Using support vector machines on photoplethysmographic signals to discriminate between hypovolemia and euvolemia.

Identifying trauma patients at risk of imminent hemorrhagic shock is a challenging task in intraoperative and battlefield settings given the variability of traditional vital signs, such as heart rate and blood pressure, and their inability to detect blood loss at an early stage. To this end, we acquired N = 58 photoplethysmographic (PPG) recordings from both trauma patients with suspected hemorrhage admitted to the hospital, and healthy volunteers subjected to blood withdrawal of 0.9 L. We propose four features to characterize each recording: goodness of fit (r2), the slope of the trend line, percentage change, and the absolute change between amplitude estimates in the heart rate frequency range at the first and last time points. Also, we propose a machine learning algorithm to distinguish between blood loss and no blood loss. The optimal overall accuracy of discriminating between hypovolemia and euvolemia was 88.38%, while sensitivity and specificity were 88.86% and 87.90%, respectively. In addition, the proposed features and algorithm performed well even when moderate blood volume was withdrawn. The results suggest that the proposed features and algorithm are suitable for the automatic discrimination between hypovolemia and euvolemia, and can be beneficial and applicable in both intraoperative/emergency and combat casualty care.

Learning curves in emergency ultrasound education.

OBJECTIVES: Proficiency in the use of bedside ultrasound (US) has become standard in emergency medicine residency training. While milestones have been established for this training, supporting data for minimum standard experience are lacking. The objective of this study was to characterize US learning curves to identify performance plateaus for both image acquisition and interpretation, as well as compare performance characteristics of learners to those of expert sonographers. METHODS: A retrospective review of an US database was conducted at a single academic institution. Each examination was scored for agreement between the learner and expert reviewer interpretation and given a score for image quality. A locally weighted scatterplot smoothing method was used to generate a model of predicted performance for each individual examination type. Performance characteristics for expert sonographers at the site were also tracked and used in addition to performance plateaus as benchmarks for learning curve analysis. RESULTS: There were 52,408 US examinations performed between May 2007 and January 2013 and included for analysis. Performance plateaus occurred at different points for different US protocols, from 18 examinations for soft tissue image quality to 90 examinations for right upper quadrant image interpretation. For the majority of examination types, a range of 50 to 75 examinations resulted in both excellent interpretation (sensitivity > 84% and specificity > 90%) and good image quality (90% the image quality benchmark of expert sonographers). CONCLUSIONS: Educational performance benchmarks occur at variable points for image interpretation and image quality for different examination types. These data should be considered when developing training standards for US education as well as experience requirements for US credentialing.

Correlation of corrected flow time in the carotid artery with changes in intravascular volume status.

PURPOSE: Assessment of volume status remains a challenge in critical care. Our purpose was to determine if Doppler waveform analysis of carotid artery blood flow correlates with changes in volume status. MATERIALS AND METHODS: Dehydrated patients receiving an intravenous fluid bolus were enrolled with exclusions including age less than 18 years, pregnancy, vasopressor administration, or atrial fibrillation. Ultrasound examination with Doppler analysis of the carotid artery was performed with measurements taken to calculate corrected flow time (FTc). Corrected flow time, mean arterial pressure, and pulse rate before and after fluid administration were compared using Wilcoxon matched-pairs signed rank test. RESULTS: Fifty-six patients were enrolled with mean fluid administration of 1110 mL. Corrected flow time increased with fluid resuscitation from prefluid mean of 299 milliseconds (95% confidence interval [CI], 282-317 milliseconds) to a postfluid mean of 340 milliseconds (95% CI, 323-358 milliseconds) (P<.0001). Mean percentage change in FTc was 14.9% (95% CI, 8.4-21.3). There were no significant changes in mean arterial pressure or heart rate from pre- to post-fluid administration. CONCLUSION: Intravenous fluid administration in dehydrated patients resulted in significant changes in FTc in the carotid artery despite no change in vital signs. Corrected flow time measured from carotid arterial blood flow may be a useful means of assessing volume status in volume-depleted patients.

Inferior vena cava displacement during respirophasic ultrasound imaging.

