Microwave applications in clinical medicine.

Over the last fifty years, energy has been applied to various human tissues for both the diagnosis and therapy of numerous diseases. However, in general, the medical community remains uninformed about the many potential applications of this energy source. We review the many areas in which microwave energy has shown clinical utility.

New connection method for isolating and disinfecting intraluminal path during peritoneal dialysis solution-exchange procedures.

Microbiological data have been collected on the performance of a new method of isolating and disinfecting the intraluminal path at the connect/disconnect site of a peritoneal dialysis (PD)-exchange pathway. High-temperature moist-heat (HTMH) disinfection is accomplished by a new device that uses microwave energy to heat the solution contained in the pressure-tight inner lumen of PD connector pairs between the transfer-set connector-clamp and the bag-connector break-away seal. An 85 degrees C (S.D. = 2.4 degrees C, n = 10) rise in solution temperature is seen in 12 seconds, thus yielding temperatures under pressure well over 100 degrees C with starting temperatures of 25 degrees C. Connector pairs were prepared by inoculation of a solution suspension containing at least 10(6) colony-forming units (CFU) of a test micro-organism. Approximately 0.4 mL of solution was contained within the mated connector pair. Using standard D-value determination methods, data were obtained for surviving organisms versus five exposure times and a positive control to obtain a population reduction curve. Four micro-organisms (S. epidermidis, P. aeruginosa, C. albicans, and A. niger) recognized to be among the most prevalent or problematic in causing peritonitis were tested. After microwave heating, the treated solution was aseptically withdrawn from the connector pair using a needle and syringe, plated in growth media, and incubated. Population counts of CFUs after incubation were used to establish survival curves. Results showed a tenfold population reduction in less than 3 seconds for all organisms tested. A 30-second cycle time safely achieves a > 10(8) population-reduction for bacteria and yeast organisms, and a > 10(7) population reduction for fungi. One potential benefit of using this new intraluminal disinfection method is that it may help reduce peritonitis resulting from the even more problematic pathogens such as the gram-negative bacteria and fungal organisms.

The limits of bloodwarming: maximally heating blood with an inline microwave bloodwarmer.

BACKGROUND: Bloodwarmers, mandatory for use in trauma resuscitation, are currently limited to a maximum temperature of 42 degrees C by the American Association of Blood Banks. Using newly available inline microwave bloodwarming technology, we sought to identify the maximal temperature to which blood may be safely heated. METHODS: Using an inline microwave bloodwarmer, we warmed refrigerated packed red blood cells to settings ranging from "Off" to 60 degrees C. We evaluated heated blood for changes in red cell structure and function by measuring hemoglobin/hematocrit, potassium, lactate dehydrogenase, plasma hemoglobin, blood smear, osmotic fragility, PO2, giving 50% O2 saturation, and hemoglobin electrophoresis. RESULTS: Measures of hemolysis showed no increase above control until temperatures of 51 to 53 degrees C were achieved (p < 0.05). Red cell size remained unaffected until temperatures of 53 degrees C were achieved (p < 0.05). Osmotic fragility was not elevated until 60 degrees C (p < 0.05). PO2 giving 50% O2 saturation was low for all samples. Hemoglobin electrophoresis remained unchanged at all temperature settings. CONCLUSION: An inline microwave bloodwarmer may be used to heat blood safely to 49 degrees C. Blood warmed to this temperature may significantly increase the amount of heat returned to the hypothermic trauma patient.

Detection of air emboli in radiographic contrast media by microwave radiometry.

To evaluate the efficacy of a microwave radiometry system in detecting in-line air emboli in radiographic contrast media, air emboli ranging in volume from 0.1 to 0.005 ml were introduced into ionic (ioxaglate) and nonionic (iohexol) contrast media at 22 or 37 degrees pumped at flow rates of 16.7, 180 or 300 ml/min through polyvinlychloride tubing with an inner diameter of 0.100 inches (2.54 mm) over which was fitted a radiometer antenna connected to a Microwave Medical System F+ radiometer and a computerized data acquisition system. A total of 400 determinations were run, with 10 replicate determinations for each unique set of experimental conditions. The success of air emboli detection was not significantly related to contrast media (p = 0.73) or contrast temperature (p = 0.68). Embolus volume (p < 0.0001) and pump speed (p < 0.0001) were significant factors affecting system performance. The system could reliably detect small (0.005 ml) emboli in both ionic and nonionic low-osmolar contrast media.

