Lab Values & ABGs Practice Test #4: ABG Basics

Answer: C. pH 7.40, PaCO2 40 mm Hg, HCO3 24 mEq/L, PaO2 92 mm Hg

Rationale: Typical adult arterial ranges are pH 7.35 to 7.45, PaCO2 35 to 45 mm Hg, HCO3 (bicarbonate) 22 to 26 mEq/L, and PaO2 about 80 to 100 mm Hg on room air. Ranges vary slightly by lab, and PaO2 is often lower in older adults, so always check your facility's reference values. Option C is normal on every value. Option A has a low pH and low bicarbonate (metabolic acidosis). Option B has a high pH and low PaCO2 (respiratory alkalosis). Option D has a pH inside the normal range, but the PaCO2 and bicarbonate are both high and the PaO2 is low, so it is not a normal gas. That pattern shows the body making up for a long-standing problem (compensation), which is covered in Test #5.

Answer: D. Look at the pH to decide whether the blood is acidotic, alkalotic or within the normal range.

Rationale: Start with the pH: below 7.35 is acidosis, above 7.45 is alkalosis. Next, look at the PaCO2 (the respiratory value) and the bicarbonate (the metabolic value) and decide which one explains the pH. A memory aid is ROME: Respiratory Opposite, Metabolic Equal. In a respiratory problem the pH and PaCO2 move in opposite directions (pH down, PaCO2 up). In a metabolic problem the pH and bicarbonate move in the same direction (pH down, HCO3 down). Only after that do you check whether the other system is compensating. The PaO2 tells you about oxygenation, not acid-base balance; it matters, but it is read separately and does not tell you whether the client is acidotic. Always read the numbers alongside the client's assessment, never on their own.

Answer: A. Uncompensated respiratory acidosis caused by hypoventilation

Rationale: The pH is low (acidosis) and the PaCO2 is high, so the problem is respiratory: pH and PaCO2 have moved in opposite directions. The bicarbonate is normal, so the kidneys have not yet compensated, which fits a sudden problem. Slow, shallow breathing from opioid sedation lets carbon dioxide build up, and CO2 acts as an acid in the blood. The nurse treats the cause: try to rouse the client and coach deep breaths, stop further opioid doses, give oxygen and naloxone according to orders or the facility's protocol, monitor respiratory rate, sedation level and oxygen saturation closely, and notify the provider or rapid response team. Remember that a normal oxygen saturation on supplemental oxygen can hide rising CO2.

Answer: D. The client has respiratory alkalosis from blowing off carbon dioxide, and alkalosis lowers ionized calcium, which causes the tingling.

Rationale: The pH is high (alkalosis) and the PaCO2 is low, moving in opposite directions, so this is respiratory alkalosis. The bicarbonate is normal, so there is no compensation yet. Breathing fast and deep removes carbon dioxide faster than the body makes it. Alkalosis makes more calcium bind to albumin, so less ionized (active) calcium is available, which causes numbness and tingling around the mouth and in the fingers, and sometimes muscle cramps. The nurse should not assume anxiety is the cause: pain, hypoxemia, fever, sepsis and pulmonary embolism can also cause rapid breathing and must be ruled out. Once physical causes are addressed, the nurse stays with the client and coaches slow breathing. Breathing into a paper bag is no longer recommended because it can lower oxygen levels.

Answer: B. Metabolic alkalosis from loss of hydrochloric acid in the gastric drainage

Rationale: The pH is high and the bicarbonate is high, moving in the same direction, so the problem is metabolic alkalosis. The PaCO2 is still within the normal range, so the lungs have not compensated in a meaningful way. Gastric fluid is rich in hydrochloric acid, so ongoing NG suction or vomiting removes acid and leaves excess bicarbonate. These clients often lose potassium and chloride as well, and low potassium makes the alkalosis harder to correct. The nurse monitors potassium and other electrolytes, measures NG output accurately, watches for weakness, dysrhythmias and confusion, and gives the fluid and electrolyte replacement that is prescribed. Lower GI losses such as diarrhea do the opposite: they remove bicarbonate and cause acidosis.

