Lightweight noninvasive trauma monitor for early indication of central hypovolemia and tissue acidosis: A review
ABSTRACT Hemorrhage is a major cause of soldier death; it must be quickly identified and appropriately treated. We developed a prototype patient monitor that noninvasively and continuously determines muscle oxygen saturation (SmO₂), muscle pH (pHm), and a regional assessment of blood volume (HbT) using near-infrared spectroscopy. Previous demonstration in a model of progressive, central hypovolemia induced by lower body negative pressure (LBNP) showed that SmO₂ provided an early indication of impending hemodynamic instability in humans. In this review, we expand the number of subjects and provide an overview of the relationship between the muscle and sublingual microcirculation in this model of compensated shock.
Healthy human volunteers (n = 30) underwent progressive LBNP in 5-minute intervals. Standard vital signs, along with stroke volume (SV), total peripheral resistance, functional capillary density, SmO₂, HbT, and pHm were measured continuously throughout the study.
SmO₂ and SV significantly decreased during the first level of central hypovolemia (-15 mm Hg LBNP), whereas vital signs were later indicators of impending cardiovascular collapse. SmO₂ declined with SV and inversely with total peripheral resistance throughout LBNP. HbT was correlated with declining functional capillary density, suggesting vasoconstriction as a cause for decreased SmO₂ and subsequently decreased pHm. CLINICAL TRANSLATION: The monitor has been miniaturized to a 58-g solid-state sensor that is currently being evaluated on patients with dengue hemorrhagic fever. Early results demonstrate significant decreases in SmO₂ similar to those observed with progressive reductions in central blood volume. As such, this technology has the potential to (1) provide a monitoring capability for both nontraumatic and traumatic hemorrhage and (2) help combat medics triage casualties and monitor patients during lengthy transport from combat areas.
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ABSTRACT: Currently available triage and monitoring tools are often late to detect life threatening clinically significant physiological aberrations and provide limited data in prioritizing bleeding patients for treatment and evacuation. The Compensatory Reserve Index (CRI) is a novel means of assessing physiologic reserve, shown to correlate with central blood volume loss under laboratory conditions. The purpose of this study was to compare the non-invasive CRI device to currently available vital signs in detecting blood loss. Study subjects were soldiers volunteering for blood donation(n=230) and the control group were soldiers who did not donate blood (n=34). Data collected before and following blood donation were compared, receiver operator characteristic curves were generated after either donation or the appropriate time interval, and areas under the curves (AUCs) were compared. Compared to pre-blood loss, blood donation resulted in a mean reduction of systolic BP by 3% (before = 123mmHg, after = 119 mmHg; P<0.01). CRI demonstrated a 16% reduction (before = 0.74, after =0.62; P<0.01). Heart rate, diastolic BP, and oxygen saturation remained unchanged. The AUC for change in CRI was 0.81, 0.56 for change in heart rate, 0.53 for change in systolic blood pressure, 0.55 and 0.58 for pulse pressure and shock index respectively. The AUCs for detecting mild blood loss at a single measurement were 0.73 for heart rate, 0.60 for systolic blood pressure, 0.62 for diastolic blood pressure, 0.45 for pulse oximetry, and 0.84 for CRI. CRI was better than standard indices in detecting mild blood loss. Single measurement of CRI may enable a more accurate triage, and CRI monitoring may allow for earlier detection of casualty deterioration.Shock (Augusta, Ga.) 03/2014; DOI:10.1097/SHK.0000000000000178 · 2.73 Impact Factor
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ABSTRACT: Humans are able to compensate for low-volume blood loss with minimal change in traditional vital signs. We hypothesized that a novel algorithm, which analyzes photoplethysmogram (PPG) wave forms to continuously estimate compensatory reserve would provide greater sensitivity and specificity to detect low-volume blood loss compared with traditional vital signs. The compensatory reserve index (CRI) is a measure of the reserve remaining to compensate for reduced central blood volume, where a CRI of 1 represents supine normovolemia and 0 represents the circulating blood volume at which hemodynamic decompensation occurs; values between 1 and 0 indicate the proportion of reserve remaining. Subjects underwent voluntary donation of 1 U (approximately 450 mL) of blood. Demographic and continuous noninvasive vital sign wave form data were collected, including PPG, heart rate, systolic blood pressure, cardiac output, and stroke volume. PPG wave forms were later processed by the algorithm to estimate CRI values. Data were collected from 244 healthy subjects (79 males and 165 females), with a mean (SD) age of 40.1 (14.2) years and mean (SD) body mass index of 25.6 (4.7). After blood donation, CRI significantly decreased in 92% (α = 0.05; 95% confidence interval [CI], 88-95%) of the subjects. With the use of a threshold decrease in CRI of 0.05 or greater for the detection of 1 U of blood loss, the receiver operating characteristic area under the curve was 0.90, with a sensitivity of 0.84 and specificity of 0.86. In comparison, systolic blood pressure (52%; 95% CI, 45-59%), heart rate (65%; 95% CI, 58-72%), cardiac output (47%; 95% CI, 40-54%), and stroke volume (74%; 95% CI, 67-80%) changed in fewer subjects, had significantly lower receiver operating characteristic area under the curve values, and significantly lower specificities for detecting the same volume of blood loss. Consistent with our hypothesis, CRI detected low-volume blood loss with significantly greater specificity than other traditional physiologic measures. These findings warrant further evaluation of the CRI algorithm in actual trauma settings. Diagnostic study, level II.Journal of Trauma and Acute Care Surgery 12/2014; 77(6):892-8. DOI:10.1097/TA.0000000000000423 · 1.97 Impact Factor
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ABSTRACT: New patient monitoring technologies can noninvasively and directly provide an assessment of the adequacy of tissue perfusion through the simultaneous determination of muscle oxygen saturation (SmO2) and muscle pH (pHm). Non-pulsatile near infrared spectroscopy is used to determine these microvascular parameters. Two separate studies were conducted using an isolated perfused swine limb preparation to widely vary venous blood oxygen saturation (SviO2) and pH (pHvi) to assess the accuracy of a noninvasive sensor with the capability to simultaneously measure both parameters. The isolated limb model is necessary to establish equilibrium between the venous output of the perfusion circuit and the venule measurement of the spectroscopic sensor. The average absolute difference between SmO2 and SviO2 determined over 50 conditions of SviO2 between 13% and 83% on 3 pig limbs was 3.8% and the coefficient of determination (R(2)) was 0.95. The average absolute difference between pHm and pHvi determined over 69 conditions of pHvi between pHvi 6.9 and pHvi 7.5 on 3 pig limbs was 0.045 pH units with an R(2) of 0.92. Measured accuracy was acceptable to support clinically relevant decision making for the assessment of impaired tissue perfusion and acidosis. Sensors were also evaluated on human subjects. There was no statistical difference in SmO2 by gender or location when multiple sensors were evaluated on the right and left calf, deltoid, and thigh of resting men and women (N = 33). SmO2 precision for subjects at rest was 5.6% over the six locations with four different sensors.Physiological Measurement 07/2013; 34(8):859-871. DOI:10.1088/0967-3334/34/8/859 · 1.62 Impact Factor