The incidence of respiratory depression in patients using patient-controlled analgesia (PCA) pumps for postoperative pain relief may be significantly underreported in current literature. Past studies have found variable rates of respiratory depression, and the FDA's Manufacturer and User Facility Device Experience Database reports 22 deaths associated with PCA pumps. In a study of 96 post-op patients, episodes of respiratory depression lasting more than 2 minutes occurred in 31% of patients, and the average patient had 19 episodes. Clearly, current methods of monitoring for respiratory depression don't capture all the potentially dangerous episodes of hypoventilation.

Using pulse oximetry with capnography (also called end-tidal carbon dioxide [ETCO₂] monitoring) can give you a better picture of your patient's respiratory status than either technique used alone. Let's look at the two options more closely.

Two are better than one

Pulse oximetry measures peripheral arterial oxygenation (SpO₂), not ventilation. An SpO₂ value below 90% is considered arterial hypoxemia. But various factors can make this monitoring technique unreliable. For example, the sensor can come off the patient's finger; also, readings may not be accurate in patients with peripheral perfusion abnormalities. In a two-part study with 45 post-op patients and 2,288 postanesthesia patients, pulse oximetry was found to be useful for monitoring only when the patient wasn't receiving supplemental oxygen. In one reported case, a post-op patient receiving high-flow oxygen administered via face mask developed severe hypercapnia, resulting in severe carbon dioxide narcosis and apnea. The patient's pulse oximetry readings were 92% to 95%, the partial pressure of arterial carbon dioxide (PaCO₂) measurement was extremely elevated at 102 mm Hg, and the arterial pH was 7.08. In this case, pulse oximetry showed adequate oxygenation but didn't clue clinicians in to the patient's hypercapnia and hypoventilation.

Capnography is a more reliable indicator of respiratory depression because it measures ETCO₂. A normal ETCO₂ value is between 35 and 45 mm Hg and should be within 6 mm Hg of the PaCO₂ value. By measuring carbon dioxide levels in the patient's expirations, capnography can be used to monitor sedation and hypoventilation postoperatively and during diagnostic procedures. It can be used for patients of all ages whether intubated or not. Readings are taken while the patient is wearing a mask or nasal cannula containing a sensor. The results are reported on a handheld device a little larger than a personal digital assistant. Given the ease of monitoring, capnography should be finding its way into many clinical areas, not just CCUs.

Figure. No caption available.

Now let's take a look at how to accurately use a capnography monitor.

Capnography how-to

When using a capnography monitor, follow these tips.

* Remove the detector from the package. As shown, make sure the indicator color in the window is the same as or darker than the purple on the frame where the word CHECK appears, indicating less than 0.5% (4 mm Hg) ETCO₂. Position the airway adapter and sensor as indicated by the manufacturer's instructions. Usually the instructions are attached to the oxygen mask or nasal cannula.

* If you're using a sidestream device, replace the water trap frequently.

* Change the airway adapter with every respiratory circuit change.

Figure. No caption available.

* Apply an ETCO₂ detector immediately after endotracheal (ET) intubation. When the ET tube is inserted and the cuff is inflated, firmly connect the detector to the tube as shown, then attach the breathing device to the detector. Position the adapter on the ET tube as directed by the manufacturer.

* Listen for breath sounds and look for symmetrical chest movement as someone else ventilates the patient.

Figure. No caption available.

* After six breaths of moderate tidal volume are delivered, compare the color in the window on full-end expirations with the colors and corresponding ranges on the frame. Interpret the results and follow up according to the manufacturer's directions.

Figure. No caption available.

* Set and turn on the alarms and adjust the volume so they're audible.

Figure. No caption available.

* If your patient's ETCO₂ and PaCO₂ readings differ, assess for factors that may affect the readings, such as tubing length, leaks, or disconnections.

Road to PCA safety

By understanding how capnography and pulse oximetry work, you can help keep your patient safe while she uses a PCA pump.

Learn more about it