Mohamad H. Tiba

Oxygen transport characterization of a human model of progressive hemorrhage

BACKGROUND Hemorrhage continues to be a leading cause of death from trauma sustained both in combat and in the civilian setting. New models of hemorrhage may add value in both improving our understanding of the physiologic responses to severe bleeding and as platforms to develop and test new monitoring and therapeutic techniques. We examined changes in oxygen transport produced by central volume redistribution in humans using lower body negative pressure (LBNP) as a potential mimetic of hemorrhage. METHODS AND RESULTS In 20 healthy volunteers, systemic oxygen delivery and oxygen consumption, skeletal muscle oxygenation and oral mucosa perfusion were measured over increasing levels of LBNP to the point of hemodynamic decompensation. With sequential reductions in central blood volume, progressive reductions in oxygen delivery and tissue oxygenation and perfusion parameters were noted, while no changes were observed in systemic oxygen uptake or markers of anaerobic metabolism in the blood (e.g., lactate, base excess). While blood pressure decreased and heart rate increased during LBNP, these changes occurred later than the reductions in tissue oxygenation and perfusion. CONCLUSIONS These findings indicate that LBNP induces changes in oxygen transport consistent with the compensatory phase of hemorrhage, but that a frank state of shock (delivery-dependent oxygen consumption) does not occur. LBNP may therefore serve as a model to better understand a variety of compensatory physiological changes that occur during the pre-shock phase of hemorrhage in conscious humans. As such, LBNP may be a useful platform from which to develop and test new monitoring capabilities for identifying the need for intervention during the early phases of hemorrhage to prevent a patients progression to overt shock.

Resonance Raman spectroscopy: a new technology for tissue oxygenation monitoring.

Objective:To evaluate resonance Raman spectroscopy for the detection of changes in sublingual mucosal hemoglobin oxygen saturation (Smo2) in response to hemorrhage and resuscitation, and to compare Smo2 with other indicators of tissue oxygenation including central venous oxygen saturation (Scvo2), lactate, base excess, and shed blood volume. Design:Prospective single group pilot study. Setting:University laboratory. Subjects:Five Sprague-Dawley rats. Interventions:Animals were anesthetized and instrumented for measurement of arterial and central venous blood gases. Raman spectroscopy was performed using a krypton ion laser providing excitation at 406.7 nm (5 mW). A 1-mm2 region of the sublingual tongue surface was chosen for investigation. Animals were subjected to stepwise hemorrhage until approximately 50% of the blood volume was removed. At each hemorrhage and resuscitation interval, Raman spectroscopy was performed and corresponding arterial and central venous blood gas and lactate measurements were made. Smo2 was calculated as the ratio of the oxygenated heme spectral peak height to the sum of the oxy- and deoxyhemoglobin spectral peak heights. Raman spectroscopy-derived Smo2 measurements were compared with Scvo2 as well as with other indicators of oxygenation. Measurements and Main Results:The mean difference between Smo2 and Scvo2 for all paired measurements was 5.8 ± 11.7 absolute saturation points. Smo2 was significantly (p < .0001) correlated with Scvo2 (r = .80), lactate (r = −.78), base excess (r = .80), and shed blood volume (r = −.75). Smo2 and Scvo2 showed similar levels of precision for predicting elevated lactate and base deficit. Conclusions:These studies demonstrate the ability of Raman spectroscopy to noninvasively track microvascular hemoglobin oxygenation in tissue and favorably correlate with other important indicators of tissue oxygenation such as Scvo2, lactate, base deficit, and shed blood volume. The technique shows promise as a method to noninvasively monitor tissue oxygenation.

Image processing and machine learning for diagnostic analysis of microcirculation

This study focuses on detection of capillaries and small blood vessels in the videos recorded from the lingual surface using Microscan SDF system. The purpose of this study is to quantitatively monitor and assess the changes that occur in microcirculation during resuscitation period. The results assist physicians in making diagnostically and therapeutically important decisions such as determination of the effectiveness of the resuscitation process. The proposed algorithm applies advanced digital image processing methods to provide quantitative assessment of video signals for detection and characterization of capillaries. The objective of the algorithm is to segment capillaries, estimate the presence and velocity of Red Blood Cells (RBCs), and identify the distribution of blood flow in capillaries for a variety of normal and abnormal cases. The algorithm first, stabilizes each frame to follow the variations in the consecutive frames. Then, time-averaging techniques are applied to the frames to reduce the motion artifact. Histogram equalization, wavelet transform, and median filtering are the subsequent steps applied to accurately detect the blood vessels in each frame. In order to estimate the velocity of RBCs, space time diagrams are obtained through cross-correlation calculations. This study aims to reduce the human interaction as well as the computation time.

Tissue oxygenation monitoring using resonance Raman spectroscopy during hemorrhage.

