Observational study of perioperative PtcCO2 and SpO2 in non-ventilated patients receiving epidural infusion or patient-controlled analgesia using a single earlobe monitor (TOSCA)

Br J Anaesth. 2007 Oct;99(4):567-71. doi: 10.1093/bja/aem206. Epub 2007 Jul 25.


Background: TOSCA, a non-invasive monitor with a single earlobe probe incorporating a Stow-Severinghaus electrode and optical sensor (Linde Medical Sensors AG, Basel, Switzerland), has previously been used with ventilated patients and in sleep laboratories. We recorded transcutaneous carbon dioxide pressures (Ptc(co(2)) and oxygen saturations (Sp(o(2)) in non-ventilated patients to investigate opioid-induced respiratory depression.

Methods: This observational cohort study included 28 ASA I and II patients, monitored between 10 p.m. and 6 a.m., before and after elective major laparotomy. After operation, patients were kept on oxygen, 4 litre min(-1), and received either bupivacaine (0.1%) containing fentanyl (2 microg ml(-1) via epidural catheter (epidural analgesia group, EPI; n = 14) or morphine via patient-controlled analgesia infusion pump (PCA-morphine group, PCA; n = 14).

Results: The preoperative median (lower/upper quartile) Ptc(co(2)) was similar in both groups at around 5.5 kPa, but significantly higher after operation in PCA with 6.9 kPa (5.6/7.3) (P = 0.02), accompanied by a longer hypercarbia time >6 kPa of 6.6 h (0.1/8.0) (P = 0.04), and lower respiratory rates of 13.9 breaths min(-1) (13.3/15.4) (P = 0.04). In EPI, the corresponding results were 5.8 kPa (5.5/6.0), 1.2 h (0.1/4.3), and 16.2 breaths min(-1) (14.8/16.7). The perioperative median Sp(o(2)) in both groups was comparable within the normal range, although generally higher when on supplemental oxygen (P = 0.26). The Sp(o(2)) time <94% was similar in both groups (P = 0.33) as were pain scores (P = 0.25).

Conclusions: Ptc(co(2)) recording in patients on PCA-morphine and supplemental oxygen revealed hypercapnia in the presence of normal respiratory rates and Sp(o(2)) values. This is recommended as an easy and sensitive monitor of respiratory depression and may have a role in the safe administration of opioid-analgesia.

Publication types

  • Comparative Study
  • Controlled Clinical Trial

MeSH terms

  • Adult
  • Aged
  • Analgesia, Epidural / adverse effects*
  • Analgesia, Patient-Controlled / adverse effects*
  • Analgesics, Opioid / adverse effects
  • Blood Gas Monitoring, Transcutaneous / instrumentation
  • Blood Gas Monitoring, Transcutaneous / methods
  • Carbon Dioxide / blood
  • Female
  • Humans
  • Hypercapnia / diagnosis*
  • Hypercapnia / etiology
  • Male
  • Middle Aged
  • Morphine / adverse effects
  • Oxygen / blood
  • Pain, Postoperative / therapy
  • Partial Pressure
  • Pilot Projects
  • Postoperative Care / instrumentation
  • Postoperative Care / methods
  • Postoperative Complications / diagnosis
  • Prospective Studies
  • Respiratory Insufficiency / blood
  • Respiratory Insufficiency / diagnosis*
  • Respiratory Insufficiency / etiology


  • Analgesics, Opioid
  • Carbon Dioxide
  • Morphine
  • Oxygen