Monitoring the anesthetized patient in the operating room.

In general, monitors used by the anesthesiologist in the operating room provide three basic functions: assessment of machine and patient status to ensure safety; assessment of depth of anesthesia; and determination of physiologic variables. Improved monitoring should reduce risk and improve patient care. Some monitoring techniques are used in only a few specialized procedures, particularly in high-risk patients or for prolonged or difficult surgery. However, many less-sophisticated techniques are used in routine daily practice and require similar philosophies of monitoring. The status of the cardiovascular system is assessed primarily by monitoring the electrocardiogram and blood pressure waveforms. Although desired effects of anesthesia impact the central nervous system, this vital organ system is not routinely monitored. Likewise, widespread monitoring of the respiratory system is not routinely accomplished. Monitoring for anesthesia in the operating room has evolved to some extent in response to what can be accomplished rather than what needs to be done. While the potential for an integrated monitoring system--including all patient and delivery system variables--clearly exists, numerous difficulties preclude its becoming a reality. What is required is the development of an integrated system that can augment the anesthesiologist's sixth sense. Initial efforts toward device interface standardization, configurability, and flexibility must be encouraged to pave the way for the integrated, automated anesthesia delivery system of the future.

A study of hospital based ambulance systems in Wisconsin.

This study was undertaken to determine the advantages and disadvantages of a hospital-based emergency medical service system with hospital employees serving as emergency medical technicians. This type of service is operating in thirteen hospitals in Wisconsin. A series of interviews was conducted to obtain pertinent information regarding the hospital, emergency medical vehicles, ambulance attendants, finances, communications and personal reactions to the system. The data indicate that hospital-based services using hospital employees can operate with minimal interference to regular patient care, and with apparent advantages to the emergency patients. In the rural, sparsely populated areas which were studied, response times from the hospital-based systems were comparable to other rural systems, and the financial and administrative aspects of the system were reviewed.

Investigation of a continuous heating/cooling technique for cardiac output measurement.

Cardiac output is frequently measured to assess patient hemodynamic status in the operating room and intensive care unit. Current research for measuring cardiac output includes continuous sinusoidal heating and synchronous detection of thermal signals. This technique is limited by maximum heating element temperatures and background thermal noise. A continuous heating and cooling technique was investigated in vitro to determine if greater thermal signal magnitudes could be obtained. A fast responding thermistor was employed to measure consecutive ejected temperature plateaus in the thermal signal. A flow bath and mechanical ventricle were used to simulate the cardiovascular system. A thermoelectric module was used to apply heating and cooling energy to the flow stream. Trials encompassing a range of input power, input frequency, and flow rate were conducted. By alternating heating and cooling, thermal signal magnitude can be increased when compared to continuous heating alone. However, the increase was not sufficient to allow for recording in all patients over the expected normal range of cardiac output. Consecutive ejected temperature plateaus were also measured on the thermal signal and ejection fraction calculations were made.

A noninvasive method of estimating mean pulmonary artery pressure in the pneumatic total artificial heart.