In-line microwave blood warming of in-date human packed red blood cells.

OBJECTIVE: To verify two hypotheses: a) In-line microwave warming of cold in-date packed red blood cells (RBCs) does not produce significant hemolysis; and b) in-line microwave warming achieves higher outlet temperatures as compared with current blood warming technology at high flow rates (> 250 mL/min). DESIGN: Multiple part, randomized, controlled study. SETTING: Surgical research laboratory of a large university medical center. SUBJECTS: Twenty-four units of cold, ready for transfusion in-date packed RBCs ranging in storage age from 6 to 16 days. INTERVENTIONS: Part I: Microwave apparatus outlet, warmed vs. unwarmed. Six units of cold packed RBCs was split into paired samples and infused at 13 mL/min through a 700-watt in-line microwave test apparatus. One paired specimen was warmed to 37 degrees C; the other was infused without warming (control). Blood was analyzed at the outlet. Part II: Microwave and countercurrent warming, inlet vs. outlet. Twelve units of cold packed RBCs was analyzed biochemically both before (inlet) and after (outlet) simulated transfusions. Six units was infused through a 900-watt in-line microwave test apparatus at > 500 mL/min. Six separate cold units were warmed at this rate using single channel countercurrent water bath warming. Part III: Microwave and countercurrent technology, inlet vs. outlet, warmed vs. unwarmed. a) Six units of cold packed RBCs was also analyzed biochemically and infused at 5 mL/min through either a microwave or countercurrent water bath warmer. b) Packed RBCs from the units used in part a) were allowed to remain stationary in the microwave heating cartridge for 15 mins with an activated heating element. Parallel stationary flow studies were done using the countercurrent blood warmer. Control unwarmed samples were also tested. MEASUREMENTS AND MAIN RESULTS: Part I: No statistical differences in hemolysis parameters were observed between microwave warmed and unwarmed packed RBCs. Part II: At high-flow rates, no statistical increases in hemolysis parameters were seen after in-line microwave or countercurrent water bath warming as compared with prewarmed cold controls. Part III: At slow-flow rates, nonstatistically significant increases were seen by passing the packed RBCs through either test apparatus unwarmed. Packed RBCs remaining stationary within microwave and countercurrent heating cartridges for 15 mins did show biochemical evidence of hemolysis. Mean plasma hemoglobin increased from 14 +/- 1.7 mg/dL in cold prewarmed units to 57.7 +/- 5.8 mg/dL (p < .05), when warmed in the microwave heating cartridge, and to 55.2 +/- 25 mg/dL (p < .05), when warmed in the countercurrent heat exchanger. Outlet Temperature Studies. Part II: The in-line 900-watt microwave device warmed cold units from a mean inlet temperature of 8.3 +/- 0.3 degrees C to a mean outlet temperature of 31.8 +/- 0.5 degrees C within 5 secs at a mean flow rate of 556 mL/min. At 30 secs, the mean outlet temperature was 33.9 +/- 0.4 degrees C (mean inlet temperature = 9.6 +/- 0.2 degrees C) for microwave warmed packed RBCs as compared with 32.1 +/- 0.5 degrees C (mean inlet temperature = 9.6 +/- 0.3 degrees C) in countercurrent water bath warmed blood (p < .05). From 20 to 30 secs, the packed RBCs warmed by microwave were statistically warmer than the countercurrent water bath warmed packed RBCs. CONCLUSIONS: a) Both in-line countercurrent warming and in-line microwave warming were associated with small increases in parameters of red cell damage representing statistically and clinically insignificant hemolysis. b) Blood sitting in any blood warming device is subject to statistically significant but clinically irrelevant increases in those parameters. c) At high-flow rates, the in-line microwave device warmed blood to higher outlet temperatures than the single channel countercurrent water bath warmer. This method may represent a clinical blood warming modality of the near future.

Early detection of extravasation of radiographic contrast medium. Work in progress.