BACKGROUND The ability to monitor the patient of hemorrhage noninvasively remains a challenge. We examined the ability of resonance Raman spectroscopy to monitor tissue hemoglobin oxygenation (RRS-StO2) during hemorrhage and compared its performance with conventional invasive mixed venous (SmvO2) and central venous (ScvO2) hemoglobin oxygen saturation as well as with near-infrared spectroscopy tissue hemoglobin oxygenation (NIRS-StO2). METHODS Five male swine were anesthetized and instrumented followed by hemorrhage at a rate of 30 mL/min for 60 minutes. RRS-StO2 was continuously measured from the buccal mucosa, and NIRS-StO2 was continuously measured from the forelimb. Paired interval measures of SmvO2, ScvO2, and lactate were made. Pearson correlation was used to quantify the degree to which any two variables are related. Receiver operating characteristic (ROC) area under the curve values were used for pooled data for RRS-StO2, NIRS-StO2, SmvO2, and ScvO2 to compare performance in the ability of tissue oxygenation methods to predict the presence of an elevated arterial blood lactate level. RESULTS Sequential RRS-StO2 changes tracked changes in SmvO2 (r = 0.917; 95% confidence interval [CI], 0.867–0.949) and ScvO2 (r = 0.901; 95% CI, 0.828–0.944) during hemorrhage, while NIRS-StO2 failed to do so for SmvO2 (r = 0.283; 95% CI, 0.04919–0.4984) and ScvO2 (r = 0.142; 95% CI, −0.151 to 0.412). ROC curve performance of oxygenation measured to indicate lactate less than or greater than 3 mM yielded the following ROC area under the curve values: SmvO2 (1.0), ScvO2 (0.994), RRS-StO2 (0.972), and NIRS-StO2 (0.611). CONCLUSION RRS-StO2 seems to have significantly better ability to track central oxygenation measures during hemorrhage as well as to predict shock based on elevated lactate levels when compared with NIRS-StO2.

Extraction of Respiratory Rate from Impedance Signal Measured on Arm: A Portable Respiratory Rate Measurement Device

In this paper, respiratory rate is extracted using signal processing and machine learning methods from electrical impedance, measured across arm. Two pairs of electrodes have been used along the arm, one for injecting the current, and one for sensing the voltage. After filtering, the frequency components and other signal features have been extracted using Short Time Fourier Transform (STFT). Then aSupport Vector Machine(SVM) model is trained to detect the breath-holding state. Frequency components and signal features of the parts of the signal that are detected to be representing the breathing state are then fed into another SVM model that extracts the respiratory rate and reduces the effect of motion artifacts. A similar method has been applied to the signal taken from end-tidal CO2 respiratory measurement device as the reference signal. This signal has been used as the ground truth for training of the SVM model and for validation of the method. The results are validated using 5-fold cross-validation method. Statistical analysis confirms the significance of the introduced features.

Effects of a combination hemoglobin based oxygen carrier–hypertonic saline solution on oxygen transport in the treatment of traumatic shock

BACKGROUND Logistics complicate fluid resuscitation of traumatic shock on the battlefield. Traumatic shock can result in oxygen debt (O(2)D) accumulation that is fatal. However, the ability of fluid strategies to repay O(2)D are not commonly reported. This pilot study examined various resuscitation fluids, including a combination of PEGylated bovine hemoglobin and hypertonic saline (AfterShock™) on their ability to repay O(2)D in traumatic shock. METHODS 41 anesthetized swine underwent hemorrhage to an O(2)D of 80 mL/kg. Animals received one of the following: 500 mL whole blood, 500 mL AfterShock™, 500 mL hypertonic (7.2%) saline, 250 mL hypertonic (7.2%) saline, 500 mL Hetastarch (6%), or 500 mL lactated Ringers. Oxygen transport variables (O(2)D, oxygen consumption, oxygen delivery, central venous hemoglobin oxygen saturation, oxygen extraction ratios), lactate clearance, and survival were monitored for 3h after treatment. Data were analyzed using mixed-model ANOVA and comparisons were made to the performance of whole blood. RESULTS Only animals receiving AfterShock™, 500 mL hypertonic saline, and 500 mL Hetastarch survived to 180 min. While not statistically significant AfterShock™ demonstrated trends in improving the repayment of O(2)D and in improving oxygen transport variables despite having lower levels of global oxygen delivery compared to whole blood, Hetastarch and 500 mL hypertonic saline groups. CONCLUSION Use of 500 mL AfterShock™, 500 mL of 7.2% saline or 500 mL of Hetastarch resulted in improved short-term survival. While not statistically significant, AfterShock™ demonstrated trends in improving O(2)D. These findings may have implications for designing resuscitation fluids for combat casualty care.

