Critical Closing Pressure by Diffuse Correlation Spectroscopy in a Neonatal Piglet Model.

The critical closing pressure (CrCP) of the cerebral vasculature is the arterial blood pressure (ABP) at which cerebral blood flow (CBF) ceases. Because the ABP of preterm infants is low and close to the CrCP, there is often no CBF during diastole. Thus, estimation of CrCP may become clinically relevant in preterm neonates. Transcranial Doppler (TCD) ultrasound has been used to estimate CrCP in preterm infants. Diffuse correlation spectroscopy (DCS) is a continuous, noninvasive optical technique that measures microvascular CBF. Our objective was to compare and validate CrCP measured by DCS versus TCD ultrasound. Hemorrhagic shock was induced in 13 neonatal piglets, and CBF was measured continuously by both modalities. CrCP was calculated using a model of cerebrovascular impedance, and CrCP determined by the two modalities showed good correlation by linear regression, median r 2 = 0.8 (interquartile range (IQR) 0.71-0.87), and Bland-Altman analysis showed a median bias of -3.5 (IQR -4.6 to -0.28). This is the first comparison of CrCP determined by DCS versus TCD ultrasound in a neonatal piglet model of hemorrhagic shock. The difference in CrCP between the two modalities may be due to differences in vasomotor tone within the microvasculature of the cerebral arterioles versus the macrovasculature of a major cerebral artery.

Observed and calculated cerebral critical closing pressure are highly correlated in preterm infants.

BACKGROUND: Cerebrovascular critical closing pressure (CrCP) is the arterial blood pressure (ABP) at which cerebral blood flow ceases. Preterm ABP is low and close to CrCP. The diastolic closing margin (diastolic ABP minus CrCP) has been associated with intraventricular hemorrhage in preterm infants. CrCP is estimated from middle cerebral artery cerebral blood flow velocity (CBFV) and ABP waveforms. However, these estimations have not been validated due to a lack of gold standard. Direct observation of the CrCP in preterm infants with hypotension is an opportunity to validate synchronously estimated CrCP. METHODS: ABP and CBFV tracings were obtained from 24 extremely low birth weight infants. Recordings where diastolic CBFV was zero were identified. The gold standard CrCP was delineated using piecewise regression of ABP and CBFV values paired by rank ordering and then estimated using a published formula. The measured and estimated values were compared using linear regression and Bland-Altman analysis. RESULTS: Linear regression showed a high degree of correlation between measured and calculated CrCP (r2 = 0.93). CONCLUSIONS: This is the first study to validate a calculated CrCP by comparing it to direct measurements of CrCP from preterm infants when ABP is lower than CrCP.

Elevated Diastolic Closing Margin Is Associated with Intraventricular Hemorrhage in Premature Infants.

OBJECTIVE: To determine whether the diastolic closing margin (DCM), defined as diastolic blood pressure minus critical closing pressure, is associated with the development of early severe intraventricular hemorrhage (IVH). STUDY DESIGN: A reanalysis of prospectively collected data was conducted. Premature infants (gestational age 23-31 weeks) receiving mechanical ventilation (n = 185) had ∼1-hour continuous recordings of umbilical arterial blood pressure, middle cerebral artery cerebral blood flow velocity, and PaCO2 during the first week of life. Models using multivariate generalized linear regression and purposeful selection were used to determine associations with severe IVH. RESULTS: Severe IVH (grades 3-4) was observed in 14.6% of the infants. Irrespective of the model used, Apgar score at 5 minutes and DCM were significantly associated with severe IVH. A clinically relevant 5-mm Hg increase in DCM was associated with a 1.83- to 1.89-fold increased odds of developing severe IVH. CONCLUSION: Elevated DCM was associated with severe IVH, consistent with previous animal data showing that IVH is associated with hyperperfusion. Measurement of DCM may be more useful than blood pressure in defining cerebral perfusion in premature infants.

The Upper Limit of Cerebral Blood Flow Autoregulation Is Decreased with Elevations in Intracranial Pressure.