BACKGROUND: Ultrasound measurement of dynamic changes in inferior vena cava (IVC) diameter can be used to assess intravascular volume status in critically ill patients, but published studies vary in accuracy as well as recommended diagnostic cutoffs. Part of this variability may be related to movements of the vessel relative to the transducer during the respiratory cycle which results in unintended comparison of different points of the IVC at end expiration and inspiration, possibly introducing error related to variations in normal anatomy. The objective of this study was to quantify both craniocaudal and mediolateral movements of the IVC as well as the vessel's axis of collapse during respirophasic ultrasound imaging. METHODS: Patients were enrolled from a single urban academic emergency department with ultrasound examinations performed by sonographers experienced in IVC ultrasound. The IVC was imaged from the level of the diaphragm along its entire course to its bifurcation with diameter measurements and respiratory collapse measured at a single point inferior to the confluence of the hepatic veins. While imaging the vessel in its long axis, movement in a craniocaudal direction during respiration was measured by tracking the movement of a fixed point across the field of view. Likewise, imaging the short axis of the IVC allowed for measurement of mediolateral displacement as well as the vessel's angle of collapse relative to vertical. RESULTS: Seventy patients were enrolled over a 6-month period. The average diameter of the IVC was 13.8 mm (95% CI 8.41 to 19.2 mm), with a mean respiratory collapse of 34.8% (95% CI 19.5% to 50.2%). Movement of the vessel relative to the transducer occurred in both mediolateral and craniocaudal directions. Movement was greater in the craniocaudal direction at 21.7 mm compared to the mediolateral movement at 3.9 mm (p < 0.001). Angle of collapse assessed in the transverse plane averaged 115° (95% CI 112° to 118°). CONCLUSIONS: Movement of the IVC occurs in both mediolateral and craniocaudal directions during respirophasic ultrasound imaging. Further, collapse of the vessel occurs not at true vertical (90°) but 25° off this axis. Technical approach to IVC assessment needs to be tailored to account for these factors.

Identification of congestive heart failure via respiratory variation of inferior vena cava diameter.

INTRODUCTION: Rapid diagnosis of volume overload in patients with suspected congestive heart failure (CHF) is necessary for the timely administration of therapeutic agents. We sought to use the measurement of respiratory variation of inferior vena cava (IVC) diameter as a diagnostic tool for identification of CHF in patients presenting with acute dyspnea. METHODS: The IVC was measured sonographically during a complete respiratory cycle of 46 patients meeting study criteria. Percentage of respiratory variation of IVC diameter was compared to the diagnosis of CHF or alternative diagnosis. RESULTS: Respiratory variation of IVC was smaller in patients with CHF (9.6%) than without CHF (46%) and showed good diagnostic accuracy with area under the receiver operating characteristic curve of 0.96. Receiver operating characteristic curve analysis showed optimum cutoff of 15% variation or less of IVC diameter with 92% sensitivity and 84% specificity for the diagnosis of CHF. CONCLUSION: Inferior vena cava ultrasound is a rapid, reliable means for identification of CHF in the acutely dyspneic patient.

Correlation of visual axis and coronal axis measurements of the optic nerve sheath diameter.

OBJECTIVE: There are several approaches to sonographic imaging and measurement of the optic nerve sheath diameter (ONSD). In this study, we sought to compare visual axis measurements of the optic nerve sheath (ONS) with traditional coronal axis measurements to assess for correlation. METHODS: The ONS was visualized in 2 views on both eyes of healthy volunteers using an 8.5-MHz probe. Coronal axis views were obtained with the probe placed at the lateral canthus directed nasally posterior to the globe. Subsequent imaging was made along the midline visual axis. The diameter in this view was measured at several points posterior to the sclera (2, 3, 6, 9, 12, and 15 mm). RESULTS: Twenty-seven subjects were enrolled (54 scans). There was a significant difference between ONSDs measured in each axis, with a coronal axis mean diameter of 3.4 mm and visual axis mean diameters at 2, 3, 6, 9, 12, and 15 mm of 4.28, 4.32, 5.15, 5.74, 6.39, and 7.42 mm, respectively (P < .05). The Pearson coefficient showed no correlation between coronal axis and visual axis measurements, with R values ranging from 0.51 to 0.69. There was a statistically significant increase in the ONSD as the nerve coursed posteriorly when measured in the visual axis. CONCLUSIONS: Visual axis measurements do not reliably correlate with coronal axis measurements. The consistently larger diameter measured in the visual axis as well as the gradually increasing diameter posteriorly suggests measurement of an artifactual shadow rather than the true ONS.

Brief apnea induces myocardial ischemic tolerance by an opioid-insensitive mechanism.

