Heat transfer coefficient: Medivance Arctic Sun Temperature Management System vs. water immersion.

BACKGROUND AND OBJECTIVE: To improve heat transfer, the Medivance Arctic Sun Temperature Management System (Medivance, Inc., Louisville, CO, USA) features an adhesive, water-conditioned, highly conductive hydrogel pad for intimate skin contact. This study measured and compared the heat transfer coefficient (h), i.e. heat transfer efficiency, of this pad (hPAD), in a heated model and in nine volunteers' thighs; and of 10 degrees C water (hWATER) in 33 head-out immersions by 11 volunteers. METHODS: Volunteer studies had ethical approval and written informed consent. Calibrated heat flux transducers measured heat flux (W m-2). Temperature gradient (DeltaT) was measured between skin and pad or water temperatures. Temperature gradient was changed through the pad's water temperature controller or by skin cooling on immersion. RESULTS: The heat transfer coefficient is the slope of W m-2/DeltaT: its unit is W m-2 degrees C-1. Average with (95% CI) was: model, hPAD = 110.4 (107.8-113.1), R2 = 0.99, n = 45; volunteers, hPAD = 109.8 (95.5-124.1), R2 = 0.83, n = 51; and water immersion, hWATER = 107.1 (98.1-116), R2 = 0.86, n = 94. CONCLUSION: The heat transfer coefficient for the pad was the same in the model and volunteers, and equivalent to hWATER. Therefore, for the same DeltaT and heat transfer area, the Arctic Sun's heat transfer rate would equal water immersion. This has important implications for body cooling/rewarming rates.

Technique to obtain positron emission mammography images in registration with x-ray mammograms.

X-ray mammograms reveal abnormal tissue densities, while metabolic images identify regions of abnormal metabolism. Conventional nuclear medicine and radiologic breast images must be acquired at different times with different patient positions making coregistration difficult. Accurate coregistration of metabolic and x-ray images of the breast is likely to be important when acquiring information about the location and diagnosis of suspicious lesions or tumors. Our PEM-1 (positron emission mammography) system detects metabolic activity within the breast. The two planar detectors are integrated into a conventional x-ray mammography unit. This arrangement simplifies the image registration process by allowing a breast metabolic image to be acquired immediately after performing an x-ray mammogram. The patient is not moved between procedures. A coregistration tool has also been developed. A thin plastic sheet with a wire frame protrudes from the side of the upper PEM detector. With the tool positioned over the suspicious area of the breast, a magnified film density image is made using the available x-ray equipment. A radio-opaque rectangular outline of the wire frame is visible on the film image. During a positron emission metabolic scan, detectors acquire a 49 x 59 mm2 image of the same region. The PEM detectors can be positioned anywhere along the width of the breast. This provides an image of a particular region of interest. Several contiguous images may be combined to provide a complete scan.

Tryptophan-morphine interactions and postoperative pain.

Patients undergoing abdominal surgery were infused with saline or the 5-hydroxytryptamine (5-HT) precursor tryptophan starting in the operating room and continuing for three hours in the recovery room. There was a nonsignificant trend for patients who received tryptophan to have higher pain scores. In the saline-treated patients, plasma tryptophan was below the range for normal healthy subjects, and there was a strong positive relationship between plasma tryptophan and morphine requirements. These data, taken together with animal data obtained using the formalin pain test, suggest that a 5-HT system in the brain can antagonize the dissociative state produced by morphine, which helps patients to tolerate pain. When plasma tryptophan falls below normal levels, brain 5-HT falls and morphine requirements are reduced. While tryptophan may potentiate spinal 5-HT function to decrease nociceptive afference in some circumstances, there may be clinical conditions in which the use of tryptophan is contraindicated.

A system for rapid identification of respiratory abnormalities using a neural network.

Potential victims of Sudden Infant Death Syndrome (SIDS) can usefully be monitored in the home environment. Conventional respiration movement monitors can be helpful but may not detect potentially dangerous hypoxaemic episodes. Thus oxygen monitoring is to be preferred but can be difficult to use in the home. In an attempt to overcome these difficulties this paper presents the results of an exploratory experiment into the use of a neural network to link the output of a respiration pressure monitor to the classification of breathing patterns as effective or otherwise. It has been shown that it is possible to predict changes in oxygen saturation, which could signify potentially dangerous episodes earlier than when other methods are employed.

Accurate segmentation of respiration waveforms from infants enabling identification and classification of irregular breathing patterns.

