Biomedical subjects
S Miodownik
Publications and source records attributed to S Miodownik.
A birdcage resonator for intracavitary MR imaging.
An intracavitary probe for magnetic resonance imaging of the pelvis has been developed that takes advantage of the "inside-out" spatial characteristics of a birdcage resonator. The probe consists of an eight-leg, birdcage resonator in a low-pass configuration operating in receive-only mode. The resonator circuit is mounted on a solid rod, is encased in Teflon, and has been used to obtain detailed images of pelvic anatomy in a male canine. The approximate cylindrical symmetry of the external sensitivity profile of this type of circuit, employed in an intracavitary application, demonstrates the potential superiority of this type of probe design over single-loop intracavitary coils. Axial, coronal, and sagittal MR images, obtained with 8 and 16 cm fields of view, are presented to illustrate the advantages of this type of intracavitary probe compared with conventional body-coil images. The prototype described in this report has been designed for clinical use in human subjects and is currently undergoing testing to determine its efficacy in the evaluation of rectal, prostate, and gynecologic pathology.
Effect of midazolam on the auditory event-related potential: measures of selective attention.
To elucidate the delayed effects of midazolam, we assessed electrophysiologic and motor responses by measuring auditory event-related potentials and a button-press reaction time response in 10 normal volunteers (aged 25-36 yr). Fifty minutes after intravenous infusion of 0.07 mg/kg of midazolam, subjects were mildly sedated, oriented, and readily responsive to verbal commands. To obtain ERPs, frequent tones (85%: 1000 Hz) and rare tones (15%: 2500 Hz) were presented at intervals of 1.5 s. Electroencephalographic signals were collected from FZ, CZ, and PZ for 1000 ms after stimulus presentation until 40 artifact-free rare-tone responses were obtained (average time, 6 min). Peak-to-peak amplitudes and latencies for N2, P3, and the subsequent negative slow wave (N3) were averaged within condition and were analyzed by repeated measures analysis of variance. After midazolam infusion, there was a 50% decrease in amplitude of P3 in response to target tones (P less than 0.006), whereas N3 latency increased by 40 ms (P less than 0.05). Event-related potential amplitudes were still significantly larger to rare (target) stimuli (P less than 0.003) after midazolam infusion. Although reaction time increased by 70 ms (P = 0.031), performance accuracy remained unchanged. Self-ratings of sleepiness and concentration show that a significant sedation effect was still present 50 min after infusion. Although routine clinical examination may be normal, full recovery from the effects of a typical intravenous dose of midazolam requires more than 50 min. The potential for adverse drug interaction, particularly with narcotics, is still present at this time.
Frequency response of the peripheral sampling sites of a clinical mass spectrometer.
Mass spectrometers are used in ICUs and ORs to measure the concentration of medical and anesthetic gases gathered from multiple sites. This investigation was designed to determine the accuracy of a clinical system, which included 12 ICU bedside stations monitored by a medical mass spectrometer (Perkin-Elmer RMS III, Pomona, CA). Each site station was connected to the analyzing unit via two Teflon tubes, one permanently installed, 30-m long, and the second disposable, 2.4-m long. A gas mixture containing 95% O2 and 5% CO2, alternating with room air, was delivered to a solenoid valve and from there to the connecting tubes. Gas flow-rate, delay time, rise time, and peak and trough concentrations were determined for each gas at solenoid cycling frequencies of 25, 50, and 100/min. After the first set of measurements, the 30-m tubes were thoroughly cleaned and all measurements repeated. In addition, the authors also measured CO2 delay and rise times when the gas was delivered to the mass spectrometer through an unused 30-m tube or a new 2.4-m tube. Gas flow-rate increased from 143 +/- 12 ml/min (mean +/- SD) to 238 +/- 9 ml/min after the tubes were cleaned. Delay time was identical for all gases at all solenoid cycling rates but decreased significantly (P less than 0.05), from 11.5 +/- 0.3 to 4.8 +/- 0.7 s after the ceiling tubes were cleaned. As solenoid valve rate increased, the difference between measured and actual gas concentration increased. The lowest accuracy was 63.6 +/- 2.1%, for CO2 at 100 cycles/min.(ABSTRACT TRUNCATED AT 250 WORDS)
An automated mechanism for protection of mass spectrometry sampling tubing.
