Robert W. Barbee

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.

Lactate profiles as a resuscitation assessment tool in a rat model of battlefield hemorrhage resuscitation.

Time profiles of arterial lactate concentrations have been proposed as markers for both the degree of physiological derangement during shock and effectiveness of clinical resuscitation, but have not been evaluated for use in short-term experimental protocols. We developed two quantitative mixed models of sequential arterial lactate concentrations to evaluate competing low-volume (<4 mL/kg) battlefield resuscitation therapies in a rat model of acute severe hemorrhagic shock: a simple linear additive model and a nonlinear mechanistic model that described lactate profiles in a continuous trajectory with a defined turning point. Data were obtained during a study evaluating a novel hemoglobin polymer (OxyVita) in a cocktail of hypertonic saline and Hextend as an alternative to standard Hextend. Fluids were either infused by titration to a mean systolic pressure of 60 mmHg or as a single bolus. Parameter estimates derived from both models were assessed for evidence of treatment efficacy and as indicators of short-term survival. A cocktail of hypertonic saline and Hextend was superior to standard Hextend in enhancing survival; however, lactate profiles did not differ between treatments. Regardless of resuscitation regimen, animals surviving to at least 60 min posthemorrhage can be discriminated from nonsurvivors by significantly lower peak lactates (a difference of at least 3 mM; P < 0.001), and all survivors exhibited a decline in lactate with resuscitation. Sequential measurements of lactate over relatively short time frames during resuscitation are of value in assessing both response to resuscitation and short-term mortality.

A pilot study to measure dynamic elasticity of the bladder during urodynamics

Previous studies using isolated strips of human detrusor muscle identified adjustable preload tension, a novel mechanism that acutely regulates detrusor wall tension. The purpose of this investigation was to develop a method to identify a correlate measure of adjustable preload tension during urodynamics.

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.

Acute length adaptation and adjustable preload in the human detrusor

The biomechanical properties of length adaptation and adjustable preload have been previously identified in detrusor smooth muscle in animal models. This in vitro study aims to show that human detrusor smooth muscle exhibits length adaptation and adjustable preload tension which could play an important role in both overactive bladder and detrusor underactivity.

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.

Metabolic Stress-Induced Activation of AMPK and Inhibition of Constitutive Phosphoproteins Controlling Smooth Muscle Contraction: Evidence for Smooth Muscle Fatigue?

Metabolic stress diminishes smooth muscle contractile strength by a poorly defined mechanism. To test the hypothesis that metabolic stress activates a compensatory cell signaling program to reversibly downregulate contraction, arterial rings and bladder muscle strips in vitro were deprived of O2 and glucose for 30 and 60 min (“starvation”) to induce metabolic stress, and the phosphorylation status of proteins involved in regulation of contraction and metabolic stress were assessed in tissues under basal and stimulated conditions. A 15–30 min recovery period (O2 and glucose repletion) tested whether changes induced by starvation were reversible. Starvation decreased basal phosphorylation of myosin regulatory light chain (MLC-pS19) and of the rho kinase (ROCK) downstream substrates cofilin (cofilin-pS3) and myosin phosphatase targeting subunit MYPT1 (MYPT1-pT696 and MYPT1-pT853), and abolished the ability of contractile stimuli to cause a strong, sustained contraction. Starvation increased basal phosphorylation of AMPK (AMPK-pT172) and 3 downstream AMPK substrates, acetyl-CoA carboxylase (ACC-pS79), rhoA (rhoA-pS188), and phospholamban (PLB-pS16). Increases in rhoA-pS188 and PLB-pS16 would be expected to inhibit contraction. Recovery restored basal AMPK-pT172 and MLC-pS19 to control levels, and restored contraction. In AMPKα2 deficient mice (AMPKα2-/-), the basal level of AMPK-pT172 was reduced by 50%, and MLC-pS19 was elevated by 50%, but AMPKα2-/- did not prevent starvation-induced contraction inhibition nor enhance recovery from starvation. These results indicate that constitutive AMPK activity participates in constitutive regulation of contractile proteins, and suggest that AMPK activation is necessary, but may not be sufficient, to cause smooth muscle contraction inhibition during metabolic stress.

Vascular smooth muscle desensitization in rabbit epigastric and mesenteric arteries during hemorrhagic shock

The decompensatory phase of hemorrhage (shock) is caused by a poorly defined phenomenon termed vascular hyporeactivity (VHR). VHR may reflect an acute in vivo imbalance in levels of contractile and relaxant stimuli favoring net vascular smooth muscle (VSM) relaxation. Alternatively, VHR may be caused by intrinsic VSM desensitization of contraction resulting from prior exposure to high levels of stimuli that temporarily adjusts cell signaling systems. Net relaxation, but not desensitization, would be expected to resolve rapidly in an artery segment removed from the in vivo shock environment and examined in vitro in a fresh solution. Our aim was to 1) induce shock in rabbits and apply an in vitro mechanical analysis on muscular arteries isolated pre- and postshock to determine whether VHR involves intrinsic VSM desensitization, and 2) identify whether net VSM relaxation induced by nitric oxide and cyclic nucleotide-dependent protein kinase activation in vitro can be sustained for some time after relaxant stimulus washout. The potencies of phenylephrine- and histamine-induced contractions in in vitro epigastric artery removed from rabbits posthemorrhage were decreased by ∼0.3 log units compared with the control contralateral epigastric artery removed prehemorrhage. Moreover, a decrease in KCl-induced tonic, relative to phasic, tension of in vitro mesenteric artery correlated with the degree of shock severity as assessed by rates of lactate and K(+) accumulation. VSM desensitization was also caused by tyramine in vivo and PE in vitro, but not by relaxant agents in vitro. Together, these results support the hypothesis that VHR during hemorrhagic decompensation involves contractile stimulus-induced long-lasting, intrinsic VSM desensitization.

Effects of vesical and perfusion pressure on perfusate flow, and flow on vesical pressure, in the isolated perfused working pig bladder reveal a potential mechanism for the regulation of detrusor compliance

Although there is evidence that deficits in bladder blood flow negatively impact bladder function, the effects of vesical, and perfusion pressures on bladder perfusion (perfusate flow), and of perfusate flow on vesical pressure, remain poorly understood. The present study used the isolated perfused working pig bladder model to examine the relationships between blood flow, and vesical and perfusion pressures.