BACKGROUND: The upper limit of cerebrovascular pressure autoregulation (ULA) is inadequately characterized. We sought to delineate the ULA in a neonatal swine model. METHODS: Neonatal piglets with sham surgery (n = 9), interventricular fluid infusion (INF; n = 10), controlled cortical impact (CCI; n = 10), or impact + infusion (CCI + INF; n = 11) had intracranial pressure monitoring and bilateral cortical laser-Doppler flux recordings during arterial hypertension until lethality. An increase in red cell flux as a function of cerebral perfusion pressure was determined by piecewise linear regression and static rates of autoregulation (SRoRs) were determined above and below this inflection. RESULTS: When identified, the ULA (median [interquartile range]) was as follows: sham group: 102 mmHg (97-109), INF group: 75 mmHg (52-84), CCI group: 81 mmHg (69-101), and CCI + INF group: 61 mmHg (52-57; p = 0.01). Both groups with interventricular infusion had significantly lower ULA compared with the sham group. CONCLUSION: Neonatal piglets without intracranial pathological conditions tolerated acute hypertension, with minimal perturbation of cerebral blood flow. Piglets with acutely elevated intracranial pressure, with or without trauma, demonstrated loss of autoregulation when subjected to arterial hypertension.

The Diastolic Closing Margin Is Associated with Intraventricular Hemorrhage in Premature Infants.

Premature infants are at an increased risk of intraventricular hemorrhage (IVH). The roles of hypotension and hyperemia are still debated. Critical closing pressure (CrCP) is the arterial blood pressure (ABP) at which cerebral blood flow (CBF) ceases. When diastolic ABP is equal to CrCP, CBF occurs only during systole. The difference between diastolic ABP and CrCP is the diastolic closing margin (DCM). We hypothesized that a low DCM was associated with IVH. One hundred eighty-six premature infants, with a gestational age (GA) range of 23-33 weeks, were monitored with umbilical artery catheters and transcranial Doppler insonation of middle cerebral artery flow velocity for 1-h sessions over the first week of life. CrCP was calculated linearly and using an impedance model. A multivariate generalized linear regression model was used to determine associations with severe IVH (grades 3-4). An elevated DCM by either method was associated with IVH (p < 0.0001 for the linear method; p < 0.001 for the impedance model). Lower 5-min Apgar scores, elevated mean CBF velocity, and lower mean ABP were also associated with IVH (p < 0.0001). Elevated DCM, not low DCM, was associated with severe IVH in this cohort.

Computerized physician order entry improves compliance with a manual exchange transfusion protocol in the pediatric intensive care unit.

AIM: To evaluate the use of a computerized physician order entry (CPOE) protocol on manual red blood cell (RBC) exchange transfusion in critically ill children with sickle cell disease. METHODS: We conducted a retrospective study of children with sickle cell disease who received a manual RBC exchange transfusion before (2001 to 2008, n=22) and after (2008 to 2009, n=11) implementation of a CPOE protocol. Outcomes included compliance with protocol, percentage reduction in sickle hemoglobin, and peak hemoglobin during exchange. RESULTS: Compliance with the manual exchange protocol improved after introduction of CPOE (pre-CPOE: 20 protocol violations vs. post-CPOE: 3 violations, P=0.02). Percentage reduction in sickle hemoglobin also improved (pre-CPOE: 55% vs. post-CPOE: 70%, P=0.04), whereas peak hemoglobin during RBC exchange was similar (pre-CPOE: 12.0 g/dL vs. post-CPOE: 11.5 g/dL, P=0.25). However, hemoglobin levels after the mean of 7 hours of exchange were significantly higher pre-CPOE (pre-CPOE: 11.5 g/dL vs. post-CPOE: 10.5 g/dL, P=0.006). CONCLUSIONS: Use of CPOE for manual RBC exchange transfusion in children is associated with improved protocol compliance, improved reduction of sickle hemoglobin, and better maintenance of hemoglobin levels in a goal range during prolonged exchanges.

Glial fibrillary acidic protein in children with congenital heart disease undergoing cardiopulmonary bypass.

