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Blood gas basics (Proceedings)

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Apr 01, 2009

• 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.

Sampling

• 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 analyzer

• The blood gas analyzer measures certain parameters in a blood sample

o pH

• Concentration of hydrogen ions (H+) in the blood (pH stands for per hydrogen)

o PaCO2

• Partial pressure of carbon dioxide (also seen as PCO2)

o PaO2

• Partial pressure of oxygen (also seen as PO2)

• With this information, the analyzer can then calculate these values...

o HCO3-

• Bicarbonate, a chemical buffer necessary to keep the pH of the blood from becoming to acidic or too basic

o BE

• 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...

o Hb

• Hemoglobin

o Electrolytes

• Na+, Cl-, K+

o SaO2

• 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.

Relevance

• 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.