Although the precise causes of Sudden Infant Death Syndrome (SIDS) are still unclear there is evidence to suggest that hypoxaemia may be a contributory factor. Transcutaneous oxygen monitors can be used but are unsatisfactory for young babies in the home. As an alternative approach, respiratory patterns can be studied but attempts at classification of individual breaths are often unsuccessful particularly during periods of extraneous noise or movement artefact. We have developed a robust algorithm which provides accurate segmentation and classification of breaths even in the presence of noise or movement. This improves on previous techniques by deferring the decision on an uncertain candidate breath until more information is available; yet the delay incurred is two breaths at most. The use of look-up tables and decision trees means that computational requirements are kept to a minimum and implementation in a simple home monitor is quite possible.

Time-frequency analysis of ECG signals using an image processing technique.

To overcome limitations that occur in time-frequency representations (TFR) of signals, a new technique that considers the ambiguity function (AF) is proposed. This work firstly introduces a filtering technique that is based on the processing of the image of the AF of the signal. Secondly it shows the potential of this technique in application to ECG signal analysis by comparing some kernel-based techniques. The analytic form of the ECG is used to eliminate aliasing and the cross terms generated by the negative spectra that appear in TFRs. This work shows that data adaptive time-frequency analysis of the ECG by image processing techniques is possible and potentially useful.

Noise coherence in closely-spaced electrodes: the implications for spatial averaged ECG recordings.

Ensemble signal averaging is conventionally used to reduce the noise content of ECG recordings. One shortcoming of this technique is that it removes the possibility of registering transient signals. As a consequence the simultaneous averaging of spatially related leads has been used. This relies on the recording electrodes receiving correlated signal components for reinforcement together with uncorrelated noise. We have developed a methodology for studying EMG noise reduction by spatial averaging and present results for different electrode spacings which suggest minimum separations for noise rejection.

Fast ECG data compression algorithms suitable for microprocessor systems.

ECG data compression techniques have received extensive attention in ECG analysis. Numerous data compression algorithms for ECG signals have been proposed during the last three decades. We describe two algorithms based on the scan-along polygonal approximation algorithm (SAPA) that are suitable for multichannel ECG data reduction on a microprocessor-based system. One represents a modification of SAPA (MSAPA) which adopts the method of integer division table searching to speed up data reduction; the other (CSAPA) combines MSAPA and TP, a turning-point algorithm, to preserve ST segment signals. Results show that our algorithms achieve a compression ratio of more than 5:1 and a percent rms difference (PRD) to the original signal of less than 3.5%. In addition, the maximum execution time of MSAPA for processing one data point is about 50 microseconds. Moreover, the CSAPA algorithm retains all of the details of the ST segment, which are important in ischaemia diagnosis, by employing the TP algorithm.

Heat loss in exposed volunteers.

Hypothermia is a common complication of major surgery and trauma. We studied this problem using Heat Flux Transducers to directly measure heat exchange between seven exposed volunteers and the environment. Heat exchange by radiation and convection was measured from the anterior chest wall and by conduction, between the back and a thermal mattress (CSZ, Blanketrol II). We determined the coefficients for: radiation = 6.6; convection = 8.3 square root of v; combined radiation and convection = 9.7; conductance = 41, all expressed in W/m2.degrees C. The clinical significance of these results is that heat loss, by radiation and convection alone, is 10 W/m2.degrees C. However, heat production under anaesthesia is only 40 W/m2, so a temperature gradient of greater than 4 degrees C between the skin and environment will cause more heat to be lost than is produced. The thermal mattress can supply 41 W/m2.degrees C, effectively doubling heat production.

Fazadinium and pancuronium: a pharmacodynamic study.

The neuromuscular blocking characteristics of fazadinium and pancuronium were compared using a general pharmacodynamic model. Since it characterizes the overall relationship between dose and effect, the model can be used to determine precisely equipotent doses, compare time-effect curves, and reconcile the apparently inconsistent clinical and pharmacokinetic characteristics of the two drugs. Fazadinium was shown to have a shorter duration than pancuronium, and appeared to exhibit less variation between subjects. The potency ratio varied between 3.4 : 1 and 7.2 : 1, depending upon the definition chosen. A general solution for the three-compartment open mammillary model, following any set of initial conditions if presented as an appendix. These equations allow explicit computation of any pattern of drug incrementation into any compartment of the model.