Accurate hemodynamic monitoring is essential for the clinical management of the recipient of a total artificial heart (TAH). The high incidence of pulmonary congestive disorders in this population complicates this already formidable task. Lack of diagnostic pulmonary artery pressure (PAP) information is recognized as a fundamental source of these problems. Because conventional methods of obtaining hemodynamic information are difficult to implement in TAH recipients, improvement of TAH case management depends on the development of innovative monitoring strategies. Noninvasive monitoring techniques have been developed for three (right atrial pressure, left atrial pressure, and aortic pressure) of the four auxiliary circulatory pressures used to quantify hemodynamic performance. Development of the fourth, for PAP, was the subject of this work. We developed a noninvasive, in vitro method of estimating mean PAP in the Jarvik-7 TAH (Symbion, Inc, Salt Lake City, UT) recipient. This information was obtained by analyzing the relationship between the pneumatic right drive pressure (RDP) and PAP waveforms produced by a Jarvik-7 (70 ml) connected to a Donovan mock circulation and driven by a Utahdrive System IIIe Controller (Symbion, Inc, Salt Lake City, UT). Total artificial heart driver parameters (i.e., heart rate, percent systole, and vacuum) were manipulated to produce a range of ventricular filling volumes (FV), from 40 to 60 ml, for three distinct states of the pulmonary vasculature: hypotensive, normal, and hypertensive. A unique multiple-linear regression equation was derived for each FV from the RDP-PAP relationship exhibited under these conditions. Comparison of computed estimates of PAP with actual measurements showed overall average correlations of greater than 0.92, with a standard error of the estimate of less than 1.9 mm Hg. The mean difference between actual and computed PAP measurements was -0.03 +/- 2.0 Hg. Estimations were accurate within 8.5% of true PAP values. Additional experimentation revealed that while the RDP-PAP relationships are dependent on FV, they are independent of the manner in which FV was obtained. Estimates proved useful over the clinical operating range of the pneumatic heart driver, as well as over the normal physiologic range of PAP in the human. This method is readily applicable to a computer-based monitoring implementation, although its effectiveness needs to be demonstrated in vivo.

Integration of monitoring for intelligent alarms in anesthesia: neural networks--can they help?

Although there has been a decrease in the number of anesthesia-related critical incidents, there are still opportunities for further improvement. We discuss the potential of integrated monitoring and artificial neural networks as a means of vigilantly watching for patterns in multiple variables to detect incidents and reduce false alarms. We estimate that half the anesthesia-related events could be detected with integrated monitoring using only 5 variables. A review of research using artificial intelligence/expert systems indicates limited potential for success using these tools alone for integrated monitoring in the operating room. We present artificial neural networks as an approach that is more suited to the type of multivariable monitoring and pattern recognition required. Along with rule-based artificial intelligence, these now have the potential to help develop innovative monitoring in the operating room.

Integrated monitoring can detect critical events and improve alarm accuracy.

A computer-based, integrated monitor system was designed and utilized to collect and interactively manage physiologic data (13 variables and 3 waveforms) from six routinely used operating room monitors. Various approaches were developed to reduce false alarms, classify waveforms, and recognize events. False alarms: false alarms in ECG heart rate detection were reduced from 37.3% to 2.6% (p=0.005) of total alarms using multi-variable analysis and rate-of-change limits. Waveform classification: using artificial neural networks (AN), CO2 waveforms were classified into (a) spontaneous, (b) mechanical, and (c) mechanical/with spontaneous breathing attempts. The system properly classified 47 of 71 spontaneous, 65 of 67 mechanical, and 37 of 44 mechanical breaths/with spontaneous breathing attempts. Another ANN was used for detection of elevated and depressed ST segments in the ECG signal. All ST segment elevations and depressions of 0.1 mV were correctly identified. Event recognition: an algorithm developed to identify endotracheal intubation correctly recognized 13 of 17 intubations. This resulted in a 42% reduction in low end-tidal-CO2 false alarms.

Cardiac output measurements. A review of current techniques and research.

Cardiac output is the volume of blood ejected by the heart per unit time. It is a useful measurement in that it can be used to evaluate overall cardiac status in both critically ill patients and patients with suspected cardiovascular disease. An ideal cardiac output measurement system would have automated continuous output capability, be minimally invasive, accurate, fast, small, low cost and clinically adaptable. This paper presents a theoretical and practical description of the variety of clinical techniques in use today and lists their advantages and shortcomings with respect to the ideal system. Included are the Fick method, indicator dilution techniques, velocity measurements and transthoracic impedance and combined Doppler ultrasound as noninvasive techniques. In addition, several experimental methods are described along with their desirable features and possible constraints. These include intravascular heating/recording, thermistor tracking of cardiac output, ejection fraction measurements and magnetic susceptibility plethysmography.

Automated measurements of isolated heart muscle contractions: effects of halothane, calcium, and magnesium on guinea pig left atrial muscle.

