Volume 5, No. 2 • Summer 1990

APSF-Sponsored Research Reveals that Capnograph Supplements Oximeter

Charles J. Cote, M. D.

A prospective evaluation of pulse oximetry and capnography in the care of anesthetized pediatric patients was undertaken in 1989 by the Pediatric Anesthesia Group at the Massachusetts General Hospital. This work was made possible by a grant from the Anesthesia Patient Safety Foundation combined with funding and equipment grants from Nellcor, Inc., Hayward, CA.

In a previous study at this institution, we found a significant reduction in the incidence of hypoxic events in children whose anesthesiologists had pulse oximetry data available to them and, compared to the entire pediatric population, and increased incidence of events in high risk patients (physical status III and IV) and in children less than two years of age. The Current study was designed to determine if capnography contributed to improved care of pediatric patients.

Following approval of the Subcommittee on Human Studies as well as Written informed consent, patients of physical status I through IV scheduled for surgical procedures under general anesthesia were studied in a randomized single-blinded prospective fashion. Patients were randomly assigned to one of four groups, A: pulse oximetry and capnography data available to those administering the anesthetic; B: only pulse oximetry data available; C. only capnography data available; and D: neither pulse oximetry nor capnography data available to the anesthesia team.

An additional anesthesiologist (whose only task was to record data) secured demographic information as well as a continuous strip chart recording of oxygen saturation, expired CO2, electrocardiogram, and the plethysmographic output from the oximeter. The observing anesthesiologist interviewed the anesthesia team and recorded events whenever a problem occurred. A major hypoxic event was defined as a decrease in the oxygen saturation to < 85% for >30 seconds and hypo- or hypercarbia were defined as an expired C02 < 25 mmHg or > 55mmHgfor >60seconds.Ifoneoftheseconditions arose during blinded data recording, the observing anesthesiologists informed the anesthesia team that a problem was Liking place and then later interviewed them. Whether or not the monitor data were available, the subject anesthesiologist doing the case was questioned as to awareness of the occurence of any problem.

35 % Have Problem

During the course of this study, 402 pediatric cases were examined and 232 problems were observed in 142 patients. There were 59 major hypoxic events in 43 patients and 73 problems related to ventilation in 58 patients. The number of patients with hypoxic events was significantly greater in groups C and D, i. e., no oximeter available, compared to groups A and M i. e., oximeter available. Patients less than two years of age and those managed with endotracheal intubation, had a higher incidence of these major hypoxic events. The pulse oximeter allowed diagnosis of 4 1 events; the diagnosis was made clinically in 13, and by the capnograph in five. Interestingly, four out of five hypoxic events diagnosed initially by capnography, were diagnosed by the observer rather than the anesthesiologist conducting the anesthetic. The capnograph was first to reveal and detect 88%of the 73 problems related to ventilation. Fifty-one of these were hypercarbia, seven hypocarbia, eight esophageal intubations, (three of which were first diagnosed by the capnograph), three circuit disconnects and four endotracheal tube obstructions. Only five of these 73 problems related to ventilation resulted in major episodes of desaturations.

This study clearly demonstrates the efficacy of pulse oximetry in detecting and preventing intraoperative hypoxemic-events in children. The incidence of hypoxemic events in this study was nearly identical to our previous study of pulse oximetry and the patient population at greatest risk was once again children less than two years of age. The capnograph did not appear to make a sufficient contribution to reducing events which lead to hypoxemia since all events leading to hypoxemia were either diagnosed first by the pulse oximeter or by the capnograph only when they were observed by a UIH party and not by the primary anesthesiologist. On the other hand, the capnography appears to make significant contribution toward the care of pediatric patients by detecting problem related to ventilation that do not necessarily lead to hypoxemia, such as hypercarbia and hypocarbia. Pulse oximetry is a more specific monitor for safeguarding against clinically important events leading to hypoxemia during pediatric anesthesia.

Other interesting observations related to ” study included a poor relationship to ventricular arrhythmias to hypercarbia, a rather frequent incidence on intra-operative endobronchial intubation diagnosed by minor but persistent changes in oxygen saturation, and a very high incidence of false alarms. False alarms of the capnography resulted in the anesthesiologist frequently ignoring the capnograph alarm. Additionally, an increased incidence of minor but not major hypoxemic events was observed in children with symptoms of a mild upper respiratory infection compared to those without symptoms. The latter observation is consistent with those of previous investigators.

The exact importance of these other observations awaits more detailed analysis of the data. Nevertheless, this study clearly suggests that pulse oximetry is the most important monitor in helping to prevent events which lead to severe “n desaturation whereas capnography is important in preventing untoward episodes related to ventilation, including those not necessarily leading to hypoxemia.

Dr. Cote is Associate Anesthetist, Massachusetts General Hospital; Associate Professor of Anesthesia, Harvard Medical School, and Norbert Rolf, M. D., Research Fellow, Massachusetts General Hospital.