Placement of esophageal stethoscope by acoustic criteria does not consistently yield an optimal location for the monitoring of core temperature

The esophageal stethoscope has evolved into a device for both acoustic and core temperature monitoring. To test whether routine placement according to acoustic criteria results in placement of the core temperature sensor in the region of contiguity between the esophagus and the heart, we determined the depth of placement electrocardiographically. All patients were undergoing nonthoracic elective operations requiring general anesthesia and tracheal intubation. First, we established that different observers selected the same esophageal depth within ±1 cm electrocardiographically, using the criterion of a symmetric biphasic P wave of maximal amplitude (7 patients). Then, in 30 more patients, we compared routine acoustic placements with the depths of the maximalamplitude biphasic P wave. Stethoscopes placed according to acoustic criteria were within ±3 cm of P-wave depths in 15 of 30 patients. In the remaining patients, measured discrepancies ranged up to 13.5 cm. We conclude that the prevailing stethoscope design, with a thermistor at the tip, below the acoustic window, does not ensure placement of the thermistor within the optimal region for monitoring of core temperature. A modification in design that would take advantage of the reliability of electrocardiographic positioning is suggested. Supported by National Institutes of Health grant HL-16910.     

The susceptibility of thermistor-based esophageal temperature probes to errors caused by electrically conductive fluids (“artificial saliva”)

Several brands of esophageal stethoscopes with thermistor-based thermometers were tested to determine the susceptibility of the probe connector to contamination by oral secretions. A solution of half normal saline and 1% carboxymethyl-cellulose was used to model the conductivity and viscosity of saliva. When 1 ml of test solution was allowed to track down the probe wires to the connector, several brands of thermistors gave erroneously elevated readings. The mean changes in temperature according to brand of thermistor were as follows: Electromedics, 0.1±0.1°C; Mallinckrodt, 1.7±0.8°C; Respiratory Products, 0.1±0.2°C; Sheridan, 3.6±1.9°C; Vital Signs, 4.8±1.3°C (peak) and 1.4±1.6°C (final); and Yellow Springs, 0.9±0.4°C. The manufacturers of the probes susceptible to this type of error should implement the appropriate design modifications. In the meantime, clinicians should be aware of this problem and may choose to prevent these errors by wrapping the connection with waterproof tape.This work was supported in part by funds from Respiratory Support Products, Inc, Santa Ana, CA.The author wishes to thank Alvin Wald, PhD, for technical advice and Mike Finster, MD, for expert editorial assistance.     

Passive warming of airway gases (artificial nose) improves accuracy of esophageal temperature monitoring

The most efficient site for monitoring heart and lung sounds by esophageal stethoscope is not the warmest segment of the esophagus. This study investigated the ability of passive warming of airway gases to increase the accuracy of temperatures measured at this site (i.e., to decrease their difference from core temperature). In 15 adult patients undergoing general anesthesia and endotrachcal intubation, esophageal temperatures were measured before and after use of a heat and moisture exchanger (an artificial nose) that passively warmed inspired gases. The resulting values were compared with nasopharyngeal temperatures, which represented core temperature. Before use of the heat and moisture exchanger, esophageal and nasopharyngeal temperatures differed significantly (mean difference ± SD, 0.9 ± 0.4‡C;P ≤ 0.001). After passive warming of inspired gases, esophageal temperatures increased significantly (mean increase ± SD, 0.5 ± 0.2‡C;P ≤ 0.001) but inconsistently (range, 0.1 to 1.2‡C). However, the mean difference between esophageal and nasopharyngeal temperatures was still significant (0.5 ± 0.3‡C;P < 0.001). Discrepancies between esophageal and core temperatures persist when a currently available esophageal stethoscope with adjacent auscultation chamber and temperature probe is used, despite passive warming of airway gases.     

The esophageal temperature gradient in anesthetized children

Objective. Our objective was to study the effect of the temperature of the anesthetic gas mixture (AGM) on esophageal temperature measurements made in children whose tracheas had been intubated for anesthesia. We also sought to establish the optimal site for the temperature sensor in the esophagus and to find a way to accurately place the sensor.Methods. Special esophageal temperature probes with thermistors located at 1-cm intervals were used for data collection on a multiplex system. Esophageal temperature measurements were made every 15 minutes for a period of 120 minutes in anesthetized children receiving heated (n=30) and unheated (n=30) anesthetic gases.Results. The temperature of the AGM (p<0.001), the site of measurement (p<0.001), and the interaction between AGM temperature and site of measurement (p<0.007) all had a significant effect on esophageal temperature measurements. This effect was greatest at a point 3 cm distal to the level of the tip of the endotracheal tube when AGMs were not heated.Conclusion. We conclude that best results are obtained when care is taken to place the thermistor in the lower quarter of the esophagus. (We provide a simple formula for calculating this placement in pediatric patients of varying ages.) Placing the probe by acoustic criteria cannot consistently be relied on to provide good thermometry.Supported in part by a grant of equipment and financial assistance from Mallinckrodt Medical, Inc, St Louis, MO.     

