What Nurses Need to Know about ABG Interpretation
Throughout the years, it has been a common notion to common folks that nurses are only physicians’ assistants and that without the doctors, nurses cannot function alone. However, this not true, in fact the functions of nurses are not dependent on doctors’ orders alone, they also have independent and collaborative functions.
Nurses, like doctors and other members of the healthcare team are all professional health care providers. Thus, we, like them, must also learn how to function alone. As doctors are not always at the bedside 24/7 as well as have other patients to cater, not only our own, we must learn to act independently. First of all, we must know how to conduct our own comprehensive assessment and not just rely on those assessment findings written by doctors on patients’ charts. Secondly, we must at least be aware of how to interpret simple laboratory findings such as ABGs.
ABG’s: What about them?
Arterial blood gas (ABG) analysis and interpretation can give you a view about a patient’s oxygenation, acid-base balance, pulmonary function, and metabolic status as by interpreting such, you may be able to assess and monitor critically ill patients in the clinical area, especially in the ICU and other critical care settings.
Usually, ABG’s are ordered for patients with the following cases: Respiratory compromise, which may then lead to hypoxia or diminished ventilation, those with Peri/postcardiopulmonary arrest or collapse; patients with medical conditions that cause significant metabolic derangement (sepsis, diabetic ketoacidosis, renal failure, heart failure, toxic substance ingestion, drug overdose, trauma, or burns); to evaluate the efficiency of therapies, and monitor the patient’s clinical status, as well as to determine treatment needs of the patient. An example of this scenario is a physician, titrating oxygenation therapy such as adjusting the level of ventilator support (FiO2, BUR, TV and PF), and make decisions about fluid and electrolyte therapy basing on ABG results. ABGs may also be indicated during the perioperative phase of major surgeries (preoperative, intraoperative, and postoperative care of the patient).
There are five components considered when analyzing ABG results, pH, PaO2, HCO3, and SaO2. SaO2, determines the percent of oxygen in hemoglobin, that is transported to tissue cells while PaO2 represents the amount of oxygen dissolved in arterial blood. Basically, an increase in PaO2 raises SaO2 and decreased PaO2 lowers the SaO2 level. Bicarbonate is a base and an important buffer of hydrogen ions (H+) in the blood. A patient’s pH tells about the concentration of hydrogen ions (H+) in arterial blood. While a low pH means more acid in the blood as the result of increased H+ concentration, lowered H+ concentration leads to a higher pH as the blood becomes more alkaline, making the relationship inversely proportional.
Metabolic acidosis is defined as pH less than 7.35 and HCO3- less than 22 mEq/L and may be caused by renal failure, diabetic ketoacidosis, lactic acidosis, sepsis, shock, diarrhea, drugs, or toxins such as ethylene glycol and methanol. On the other hand, metabolic alkalosis has a pH greater than 7.45 and an HCO3- greater than 26 mEq/L and may be caused by diuretics, corticosteroids, excessive vomiting, dehydration, Cushing syndrome, liver failure, or hypokalemia.
In terms of respiration, Respiratory acidosis has a PaCO2 above 45 mm Hg due to hypoventilation and a pH below 7.35 and may be caused by respiratory infections, severe airflow obstruction like COPD or asthma, neuromuscular disorders such as multiple sclerosis, massive pulmonary edema, pneumothorax, central nervous system depression, spinal cord injury, or chest wall injury; while Respiratory alkalosis may occur and noted with a PaCO2 below 35 mm Hg and pH more than 7.45 due to hyperventilation. It may be caused by pain, anxiety, early stages of pneumonia or pulmonary embolism, hypoxia, brainstem injury, severe anemia, or excessive mechanical ventilation.
In cases when the respiratory system falters, the metabolic system can attempt to compensate, and vice versa. For example, if compensatory mechanisms restore a normal pH, then the acid-base imbalance is fully compensated. While if the compensation involves the opposite direction of respiratory and metabolic processes, and is verified by abnormal PaCO2 and HCO3- parameters (If there are three abnormal parameters such as pH, PaCO2, and HCO3), then it is partially compensated. Lastly, when the pH and either PaCO2 or HCO3- are abnormal, but the counterpart is normal, then there is no compensation.
Interpreting ABG results may seem complicated however we nurses must at least have a clue and basic knowledge about how to interpret such. By doing so, not only are we expanding our knowledge, we are also allowing ourselves to know more about our patient’s condition and modify our nursing care plans according to specific results.