Search

Background: Pulse oximetry is considered a standard of care in both the operating room and the postanesthetic care unit, and it is widely used in all critical care settings. Pulse oximeters may fail to provide valid SpO2 data in various situations that produce low signal-to-noise ratio. Motion artifact is a common cause of oximeter failure and loss of accuracy. This study compares the accuracy and data dropout rates of three current pulse oximeters during standardized motion in healthy volunteers., Methods: Ten healthy volunteers were monitored by three different pulse oximeters: Nellcor N-200, Nellcor N-3000, and Masimo SET (prototype). Sensors were placed on digits 2, 3, and 4 of the test hand, which was strapped to a mechanical motion table. The opposite hand was used as a stationary control and was monitored with the same pulse oximeters and an arterial cannula. Arterial oxygen saturation was varied from 100% to 75% by changing the inspired oxygen concentration. While SpO2 was both constant and changing, the oximeter sensors were connected before and during motion. Oximeter errors and dropout rates were digitally recorded continuously during each experiment., Results: If the oximeter was functioning before motion began, the following are the percentages of time when the instrument displayed an SpO2 value within 7% of control: N-200 = 76%, N-3000 = 87%, and Masimo = 99%. When the oximeter sensor was connected after the beginning of motion, the values were N-200 = 68%, N-3000 = 47%, and Masimo = 97%. If the alarm threshold was chosen SpO2 less than 90%, then the positive predictive values (true alarms/total alarms) are N-200 = 73%, N-3000 = 81%, and Masimo = 100%. In general, N-200 had the greatest SpO2 errors and N-3000 had the highest dropout rates., Conclusions: The mechanical motions used in this study significantly affected oximeter function, particularly when the sensors were connected during motion, which requires signal acquisition during motion. The error and dropout rate performance of the Masimo was superior to that of the other two instruments during all test conditions. Masimo uses a new paradigm for oximeter signal processing, which appears to represent a significant advance in low signal-to-noise performance.

Background: Pulse oximetry is considered a standard of care in both the operating room and the postanesthetic care unit, and it is widely used in all critical care settings. Pulse oximeters may fail to provide valid pulse oximetry data in various situations that produce low signal-to-noise ratio. Motion artifact is a common cause of oximeter failure and loss of accuracy. This study compares the accuracy and data dropout rates of three current pulse oximeters during standardized motion in healthy volunteers., Methods: Ten healthy volunteers were monitored by three different pulse oximeters: Nellcor N-200, Nellcor N-3000, and Masimo SET (prototype). Sensors were placed on digits 2, 3, and 4 of the test hand, which was strapped to a mechanical motion table. The opposite hand was used as a stationary control and was monitored with the same pulse oximeters and an arterial cannula. Arterial oxygen saturation rate varied from 100% to 75% by changing the inspired oxygen concentration. While pulse oximetry was both constant and changing, the oximeter sensors were connected before and during motion. Oximeter errors and dropout rates were digitally recorded continuously during each experiment., Results: If the oximeter was functioning before motion began, the following are the percentages of time when the instrument displayed a pulse oximetry value within 7% of control: N-200 = 76%, N-3000 = 87%, and Masimo = 99%. When the oximeter sensor was connected after the beginning of motion, the values were N-200 = 68%, N-3000 = 47%, and Masimo = 97%. If the alarm threshold was chosen as pulse oximetry less than 90%, then the positive predictive values (true alarms/ total alarms) are N-200 = 73%, N-3000 = 81%, and Masimo = 100%. In general, N-200 had the greatest pulse oximetry errors and N-3000 had the highest dropout rates., Conclusions: The mechanical motions used in this study significantly affected oximeter function, particularly when the sensors were connected during motion, which requires signal acquisition during motion. The error and dropout rate performance of the Masimo was superior to that of the other two instruments during all test conditions. Masimo uses a new paradigm for oximeter signal processing, which appears to represent a significant advance in low signal-to-noise performance.

Background: Although the effects of propofol on cerebral metabolism have been studied in animals, these effects have yet to be directly examined in humans. Consequently, we used positron emission tomography (PET) to demonstrate in vivo the regional cerebral metabolic changes that occur in humans during propofol anesthesia., Methods: Six volunteers each underwent two PET scans; one scan assessed awake-baseline metabolism, and the other assessed metabolism during anesthesia with a propofol infusion titrated to the point of unresponsiveness (mean rate +/- SD = 7.8 +/- 1.5 mg.kg-1.h-1). Scans were obtained using the 18fluorodeoxyglucose technique., Results: Awake whole-brain glucose metabolic rates (GMR) averaged 29 +/- 8 mumoles.100 g-1.min-1 (mean +/- SD). Anesthetized whole-brain GMR averaged 13 +/- 4 mumoles.100 g-1.min-1 (paired t test, P < or = 0.007). GMR decreased in all measured areas during anesthesia. However, the decrease in GMR was not uniform. Cortical metabolism was depressed 58%, whereas subcortical metabolism was depressed 48% (P < or = 0.001). Marked differences within cortical regions also occurred. In the medial and subcortical regions, the largest percent decreases occurred in the left anterior cingulate and the inferior colliculus., Conclusion: Propofol produced a global metabolic depression on the human central nervous system. The metabolic pattern evident during anesthesia was reproducible and differed from that seen in the awake condition. These findings are consistent with those from previous animal studies and suggest PET may be useful for investigating the mechanisms of anesthesia in humans.

