Michael L. Gimbel

Brain tissue oxygenation during hemorrhagic shock, resuscitation, and alterations in ventilation.

OBJECTIVES Recently developed polarographic microelectrodes permit continuous, reliable monitoring of oxygen tension in brain tissue (PbrO2). The aim of this study was to investigate the feasibility and utility of directly monitoring PbrO2 in cerebral tissue during changes in oxygenation or ventilation and during hemorrhagic shock and resuscitation. We also sought to develop a model in which treatment protocols could be evaluated using PbrO2 as an end point. METHODS Licox Clark-type polarographic probes were inserted in the brain tissue of 16 swine to monitor PbrO2. In eight swine, changes in PbrO2 were observed over a range of fractional concentrations of inspired O2 (FiO2) as well as during periods of hyperventilation and hypoventilation. In eight other swine, PbrO2 was monitored during a graded hemorrhage of up to 70% estimated blood volume and during the resuscitation period. RESULTS When FiO2 was elevated to 100%, PbrO2 increased from a baseline of 15+/-2 mm Hg to 36+/-11 mm Hg. Hyperventilation while breathing 100% oxygen resulted in a 40% decrease in PbrO2 (p < 0.05), whereas hypoventilation increased PbrO2 to 88 mm Hg (p < 0.01). A graded hemorrhage to 50% estimated blood volume significantly reduced PbrO2, mean arterial pressure, and intracranial pressure (p < 0.01). Continued hemorrhage to 70% estimated blood volume resulted in a PbrO2 of 2.9+/-1.5 mm Hg. After resuscitation, PbrO2 was significantly elevated, reaching 65+/-13 mm Hg (p < 0.01), whereas mean arterial pressure and cerebral perfusion pressure simply returned to baseline. CONCLUSION Directly measured PbrO2 was highly responsive to changes in FiO2, ventilatory rate, and blood volume in this experimental model. In particular, hypoventilation significantly increased PbrO2, whereas hyperventilation had the opposite effect. The postresuscitation increase in PbrO2 may reflect changes in both O2 delivery and O2 metabolism. These experiments set the stage for future investigations of a variety of resuscitation protocols in both normal and injured brain.

Monitoring partial and full venous outflow compromise in a rabbit skin flap model.

Background: Free flap failure often results from venous thrombosis. The authors developed a rabbit flap model of partial venous obstruction and evaluated four monitoring devices in detecting partial and full venous compromise. Methods: Nine skin flaps were elevated on their arteriovenous pedicles in rabbits. The flap was assessed with quantitative Doppler of arterial inflow, transcutaneous oxygen and carbon dioxide tension, near-infrared spectroscopy tissue oxygen saturation, and scanning laser Doppler imaging. After a stable baseline was achieved, the outflow vein was subjected to partial and full venous obstruction followed by release. Results: Pedicle arterial flow decreased significantly from baseline (5.9 ± 3.0 ml/minute) during partial (4.1 ± 2.4 ml/minute; 30.5 percent; p < 0.01) and full obstruction (0.3 ± 0.4 ml/minute; 94.9 percent; p < 0.01). All other measures changed significantly with full obstruction: transcutaneous oxygen tension decreased by 79.6 percent; transcutaneous carbon dioxide tension increased by 69.0 percent; near-infrared spectroscopy tissue oxygen saturation decreased by 35.7 percent; and scanning laser Doppler imaging decreased by 78.8 percent. Laser Doppler imaging was the only noninvasive device that decreased significantly (p < 0.01) with partial obstruction, from 222.8 ± 77.3 units to 186.5 ± 73.2 units (16.3 percent). Conclusions: The authors established a venous obstruction flap model and evaluated four clinically relevant monitoring devices during partial and full venous occlusion. All devices detected full occlusion, but only scanning laser Doppler imaging and arterial Doppler detected partial occlusion. Scanning laser Doppler imaging monitoring may allow warning of impending venous obstruction. Near-infrared spectroscopy tissue oxygen saturation varied the least between flaps and therefore may be the most easily interpreted device for full venous occlusion. Both characteristics are important for clinical application.

Comparative analysis of compressive sensing approaches for recovery of missing samples in implantable wireless Doppler device

An implantable wireless Doppler device used in microsurgical free flap surgeries can suffer from lost data points. To recover the lost samples, the authors considered the approaches based on a recently proposed compressive sensing. In this paper, they performed a comparative analysis of several different approaches by using synthetic and real signals obtained during blood flow monitoring in four pigs. They considered three different bases functions: Fourier bases, discrete prolate spheroidal sequences and modulated discrete prolate spheroidal sequences, respectively. To avoid the computational burden, they considered the approaches based on the l 1 minimisation for all the three bases. To understand the trade-off between the computational complexity and the accuracy, they also used a recovery process based on a matching pursuit and modulated discrete prolate spheroidal sequences bases. For both the synthetic and the real signals, the matching approach with modulated discrete prolate spheroidal sequences provided the most accurate results. Future studies should focus on the optimisation of the modulated discrete prolate spheroidal sequences in order to further decrease the computational complexity and increase the accuracy.

