Blood gas analysis is used to evaluate respiratory function and provide a measure for determining acid-base balance. Respiratory, renal, and cardiovascular system functions are integrated in order to maintain normal acid-base balance.
Therefore, respiratory or metabolic disorders may cause abnormal blood gas findings. The blood gas measurements commonly reported are pH, partial pressure of carbon dioxide in the blood (Pco2), partial pressure of oxygen in the blood (Po2), bicarbonate (HCO3 –), O2 saturation, and base excess (BE) or base deficit (BD). pH reflects the number of free hydrogen ions (H+) in the body. A pH less than 7.35 indicates acidosis.
A pH greater than 7.45 indicates alkalosis. Changes in the ratio of free H+ to HCO3 will result in a compensatory response from the lungs or kidneys to restore proper acid-base balance. Pco2 is an important indicator of ventilation. The level of Pco2 is controlled primarily by the lungs and is referred to as the respiratory component of acid-base balance.
The main buffer system in the body is the bicarbonate– carbonic acid system. Bicarbonate is an important alkaline ion that participates along with other anions, such as hemoglobin, proteins, and phosphates, to neutralize acids. For the body to maintain proper balance, there must be a ratio of 20 parts bicarbonate to one part carbonic acid (20:1). Carbonic acid level is indirectly measured by Pco2.
This group of tests is used to assess conditions such as asthma, chronic obstructive pulmonary disease (COPD), embolism (e.g., fatty or other embolism) during coronary arterial bypass surgery, and hypoxia. It is also used to assist in the diagnosis of respiratory failure, which is defined as a Po2 less than 50 mm Hg and Pco2 greater than 50 mm Hg.
Blood gases can be valuable in the management of patients on ventilators or being weaned from ventilators. Blood gas values are used to determine acid-base status, the type of imbalance, and the degree of compensation as summarized in the following section.
Restoration of pH to near-normal values is referred to as fully compensated balance. When pH values are moving in the same direction (i.e., increasing or decreasing) as the Pco2 or HCO3 −, the imbalance is metabolic.
When the pH values are moving in the opposite direction from the Pco2 or HCO3 −, the imbalance is caused by respiratory disturbances. To remember this concept, the following mnemonic can be useful: MeTRO = Metabolic Together, Respiratory Opposite.