Background: Previous studies have shown that pulse oximeters whose sensors are positioned improperly may yield erroneously low saturation (SpO2) values on normoxemic subjects. The behavior of oximeters with malpositioned sensors during hypoxemia has not been studied. The current study is aimed at determining the behavior of several different pulse oximeters over a wide range of arterial oxygen saturation (SaO2)., Methods: In each of 12 healthy volunteers, a radial artery cannula was inserted, and eight different pulse oximeters, five of which had malpositioned sensors, were applied. Subjects breathed controlled mixtures of nitrogen and oxygen to slowly vary their SaO2 from 100% to 70%. Arterial blood samples were analyzed and pulse oximeter data were recorded at five stable SaO2 values for each subject., Results: The oximeters with malpositioned sensors vary greatly in their behavior, depending on both the actual SaO2 and the manufacturer and model. One oximeter underestimated saturation at all SaO2 values, while three others underestimated at high SaO2 and overestimated at low SaO2. Linear regression analysis shows a decrease in the slope of SpO2 versus SaO2 in most cases, indicating a loss of sensitivity to SaO2 changes. Between-subject variation in response curves was significant., Conclusions: The calibration curves of the pulse oximeters studied were changed greatly by sensor malpositioning. At low SaO2 values, these changes could cause the oximeter to indicate that a patient was only mildly hypoxemic when, in fact, hypoxemia was profound. It is recommended that sensor position be checked frequently and that inaccessible sensor locations be avoided whenever possible.

Background: We describe the anesthetic management for a new surgical procedure: laser ablation of emphysematous bullae via thoracoscope. Although thoracoscopy is not new, this is the first description of a series of patients with bilateral, chronic lung disease who underwent long periods of one-lung ventilation (OLV) during thoracoscopic therapy., Methods: Twenty-six laser ablation procedures were performed in 22 patients. The patients were elderly (mean age 63 yr) with a large incidence of coexisting cardiovascular disease. Most required chronic home oxygen therapy. Patients were monitored invasively, and hemodynamic data were recorded every 5 min. Arterial blood gas analyses were performed every 15 min. Comparisons were made between three intraoperative periods: two-lung ventilation (TLV) before thoracoscopy, OLV during thoracoscopy, and TLV after thoracoscopy., Results: All patients survived the operation despite a mean OLV duration of 170 min, but several experienced serious intraoperative problems, such as hypoxemia or hypotension. Hypoxemia was treated with nondependent lung continuous positive airway pressure and dependent lung positive end-expiratory pressure. In all patients the lungs were adequately ventilated, but bronchopleural fistulae occurred upon return to TLV in every case. The resulting air leaks, often 50% of inspired tidal volume, required the use of a pressure-cycled ventilator to maintain oxygenation. Postoperative air leaks greater than 50% of inspired tidal volume usually required subsequent surgical correction, while smaller leaks resolved spontaneously. Mechanical ventilation was required for an average of 5 days. Eighty-four percent have survived at least 6 months, and nearly all survivors report symptomatic improvement., Conclusions: Ablation of bullae appears to provide symptomatic improvement, and thoracoscopy might be better tolerated than thoracotomy, especially in patients with severe bullous emphysema.

Transcutaneous oxygen tension (PtcO2) is often used to monitor neonates and infants in special care units and the operating room. The transcutaneous index (TCI = PtcO2/arterial oxygen tension [PaO2]) is known to depend both on age and on cardiac index but is assumed to be independent of other physiologic variables. In this study we have shown that TCI also depends upon arterial carbon dioxide tension (PaCO2). Five young pigs were anesthetized and paralyzed and their lungs mechanically ventilated while they were monitored with PtcO2 electrodes and serial arterial blood gas analyses. For a 45 degrees C PtcO2 sensor, the mean TCI during normocapnia was 0.78, whereas during hyperventilation (PaCO2 = 20 mmHg) the mean TCI was reduced 65%, to 0.27. The corresponding TCI values for a 43 degrees C sensor were 0.33 and 0.065, representing an 80% decrease in TCI during hyperventilation. Hypoventilation had little effect upon TCI as long as hypoxemia was avoided. Twelve awake adult volunteers with radial artery cannulas were monitored with PtcO2 sensors at several body sites and two sensor temperatures. For a 44 degrees C sensor on the chest, the mean TCI decreased from 0.77 at normocapnia to 0.60 at a PaCO2 of 17 mmHg, a 22% change. For the same sensor on the foot, TCI decreased from 0.63 to 0.32, a 49% change. For a 42 degrees C sensor under the same conditions, the corresponding TCI decreases were 51 and 64%. Six of the volunteers were also monitored with laser-Doppler skin blood flow probes located on the chest, hand, and foot.(ABSTRACT TRUNCATED AT 250 WORDS)

