Pulse oximetry and bedside capnography are important tools in monitoring sedated patients. Respiratory therapists should be key consultants on multidisciplinary care teams in non-critical care inpatient units.
By Patricia Carroll, RRT, RN, BC, CEN, MS
In January 2003, the Joint Commission on Accreditation of Healthcare Organizations (JCAHO) began assessing how well hospitals had implemented plans to meet specified patient-safety goals. Two goals relate directly to monitoring patients receiving opioids by infusion for pain control: improving the safety of using infusion pumps and improving the effectiveness of clinical alarm systems.1
To provide safe care, all clinicians must understand how infused opioids affect respiration and how to use monitors and clinical alarm systems appropriately for continuous assessment of these at-risk patients. This is particularly important in a medical-surgical setting in which physical assessments are periodic. Today, pulse oximetry is not enough; bedside capnography is easy to use and has a favorable cost-benefit profile.
RTs should be key members of multidisciplinary teams in general inpatient units, not just critical care areas. They can help other team members understand how adding capnography to monitoring will optimize patient safety.
|• Long duration
• Noninvasive nature
|• High risk of respiratory depression in patients not already taking high-dose opioids
• More difficult to reverse if respiratory depression occurs because the drug is in the subcutaneous tissue
• Delayed onset of action (12 hours)
|• Allows rapid pain relief without intravenous access
• Can be used for PCA (patient-controlled analgesia)
• Easy to maintain as an outpatien
|• Limited to 2 to 4 mL per hour
• Irritation at injection site
• Can be difficult to reverse using naloxone
|• Best for rapid relief of severe, acute pain
• Allows PCA
|• Patient must be awake in order to trigger PCA
• Airway obstruction is intermittent and associated with sleep
|• Fewer systemic side effects
• Decreased respiratory complications and/or improved pulmonary function, compared with systemic infusion
• Shorter time to extubation
• Lower doses are needed to provide the same analgesia obtained using other routes
• Decreased incidence of respiratory depression, compared with intravenous opioids
• Catheter can be placed at the level of pain
• Fewer thrombotic events
|• Typically used with a local anesthetic agent, which can cause lower-extremity weakness and urinary retention
• More invasive, with a higher risk of infection
• Risk of epidural hematoma (particularly with anticoagulants)
• Overdose can cause respiratory depression
|• As above, but limited to lumbar region
|• Limited to lumbar region to avoid spinal-cord injury
• Least studied route
• Risk of spinal headache
• Overdose can cause respiratory depression
Intravenous opioids depress respiration and cause alveolar hypoventilation by acting directly on the brain stem and reducing the sensitivity of the brain’s respiratory centers to increased carbon dioxide levels.2 Many clinicians, however, are unaware that this central depression is not the only mechanism by which opioids can impair respiratory effort. Airway obstruction also contributes to respiratory insufficiency. Opioids decrease muscle tone in the oropharynx, leading to intermittent airway obstruction, particularly during sleep. This process is similar to that seen in sleep apnea.2 Since this airway obstruction is intermittent, it is rarely identified when respiratory rate is recorded on an hourly schedule.
Opioids can be administered in many ways. The table (page 40) lists advantages and disadvantages of five common routes. The goal of intrathecal opioid administration, in which the drug is infused into the subarachnoid space (as it is for spinal anesthesia), is to achieve effective pain control with less sedation and fewer side effects than seen with systemic administration. To evaluate dose-related effects on respiration and thus maximize the safety of this pain-management approach, researchers3 gave varying doses of intrathecal morphine to young, healthy volunteers who were opioid naive. Increasing the dose of intrathecal morphine from 0.2 mg to 0.4 mg and 0.6 mg provided enhanced analgesia (for pain caused by pressure applied to the tibia); however, it also significantly increased respiratory depression. A key finding in this study was that respiratory depression was identified by rising PaCO2 alone. Dose-related hypercapnia did not change heart rate, blood pressure, respiratory rate, pupillary response to light, or sedation level. Thus, respiratory depression was invisible during routine clinical assessment.