Microwave radiometry is a passive and noninvasive technique that allows quick detection of subcutaneous temperature changes. The feasibility of this technique for differentiating normal intravenous infusions of radiographic contrast medium from extravasations of contrast medium was tested in anesthetized dogs. Room-temperature and heated ionic and nonionic contrast media were administered at flow rates ranging from 0.2 to 9.9 mL/sec by means of a power injector. On the basis of these experiments, an algorithm to adjust for extravasation detection thresholds as a function of injection flow rates was developed. With this algorithm, results showed a false-positive rate of 0% at all infusion rates and false-negative rates of 2%, 2%, and 4% at pump speeds of 0.2, 1.0, and 9.9 mL/sec, respectively. The times of these extravasation "alarms" corresponded to maximum extravasated volumes, respectively, of 4, 6.5, and 8 mL. Microwave radiometry has clinical potential for early detection of extravasation of contrast medium administered with power injectors.

Rapid in-line blood warming using microwave energy: preliminary studies.

The management of massive blood loss resulting from trauma or surgery necessitates rapid transfusion capability. Hypothermia secondary to shock, transfusion, and prolonged surgical procedures significantly increases morbidity and mortality in these patients. Transfusion at high flow rates frequently exceeds the warming capacity of conventional blood-warming devices, whose inherent resistance also limits the maximal flow rates. Microwave ovens are capable of blood warming, but have been associated with unacceptable hemolysis. We have investigated the possibility of using microwave energy to provide rapid in-line blood warming. Fresh blood from 10 human subjects was warmed from an average of 18 degrees C to temperatures ranging from 37 to 39 degrees C at flow rates from 250 to 500 mL/min. Laboratory analysis of free plasma hemoglobin, haptoglobin, hematocrit, hemoglobin, and electrolytes showed no difference between heated and control samples. LDH was elevated in those samples warmed repeatedly, but remained within the normal range. These data indicate the potential for further investigation utilizing properly controlled microwave energy for in-line blood and fluid warming.

Effectiveness of microwave moist-heat intraluminal disinfection of CAPD connectology.

Studies were performed to quantify the microbial population reduction achieved with the Peritoneal Dialysis Moist Heat Device (PDM-1). This microwave method is used to disinfect the inner lumen of connectors used in the exchange process for continuous ambulatory peritoneal dialysis (CAPD). The microwave heating technique was evaluated on different connector systems containing a suspension of 10(6) microorganisms. The most prevalent and most problematic peritonitis-causative microorganisms were tested. After heating, the degree of disinfection was measured by enumerating bacterial colonies of the treated suspension. D-value determinations were then performed. The D-value for spores of A. niger was found to be 7.1 seconds for one type of connector system. Two other connector systems containing different intraluminal solution volumes were also tested using spores of A. niger and D-values were determined to be 7.6 seconds and 9.5 seconds, respectively. Other organisms tested were determined to have D-value times shorter than that for A. niger. Rapid heating of the solution contained within the CAPD connectors is a key characteristic of the microwave technique since temperatures rise high enough for destruction of microorganisms while leaving the plastic safe to touch. Thus, this technique is a safe and effective method for providing intraluminal disinfection.

Spatial resolution measurements for passive microwave radiometry using a tissue-equivalent phantom.

A tissue-equivalent "hot" line source phantom is described for assessing spatial resolution in passive microwave radiometry systems. LSFs were measured for two rectangular waveguide antennas connected to a 4.7-GHz radiometer. The normalized LSFs and corresponding modulation transfer functions were found to be independent of line source temperature, but dependent upon antenna size, orientation, and line source depth.

Moist heat intraluminal disinfection of CAPD connectors.

A moist heat technique for disinfecting the inner lumen of commercially available connectology used in the exchange process for Continuous Ambulatory Peritoneal Dialysis (CAPD) was evaluated. Moist heat was generated by a device (PDM-1) that directed microwave energy to heat a sample solution containing a concentration of 10(6) microorganisms inside a pair of mated plastic CAPD connectors. Microorganisms tested included those most prevalent and most problematic in causing peritonitis. Testing, performed according to F.D.A. approved standards, involved heating a sample solution and then placing the sample solution into vials which were then sealed and incubated. Absolute determination of growth versus no growth was measured by macroscopic observation. Positive control samples were performed in the same manner but were not exposed to heat. Negative controls were performed in the same manner in the absence of test organisms. At temperatures of approximately 100 degrees C a D-value of 6.6 seconds was determined using the organism found the most thermoresistant. A cycle time of 54 seconds appeared sufficient to achieve a 10(6) population reduction of all microorganisms tested. The moist heat technique offers a safe, effective method for disinfection of the inner lumen of CAPD connectors.