The timing of preoperative prophylactic low-molecular-weight heparin administration in breast reconstruction.

Background: Venous thromboembolism continues to be problematic despite increased recognition and advancements in venous thromboembolism prophylaxis. Although migration toward preoperative chemoprophylaxis increases, plastic surgeons seem reticent to adopt this practice. This study evaluates preoperative enoxaparin administration in breast reconstruction patients. Methods: Patients undergoing breast reconstruction performed by a single surgeon over a 5-year period were evaluated retrospectively. The authors introduced preoperative chemoprophylaxis with enoxaparin in all breast reconstructions during this time. Prosthetic-based and microsurgical breast reconstructions were examined. Patients were divided into two groups: those who did and those who did not receive preoperative enoxaparin. The authors reviewed patient demographics, comorbidities, and complications, focusing on bleeding complications. Results: Three hundred patients (450 breasts) were included. One hundred fifty-four patients (244 breasts) underwent reconstruction with tissue expanders, and 146 (206 breasts) underwent free flap reconstructions. One hundred seventy-nine of 300 were given preoperative enoxaparin. Eleven hematomas occurred, eight (4.5 percent) in the enoxaparin group and three (2.5 percent) without enoxaparin (p = 0.399). Blood transfusions were given to four patients (2.2 percent) who received enoxaparin and one patient (0.8 percent) who did not (p = 0.652). Finally, any type of bleeding complication occurred in 11 patients (6.1 percent) with enoxaparin and in four (3.3 percent) without (p = 0.419). Larger breasts were more likely to receive enoxaparin (p = 0.011), which did not result in higher bleeding complications. Conclusion: In this retrospective study, the authors found that preoperative chemoprophylaxis in breast reconstruction was associated with an acceptable rate of postoperative bleeding complications. CLINICAL QUESTION/LEVEL OF EVIDENCE: Therapeutic, III.

The Development of a Wireless Implantable Blood Flow Monitor.

Summary: Microvascular anastomotic failure remains an uncommon but devastating problem. Although the implantable Doppler probe is helpful in flap monitoring, the devices are cumbersome, easily dislodged, and plagued by false-positive results. The authors have developed an implantable wireless Doppler monitor prototype from off-the-shelf components and tested it in a swine model. The wireless probe successfully distinguished between femoral vein flow, occlusion, and reflow, and wirelessly reported the different signals reliably. This is the first description of a wireless implantable blood flow sensor for flap monitoring. Future iterations will incorporate an integrated microchip-based Doppler system that will decrease the size to 1 mm2, small enough to fit onto an anastomotic coupler.

Analysis of the vertical mammaplasty design in skin-sparing mastectomy and immediate autologous reconstruction

BACKGROUND Skin-sparing mastectomy designs for immediate autologous breast reconstruction include racquet, Wise, and vertical mammaplasty incisions. The vertical design addresses ptosis while maintaining viable skin flaps. This study compares the racquet to the vertical incision. METHODS Immediate skin-sparing autologous breast reconstructions by a single surgeon using either vertical or racquet incisions from August 2006 to September 2011 were analyzed. Aesthetic scoring was based on a Likert-scale assessment of scar appearance, shape, preoperative versus postoperative aesthetic comparison, and overall aesthetic outcome. Responses were analyzed using the Mann-Whitney test. RESULTS Seventy-seven patients (48 racquets and 29 vertical) were included. Patient demographics and complications did not differ. Vertical design reconstructions were used for patients with higher-grade ptosis (p < 0.001). Significantly better-appearing scars (3.8 vs. 3.5; p = 0.04) were observed in the vertical group. Vertical reconstructions showed a trend toward significance in cosmetic improvement compared with preoperative appearance (3.2 vs. 3.0; p = 0.06). There was no difference in shape (vertical 3.6, racquet 3.6; p = 0.86) or in postoperative aesthetic result (vertical 3.6, racquet 3.4; p = 0.41). CONCLUSIONS Shape and overall postoperative aesthetic appearance did not significantly differ despite greater ptosis preoperatively in the vertical group, demonstrating the efficacy of the vertical design in reconstruction. Vertical design reconstructions were rated aesthetically superior to their pre-mastectomy appearance. Significantly better scar scores in the vertical group reflect the camouflaged nature of vertical incisions. These results demonstrate that the aesthetic outcome of the vertical design reconstruction in ptotic breasts is as good as and potentially superior to the racquet design reconstruction in non-ptotic breasts.

