The test may be preformed following a positive Allen test. An ABG syringe contains approximately 0.2ml of heparin to prevent coagulation of sample. Insert syringe at a 45-75 degree angle. Do not pull back on syringe; allow the arterial blood pressure to fill the syringe. After withdrawing 3ml, remove syringe and apply pressure. Because the vessel is an artery, pressure must be applied for about 5 minutes.
Contraindications:
The ABG test can occlude the artery. This is why the Allen test is so important in all patients. Also, during the administration of the test, the needle can puncture nearby anatomical areas, such as nerves or ligaments. This is why only specially trained individuals may administer the test. Because the test is drawn arterially, the risk of bleeding is much higher. Pressure must be held for much longer than a venous blood draw. Also, the ABG is more painful than a venous blood draw; lidocaine may be administered before-hand to decrease pain.
Average Cost of Test:
According to the ORMC respiratory therapy department, an ABG costs approximately $235.
Length of Time required for obtaining Results:
After the blood reaches the blood gas analyzer, the results can be obtained in minutes
Normal Lab Values:
Various ABG’s detect different values. These are the main values used to detect acid/base imbalance. Most of these will be used to discuss various imbalances and their causation.
pH- 7.35-7.45
HCO3- 22-26
PaO2- 70-100mmHg
PaCO2- 35-45mmHg
Base excess- -2/+2
SaO2- 95-100%
How to Interpret Results:
pH- measures the amount of hydrogen ions in the blood, which indicates the acid / base balance in the blood. When the pH increases, the body is alkaline. When the pH decreases, the body is more acidic.
HCO3- the amount of bicarb in the blood, this indicates the amount of base. As HCO3 rises, so does pH.
PaCO2- the partial pressure of CO2, this indicates the amount of acid in the blood. This has an inverse relationship with pH.
PaO2- the partial pressure of oxygen molecules dissolved in the blood
BE- represents the amount of metabolic buffering agents in the blood, try to compensate with imbalances in pH, indicates your buffering capacity
This is a brief discussion of respiratory and metabolic acidosis/alkalosis.
Respiratory Acidosis- occurs due to respiratory depression, such as pneumonia, COPD, and over-sedation. This is represented by decrease in pH and an increase in CO2. This is also accompanied by a decreased PaO2 and possibly a decreased SaO2.
Respiratory Alkalosis- caused by increased respiratory rate, such as hyperventilation. This is represented by an increased pH and a decreased CO2.
Metabolic Acidosis- occurs due to too much acid in the body, usually due to problems filtering out waste, such as kidney disease, diabetes, and shock. Indicated by decreased pH and decreased HCO3.
Metabolic Alkalosis- seen in hypokalemia and chronic vomiting. Shown by increased pH and increased HCO3.
When in these irregular states, the body attempts to compensate for the acid/base imbalance with the opposite system that s unaffected by the disease. Such as when the body is in metabolic acidosis, the body will attempt to rid itself of the extra acid by releasing CO2. This will occur with hyperventilating, thus decreasing CO2 below normal, but normalizing pH. Another example is in respiratory acidosis, the body will rid itself of extra acids by secreting them out through the kidneys. Thus causing an increase in HCO3 levels and normalizing pH.
Follow up of Test Results:
The goal of an abnormal ABG would be to correct the imbalance by treating the disease. For example, in respiratory acidosis, the patient is holding in CO2, so treatment would be to help blow off CO2 by using a BiPAP or if severe intubation. In metabolic acidosis, the treatment would be to neutralize the acid by decreasing blood sugar if caused by diabetes or dialysis if caused by renal failure.
Additional information for a nurse practitioner:
If a patient has COPD, their oxygen, CO2 exchange is impaired in the lungs. They live in a state of respiratory acidosis. This causes a change in their respiratory drive, change it from one of CO2 release, to one of O2 consumption, thus treating a COPD patient with high flow O2 can suppress their respiratory drive. If their PaCO2 is 55mmHg, do not treat it unless they are symptomatic.
Carboxyhemoglobin
One consideration that must be made when drawing an ABG is carboxyhemogloblin. Carboxyhemoglobin is the binding of carbon monoxide (CO) with the hemoglobin molecules in the blood. This occurs with carbon monoxide poisoning, fire, and long-term heavy smokers. The hemoglobin molecules bond much easier to CO than O2, thus causing the carboxyhemoglobin level to rise quickly.
An increased carboxyhemoglobin level can cause a decreased pH, but a low PaCO2. Thus, if a practitioner suspects CO poisoning or fire a carboxyhemoglobin level must be drawn.
Normal Level: 5%
Interpreting Results: >5% and symptomatic, such as confusion, dyspnea and severe headache
Treatment: High flow O2 and removal of causative agent. O2 will counteract the CO level quickly.
