Arterial blood gases: Nurses' guide to interpreting test results
ABGs provide essential insight into the respiratory and metabolic systems. Fast and accurate interpretation of ABGs is the key to appropriate treatment.
STAFF EDITOR: Martha K. Raymond, RN,BSN,BS
TWENTY-SEVEN-YEAR-OLD JOEL NUNEZ was brought to the emergency room complaining of shortness of breath, weakness, and restlessness. He had no medical history and no known drug allergies. His oxygen saturation (SaO2) was 65%. Nunez was quickly intubated and placed on mechanical ventilation. Routine labs revealed glucose of 743 mg/dL, sodium of 142 mEq/L, potassium of 2.8 mEq/L, bicarbonate (HCO3) of 6 mEq/L, blood urea nitrogen of 23 mg/dL, creatinine of 2.1 mg/kg, calcium of 7.7 mg/dL, magnesium of 2 mEq/L, ionized calcium of 1.23 mg/dL, and phosphorus of less than 0.5 mg/dL. The complete blood count showed a white blood cell count of 2.9 x 103 /μL, hemoglobin of 15 g/dL, hematocrit of 42%, and platelets of 152,000/μL. In the emergency department, the nurse administered 10 units of regular insulin and started an insulin drip at 7 units/hour. Nunez also received one ampule of bicarbonate. Intravenous fluids were a mixture of 5% dextrose in water (D5W), two ampules of bicarbonate, and 40 mEq of potassium chloride, which ran at 150 mL/hr.
When the nurse called the report to intensive care, the patient's vitals were blood pressure of 108/77 mm Hg, heart rate of 129 beats per minute, and a respiratory rate of 18 breaths per minute while on a ventilator. The first arterial blood gas (ABG) values revealed a pH of 6.84, partial pressure of carbon dioxide (PaCO2) of 33 mm Hg, partial pressure of oxygen (PaO2) of 246 mm Hg, HCO3 of 6 mEq/L, base excess of –28, and a SaO2 of 99% on 100% oxygen, while ventilated.
Number soup, right? While it's easy to be overwhelmed by a litany of lab values, these num-bers hold the key to appropriate intervention. Patients today are sicker than ever, with the high census and acuity of critical-care units filtering into acute care and outpatient areas. Nurses need to anticipate these high-acuity patients and be equipped to act in order to ensure positive outcomes and patient safety.
ABG analysis is a diagnostic tool that reflects the patient's oxygenation, ventilation, and acid-base balance.1 While ABGs once were the province of critical-care doctors and nurses, it's important for all nurses to be proficient at ABG interpretation. Rapid interpretation and treatment can mean the difference between a negative or positive patient outcome.
ACID-BASE BALANCE AND PH The adult body is made up of 55% to 60% water. About two-thirds of this water is intracellular fluid (ICF), and the other third is extracellular fluid (ECF).2 Particular amounts of ions are dissolved in these bodily fluids to maintain homeostasis. The hydroxide ion (OH- ) makes bodily fluid more alkaline, or basic; while the hydrogen ion (H+ ) makes it more acidic. Thus, hydroxide may be considered a base, and hydrogen an acid. These ions coexist in a 20-to-1 ratio, respectively. When this ratio is shifted one way or another, homeostasis is disrupted, and the body must compensate to maintain a normal pH.
The pH scale of 1 to 14 is used to reflect the degree of acid and base in a solution. Homeostasis for the human body is only 7.35 to 7.45; any variation from this range can cause detrimental effects. A pH below 7.35 reflects a state of acidosis, and a pH greater than 7.45 reflects a state of alkalosis.
BUFFER SYSTEMS Buffer systems convert strong acids or bases into weaker ones by adding or subtracting hydrogen ions, helping the body maintain homeostasis. Acids are substances that can readily give up a hydrogen ion, while a base can readily accept a hydrogen ion.