BACKGROUND: Intermittent brief "preconditioning" (PC) ischemia has been shown to render the heart resistant to a subsequent sustained ischemic insult, in part through an opioid-dependent mechanism. Using the rabbit model, we tested the hypothesis that intermittent in vivo apnea elicits a cardioprotective response similar to that achieved with conventional PC ischemia. In addition, we sought to determine if infarct size reduction seen in this model was stimulated via opioid receptor activation. METHODS: Anesthetized, intubated rabbits (n=35) were randomized to receive three 4.5-min bouts of apnea interspersed with 5 min normal ventilation or time-matched standard ventilation (controls). Upon completion of the in vivo PC/control period, the hearts were excised and assessed for ischemic tolerance on a modified Langendorff apparatus (40 min global ischemia+2h reperfusion). To assess the contribution of opioid receptor stimulation, two additional control and PC groups received the nonspecific opioid antagonist naloxone (10 mg/kg) prior to the in vivo intervention phase. Infarct size (delineated by tetrazoliam staining and expressed as a percentage of the left ventricle [LV]) was compared among the four groups by ANOVA. RESULTS: Infarct size was significantly reduced in hearts that received antecedent apneic PC when compared with controls (63+/-5% vs. 34+/-8%) of the LV, respectively; P<.05). Pretreatment with naloxone had no significant effect on infarct size in nonpreconditioned hearts (80+/-6%) and did not inhibit the protective effects of apnea-induced PC (52+/-10% in naloxone+PC group). CONCLUSIONS: Intermittent apnea evokes significant myocardial ischemic tolerance through an opioid-insensitive mechanism.

Myocardial preconditioning factors evoke mesenteric ischemic tolerance via opioid receptors and K(ATP) channels.

We have shown that a reverse-phase concentrate generated from the effluent of preconditioned (PC) rabbit hearts evokes a cardioprotective effect in virgin acceptor hearts. With the use of a model of sustained (1 h) simulated ischemia in isolated, spontaneously contracting rabbit jejunum, our current aims were to 1) determine whether protective factor(s) released from PC hearts can improve ischemic tolerance in noncardiac tissue; and 2) obtain preliminary insight into the mediator(s) involved in triggering and eliciting this remote protection. Recovery of contractile force following reoxygenation (our index of ischemic tolerance) was enhanced in jejunal segments pretreated with concentrate generated from PC hearts (33 +/- 3% of baseline, P < 0.01) versus segments that received no concentrate (21 +/- 2%) and segments treated with concentrate from normoxic hearts (16 +/- 3%; P < 0.01). Protection achieved with PC concentrate was attenuated by coadministration of naloxone or glibenclamide, thereby implicating the involvement of opioids and ATP-sensitive potassium channels. Moreover, evaluation of purified subfractions of the crude PC concentrate identified a specific bioactive fraction that may participate in triggering the improved jejunal ischemic tolerance.

Delta opiates increase ischemic tolerance in isolated rabbit jejunum.

UNLABELLED: Mammalian hibernation is mediated by humoral agonists of the delta opioid receptor (DOR). Moreover, transfer of either humoral or synthetic DOR agonists to non-hibernators reportedly induces a state of improved myocardial ischemic tolerance. OBJECTIVE: To determine whether the DOR agonist D-Ala 2, D-Leu 5, enkephalin (DADLE) similarly elicits protection in noncardiac-i.e., mesenteric-tissue. METHODS: In Protocols 1 and 2, the authors developed and characterized an in vitro model of mesenteric ischemia/reperfusion in isolated rabbit jejunum by documenting the effect of increasing ischemic duration (0 to 120 minutes) and the relative importance of glucose and/or oxygen deprivation on the evolution of jejunal injury. In Protocol 3, jejunal segments were randomized to receive either no treatment (controls) or 15 minutes of pretreatment with 1 microM DADLE, followed by 60 minutes of simulated ischemia and 30 minutes of reperfusion. Jejunal injury was quantified by repeated, time-matched assessment of peak contractile force evoked by 1 microM acetylcholine (all protocols) and delineation of tissue necrosis (Protocol 1). RESULTS: Development of significant jejunal injury required combined oxygen/glucose deprivation. Moreover, there was a direct relationship between ischemic duration and tissue injury, and a significant inverse correlation between reperfusion contractile force (% of baseline) and the extent of smooth muscle necrosis (r(2) = 0.87; p < 0.01). Most notably, mesenteric ischemia/reperfusion injury was attenuated by DADLE: reperfusion contractile force was 47 +/- 5% versus 36 +/- 5% in DADLE-treated versus control segments (p < 0.01). CONCLUSIONS: Treatment with the delta opioid agonist DADLE increases ischemic tolerance of isolated rabbit jejunum.