Arterial blood gas is a type of blood test. This article provides information on the arterial blood gas test and how the results are interpreted.
Blood from an artery is examined to determine the pH of the blood. An artery is punctured with a fine thin needle, and then with the help of a syringe, a small amount of blood is drawn out. A small amount of heparin in the syringe prevents clotting of the blood. Uninterrupted flow of blood into the syringe shows that the sample is truly from the artery. Mostly, the radial artery at the wrist is punctured for this. The Use of other arterial sites is also common, for instance, femoral artery in the groin. An arterial catheter is also used to draw the blood.
Why are Arterial Blood Gas Tests Done?
- The test helps you determine the pH of the blood, which in turn tells you about the partial pressure of oxygen and carbon dioxide and the bicarbonate level.
- The test provides information on how much oxygen and carbon dioxide is present in your blood.
- Concentrations of lactate, hemoglobin, several electrolytes, oxyhemoglobin, carboxyhemoglobin, and methemoglobin can also be checked with the help of this test.
- Blood gas tests are mainly done to assess the oxygenation capacity of the lungs for diagnostic reasons.
- The oxygen pressure in the blood can be assessed for therapeutic reasons.
- Arterial blood gases can be used to assess respiratory adequacy.
- This test helps assess acid-base status.
- The normal blood pH range is 7.35 to 7.45. Acidity or alkalinity of the blood can be measured with pH values.
- The normal range of PaO2 is 80 to 100 mm Hg. PaO2 tells us about the partial pressure of oxygen dissolved in arterial blood.
- The normal range of SaO2 is 95% to 100%. The arterial oxygen saturation can be assessed with SaO2.
- The normal range of PaCO2 is 35 to 45 mm Hg. PaCO2 tells us about the amount of carbon dioxide dissolved in arterial blood.
- The normal range of HCO3 is 22 to 26 mEq/liter and HCO3 indicates the calculated value of the amount of bicarbonate in the bloodstream.
- The normal range of B.E. (base excess) is -2 to +2 mEq/liter. This value indicates the amount of excess or insufficient level of bicarbonate in the system.
The Ph is inversely proportional to the number of hydrogen ions (H+) in the blood. The pH of a solution is measured on a scale from 1 (very acidic) to 14 (very alkaline). A liquid with a pH of 7, for instance, water, is neutral (neither acidic nor alkaline). For normal metabolism, normal level of pH has to be maintained. If the blood is acidic (pH below 7.35), a decrease in the force of cardiac contractions, a decrease in the vascular response to catecholamines, and a diminished response to actions of certain medications is noticed.
When the blood is alkaline (pH above 7.45), tissue oxygenation and neurological and muscular functioning is affected. The pH range of 7.8 to 6.8 will severely affect cellular functioning, and if left unattended, may result in death. The body maintains pH within normal range using delicate buffer mechanisms between the respiratory and renal systems.
The lungs are triggered to either increase or decrease the rate and depth of ventilation until the appropriate amount of CO2 has been re-established. This way, they compensate for the imbalance within 1 to 3 minutes.
Decrease in blood pH triggers the kidneys to retain HCO3 so as to compensate, and the rise in blood pH forces the kidneys to excrete HCO3 through the urine. The kidneys are an excellent means of regulating acid-base balance, but the system may take from hours to days to correct the imbalance.
Acid base disorders can lead to ‘respiratory acidosis‘, which is defined as a pH less than 7.35 with a PaCO2 greater than 45 mm Hg. Acidosis is said to occur when the accumulated CO2 combines with water in the body to produce carbonic acid, and thus, lowers the pH of the blood. Headache, restlessness, confusion, dyspnea, respiratory distress, and/or shallow respiration, tachycardia and dysrhythmias, even drowsiness and unresponsiveness may be noted. Although patients will require oxygen supply, oxygen alone will not correct the problem.
A pH greater than 7.45 with a PaCO2 less than 35 mm Hg is known as ‘respiratory alkalosis’. Lightheadedness, numbness, tingling, confusion, inability to concentrate, blurred vision, dysrhythmias and palpitations, dry mouth, diaphoresis, and tetanic spasms of the arms and legs are the symptoms of respiratory alkalosis. Dramatically increased work of breathing is observed in such cases. Patients need to be monitored closely for respiratory muscle fatigue, as the exhausted respiratory muscles can result in acute respiratory failure.
A bicarbonate level of less than 22 mEq/L with a pH of less than 7.35 is known as ‘metabolic acidosis’, which is caused by either decreased level of base in the bloodstream or increased level of acids, other than CO2. Headache, confusion, restlessness progressing to lethargy, then stupor or coma are the symptoms of metabolic acidosis. Cardiac dysrhythmias, warm, flushed skin, as well as nausea and vomiting are also commonly noticed.
A bicarbonate level greater than 26 mEq/liter with a pH greater than 7.45 is known as ‘metabolic alkalosis’. Dizziness, lethargy, disorientation, seizures and coma, weakness, muscle twitching, muscle cramps, nausea, vomiting, and respiratory depression are the symptoms of metabolic alkalosis. When an acid-base imbalance occurs, the body naturally attempts to compensate. The lungs and kidneys play an important role in this attempt of compensating. An uncompensated or partially compensated pH requires prompt treatment. The arterial blood gas test helps determine the course of the treatment.