An in vitro method for automatically measuring muscle contraction force has been demonstrated in a study of the effects of the inhalation anesthetic halothane followed by calcium chloride or magnesium sulfate on isolated guinea pig left atrial muscle. An automated computer-controlled system was used to collect muscle contraction force waveforms and to analyze contraction waveforms for comparison of variables before and after drug administration. Two concentrations of halothane (0.5 and 1.5%) were administered to the atrial preparation for 30 minutes and followed by calcium chloride or magnesium sulfate. Six variables (latency, time to peak tension, peak tension, maximum rate of change of pressure, force time integral, and relaxation time) were automatically determined from averaged stimulus-response curves. Results were normalized and compared with controls administered only calcium and magnesium and with controls administered no drugs. The automated system greatly simplified data collection and accumulation and statistical analysis of multiple responses. The system made possible averaging and analysis of more data with less variability than is normally obtained with manual systems. The results confirm several known actions of these agents. Halothane prolongs latency (9 and 21% for 0.5 and 1.5% halothane, respectively) and shortens time to peak tension (6 and 17% for 0.5 and 1.5% halothane, respectively) and relaxation time (17 and 39% for 0.5 and 1.5% halothane, respectively). At high halothane concentrations (1.5%) calcium chloride shortens latency (10%) and prolongs time to peak tension (11%); magnesium sulfate prolongs latency (14%) and shortens time to peak tension (10%).

ECG lead with the endotracheal tube.

We evaluated the potential of obtaining ECG signals from electrodes attached to an endotracheal tube. Tests during surgery using a 2-electrode system with 3 pediatric patients produced recordings suitable for rate monitoring and arrhythmia detection.

Clinical engineering as an academic discipline.

This paper includes sections written by the current or former Clinical Engineering coordinators of five universities on common problems faced by Clinical Engineering (CE) educational programs and the different solutions adopted on various campuses. The problems discussed include student recruitment, financial support, containment of student credit hours and faculty time, retention of CE graduates in the profession, and differentiation between Clinical Engineering and Biomedical Engineering Technology.

Effects of pulse duration on neuromuscular blockade monitoring: implications for supramaximal stimulation.

Questions have been raised concerning the reliability of surface electrodes in achieving supramaximal stimulation during the monitoring of neuromuscular blockade; needle electrodes are considered reliable in this respect. This study compares interelectrode impedances of needle and surface electrodes during neuromuscular blockade monitoring and suggests those characteristics of the stimulation pulse that can ensure reliable supramaximal stimulation with either type of electrode. Interelectrode voltage and current for surface and needle electrodes were measured by using 1.0-ms pulses at low, medium, and high stimulation levels on 22 surgical patients during anesthesia. Data were collected immediately after electrode application, and again at 10 minutes after application. Stimulation with surface electrodes produced an initial, transient surge of current, followed by a lower steady-state value. At high stimulation levels, the peak transient current was 87% higher than the steady-state current. Needle electrodes produced a constant high-current response. At high stimulation levels the transient impedance of the surface electrode and the impedance of the needle electrode were essentially equal (0.7 k omega and 0.75 k omega, respectively). The transient impedance was significantly lower (P less than 0.001) and was associated with less interpatient variation (P less than 0.001) and less sensitivity to the duration of electrode application than was the steady-state impedance of the surface electrode. These data suggest that high-current pulses with widths of less than 0.2 ms could provide reliable supramaximal stimulation with either type of electrode.

Evaluation of peripheral nerve stimulators and relationship to possible errors in assessing neuromuscular blockade.

Voltage and current output characteristics were measured on six commercially available peripheral nerve stimulator devices. The results are evaluated as possible sources of variability in peripheral nerve stimulator function and neuromuscular blockade assessment. The authors found significant differences in output voltage waveform and in maximum current into a 470 ohm load (21.4 to 128 mA.). Output current decreased from 25 to 88% in the different devices, with a load impedance increase from 470 to 10,000 ohms. Due to the variability in peripheral nerve stimulation units and the decrease in current output at higher load impedance, less than supramaximal stimulation is possible with erroneous interpretation of neuromuscular blockade.