June 10, 2008
Monitoring the Postoperative COPD Patient
In our April 8, 2008 Patient Safety Tip of the Week “Oxygen as a Medication” we discussed both the benefits and risks of oxygen therapy. While oxygen is obviously very helpful or lifesaving in many circumstances, it does have potential adverse effects as well. In the patient with COPD, precipitation of hypercapnia is a well-known potential complication of oxygen therapy. In days of old we were taught that oxygen therapy in the COPD patient depressed respiratory drive. However, most now feel that oxygen therapy in the COPD leads to hypercapnia by impaired ability to eliminate CO2 due to increased ventilation/perfusion mismatch.
The problem is nicely illustrated by a March 2008 AHRQ Web M&M case. That was a case of an elderly patient with COPD who developed an exacerbation of COPD with community-acquired pneumonia. On oxygen 4L/min. she developed obtundation due to hypercapnia and respiratory acidosis while her O2 saturation was 98%. That article elegantly makes the case that an overdose of oxygen in that patient led to the untoward event. The author points out that most COPD patients have become quite tolerant of lesser degrees of oxygen saturation and points out that the currently recommended target oxygen tension in exacerbated COPD is about 60–65 mm Hg, which is equivalent to a saturation of approximately 90%–92%. He points out that most patients with exacerbations of COPD are adequately oxygenated if the oxygen tension can be maintained above 50 mm Hg, corresponding to an oxygen saturation above about 85%. He states that an about-to-be-published British Thoracic Society Emergency Oxygen Guideline for the United Kingdom will recommend that the saturation should be maintained above 88% in most cases of exacerbated COPD to allow an additional margin of safety but that high-risk patients with prior hypercapnic respiratory failure may be safely managed with an oxygen saturation in the range of 85%–88%.
A scenario we commonly encounter is the COPD patient who undergoes surgery and needs postoperative opioids for pain relief. Those patients are especially at risk for hypercapnia due to both the oxygen effect and the respiratory depressant effect of the opioids. Such patients may be at risk for hypercapnia even if they have not had a prior episode of hypercapnia.
Monitoring the effectiveness of oxygen therapy is usually accomplished noninvasively by pulse oximetry. But this only measures oxygen saturation and does not provide any assessment of the pCO2 and pH status. You’d be surprised how often even COPD patients who have a history of hypercapnia fail to be adequately monitored for hypercapnia.
The gold standard for monitoring nonintubated patients for hypercapnia is still arterial blood gas analysis. However, ABG’s are an invasive test and their risk may be further increased in the postop patient because of pharmacologic DVT prophlylaxis.
Therefore, we often need to look at ways to nonivasively monitor such patients. Don’t forget that the patient’s level of arousal/mental status is really a vital sign! It is critical that monitoring the patient’s level of arousal be built into the monitoring protocols. Our nursing colleagues in the field of pain management have used some practical scales in monitoring patients on PCA pump treatment.
We can learn from our colleagues in the field of pain management. Pasero and McCaffery (2002) discuss risk factors for opioid-induced respiratory depression and keys to monitoring such patients. Infants less than 6 months old, opioid-naïve elderly patients, and patients with coexisting conditions such as COPD, sleep apnea, or major organ failure are at increased risk of respiratory depression. In addition, drugs such as intramuscular opioids, muscle relaxants and anxiolytics, benzodiazepines, sedating antihistamines, and some antiemetics may increase the risk for opioid-induced respiratory depression. In their discussion of monitoring, they provide the following very practical sedation scale:
S = Sleep, easy to arouse (acceptable; no action necessary)
1 = Awake and alert (acceptable; no action necessary)
2 = Slightly drowsy, easily arousable (acceptable; no action necessary)
3 = Frequently drowsy, arousable, drifts off to sleep during conversation (unacceptable; decrease opioid dose by 25-50%, add an opioid-sparing analgesic, and monitor the patient’s level of sedation and respiratory status closely)
4 = Somnolent, minimal or no response to physical stimulation (unacceptable; stop opioid, consider administering naloxone)
But it should be kept in mind that an oversedated patient may respond to physical stimulation by becoming aroused and answering questions and their respiratory rate may improve temporarily as well (Cohen et al 2006). But they fall back into the state of oversedation when the stimulation ceases. Therefore, the level of arousal and rate and depth of respiration should first be assessed by simply observing the patient before stimulation.