Answer: A. Metabolic acidosis caused by loss of bicarbonate in the stool

Rationale: The pH is low and the bicarbonate is low, moving in the same direction, so this is metabolic acidosis. The PaCO2 is still normal, so the lungs have not yet compensated by breathing off more CO2. Intestinal fluid below the stomach is rich in bicarbonate, so heavy diarrhea removes base and leaves the blood more acidic. Other common causes of metabolic acidosis include diabetic ketoacidosis, lactic acidosis from shock or sepsis, and kidney failure. Option D is wrong because the bicarbonate is also low. The nurse watches for deep, rapid breathing, confusion and dysrhythmias, monitors potassium (diarrhea also removes potassium), tracks intake and output and weight, and gives the fluids and electrolytes that are prescribed.

Answer: B. Hold firm, direct pressure on the site for at least 5 minutes, and longer because the client is on heparin, then check the site and the pulse and circulation below it.

Rationale: Arteries are under high pressure, so an arterial puncture needs firm, direct pressure until bleeding has fully stopped. Many protocols call for at least 5 minutes, and longer, often 10 minutes or more, for clients on anticoagulants or with bleeding problems. Follow your facility's policy and keep holding until no bleeding or swelling is seen. Afterward the nurse checks the site for bleeding or a hematoma and checks color, warmth, sensation and the pulse below the site. Thirty seconds of light pressure is not enough and risks a hematoma. Moving the wrist and applying warmth encourage bleeding. Before a radial puncture, many facilities also require a check of collateral blood flow to the hand, such as the modified Allen test.

Answer: C. The acid-base values are within normal range, but the client is hypoxemic; apply oxygen as prescribed or per protocol and notify the provider.

Rationale: Acid-base balance and oxygenation are two separate questions. Here the pH, PaCO2 and bicarbonate are all within typical ranges, so there is no acid-base imbalance. The PaO2, though, is well below the usual 80 to 100 mm Hg on room air, and together with an SpO2 of 89% it shows hypoxemia. The nurse starts oxygen according to the order or the facility's protocol, raises the head of the bed, assesses work of breathing and mental status, and reports the findings. Oxygen saturation targets differ between clients, for example in some clients with chronic lung disease, so follow the prescribed target. Option B misses the low PaO2. Options A and D describe acid-base problems that are not present.

Answer: D. The client is tiring and moving toward respiratory failure; the nurse should call the rapid response team or provider immediately and stay with the client.

Rationale: Early in an asthma attack, clients breathe fast and the PaCO2 falls, causing respiratory alkalosis, as the first result shows. When the PaCO2 climbs back to normal or above during a severe attack, it means the client can no longer move enough air, usually from fatigue and severe airway narrowing. The new result is now respiratory acidosis (pH below 7.35, PaCO2 above 45). Quieter wheezing with more drowsiness and falling saturation is an ominous sign, sometimes called a silent chest, because too little air is moving to make a wheeze. The nurse calls for help at once, stays with the client, keeps the head of the bed up, gives the treatments that are ordered, and prepares for possible ventilatory support.

Answer: A. The oxygen delivery device and flow rate (or FiO2), and the client's temperature

Rationale: The PaO2 can only be judged correctly if the lab and provider know how much oxygen the client was receiving when the sample was drawn, so record the device and flow rate or FiO2 (and ventilator settings if used). Many labs also ask for the client's temperature, because temperature affects blood gas values, along with the time the sample was drawn. If oxygen settings were just changed or the client was just suctioned, many facilities wait a set time, often 20 to 30 minutes, before drawing the sample; follow your policy. The sample is labeled, any air bubbles are removed, the syringe is capped, and it is sent right away. If there will be a delay, follow the lab's instructions, which often call for placing it on ice. Yesterday's saturation and meal time do not help interpret this sample.