The usefulness of medical mass spectrometers in intensive care units can be limited by too frequent obstruction of the tubing that transports gases from the patients to the analyzing unit. To overcome this problem, we developed an automated system consisting of an infrared light sensor and a three-way valve. One port of the three-way valve connects to 2.4-m disposable tubing that collects gases from the patient's airway. The second port is connected to a mass spectrometer analyzing unit through 30-m permanently installed tubing. The third port is connected to a pressurized oxygen source. An infrared light sensor is placed on the shorter tubing, between the patient and the three-way valve. When increased optical density is detected, due to entrainment of respiratory secretions, the three-way valve is activated. Gas flow is closed between the patient and the mass spectrometer and opened between the pressurized oxygen source and patient tubing to flush its contents. During the six years that the protection system has been in use, the frequency with which the disposable gas collection tubing is changed has been halved. Furthermore, periodic tests of delay and response times, performed at each bedside station, indicate that permanent connection tubing only needed cleaning at 2- to 3-year intervals. The system has decreased the cost of operating our mass spectrometers while also reducing periods of unavailability due to equipment failure.
Capnography in mechanically ventilated patients.
Capnography, the science of CO2 waveforms analysis, can play a role in the management of mechanically ventilated patients. Mass spectrometers are the devices most commonly used to collect sequentially and examine CO2 waveforms from multiple patients in the ICU or operating rooms. We present here a review of some clinical and technical problems, which may be resolved efficiently and expeditiously through the use of mass spectrometry and capnography. Mechanical failures, especially those that lead to rebreathing of exhaled gases, can be readily detected. The patient's progress during weaning and the consequences of changes in mechanical assistance can be virtually and noninvasively determined. An expanded role of capnography in mechanically ventilated patients can increase the use of mass spectrometers in the ICU.
Indirect calorimetry in the mechanically ventilated patient.
We used indirect calorimetry to measure oxygen consumption (VO2) and carbon dioxide production in 29 mechanically ventilated patients. These data were compared to VO2 measured simultaneously by a standard thermodilution technique. A good correlation was demonstrated between the methods, but VO2 measured by indirect calorimetry was 15% higher than VO2 measured by thermodilution.
Pneumatic-to-electrical analog for high-frequency jet ventilation of disrupted airways.
A pneumatic-to-electrical circuit analog is used to describe 2 separate mechanisms by which high-frequency jet ventilators sustain ventilation and oxygenation in the presence of large airway disruptions. The frequency-dependent mechanism is based on variations in the pneumatic equivalent to capacitive reactance. The pressure-dependent mechanism models lung defects on a voltage-controlled resistor. The electrical circuit model is also used to explain the factors leading to gas trapping and inadvertent positive end-expiratory pressure during high-frequency jet ventilation.
High-frequency jet ventilation: technical implications.
A variety of technical decisions are required for the proper selection and safe and efficacious application of high-frequency jet ventilation (HFJV). Criteria for analyzing the performance of an HFJV system are presented, along with discussions of some of the more common respiratory measurements and their applicability to HFJV.
Experimental evaluation of high-frequency jet ventilation.
The consensus of available studies indicates that high-frequency jet ventilation (HFJV) can adequately ventilate animals in respiratory failure, although a clear superiority to volume-cycled ventilation (VCV) cannot be established. HFJV is probably useful in the presence of airway disruption and in tracheal or pulmonary surgery. Clinical trials and additional bench and animal studies must be performed, to reach a full understanding of the potential benefits of this technique.
ECG infusion artifact.
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Role of high-frequency jet ventilation in the management of respiratory failure.
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Physiologic implications of high frequency jet ventilation techniques.
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High-frequency jet ventilation. A prospective randomized evaluation.
