altered mental status.
Hyperventilation due to anxiety (or resulting from the body's compensatory mechanism when the sympathetic nervous system is activated) may cause excessive loss of CO2, leading to respiratory alkalosis. This condition manifests with symptoms such as tingling around the mouth and dizziness. Having the patient breathe into a paper bag to rebreathe the expired carbon dioxide may offset respiratory alkalosis to some degree.
Remind the patient who is receiving oxygen to be aware of the risks of fire associated with oxygen use: Oxygen supports the combustion process. If the patient is receiving low-dose oxygen and an item nearby catches on fire, first make sure the patient is not in immediate danger, and then turn off the oxygen.