Observer reliability in detecting surreptitious random occlusions of the monaural esophageal stethoscope

The esophageal stethoscope is used often during anesthesia to monitor ventilation and cardiac function. Deficiencies in observer vigilance may limit the effectiveness of this monitoring instrument. The aim of this study was to determine how long it took for an observer to detect a surreptitiously occluded monaural esophageal stethoscope in the setting of clinical anesthesia. During routine anesthesia, where an esophageal stethoscope was in use, a computer-guided device would artificially, silently, and at random time intervals, occlude the stethoscope tubing. Personnel using the stethoscope noted when they perceived the absence of stethoscope sounds. We studied 320 stethoscope occlusions in 32 patients. The time between stethoscope occlusion and detection was 34 ±59 seconds (mean ±SD). Eighty-seven percent of detections were made in less than 60 seconds. However, 13% of detections were delayed for more than 60 seconds, and 2.3% for more than 240 seconds. While anesthesia personnel using an esophageal stethoscope could detect most stethoscope occlusions, failure to appreciate such episodes occurred in a small but significant number of cases. This suggests that the esophageal stethoscope has some definite limitations as a continuous monitor and that other monitoring techniques, such as oximetry, capnography, and ventilator disconnect alarms, as well as visual/tactile inspection of the patient, should be used as well.     

Response of lower esophageal contractility to changing concentrations of halothane or isoflurane: A multicenter study

A multiple-center study was performed to determine the relationship between lower esophageal contractility, clinical signs, and anesthetic concentration as expressed by minimum alveolar concentration (MAC). One hundred four American Society of Anesthesiologists Class I through III patients were exposed to isoflurane (with and without nitrous oxide) or halothane in concentrations of 0.5, 1.0, and 1.5 MAC. Heart rate and systolic blood pressure were continuously monitored. Both the amplitude and frequency of spontaneous and provoked lower esophageal contractions were measured in situ by using a 24-F probe equipped with provoking and measuring balloons. Combined results demonstrated statistically significant correlations (P<0.001) between lower esophageal contractility and MAC. Spontaneous lower esophageal contractions decreased from 1.10±0.12 (SEM) contractions per minute (0.5 MAC) to 0.42±0.05 (1 MAC) to 0.18±0.05 (1.5 MAC). Provoked lower esophageal contractility values decreased from 45±4 mm Hg (0.5 MAC) to 29±3 (1 MAC) to 19±2 (1.5 MAC). Heart rate changes did not correlate with MAC, and systolic blood pressure correlated in only one of three centers. Intracenter and intercenter analyses failed to demonstrate a significant relationship between lower esophageal contractility and heart rate or systolic blood pressure. No intracenter differences in either amplitude or frequency of lower esophageal contractions were observed, despite differences in volatile agents, induction techniques and agents, patient populations, and duration of anesthesia. Our studies indicate that lower esophageal contractility may be an indicator of anesthetic depth as reflected by MAC, but further studies are needed to quantify the effects of surgical stimulus, intravenous anesthetics, vasodilators, anticholinergics, calcium channel blockers, beta-adrenergic agonists, and the presence of a nasogastric tube.     

An anesthesia information system for monitoring and record keeping during surgical anesthesia

We have developed an anesthesia information system (AIS) that supports the anesthesiologist in monitoring and recording during a surgical operation. In development of the system, emphasis was placed on providing an anesthesiologist-computer interface that can be adapted to typical situations during anesthesia and to individual user behavior. One main feature of this interface is the integration of the input and output of information. The only device for interaction between the anesthesiologist and the AIS is a touch-sensitive, high-resolution color display screen. The anesthesiologist enters information by touching virtual function keys displayed on the screen. A data window displays all data generated over time, such as automatically recorded vital signs, including blood pressure, heart rate, and rectal and esophageal temperatures, and manually entered variables, such as administered drugs, and ventilator settings. The information gathered by the AIS is presented on the cathode ray tube in several pages. A main distributor page gives an overall view of the content of every work page. A one-page record of the anesthesia is automatically plotted on a multicolor digital plotter during the operation. An example of the use of the AIS is presented from a field test of the system during which it was evaluated in the operating room without interfering with the ongoing operation. Medical staff who used the AIS imitated the anesthesiologist’s recording and information search behavior but did not have responsibility for the conduct of the anesthetic.     