Three hundred nine patients were randomly allocated to two ventilatory protocols; 157 patients were supported with a volume-cycled ventilator (VCV) (Bear Medical BEAR 1) and 152 with a high-frequency jet ventilatory (HFJV) developed at our institution. The two ventilators were compared for safety, reliability, ease of use, and efficacy in maintaining gas exchange. On VCV, end points of therapy were: fractional concentration of oxygen in the inspired gas (FIo2) less than or equal to 0.40; arterial oxygen pressure (PaO2) greater than or equal to 70 mm Hg; cardiac index (CI) greater than or equal to 3.5 L/min/sq m; and spontaneous respiratory rate less than or equal to eight breaths per minute. On HFJV, end points were: FIo2 less than or equal to 0.45; arterial oxygen saturation greater than or equal to 0.90; and CI greater than or equal to 3.5 L/min/sq m. Spontaneous ventilation and pulmonary venous admixture reduction were the goals on VCV, with oxygen transport the goal on HFJV, Total duration of use of the ventilators was approximately 800 days with both types of devices; there were no technical failures, and the incidence of barotrauma was less than 5 percent. The end point of mechanical ventilation was reached by a significantly higher percentage of the patients randomized to HFJV. Patients who failed to reach the therapeutic goal within 24 hours were crossed over to the other form of support. Those crossed from VCV to HFJV improved more rapidly and in greater number than those crossed from HFJV to VCV. When survival and total duration of stay in the intensive care unit were considered, there was no difference between VCV and HFJV. Considering data on gas exchange, VCV provided a higher PaO2 at equivalent positive end-respiratory pressure than HFJV. Alveolar ventilation was slightly better on HFJV. Differences were statistically but not clinically significant. On HFJV, oxygenation and ventilation were maintained with lower peak inspiratory pressures and smaller tidal volumes than those required for VCV. This investigation proves that HFJV is a safe and reliable method to provide mechanical support which does not, at this time, offer obvious benefits over VCV.
Technical aspects and clinical implications of high frequency jet ventilation with a solenoid valve.
High frequency jet ventilation (HFJV) is an incompletely studied technique of mechanical respiratory support. The authors have built a ventilator based on a solenoid valve, that allows independent selection of respiratory rate and inspiratory/expiratory ratio. The ventilator can be synchronized to the heart rate. Humidification is provided by warm saline dripped in front of the injector nozzle, so that the jet stream itself acts as a nebulizer. Tube diameter, length, and deformability are fundamental determinants of inspiratory flow rate and wave form. Cannula kinking and inadequate humidification were the most significant sources of complications.
Criteria for selective positive end-expiratory pressure and independent synchronized ventilation of each lung.
Respiratory failure of different origins often requires therapy with mechanical ventilation and positive end-expiratory pressure (PEEP). These supports are occasionally inadequate if the damage to one lung is significantly more pronounced than that to the other lung. Technical means exist to ventilate each lung independently or to provide a different PEEP to each lung. The findings from nine patients in whom one of these techniques was applied are presented, and advantages and disadvantages are discussed.
Doubly tuned solenoidal resonators for small animal imaging and spectroscopy at 1.5 Tesla.
The design and construction of solenoidal resonators for use with small animals in a 1.5-Tesla clinical imaging system are described. The coils have been designed to exploit the B1 distributions of two resonant modes of a four-turn solenoid whose windings are in parallel. Both singly and doubly tuned versions have been constructed. 1H images of normal and pathologic anatomy in mice and rats as well as a 31P spectrum of a Walker 256 rat sarcoma are presented. A primary advantage of this design is that the coils are easy to build and implement while providing the necessary sensitivity to allow high quality images to be obtained with no changes to the hardware or software of the clinical unit.
Design of apparatus for precise x-ray dose chamber calibrations.
An apparatus for precision calibration of ion chambers in the x-ray region from 16 to 320 kV is described. The development of a fast-acting shutter with "opening" and "closing" times of less than 3-ms eliminates the requirement for operating time corrections. Controls from outside the radiation room permit changing x-ray filters and alternately positioning both test and standard ion chambers in the x-ray beam. Thus, the remote controls eliminate the need to enter the x-ray room in the course of a series of calibrations. The potential advantages resulting from the capabilities of this apparatus are described in this Technical Report.