In a follow-up study,4 volunteers were evaluated under different dose regimens comparing intravenous and intrathecal routes and placebo. Intrathecal morphine and an equianalgesic dose of intravenous morphine both depress the respiratory response to hypoxia, and the intrathecal effects are longer lasting. This depression is due to the direct effect of the opioid on the brain, not on peripheral chemoreceptors, as was once thought.
Pulse oximetry is a widely used method of monitoring patients’ respiratory response to opioid analgesia infusions, despite a lack of evidence of its effectiveness for this use. RTs play a critical role in pointing out that a complete respiratory assessment in at-risk patients must include assessment of both oxygenation and ventilation. Results of a survey5 at a major medical center showed that only 35% of nurses and 39% of physicians knew that pulse oximetry monitoring does not reflect changes in ventilation. In addition, nurses and physicians need to learn that pulse oximetry is not an accurate apnea alarm, since it can take as long as 3 minutes after complete airway obstruction for saturation to drop.6
One of the six key goals of the JCAHO safety initiative is to improve the effectiveness of clinical alarm systems. Historically, pulse oximetry has been associated with multiple, repeated false alarms. One study7 of alarm soundings in a pediatric intensive care unit during a 7-day period found that pulse-oximetry alarms made up 44% of all alarm soundings (the greatest number); of those, only 7% were clinically significant.
The very nature of opioid-induced hypoventilation, which is intermittent and often associated with airway obstruction, can trigger a pulse-oximeter alarm that corrects itself, particularly if the sound of the alarm awakens the patient enough so that he opens his airway and takes a breath. Since as few as 7% of pulse-oximetry alarms might be clinically significant, it is easy to see how they can be ignored on a busy medical-surgical unit.
In the past few years, capnography has made the transition from an expensive, complicated technology for patients with artificial airways to a simple monitoring technology that can measure end-tidal carbon dioxide (ETCO2) effectively in patients who are breathing spontaneously (or should be, but are hypoventilating).8 In particular, a newer form of sidestream capnography has made it possible for any patient at risk for hypoventilation to benefit from continuous ETCO2 monitoring.
This type of capnography uses a device similar to a nasal cannula as a patient interface. Oxygen can be administered without interfering with collection of exhaled gas for carbon dioxide measurement. Conventional technology uses narrow-band infrared technology that is not carbon dioxide specific. Contamination by other gases, such as nitrous oxide and oxygen, can cause artifacts requiring correction algorithms in the sensor and, in many cases, manual recalibration. In contrast, the newer sidestream system uses a laser-based technology that creates an infrared emission precisely matching the absorption spectrum of carbon dioxide. This results in a specificity for carbon dioxide that eliminates the need for manual calibration or corrections by the user, along with a very short light path that reduces the sample size to 15 L, allowing accurate ETCO2 measurements using small volumes of exhaled gas at low flow rates.9,10
The tubing connecting the patient interface to the sensor has a multichannel design with narrow, hydrophobic openings that face different directions. This further enhances accuracy by minimizing occlusion by condensation or secretions. Capnography technology now allows clinicians to measure ETCO2 accurately and continuously at the bedside with ease.
A Proactive Approach
To be advocates for patient safety in settings where patients are receiving infused opioids, RTs must ensure that clinicians understand that pulse oximetry does not provide information about ventilation and pulse oximeters should not be used as apnea monitors (in fact, if patients are receiving supplemental oxygen, pulse oximetry readings may remain normal for at least 3 minutes after a patient stops breathing). RTs also should note that spot checks using pulse oximeters are not supported by research; hypoventilation can occur with no change in physical assessment, even in the presence of normal SpO2 readings; and the most effective way to monitor infusions that can put patients at risk for respiratory depression is to monitor oxygenation using pulse oximetry and to monitor ventilation using capnography.8
RTs form a critical link in the patient-safety chain by teaching nurses and physicians the indications for and limitations of pulse oximetry and the technology available to ensure assessment of both oxygenation and ventilation. Then, all clinicians will be able to see the whole picture clearly.
Patricia Carroll, RRT, RN, BC, CEN, MS, is a member RT’s editorial advisory board. For more information, contact [email protected]
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