Electroencephalogram spectral edge frequency,lower esophageal contractility,and autonomic responsiveness during general anesthesia

Both the electroencephalogram (EEG) spectral edge frequency (SEF) and lower esophageal contractility (LEC) indices have been reported to be useful indicators of anesthetic depth. We designed a prospective study to evaluate the relationship between changes in these two variables and objective measurements of physiologic responsiveness to surgical stress (i.e., changes in hemódynamic variables and plasma levels of norepinephrine, epinephrine, total catecholamines, and vasopressin). Eighty-nine consenting adult males undergoing radical prostatectomy procedures under a standardized general anesthetic technique were studied according to a randomized, single-blinded protocol. General anesthesia was induced with 30 µg/kg intravenous (IV) alfentanil, 2.5 mg/kg IV thiopental, and 0.1 mg/kg IV vecuronium, and subsequently maintained with 0.5 µg/kg/min alfentanil, nitrous oxide (N₂O) 67% in oxygen, and 0.8 µg/kg/min vecuronium. Following retropubic dissection, 81 patients (92%) manifested acute hypertensive responses, with mean arterial pressure increasing from 90±14 to 122±14 mm Hg (mean ± SD). This acute hypertensive response was treated with one of three different treatment modalities (20 to 60 µg/kg IV alfentanil, 0.5 to 2.0% inspired isoflurane, or 0.05 to 0.15 mg/kg IV trimethaphan) to return the mean arterial pressure to within 10% of the preincisional (baseline) value within 5 to 10 minutes. Although the mean arterial pressure, heart rate, and plasma levels of catecholamines and vasopressin significantly increased following the surgical stimulus, and decreased after adjunctive therapy, the EEG-SEF and LEC index (LECI) values did not significantly change during these study intervals. Furthermore, using a logistic regression analysis, we observed that preincision EEG-SEF and LECI values could not predict whether patients would manifest a hypertensive response. Therefore, the EEG-SEF and LECI were unreliable indicators of anesthetic depth.This study was supported in part by a grant from the Ambulatory Anesthesia Research Foundation, Los Altos, CA. (Dr White is a member of the Board of Directors.)The authors would like to thank Dan Kuni (Baxter Healthcare) for his assistance in obtaining the equipment used to perform the study; Vinod Kothapa, MD, for his valuable assistance with the anesthetic management of the study patients; Alex K. Mills, MD, for his assistance with the EEG interpretation; and Steven A. Bai, PhD, for his assistance with the plasma alfentanil analyses.     

Frequently repeated fick cardiac output measurements during anesthesia

A computer-based system was developed for monitoring cardiac output using the Fick principle during general anesthesia. The variables of the oxygen-consumption Fick equation were measured using the following system: oxygen uptake by an originally developed respiratory gas monitoring system, arteriovenous oxygen saturation difference by pulse and fiberoptic oximetry, and hemoglobin concentration by an in vitro oximeter. Fick cardiac output and systemic vascular resistance were calculated every 30 seconds. Fick cardiac output was compared with thermodilution cardiac output in 11 anesthetized patients. A total of 208 corresponding cardiac output measurements showed a range of 2 to 9 L · min^-1. The correlation coefficient between the thermodilution and Fick cardiac outputs was 0.961, with a regression equation of Fick cardiac output = 1.058 thermodilution cardiac output 0.359. The difference between the thermodilution and Fick cardiac outputs was 0.103 ± 0.395. The Fick cardiac output was significantly lower than the thermodilution cardiac output, especially in the low flow range. We demonstrated that this new monitoring system was clinically feasible and sufficiently accurate, under the limited circumstances of our study. The integration of routinely used equipment has made possible a frequently repcatable method for estimating cardiac output in patients.     

Inexpensive temperature monitor for heated humidifiers

A commercially available indoor/outdoor electronic thermometer has been adapted to monitor both airway gas temperature and operating room temperature when heated humidifiers are used. Heated humidifiers that do not have temperature monitoring capabilities pose a risk of dangerously high inspired gas temperatures.     