Low voltage electrical activity preceding right atrial depolarisation in man.

Electrical recordings were made in the high right atrium in 28 patients undergoing cardiac catheterisation and in 3 healthy volunteers. After filtering and amplification by 3 to 10 million times, the signals were passed through a signal averaging process in a digital computer. Of the 28 subjects who had technically satisfactory recordings, 23 showed low voltage electrical activity preceding the conventionally-recorded atrial depolarisation. The low voltage activity started 50 to 200 ms before the atrial deflection and was variable in shape. These early signals may be the result of activity in the region of the sinus node.

Spirometry in healthy men: a correlation with smoking and with mild symptoms.

Symptoms, smoking history, and variates taken from the forced expiratory manoeuvre were studied in a group of 271 healthy men attending a mass radiography unit. It was found that correlation of the forced expiratory ratio (FEV1/FVC) with the presence of mild or moderate respiratory symptoms was at least as satisfactory as that of technically more complicated measurements such as MEF50 and MEF75. In asymptomatic individuals those differences associated with smoking were the same for FEV1/FVC as for MEF75. Asymptomatic abnormalities of lung function were not observed. Simple measurements such as FEV1/FVC can therefore provide as much information from a forced expiratory manoeuvre to screen for early airways obstruction as MEF50 and MEF75.

Reduction of postoperative morbidity following patient-controlled morphine.

The present study examined the impact of two methods of pain management on recovery in 38 women undergoing hysterectomy. One group received IV morphine in the recovery room and IM morphine on the ward on a PRN basis (PRN group). In the other group, a loading dose of morphine 8 mg IV was given when the patient first complained of pain and patient-controlled IV morphine (PCA) was initiated and continued for 48 h (PCA group). Both groups received similar amounts of morphine overall, differently distributed over time. The PCA patients received 8 mg.h-1 in the recovery room (approximately 2.5 hrs) and less thereafter. The PRN patients received approximately 2 mg.h-1 for the entire 48-hr period. Pain control was better throughout convalescence and less variable across time with PCA management. Minute ventilation also recovered faster and by day four was 25 per cent above the preoperative baseline in the PCA group. In addition, oral temperature became normal one day earlier, ambulation recovered more rapidly and patients were discharged from hospital earlier. The data suggest that early treatment with relatively high, self-titrated morphine doses may alter the course of the metabolic response to surgery.

Perioperative thermal insulation: minimal clinically important differences?

BACKGROUND: Reduction of heat losses from the skin by thermal insulation is used to avoid perioperative hypothermia. However, there is little information about the physical properties of various insulating materials used in the operating room. METHODS: The following insulation materials were tested using a validated manikin: cotton surgical drape tested in two and four layers; Allegiance drape; 3M Steri-Drape; metallized plastic sheet; Thermadrape Barkey thermcare 1 tested in one and two layers; hospital duvet tested in one and two layers. Heat loss from the surface of the manikin can be described as: Q(*);= h.DeltaT.A where Q(*); is heat flux, h is the heat exchange coefficient, DeltaT is the temperature gradient between the environment and surface and A is the area covered. The heat flux per unit area (Q(*); A(-1)) and surface temperature were measured with nine calibrated heat-flux transducers. The environmental temperature was measured using a thermoanemometer. DeltaT was varied and h was determined by linear regression analysis as the slope of DeltaT vs Qdot; A(-1). The reciprocal of h defines the insulation. RESULTS: The insulation value of air was 0.61 Clo. The insulation values of the materials varied between 0.17 Clo (two layers of cotton surgical drapes) to 2.79 Clo (two layers of hospital duvet). CONCLUSIONS: There are relevant differences between various insulating materials. The best commercially available material designed for use in the operating room (Barkey thermcare 1) can reduce heat loss from the covered area by 45% when used in two layers. Given the range of insulating materials available for outdoor activities, significant improvement in insulation of patients in the operating room is both possible and desirable.

Heat loss in an animal experimental model.

Heat loss leading to hypothermia is common during surgery, particularly when a body cavity is exposed. To determine the principal site of heat loss we used heat flux transducers to measure heat loss from the skin and the exposed abdominal cavity of seven pigs. Heat loss from the skin was 74 +/- 15 W/m2, and from the abdominal cavity, 350 +/- 122 W/m2 (p less than 0.002; ratio = 1:4.7). The area of the skin was 0.52 +/- 0.06 m2, and of the abdominal cavity, 0.04 +/- 0.006 m2 (ratio = 13.6:1). For every 13.6 watts lost from the skin, 4.7 watts were lost from the abdominal cavity; i.e. skin heat loss accounted for 74% of total heat loss. Therefore adequate insulation would reduce the incidence of hypothermia. Evaporation accounted for the largest heat loss from the abdominal cavity. Evaporative losses could be minimised by enclosing the bowel in a plastic bag.

