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Postanesthesia monitoring revisited II
Letters to the Editor
Postanesthesia Monitoring Revisited II To the Editor: In the editorials by Kevin Tremper’ and John Downs,2 several important issues were raised, perhaps the most important being the recognition of the lack of objective knowledge in all aspects of patient monitoring in the postanesthesia care unit (PACU) and the need for further study and development. Tremper correctly criticized our study3 for being too small and for the “nearly self-fulfilling prophecy of the question asked.” We are, however, proud to share this criticism with almost all originators of novel methodology. Prototype devices need to be tested and compared with state-of-the-art techniques in a clinical setting, and if the originators are not able to organize such tests, progress will be impeded. As we pointed out in our article, we welcome investigators who wish to perform independent studies, and we commit ourselves to be of assistance in any possible way. Tremper misconceived our sensor as being capnometric. A capnometer has many but not all of the virtues of a flow sensor (like the one we used). However, when it comes to reliable detection of apnea, capnometry is known to provide ambiguous signals due to improper gas mixing.4.” Tremper found it difficult to conclude that pulse oximetry produces too many false positive alarms due to the lack of an objective reference method. In principle, he is right, of course. But the indisputable fact that we observed a mean time of 8 minutes between alarms should be enough to conclude that something is wrong. The viewpoints of Downs regarding oxygen (0,) therapy are certainly worth noting. Furthermore, we fully support his suggestion of using the pulse oximeter as a measuring instrument rather than an alarm device. However, this situation requires that the nurses be properly trained to interpret pulse oximeter signals and to know when to disregard a reading due to motion artifacts and/or poor perfusion. The main issue raised in our article was whether monitoring equipment provides adequate assistance of the PACU nurses in terms of early warning against dangerous patient conditions. The high frequency of upper airway and ventilatory problems urged us to emphasize this aspect. The (expected) conclusion was that pulse oximetry would not provide adequate sensitivity to these problems, especially when combined with O2 therapy. The high alarm rate and low coincidence with apnea that we ob served in our study was not postulated. Without being presumptuous, however, we suggested methods and devices that seem, at least in our own hands, to perform adequately.
Lars Wiklund, MD, PhD Professor and Chairman of Anesthesiology Uppsala University S-751 85 Uppsala, Sweden
Bertil H6k, PhD Associate Professor of Technology Uppsala University and Hijk Instrument S-721 31 Vasteras, Sweden
AB
References 1. Tremper KK: Necessary monitoring in the postanesthesia care unit? J Clin An&h 1994;6:178-9. 2. Downs JB: Prevention of hypoxemia: the simple, logical, but incorrect solution. J Clin An&h 1994;6:180-1. 3. Wiklund L, HBk B, Stahl K, Jordeby-Jonsson A: Postanesthesia monitoring revisited: frequency of true and false alarms from different monitoring devices. J Clin Anesth 1994;6:182-8. 4. Lena G, Heipertz W, Epple E: Capnometry for continuous postoperative monitoring of non-intubated, spontaneously breathing patients. J CZin Monit 1991;7:245-9. 5. Vegfors M, Ugnell H, Hok B, Oberg PA: Experimental evaluation of two new sensors for respiratory rate monitoring. Physiol Mem 1993;14:171-81. Editori note: Dr. Tremper and Dr. Downs both declined to reply to Drs. Wiklund and Hok’s letter.
