Blood gas basics (Proceedings)
• Blood gas analysis gives us information about a patient's acid-base status and pulmonary function. The usefulness of this information is dependent on the users ability to accurately interpret the results.
• Arterial samples can be taken from any artery under anesthesia. Those used most frequently include the dorsal metatarsal and coxyggeal arteries. They are also easiest to maintain a catheter for invasive blood pressure monitoring or serial sampling. Other arteries that can be used include the femoral and lingual.• In the awake patient, the dorsal metatarsal artery is used most frequently for arterial blood gas sampling- it is recommended that a local anesthetic cream or local block be used prior to performing and arterial stick on an awake patient.
• Venous samples can be taken from any vein; central vs. peripheral is a personal preference. Venous samples give us minimal information on oxygenation status and pulmonary function but are useful in determining the metabolic status of the patient.
• The blood gas analyzer measures certain parameters in a blood sample
• Concentration of hydrogen ions (H+) in the blood (pH stands for per hydrogen)
• Partial pressure of carbon dioxide (also seen as PCO2)
• Partial pressure of oxygen (also seen as PO2)
• With this information, the analyzer can then calculate these values...
• Bicarbonate, a chemical buffer necessary to keep the pH of the blood from becoming to acidic or too basic
• Base excess. This value indicates the amount of excess or insufficient level of bicarbonate in the blood.
• Other analyzers have the ability to measure and calculate even more...
• Na+, Cl-, K+
• Arterial oxygen saturation
o A-a gradient
• Alveolar-arterial gradient. This is a measure of the difference between the alveolar concentration of oxygen, and the arterial concentration of oxygen. It is used in diagnosing the source of hypoxemia.
o Anion gap
• The concentration of unmeasured anions in the blood. A high anion gap may be another indicator of metabolic acidosis; a low anion gap is rare.
• The body functions at a blood pH between 7.36-7.44 (the range is slightly different for each species but most mammals are close to this range so it is acceptable to use this range as a guide)
o The body must maintain this range at all times in order for normal metabolism to take place
o When blood pH drops below 7.36 it is considered an acidemia (acidosis is the process, acidemia is the result)
o When the blood pH rises above 7.44 it is considered an alkalemia (alkalosis is the process, alkalemia is the result)
o pH values below 6.8 or above 7.8 are significant and will interfere with cellular functioning and if left untreated, death.