To the Editor
The Summer 1990 article “APSF-Sponsored Research Reveals that Capnograph Supplements Oximeter” was of particular interest to us. That study involved pediatric patients undergoing general anesthesia and suggested that capnography “appears to make significant contribution toward the care of pediatric patients by detecting problems related to ventilation that do not necessarily lead to hypoxemia, such as hypercarbia and hypocarbia.”
The merits of capnography during intubation and general anesthesia have been universally accepted, specifically its ability to detect esophageal tube placement, disconnection, obstruction, mechanical failure, and air embolism. Problems haw been encountered, however, when used with pediatric patients or in situations where intubation was not required.
Our own research interests involve the use of capnography to follow respiratory status during regional and monitored anesthesia care (MAC) in the operating room. Additionally, me am examining the use of capnography for ventilatory monitoring during post anesthesia cam Being in a Veterans Administration hospital, our patient population is very different from that of the pediatric study. However, with the extremes of age, there are some pulmonary and cardiac similarities between the two patient groups, specifically with regards to limited reserves. The majority of our patients are chronic smokers with both cardiovascular and pulmonary disease which complicate their management compared to nonsmokers. We are interested in the possible alterations in ventilatory response in these patients with COPD in the presence of sedation, anesthetic agents, and supplemental oxygen use during MAC and post anesthesia care.
Until the advent of end-tidal C02 (ETC02) and/or mass spectroscopy, pulse oximetry, and ventilatory rate have been the only convenient means to follow ventilation in non-intubated, spontaneously breathing patients. An arterial line is invasive and provides only intermittent data. Recently methods have been developed for use with nasal prongs to follow ETC02. (1,2,3) They provide a somewhat accurate and reproducible means to follow trends in ventilation. Transcutaneous C02 electrodes have also been developed and correlated with arterial C02 but they require a long equilibration time and can produce cutaneous bums. (4)
Our preliminary data for patients in the operating room and the post anesthesia room suggests that there is a small but significant percentage of these patients who experience episodes of hypoventilation (hypercarbia) which are not detected by respiratory rate and pulse oximetry since rate and oxygen saturation can remain satisfactory. The majority of these episodes were transient. However, occasionally the trend continued or progressed. They occurred predominantly in the post anesthesia room where numerous etiological factors could play a role.
Although the final analysis of our data is pending and will be reported in dew, we presently believe that the statement quoted in the first paragraph above may also be applicable to our patient population and possibly to surgical patients in general.
Dean L. Melnyk, M.D., Ph.D. Shep Cohen, M.D. St. Paul, MN
- Bowe EA, Boyen PG, Broome JA, and Klein EF. Accurate determination of end-tidal carbon dioxide during administration of oxygen by nasal cannulae. J. Clin. Monitoring 1989;5:105-15.
- Desniarattes R, Kennedy R, Davis DR. Inexpensive capnography during monitored anesthesia care. Anesth Analg 1990;71:100-6.
- Ibarra E. Len DE. Mass spectrometer monitoring of patients with regional anesthesia (letter). Anesthesiology 1985;63:572-3.
- Shapiro BA and Cane RD. Blood gas monitoring: Yesterday, today and tomorrow, Crit. Care Mod. 1989; 17:573-8 1.