Automated anesthesia records and anesthetic incidents

A case report is presented in which the cause of an anesthetic mishap would have remained a mystery had it not been for an automated anesthesia record.     

Automated anesthesia records and anesthetic incidents

A case report is presented in which the cause of an anesthetic mishap would have remained a mystery had it not been for an automated anesthesia record.     

Acceptable ranges for vital signs during general anesthesia

Objective. Define the ranges for normal vital signs during general anesthesia.Methods. We studied 50 patients undergoing general anesthesia. We asked residents to state desirable ranges for each patient's systolic and diastolic blood pressure (BP), heart rate (HR), SpO₂, andPetCO₂ during induction, intubation, maintenance, and emergence from anesthesia. We called these ranges the clinical operating range (COR) and observed the frequency, duration, and magnitude of transgressions of these CORs. We also recorded whether the transgressions were treated or tolerated, or whether the COR values were changed.Results. Upper COR values in the maintenance phase for systolic BP were 38%±20% above the preoperative values and 30%±20% above the values recorded just before induction of anesthesia. Lower COR values in the maintenance phase for systolic BP were 27%±9% below preoperative, and 31%±11% below pre-induction values. For HR, upper and lower COR values in the maintenance phase were 53%±44% above and 38%±17% below preinduction values, respectively. Transgressions of COR values for BP and HR were common, treatment frequent, and redefinition of COR values rare.Conclusion. Clinicians recognize ranges for vital signs during uneventful anesthesia. These CORs may differ from one stage of anesthesia to the next. Transgressions of these ranges are common. Not all transgressions are treated.Reprints are not available.     

Wakeful response to command indicates memory potential during emergence from general anesthesia

Objective. An important aspect of assessing anesthetic depth is determining whether a patient will remember events during surgery. We looked for a clinical sign that would indicate a patient's potential for memory formation during emergence from anesthesia. A clinical sign indicating memory potential could be a useful endpoint for measuring the performance of anesthetic depth monitors and for titrating administration of anesthetic agents.Methods. We evaluated patients' responses to commands to open the eyes, squeeze the hand four times, and count 20 numbers. These responses were correlated with results on recall, cued recall, and multiple-choice memory tests.Main Results. Patients did not have evidence of memory formation until they sustained wakefulness sufficiently long to complete at least four hand squeezes or count four numbers. Of 28 patients, 13 (46%) with this sustained wakeful response had memory. Of 22 patients, 0 (0%) had evidence of memory formation when they demonstrated a brief wakeful response, defined as being responsive to command but unable to complete more than one hand squeeze or count, or an intermediate response, defined as two or three hand squeezes or counts.Conclusions. We conclude that a brief wakeful response to command indicates that a patient is unlikely to form memories, while a sustained wakeful response indicates that a patient may form memories. Thus, a patient's wakeful response to command could be a useful indicator of potential for memory.This work was supported by the Kaiser Foundation Research Institute. Elements of this work were presented at the Society for Technology in Anesthesia, January 1992, San Diego, CA.     

Continuous fiberoptic arterial oxygen tension measurements in dogs

An experimental study using a new fiberoptic sensor for the continuous intraarterial measurement of oxygen tension is described. This optode sensor uses the phenomenon of fluorescence quenching to determine the oxygen tension of the surrounding medium. To assess the accuracy of this device, we anesthetized 4 dogs and monitored them continuously with arterial catheters and an intraarterial optode probe, and intermittently with arterial blood gas analysis. The inspired oxygen fraction was varied from 1.0 to 0.1, and arterial blood gases were measured for comparison with the optode reading. Two hundred ninety data sets yielded a correlation coefficient of 0.96, with a linear regression slope of 0.98 and intercept of 5.1 mm Hg. In the 72 data sets from the last dog, the bias and precision of the optode arterial oxygen tension values were –10.3 mm Hg and 20.0 mm Hg, respectively. The optode probe was easily inserted through a 20-gauge catheter and did not interfere with continuous arterial pressure measurement or blood sampling. This study suggests that the optode has great potential as a continuous, real-time monitor of arterial oxygen tension.     