The accuracy of pulse oximetry (for pulse hemoglobin oxygen saturation [SpO2]) and mixed venous oximetry (for mixed venous hemoglobin oxygen saturation [SvO2]) was assessed during progressive normovolemic anemia in dogs. Splenectomized mongrel dogs under general anesthesia were monitored with a three-wavelength pulmonary artery oximeter catheter (10 dogs) and a pulse oximeter (11 dogs). Data were collected while fractional inspired oxygen concentration (FIO2) was varied from 1.00 to 0.05 in seven steps. The dogs then underwent isovolemic hemodilution, and the FIO2 was again varied. This sequence continued until data no longer could be obtained. The accuracy of each device was assessed by determining the bias (the average difference between the continuous monitor oximeter and the bench oximeter) and the precision (the standard deviation of the difference). For the three-wavelength Oximetrix catheter (for hemoglobin oxygen saturation denoted here SoxO2), the overall bias (SoxO2 - SvO2) and precision were -0.7 +/- 8.6% for the 193 data points. The accuracy as assessed by bias and precision for SoxO2 was similar for hematocrits of 40-15%. (Bias +/- precision was 2.1 +/- 5.7% for hematocrits greater than 40%, and -1.1 +/- 7.5% for hematocrits of 15% to 19%). At hematocrits between 10 and 14%, the precision worsened to 12%, and for hematocrits less than 10% the bias +/- precision was -11.5 +/- 11.8%. The overall SpO2 accuracy was 0.2 +/- 7.6% for 178 points. The pulse oximeter's accuracy was similar, down to hematocrits of 10%. Below 10%, the bias and precision worsened to -5.4 +/- 18.8%.(ABSTRACT TRUNCATED AT 250 WORDS)

The performance of three commercially available pulse oximeters was assessed in five anesthetized dogs in which increasing levels of methemoglobin were induced. Hemoglobin oxygen saturation in each dog was monitored with three pulse oximeters (Nellcor N-100, Ohmeda 3700, and Novametrix 500) and a mixed venous saturation pulmonary artery catheter (Oximetrix Opticath). Arterial and mixed venous blood specimens were analyzed for PaO2, PaCO2, and pHa using standard electrodes. An IL-282 Co-oximeter was used on the same specimens to determine oxyhemoglobin and methemoglobin as percentages of total hemoglobin. Methemoglobin levels of up to 60% were induced by intratracheal benzocaine. As MetHb gradually increased while the dogs were breathing 100% inspired oxygen, the pulse oximeter saturation (SpO2) overestimated the fractional oxygen saturation (SaO2) by an amount proportional to the concentration of methemoglobin until the latter reached approximately 35%. At this level the SpO2 values reached a plateau of 84-86% and did not decrease further. When, at fixed methemoglobin levels, additional hemoglobin desaturation was induced by reducing inspired oxygen fraction, SpO2 changed by much less than did SaO2 (regression slopes from 0.16 to 0.32). Thus, at high methemoglobin levels SpO2 tends to overestimate SaO2 by larger amounts at low hemoglobin saturations. Plots of SpO2 versus functional saturation (oxyhemoglobin/reduced hemoglobin plus oxyhemoglobin) show an improved but still poor relationship (regression slopes from 0.32 to 0.46). The Oximetrix Opticath pulmonary artery catheter behaves similarly but provides somewhat better agreement with functional saturation than do the pulse oximeters in the presence of methemoglobinemia. Pulse oximetry data (SpO2) should be used with caution in patients with methemoglobinemia.

Five dogs were anesthetized, intubated, and ventilated with various mixtures of oxygen, nitrogen, and carbon monoxide. Each dog was monitored with arterial and pulmonary artery catheters, a transcutaneous PO2 analyzer, and two pulse oximeters. An IL-282 Co-oximeter was used to periodically measure arterial oxyhemoglobin (O2Hb) and carboxyhemoglobin (COHb) as percentages of the total hemoglobin. The PaO2, PaCO2, and pHa were measured in the same blood specimens using standard electrodes. When the inspired oxygen concentration was reduced in the absence of COHb, the pulse oximeter saturation (SpO2) estimated O2Hb with reasonable accuracy. COHb levels were then varied slowly from 0-75% in each dog. As the COHb level increased and oxyhemoglobin decreased, both pulse oximeters continued to read an oxygen saturation of greater than 90%, while the actual O2Hb fell below 30%. In the presence of COHb, the SpO2 is approximately the sum of COHb and O2Hb, and, thus, may seriously overestimate O2Hb. The pulse oximeter, as the sole indicator of blood oxygenation, should, therefore, be used with caution in patients with recent carbon monoxide exposure. On the other hand, transcutaneous PO2 falls linearly as COHb increases, and reaches about one-fifth of its initial value at the highest COHb levels despite the maintenance of constant arterial PO2.