Construction and evaluation of a manikin for perioperative heat exchange.

BACKGROUND: During surgery hypothermia can be avoided only if the heat exchange between the body surface and the environment can be controlled. To allow a systematic analysis of this heat exchange, we constructed and evaluated a copper manikin of the human body. METHODS: The manikin consists of six tubes (head, trunk, two arms and two legs) painted matt-black to simulate the emissivity of the human skin. Hot-water mattresses are bonded to the inner surface of the copper tubes to set the surface temperature. Calibrated heat flux transducers were placed on the following points to determine the heat exchange coefficient for radiation and convection (hRC) of the manikin: Forehead, chest, abdomen, upper arm, forearm, dorsal hand, anterior thigh, anterior leg and foot. Room temperature was set to 22 degrees C. Surface temperature of the manikin was set between 22 degrees C and 38 degrees C. The hRC was determined by linear regression analysis as the slope of the temperature gradient between the manikin and the room versus the measured heat flux. Subsequently we studied five minimally clothed volunteers in a climate chamber. Initial chamber temperature was set to 29 degrees C and was lowered slowly to 12 degrees C. The hRC was determined as described above for each volunteer. RESULTS: The hRC of the manikin was 11.0 W m(-2) degrees C(-1) and hRC of the volunteers was 10.8 W m(-2) degrees C(-1). CONCLUSION: The excellent correlation of hRC between the volunteers and the manikin will allow the manikin to be used for standardised studies of perioperative heat exchange.

A flexible signal-averaging system for cardiac waveforms.

The repetitive nature of cardiac waveforms renders them suitable for processing by signal averaging. A flexible system is described, based on a general-purpose digital computer, capable of averaging a variety of cardiac signals in excess of 0.5 microV containing frequency components below a theoretical Nyquist limit of 5 kHz. Important features of the system include real-time processing capability at a high level of interactive control, and the facility to trigger the averaging process accurately from any part of either the data or a synchronous reference waveform. Application of the system to electrocardiographic and phonocardiographic signals at conventional gain has enabled clinically useful records to be obtained in the face of obtrusive environmental noise. The averaging of signals of a similar nature after considerably increased amplification has revealed cardiac activity unseen in conventional records in a total of 81 subjects.

Comparison of forced-air warming systems with lower body blankets using a copper manikin of the human body.

BACKGROUND: Forced-air warming has gained high acceptance as a measure for the prevention of intraoperative hypothermia. However, data on heat transfer with lower body blankets are not yet available. This study was conducted to determine the heat transfer efficacy of six complete lower body warming systems. METHODS: Heat transfer of forced-air warmers can be described as follows:[1]Qdot;=h.DeltaT.A where Qdot; = heat transfer [W], h = heat exchange coefficient [W m-2 degrees C-1], DeltaT = temperature gradient between blanket and surface [ degrees C], A = covered area [m2]. We tested the following forced-air warmers in a previously validated copper manikin of the human body: (1) Bair Hugger and lower body blanket (Augustine Medical Inc., Eden Prairie, MN); (2) Thermacare and lower body blanket (Gaymar Industries, Orchard Park, NY); (3) WarmAir and lower body blanket (Cincinnati Sub-Zero Products, Cincinnati, OH); (4) Warm-Gard(R) and lower body blanket (Luis Gibeck AB, Upplands Väsby, Sweden); (5) Warm-Gard and reusable lower body blanket (Luis Gibeck AB); and (6) WarmTouch and lower body blanket (Mallinckrodt Medical Inc., St. Luis, MO). Heat flux and surface temperature were measured with 16 calibrated heat flux transducers. Blanket temperature was measured using 16 thermocouples. DeltaT was varied between -10 and +10 degrees C and h was determined by a linear regression analysis as the slope of DeltaT vs. heat flux. Mean DeltaT was determined for surface temperatures between 36 and 38 degrees C, because similar mean skin temperatures have been found in volunteers. The area covered by the blankets was estimated to be 0.54 m2. RESULTS: Heat transfer from the blanket to the manikin was different for surface temperatures between 36 degrees C and 38 degrees C. At a surface temperature of 36 degrees C the heat transfer was higher (between 13.4 W to 18.3 W) than at surface temperatures of 38 degrees C (8-11.5 W). The highest heat transfer was delivered by the Thermacare system (8.3-18.3 W), the lowest heat transfer was delivered by the Warm-Gard system with the single use blanket (8-13.4 W). The heat exchange coefficient varied between 12.5 W m-2 degrees C-1 and 30.8 W m-2 degrees C-1, mean DeltaT varied between 1.04 degrees C and 2.48 degrees C for surface temperatures of 36 degrees C and between 0.50 degrees C and 1.63 degrees C for surface temperatures of 38 degrees C. CONCLUSION: No relevant differences in heat transfer of lower body blankets were found between the different forced-air warming systems tested. Heat transfer was lower than heat transfer by upper body blankets tested in a previous study. However, forced-air warming systems with lower body blankets are still more effective than forced-air warming systems with upper body blankets in the prevention of perioperative hypothermia, because they cover a larger area of the body surface.

