# A 65-year-old client with obstructive sleep apnea and BMI 38 is being initiated on PCA hydromorphone after abdominal surgery. The unit standard of care for high-risk PCA includes continuous capnography. Which monitoring parameter does the nurse prioritize that distinguishes capnography from pulse oximetry? > source: MyMerci (mymerci.kr) > url: https://mymerci.kr/pages/nclex_q.php?qn_id=375173&lang=en > language: en > subject: Adverse Effects/Contraindications/Interactions > category: PA ## Question A 65-year-old client with obstructive sleep apnea and BMI 38 is being initiated on PCA hydromorphone after abdominal surgery. The unit standard of care for high-risk PCA includes continuous capnography. Which monitoring parameter does the nurse prioritize that distinguishes capnography from pulse oximetry? ## Option 1. Capnography measures end-tidal CO2 (ETCO2) and respiratory rate continuously and detects opioid-induced hypoventilation earlier than pulse oximetry, which lags because supplemental oxygen masks falling SpO2 until significant hypoventilation has already occurred; the nurse acts on a rising ETCO2, falling RR, or sedation increase before SpO2 drops. **✔ Correct answer** 2. Capnography measures end-tidal carbon dioxide and respiratory rate continuously, making it a comprehensive substitute for sedation assessment and pulse oximetry because it directly indicates the patient's ventilatory status without the need for additional assessments like the Pasero Sedation Scale. 3. Capnography measures the partial pressure of oxygen in the bloodstream by analyzing exhaled gases, which provides a more precise assessment of oxygenation than pulse oximetry in obese patients with obstructive sleep apnea, thereby rendering continuous capnography superior to intermittent pulse oximetry checks. 4. Capnography is unnecessary as long as pulse oximetry readings stay above 92% because supplemental oxygen ensures adequate ventilation and prevents hypercapnia, making continuous ETCO2 monitoring redundant in patients receiving PCA opioids who are on supplemental oxygen, and the nurse can rely on SpO2 trends alone to detect respiratory depression. **Correct answer: 1** ## Explanation Continuous capnography measures the partial pressure of CO2 in exhaled breath (end-tidal CO2 or ETCO2, normal 35 to 45 mmHg) and the respiratory rate. Compared with pulse oximetry, capnography provides the earliest objective signal of opioid-induced hypoventilation: rising ETCO2 reflects accumulating CO2 from inadequate ventilation, and a flattening or absent waveform reflects apnea. Pulse oximetry lags significantly, especially when supplemental oxygen is in place, because oxygen saturation can remain in the 90s even when minute ventilation has fallen and CO2 has climbed; SpO2 drop is therefore a late sign. Recommended monitoring for high-risk PCA patients (sleep apnea, obesity, comorbid sedatives, opioid-naive, elderly, OSA risk score, prior respiratory event): (1) continuous capnography with ETCO2 and waveform, (2) continuous pulse oximetry, (3) frequent sedation scale (POSS) and RR observations, (4) two-nurse double check at every PCA program change. The nurse intervenes when ETCO2 rises above 50 to 55 mmHg or 10 mmHg above baseline, RR drops below 10, sedation rises to POSS 3 or 4, or waveform becomes erratic — without waiting for SpO2 to fall. Believing capnography is unnecessary above 92 percent SpO2 reflects the exact knowledge gap that has caused PCA deaths. ## In-depth explanation Capnography catches hypoventilation before SpO2 drops. 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