A novel noninvasive impedance-based technique for central venous pressure measurement

Knowledge of central venous pressure (CVP) is considered valuable in the assessment and treatment of various states of critical illness and injury. We tested a noninvasive means of determining CVP (NICVP) by monitoring upper arm blood flow changes in response to externally applied circumferential pressure to the upper arm veins. Thirty-six patients who were undergoing CVP monitoring as part of their care had NICVP determined and compared with CVP. Volume changes were measured in the upper arm using tetra-polar impedance plethysmography underneath a blood pressure cuff. The cuff was inflated over 5 s to a pressure greater than CVP but less than diastolic arterial pressure. After 45 to 60 s, the cuff was rapidly deflated. Noninvasive CVP was determined as the cuff pressure noted at the maximum derivative of the volume increase under the cuff during deflation. Noninvasive CVP was then compared with invasively measured CVP taken during the same period by Bland-Altman analysis. A total of 108 trials (three per subject) were performed on 36 patients. Mean bias was −0.26 mmHg (95% confidence interval [CI]: −0.67, 0.15). Limits of agreement were −2.7 and 2.2 mmHg with the 95% CI for the lower limit of agreement (−3.4, −2.0 mmHg) and for the upper limit of agreement (1.5, 2.9 mmHg). Correlation between CVP and NICVP was 0.95 (95% CI: 0.93 to 0.97; P < 0.0001). Noninvasive CVP as determined in this study may be a clinically useful substitute for traditional CVP measurement and may offer a tool for early diagnosis and treatment of acute states in which knowledge of CVP would be helpful.

Use of pelvic hemostasis belt to control lethal pelvic arterial hemorrhage in a swine model.

BACKGROUND Hemorrhage is the leading cause of death for both civilian and battlefield injuries. Hemorrhage from pelvic vascular wounds is of concern since it is difficult to control before surgical intervention. This has resulted in renewed interest in developing presurgical endovascular approaches to hemorrhage control. However, it is likely that other short-term techniques may be needed as a bridge to such approaches. We tested a prototype device called the pelvic hemostasis belt (PHB) for its ability to reduce or halt blood flow in a lethal model of pelvic arterial injury. METHODS Seventeen male swine, 42 (5.2)kg were anesthetized, instrumented, and then randomized into three groups (control, military antishock trousers [MAST], and PHB). Animals underwent laparotomy with placement of a 4-0 stainless steel monofilament suture through the right iliac artery. The laparotomy was closed, and the iliac suture was exteriorized. Hemorrhage was produced by pulling the suture through the iliac artery. In both PHB and MAST groups, the devices were applied over the pelvis and lower abdomen for 60 minutes, followed by release and monitoring for 30 minutes or until the animal expired. Hetastarch (500 mL) was infused immediately after commencement of hemorrhage. RESULTS All PHB group animals and only two from the MAST group survived for 60 minutes. Mean (SD) survival time for the control group was 13 (12.3) minutes. Log-rank (Mantel-Cox) survival analysis demonstrated a significant difference in survival time when comparing all groups (p < 0.0001) as well as when comparing PHB and MAST groups (p = 0.018). Significant differences were noted between groups in mean arterial pressure, lactate, and central venous hemoglobin oxygen saturation levels. CONCLUSION The PHB was successful in improving survival for 60 minutes after a lethal vascular injury. Such a device may be helpful to bridge endovascular methods of hemorrhage control.

Dynamic Limb Bioimpedance and Inferior Vena Cava Ultrasound in Patients Undergoing Hemodialysis

Assessment of volume status in critically ill patients poses a challenge to clinicians. Measuring changes in the inferior vena cava (IVC) diameter using ultrasound is becoming a standard tool to assess volume status. Ultrasound requires physicians with significant training and specialized expensive equipment. It would be of significant value to be able to obtain this measurement continuously without physician presence. We hypothesize that dynamic changes in limb’s bioimpedance in response to respiration could be used to predict changes in IVC. Forty-six subjects were tested a hemodialysis session. Impedance was measured via electrodes placed on the arm. Simultaneously, the IVC diameter was assessed by ultrasound. Subjects were asked to breathe spontaneously and perform respiratory maneuvers using a respiratory training device. Impedance (dz) was determined and compared with change in IVC diameter (dIVC; r = 0.76, p < 0.0001). There was significant relationship between dz and dIVC (p< 0.0001). Receiver-operator curves for dz at thresholds of dIVC (20% to70%) demonstrated high predictive power with areas under the curves (0.87–0.99, p < 0.0001). This evaluation suggests that real-time dynamic changes in limb impedance are capable of tracking a wide range of dynamic dIVC. This technique might be a suitable surrogate for monitoring real-time changes in dIVC to assess intravascular volume status.

Continuous Inferior Vena Cava Diameter Tracking through an Iterative Kanade–Lucas–Tomasi-Based Algorithm

Ultrasound assessment of the respiratory-induced change in size of the inferior vena cava is a useful technique in the evaluation and management of critically ill patients. We have developed an automated technique based on the Kanade-Lucas-Tomasi feature tracker and pyramidal segmentation to continuously track the diameter of the inferior vena cava during ultrasound. To test the accuracy of this automated process, the inferior vena cava of 47 spontaneously breathing patients were measured by trained ultrasound physicians and compared against the results obtained via the automated tracking. Good agreement between the techniques was found, with intra-class correlation coefficients for maximum vessel diameter, minimum diameter and caval index of 0.897, 0.967 and 0.975, respectively. More than 95% of the difference between physicians and automated measurements agreed to within 10% of the inferior vena cava collapse. Furthermore a phenomenon of cardiac collapsibility index variability was observed and reported. The accuracy and precision of this algorithmic technique provide a foundation for future automated measures for critical care ultrasound.