Monitoring Anesthetic Depth during Laryngeal Mask Anesthesia To the Editor: It is widely considered that inadequate anesthesia increases the risk of complications with the laryngeal mask airway (LMA) ,‘S including aspiration.3,4 The importance of monitoring anesthetic depth in complication prevention has been highlighted,” and every anesthetist is aware that respiratory rate and pattern can be useful clinical signs. The problems of using these signs are the variable effects of the different anesthetic and opioid drugs and the fact that changes in respiratory pattern are a highly subjective observation. Depending on the pharmacokinetics of the particular drugs used, they may herald either immediate or slow onset of awakening, and thus are unreliable as a diagnostic tool for depth of anesthesia, particularly where propofol and desflurane are used. The supraglottic location of the LMA cuff implies that it is uniquely positioned to detect laryngeal and pharyngeal activity. We would like to discuss the possibility of using this activity as an aid to monitoring anesthetic depth. Vocal cord activity is a commonly used clinical sign of inadequate anesthetic depth in nonintubated patients. Sudden cord closure can be precipitated by intense surgical stimulation,‘j but milder noxious stimuli may be associated with a lesser degree of cord closure. Subtle glottic movement can be readily observed, and perhaps quantiJ. Clin. Anesth.,
vol. 6, November/December
1994
525
Postanesthesia Monitoring Revisited II To the Editor: In the editorials by Kevin Tremper’ and John Downs,2 several important issues were raised, perhaps the most important being the recognition of the lack of objective knowledge in all aspects of patient monitoring in the postanesthesia care unit (PACU) and the need for further study and development. Tremper correctly criticized our study3 for being too small and for the “nearly self-fulfilling prophecy of the question asked.” We are, however, proud to share this criticism with almost all originators of novel methodology. Prototype devices need to be tested and compared with state-of-the-art techniques in a clinical setting, and if the originators are not able to organize such tests, progress will be impeded. As we pointed out in our article, we welcome investigators who wish to perform independent studies, and we commit ourselves to be of assistance in any possible way. Tremper misconceived our sensor as being capnometric. A capnometer has many but not all of the virtues of a flow sensor (like the one we used). However, when it comes to reliable detection of apnea, capnometry is known to provide ambiguous signals due to improper gas mixing.4.” Tremper found it difficult to conclude that pulse oximetry produces too many false positive alarms due to the lack of an objective reference method. In principle, he is right, of course. But the indisputable fact that we observed a mean time of 8 minutes between alarms should be enough to conclude that something is wrong. The viewpoints of Downs regarding oxygen (0,) therapy are certainly worth noting. Furthermore, we fully support his suggestion of using the pulse oximeter as a measuring instrument rather than an alarm device. However, this situation requires that the nurses be properly trained to interpret pulse oximeter signals and to know when to disregard a reading due to motion artifacts and/or poor perfusion. The main issue raised in our article was whether monitoring equipment provides adequate assistance of the PACU nurses in terms of early warning against dangerous patient conditions. The high frequency of upper airway and ventilatory problems urged us to emphasize this aspect. The (expected) conclusion was that pulse oximetry would not provide adequate sensitivity to these problems, especially when combined with O2 therapy. The high alarm rate and low coincidence with apnea that we ob served in our study was not postulated. Without being presumptuous, however, we suggested methods and devices that seem, at least in our own hands, to perform adequately.
Lars Wiklund, MD, PhD Professor and Chairman of Anesthesiology Uppsala University S-751 85 Uppsala, Sweden
Bertil H6k, PhD Associate Professor of Technology Uppsala University and Hijk Instrument S-721 31 Vasteras, Sweden
AB
References 1. Tremper KK: Necessary monitoring in the postanesthesia care unit? J Clin An&h 1994;6:178-9. 2. Downs JB: Prevention of hypoxemia: the simple, logical, but incorrect solution. J Clin An&h 1994;6:180-1. 3. Wiklund L, HBk B, Stahl K, Jordeby-Jonsson A: Postanesthesia monitoring revisited: frequency of true and false alarms from different monitoring devices. J Clin Anesth 1994;6:182-8. 4. Lena G, Heipertz W, Epple E: Capnometry for continuous postoperative monitoring of non-intubated, spontaneously breathing patients. J CZin Monit 1991;7:245-9. 5. Vegfors M, Ugnell H, Hok B, Oberg PA: Experimental evaluation of two new sensors for respiratory rate monitoring. Physiol Mem 1993;14:171-81. Editori note: Dr. Tremper and Dr. Downs both declined to reply to Drs. Wiklund and Hok’s letter.
Monitoring Anesthetic Depth during Laryngeal Mask Anesthesia To the Editor: It is widely considered that inadequate anesthesia increases the risk of complications with the laryngeal mask airway (LMA) ,‘S including aspiration.3,4 The importance of monitoring anesthetic depth in complication prevention has been highlighted,” and every anesthetist is aware that respiratory rate and pattern can be useful clinical signs. The problems of using these signs are the variable effects of the different anesthetic and opioid drugs and the fact that changes in respiratory pattern are a highly subjective observation. Depending on the pharmacokinetics of the particular drugs used, they may herald either immediate or slow onset of awakening, and thus are unreliable as a diagnostic tool for depth of anesthesia, particularly where propofol and desflurane are used. The supraglottic location of the LMA cuff implies that it is uniquely positioned to detect laryngeal and pharyngeal activity. We would like to discuss the possibility of using this activity as an aid to monitoring anesthetic depth. Vocal cord activity is a commonly used clinical sign of inadequate anesthetic depth in nonintubated patients. Sudden cord closure can be precipitated by intense surgical stimulation,‘j but milder noxious stimuli may be associated with a lesser degree of cord closure. Subtle glottic movement can be readily observed, and perhaps quantiJ. Clin. Anesth.,
vol. 6, November/December
1994
525