Comparison of forced-air warming systems with upper body blankets using a copper manikin of the human body.

BACKGROUND: Forced-air warming with upper body blankets has gained high acceptance as a measure for the prevention of intraoperative hypothermia. However, data on heat transfer with upper body blankets are not yet available. This study was conducted to determine the heat transfer efficacy of eight complete upper body warming systems and to gain more insight into the principles of forced-air warming. METHODS: Heat transfer of forced-air warmers can be described as follows: Qdot;=h. DeltaT. A, where Qdot;= heat flux [W], h=heat exchange coefficient [W m-2 degrees C-1], DeltaT=temperature gradient between the blanket and surface [ degrees C], and A=covered area [m2]. We tested eight different forced-air warming systems: (1) Bair Hugger and upper body blanket (Augustine Medical Inc. Eden Prairie, MN); (2) Thermacare and upper body blanket (Gaymar Industries, Orchard Park, NY); (3) Thermacare (Gaymar Industries) with reusable Optisan upper body blanket (Willy Rüsch AG, Kernen, Germany); (4) WarmAir and upper body blanket (Cincinnati Sub-Zero Products, Cincinnati, OH); (5) Warm-Gard and single use upper body blanket (Luis Gibeck AB, Upplands Väsby, Sweden); (6) Warm-Gard and reusable upper body blanket (Luis Gibeck AB); (7) WarmTouch and CareDrape upper body blanket (Mallinckrodt Medical Inc., St. Luis, MO); and (8) WarmTouch and reusable MultiCover trade mark upper body blanket (Mallinckrodt Medical Inc.) on a previously validated copper manikin of the human body. Heat flux and surface temperature were measured with 11 calibrated heat flux transducers. Blanket temperature was measured using 11 thermocouples. The temperature gradient between the blanket and surface (DeltaT) was varied between -8 and +8 degrees C, and h was determined by linear regression analysis as the slope of DeltaT vs. heat flux. Mean DeltaT was determined for surface temperatures between 36 and 38 degrees C, as similar mean skin surface temperatures have been found in volunteers. The covered area was estimated to be 0.35 m2. RESULTS: Total heat flow from the blanket to the manikin was different for surface temperatures between 36 and 38 degrees C. At a surface temperature of 36 degrees C the heat flows were higher (4-26.6 W) than at surface temperatures of 38 degrees C (2.6-18.1 W). The highest total heat flow was delivered by the WarmTouch trade mark system with the CareDrape trade mark upper body blanket (18.1-26.6 W). The lowest total heat flow was delivered by the Warm-Gard system with the single use upper body blanket (2.6-4 W). The heat exchange coefficient varied between 15.1 and 36.2 W m-2 degrees C-1, and mean DeltaT varied between 0.5 and 3.3 degrees C. CONCLUSION: We found total heat flows of 2.6-26.6 W by forced-air warming systems with upper body blankets. However, the changes in heat balance by forced-air warming systems with upper body blankets are larger, as these systems are not only transferring heat to the body but are also reducing heat losses from the covered area to zero. Converting heat losses of approximately 37.8 W to heat gain, results in a 40.4-64.4 W change in heat balance. The differences between the systems result from different heat exchange coefficients and different mean temperature gradients. However, the combination of a high heat exchange coefficient with a high mean temperature gradient is rare. This fact offers some possibility to improve these systems.