A System for Simple Real-Time Anastomotic Failure Detection and Wireless Blood Flow Monitoring in the Lower Limbs

Current totally implantable wireless blood flow monitors are large and cannot operate alongside nearby monitors. To alleviate the problems with the current monitors, we developed a system to monitor blood flow wirelessly, with a simple and easily interpretable real-time output. To the best of our knowledge, the implanted electronics are the smallest in reported literature, which reduces bio-burden. Calibration was performed across realistic physiological flow ranges using a syringe pump. The devices sensors connected directly to the bilateral femoral veins of swine. For each 1 min, blood flow was monitored, then, an occlusion was introduced, and then, the occlusion was removed to resume flow. Each vein of four pigs was monitored four times, totaling 32 data collections. The implant measured 1.70 cm3 without battery/encapsulation. Across its calibrated range, including equipment tolerances, the relative error is less than ±5% above 8 mL/min and between -0.8% and +1.2% at its largest calibrated flow rate, which to the best of our knowledge is the lowest reported in the literature across the measured calibration range. The average standard deviation of the flow waveform amplitude was three times greater than that of no-flow. Establishing the relative amplitude for the flow and no-flow waveforms was found necessary, particularly for noise modulated Doppler signals. Its size and accuracy, compared with other microcontroller-equipped totally implantable monitors, make it a good candidate for future tether-free free flap monitoring studies.Current totally implantable wireless blood flow monitors are large and cannot operate alongside nearby monitors. To alleviate the problems with the current monitors, we developed a system to monitor blood flow wirelessly, with a simple and easily interpretable real-time output. To the best of our knowledge, the implanted electronics are the smallest in reported literature, which reduces bio-burden. Calibration was performed across realistic physiological flow ranges using a syringe pump. The device’s sensors connected directly to the bilateral femoral veins of swine. For each 1 min, blood flow was monitored, then, an occlusion was introduced, and then, the occlusion was removed to resume flow. Each vein of four pigs was monitored four times, totaling 32 data collections. The implant measured 1.70 cm3 without battery/encapsulation. Across its calibrated range, including equipment tolerances, the relative error is less than ±5% above 8 mL/min and between −0.8% and +1.2% at its largest calibrated flow rate, which to the best of our knowledge is the lowest reported in the literature across the measured calibration range. The average standard deviation of the flow waveform amplitude was three times greater than that of no-flow. Establishing the relative amplitude for the flow and no-flow waveforms was found necessary, particularly for noise modulated Doppler signals. Its size and accuracy, compared with other microcontroller-equipped totally implantable monitors, make it a good candidate for future tether-free free flap monitoring studies.

Abstract 109: venous flow monitoring using an entirely implanted, wireless Doppler sensor.

PurPose: Successful salvage of the threatened free flap is dependent upon prompt diagnosis of vascular occlusion and timely restoration of blood flow. Many monitoring systems can be used to augment clinical exam, but all suffer from drawbacks. The implantable venous Doppler offers rapid diagnosis of vascular compromise, but has a cumbersome transcutaneous wire and is reported to have high false positive rates largely due to inadvertent internal probe dislodgement. A wireless device would avoid these problems. This study describes the implementation of an entirely implanted Doppler sensor with wireless transmission of flow data in a pig femoral vein model.

Automatic Patency Discrimination in the Pig Bilateral Femoral Veins for Biomedical Implants

Free flap surgeries require hourly monitoring to detect vascular compromise. If not caught promptly, the flap can be lost. Monitoring free flaps using the gold standard requires experienced operators to interpret blood flow signatures, which are often difficult to distinguish from background noise. Previously reported hardware-only automatic patency classification showed a high sensitivity, specificity, and a low false-positive rate, but it was demonstrated using bulky discrete electronics and a syringe pump to generate the expected flow rates. In this paper, we investigate automatic hardware-only patency classification on blood flow data collected from the bilateral femoral veins during flow and occluded states using SPICE simulations in an IBM 130-nm CMOS process with a 1-V supply voltage and a 200-ms window length. Experimental results show a very high sensitivity (99.45%), specificity (99.93%), and very low false-positive rate (0.07275%) at just 8.715

Analysis of the Use of Low Molecular Weight Heparin for Thromboembolic Complication Chemoprophylaxis in Microsurgical Breast Reconstruction

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