Measurement of end-tidal CO2 has been long utilized in intubated patients, but capnography has been gaining interest in multiple settings for non-intubated patients. D’Arcy (2008) discusses use of capnography to monitor for hypoventilation in patients on PCA. Other studies have documented the utility of capnography in PCA patients (Overdyk 2007) (Maddox 2006), monitored anesthesia care (Soto 2004), and postoperative orthopedic patients at risk for sleep apnea (Hutchison 2008), though the usefulness was often due to the equipment’s ability to identify apneic periods rather than identifying high end-tidal CO2 levels. Newer methods using isotopes for monitoring for hypercapnia are under investigation (Modak 2007).
Facilities need to establish methods for identifying patients at high risk for respiratory depression when on opioids and implement protocols for monitoring them. Understanding the role of oxygen therapy in the COPD patient and its potential for precipitating hypercapnia and respiratory arrest is an important part of this process. Keep your eyes open for the British Thoracic Society guideline "Emergency Oxygen Use in Adult Patients" expected to be published in Thorax this summer.
Update: See our January 27, 2009 Patient Safety Tip of the Week “Oxygen Therapy: Everything You Wanted to Know and More!”. This summarizes the British Thoracic Society Guideline Emergency Oxygen Use in Adult Patients that was published in the Journal Thorax in October 2008. It has an excellent section on management of patients at risk for hypercapnic respiratory failure. It does not, at this time, make recommendations about noninvasive monitoring techniques for CO2 retention. It points out that measurement of end-tidal CO2 does not correlate well with arterial pCO2, hence is of limited value in monitoring the COPD patient or other patient at risk for hypercapnic respiratory failure. It does offer optimism that newer devices that noninvasively measure both oxygen saturation and pCO2 are in the research pipeline. They also stress observation for the clinical signs of hypercapnia (vasodilation, bounding pulse, flapping tremor, drowsiness, confusion and coma).
O’Driscoll BR, Howard LS, Davison AG and the British Thoracic Society. Emergency Oxygen Guideline Group. BTS Guideline Emergency Oxygen Use in Adult Patients. Thorax 2008; 63 (suppl. VI): 1-68 http://www.brit-thoracic.org.uk/ClinicalInformation/EmergencyOxygen/EmergencyOxygenuseinAdultPatients/tabid/327/Default.aspx
O'Driscoll BR. AHRQ Web M&M Case & Commentary. Overdose on Oxygen?. March 2008 http://www.webmm.ahrq.gov/case.aspx?caseID=172
Pasero C, McCaffery M. Monitoring Sedation: It's the key to preventing opioid-induced respiratory depression. American Journal of Nursing. 2002; 102(2):67-69
Cohen MR, Weber RJ, Moss J (Institute for Safe Medication Practices). Patient-Controlled Analgesia: Making it Safer for Patients. A continuing education program for pharmacists and nurses. ISMP. April 2006 http://www.ismp.org/profdevelopment/PCAMonograph.pdf
D’Arcy Yvonne. Keep your patient safe during PCA. Nursing2008 2008; 38: 50-55 http://www.nursingcenter.com/prodev/ce_article.asp?tid=762689
Overdyk FJ, Rickey Carter R, Maddox RR, Callura J, Herrin AE, Craig H
Continuous Oximetry/Capnometry Monitoring Reveals Frequent Desaturation and Bradypnea During Patient-Controlled Analgesia. Anesth Analg 2007; 105: 412-418
Maddox RR, Williams, CK, Oglesby H, Butler B, Colcasure B.
Clinical experience with patient-controlled analgesia using continuous
respiratory monitoring and a smart infusion system. American Journal of
Health-System Pharmacy. 63(2): 157-164, January 15, 2006.
Soto RG, Fu ES, Vila H, Miguel RV. Capnography Accurately Detects Apnea During Monitored Anesthesia Care. Anesth Analg 2004;99:379-382 http://www.anesthesia-analgesia.org/cgi/reprint/99/2/379
Hutchison R, Rodriguez L. Capnography and Respiratory Depression. Am J Nurs 2008 Feb;108(2):35-9 http://www.ncbi.nlm.nih.gov/pubmed/18227667?ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
Modak AS, Reid JM, Williams LA, Hobbs GR. A simple non-invasive method to detect and monitor hypercapnia: the sodium [13C]bicarbonate breath test. Isotopes Environ Health Stud 2007 Mar;43(1): 23-9 http://www.ncbi.nlm.nih.gov/pubmed/17454270