OBJECTIVE: To determine whether blood levels of the brain-specific biomarker glial fibrillary acidic protein rise during cardiopulmonary bypass for repair of congenital heart disease. METHODS: This is a prospective observational pilot study to characterise the blood levels of glial fibrillary acidic protein during bypass. Children <21 years of age undergoing bypass for congenital heart disease at Johns Hopkins Hospital and Texas Children's Hospital were enrolled. Blood samples were collected during four phases: pre-bypass, cooling, re-warming, and post-bypass. RESULTS: A total of 85 patients were enrolled between October, 2010 and May, 2011. The median age was 0.73 years (range 0.01-17). The median weight was 7.14 kilograms (range 2.2-86.5). Single ventricle anatomy was present in 18 patients (22%). Median glial fibrillary acidic protein values by phase were: pre-bypass: 0 ng/ml (range 0-0.35); cooling: 0.039 (0-0.68); re-warming: 0.165 (0-2.29); and post-bypass: 0.112 (0-0.97). There were significant elevations from pre-bypass to all subsequent stages, with the greatest increase during re-warming (p = 0.0001). Maximal levels were significantly related to younger age (p = 0.03), bypass time (p = 0.03), cross-clamp time (p = 0.047), and temperature nadir (0.04). Peak levels did not vary significantly in those with single ventricle anatomy versus two ventricle repairs. CONCLUSION: There are significant increases in glial fibrillary acidic protein levels in children undergoing cardiopulmonary bypass for repair of congenital heart disease. The highest values were seen during the re-warming phase. Elevations are significantly associated with younger age, bypass and cross-clamp times, and temperature nadir. Owing to the fact that glial fibrillary acidic protein is the most brain-specific biomarker identified to date, it may act as a rapid diagnostic marker of brain injury during cardiac surgery.

Optic nerve sheath diameter measurement techniques: examination using a novel ex-vivo porcine model.

BACKGROUND: Ultrasound (U/S) and MRI measurements of the optic nerve sheath diameter (ONSD) have been proposed as intracranial pressure measurement surrogates, but these methods have not been fully evaluated or standardized. The purpose of this study was to develop an ex-vivo model for evaluating ONSD measurement techniques by comparing U/S and MRI measurements to physical measurements. METHODS: The left eye of post mortem juvenile pigs (N = 3) was excised and the subdural space of the optic nerve cannulated. Caliper measurements and U/S imaging measurements of the ONSD were acquired at baseline and following 1 cc saline infusion into the sheath. The samples were then embedded in 0.5% agarose and imaged in a 7 Tesla (7T) MRI. The ONSD was subsequently measured with digital calipers at locations and directions matching the U/S and direct measurements. RESULTS: Both MRI and sonographic measurements were in agreement with direct measurements. U/S data, especially axial images, exhibited a positive bias and more variance (bias: 1.318, 95% limit of agreement: 8.609) compared to MRI (bias: 0.3156, 95% limit of agreement: 2.773). In addition, U/S images were much more dependent on probe placement, distance between probe and target, and imaging plane. CONCLUSIONS: This model appears to be a valid test-bed for continued scrutiny of ONSD measurement techniques. In this model, 7T MRI was accurate and potentially useful for in-vivo measurements where direct measurements are not available. Current limitations with ultrasound imaging for ONSD measurement associated with image acquisition technique and equipment necessitate further standardization to improve its clinical utility.

Postnatal Brain Trajectories and Maternal Intelligence Predict Childhood Outcomes in Complex CHD.

Objective: To determine whether early structural brain trajectories predict early childhood neurodevelopmental deficits in complex CHD patients and to assess relative cumulative risk profiles of clinical, genetic, and demographic risk factors across early development. Study Design: Term neonates with complex CHDs were recruited at Texas Children's Hospital from 2005-2011. Ninety-five participants underwent three structural MRI scans and three neurodevelopmental assessments. Brain region volumes and white matter tract fractional anisotropy and radial diffusivity were used to calculate trajectories: perioperative, postsurgical, and overall. Gross cognitive, language, and visuo-motor outcomes were assessed with the Bayley Scales of Infant and Toddler Development and with the Wechsler Preschool and Primary Scale of Intelligence and Beery-Buktenica Developmental Test of Visual-Motor Integration. Multi-variable models incorporated risk factors. Results: Reduced overall period volumetric trajectories predicted poor language outcomes: brainstem ((ß, 95% CI) 0.0977, 0.0382-0.1571; p = 0.0022) and white matter (0.0023, 0.0001-0.0046; p = 0.0397) at 5 years; brainstem (0.0711, 0.0157-0.1265; p = 0.0134) and deep grey matter (0.0085, 0.0011-0.0160; p = 0.0258) at 3 years. Maternal IQ was the strongest contributor to language variance, increasing from 37% at 1 year, 62% at 3 years, and 81% at 5 years. Genetic abnormality's contribution to variance decreased from 41% at 1 year to 25% at 3 years and was insignificant at 5 years. Conclusion: Reduced postnatal subcortical-cerebral white matter trajectories predicted poor early childhood neurodevelopmental outcomes, despite high contribution of maternal IQ. Maternal IQ was cumulative over time, exceeding the influence of known cardiac and genetic factors in complex CHD, underscoring the importance of heritable and parent-based environmental factors.