Automating the recording and improving the presentation of the anesthesia record

Although anesthesia records have been kept for over a hundred years, there is still discussion of their value and content. Two uses of the record are widely accepted: (1) review after the anesthetic event (as in medicolegal disputes), and (2) support of patient care during the delivery of an anesthetic. Although the anesthetic record is mandatory in much of the world, there is not a single standard for its format. Automating the generation and presentation of the record will enhance its value and help develop a consensus as to content. Merely automating the steps used to produce the manually generated record does not realize the full benefit of automation. For maximum benefit, the primary goal of automation should be to support the uses of the record. Specific techniques that are discussed include increasing time resolution, optimizing the type and location of input and display equipment, and tailoring the human interface. Particular attention is paid to the issue of how much detail is acceptable in the record, how to use visual cues to present detail properly, how to exclude extraneous detail, and how to avoid misleading presentations (erroneous interpretation of the data). Specific elements discussed include line width, the use of color, presentation of gradients, statistical summaries, contexts for reporting data, graphical techniques for increasing data content, and pictorial presentations. Current records are more often confusing because presented information is inconsistently displayed or irrelevant than because too much information is offered, and automation can ameliorate this problem.Electrical EngineeringPhysiology     

Use of the pacing pulmonary arterial catheter to detect endocardial electrical activity during hypothermic cardioplegic arrest

The pacing Swan-Ganz catheter was evaluated for its ability to monitor atrial and ventricular electrical activity during cardioplegic arrest on cardiopulmonary bypass. This endocardial electrical activity was compared with the activity found on the standard electrocardiogram (ECG). The atrial electrodes detected activity that was noted also by visual inspection. The ventricular electrodes detected recurring electrical activity in 7 of 18 patients. Three of these 7 patients did not have simultaneous standard ECG activity, indicating that, in the usual monitoring circumstances, this ventricular electrical activity would not have been treated with repeat cardioplegia. If the pacing Swan-Ganz catheter is used for clinical care, it can be used also to monitor myocardial electrical activity during cardioplegic arrest.     

Recognition of mixed anesthetic agents by mass spectrometer during anesthesia

Anesthetic agents are sometimes added to the wrong vaporizer on an anesthesia machine. As a result, the vaporizer may deliver a mixture of anesthetic agents at concentrations inappropriate for use on a patient. However, untoward clinical complications related to vaporizers can be prevented with a time-shared mass spectrometer. This device accurately and rapidly indicates the gases and gas concentrations present in a vaporizer.     

Esophageal apoplexy: Case report, review, and comparison with other esophageal disorders

Esophageal injuries are potentially serious disorders requiring prompt recognition and management. In addition to the well-recognized Mallory-Weiss and Boerhaave's syndromes, there exists a condition of spontaneous intramural esophageal hemorrhage: esophageal apoplexy. A case of esophageal apoplexy is presented as well as an evaluation of clinical presentations based on a collected review of the 66 cases previously reported. In addition, esophageal apoplexy is contrasted with the Mallory-Weiss and Boerhaave's syndromes, focusing on distinguishing attributes of utility to the emergency practitioner.     

Preoperative evaluation of gastric cancer: value of spiral CT during gastric arteriography (CTGA)

Background: To evaluate the utility of dual-phase spiral computed tomography during gastric arteriography (CTGA) in the preoperative staging of gastric cancers. Methods: We performed CTGA in 21 patients with pathologically proven gastric cancers. CTGA findings were prospectively analyzed and correlated with surgical and pathologic findings. Dual-phase scans were performed at 10 s (early) and 60–100 s (delayed) after injection of 120 mL of contrast medium at an injection rate of 6 mL/s through a preset 5-Fr catheter positioned in the celiac trunk. Spiral CT scans were assessed for enhancing pattern of the normal gastric wall, tumor detectability, and accuracy of tumor staging. Results: Normal gastric mucosa was clearly visible as two or three layers in all patients on early-phase scans and in eight patients on delayed-phase scans. The primary tumors were correctly detected with CTGA in seven (88%) of the eight early gastric cancers and in all 13 (100%) advanced gastric cancers. The accuracy of CTGA for T staging was 50% and 77% in early and advanced gastric cancers, respectively. The overall accuracy for tumor detection and T staging was 95% and 67%, respectively. The accuracy of CTGA for the degree of serosal invasion and regional lymph node metastasis was 77% and 76%, respectively. Conclusion: The CTGA technique improved tumor detection rate and accuracy of tumor staging, especially in early gastric cancer, and may be very useful in the preoperative staging of gastric cancer. Received: 31 August 2000/Accepted: 20 September 2000