Blood pressure excursions below the cerebral autoregulation threshold during cardiac surgery are associated with acute kidney injury.

OBJECTIVES: To determine whether mean arterial blood pressure excursions below the lower limit of cerebral blood flow autoregulation during cardiopulmonary bypass are associated with acute kidney injury after surgery. SETTING: Tertiary care medical center. PATIENTS: Four hundred ten patients undergoing cardiac surgery with cardiopulmonary bypass. DESIGN: Prospective observational study. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Autoregulation was monitored during cardiopulmonary bypass by calculating a continuous, moving Pearson's correlation coefficient between mean arterial blood pressure and processed near-infrared spectroscopy signals to generate the variable cerebral oximetry index. When mean arterial blood pressure is below the lower limit of autoregulation, cerebral oximetry index approaches 1, because cerebral blood flow is pressure passive. An identifiable lower limit of autoregulation was ascertained in 348 patients. Based on the RIFLE criteria (Risk, Injury, Failure, Loss of kidney function, End-stage renal disease), acute kidney injury developed within 7 days of surgery in 121 (34.8%) of these patients. Although the average mean arterial blood pressure during cardiopulmonary bypass did not differ, the mean arterial blood pressure at the limit of autoregulation and the duration and degree to which mean arterial blood pressure was below the autoregulation threshold (mm Hg × min/hr of cardiopulmonary bypass) were both higher in patients with acute kidney injury than in those without acute kidney injury. Excursions of mean arterial blood pressure below the lower limit of autoregulation (relative risk 1.02; 95% confidence interval 1.01 to 1.03; p < 0.0001) and diabetes (relative risk 1.78; 95% confidence interval 1.27 to 2.50; p = 0.001) were independently associated with for acute kidney injury. CONCLUSIONS: Excursions of mean arterial blood pressure below the limit of autoregulation and not absolute mean arterial blood pressure are independently associated with for acute kidney injury. Monitoring cerebral oximetry index may provide a novel method for precisely guiding mean arterial blood pressure targets during cardiopulmonary bypass.

Intracranial pressure and optic nerve sheath diameter as cephalic venous pressure increases in swine.

BACKGROUND: Nontraumatic, nonhydrocephalic increases in intracranial pressure (ICP) are often difficult to diagnose and may underlie spaceflight-related visual changes. This study looked at the utility of a porcine animal model of increasing cephalic venous pressure to mimic acute changes in ICP and optic nerve sheath diameter (ONSD) from cephalic venous fluid shifts observed during spaceflight. METHODS: Anesthetized juvenile piglets were assigned to groups of either naïve (N = 10) or elevated superior vena cava pressure (SVCP; N = 20). To elevate SVCP, a 6F custom latex balloon catheter was inserted and inflated to achieve SVCP of 20 and 40 mmHg for 1 h at each pressure. In both groups, serial measurements of ICP, internal jugular pressure (IJP), and external jugular pressure (EJP) were made hourly for 3 h, and ONSD of the right eye was measured hourly by ultrasound (US). RESULTS: There was a significant linear correlation between IJP and ICP (slope: 0.9614 +/- 0.0038, r = 0.9683). With increasing SVCP, resulting ONSD was also well correlated with the ICP (slope: 0.0958 +/- 0.0061, r = 0.7841). The receiver operating characteristic curve for ONSD in diagnosing elevated ICP had an area under the curve of 0.9632 with a sensitivity and specificity of 92% and 91%, respectively, for a cutoff of 5.45 mm. CONCLUSIONS: Increases in SVCP result in ICP changes that are well correlated with alteration in ONSD. These changes are consistent with observed ONSD changes monitored during spaceflight.

Predicting the limits of cerebral autoregulation during cardiopulmonary bypass.

BACKGROUND: Mean arterial blood pressure (MAP) targets are empirically chosen during cardiopulmonary bypass (CPB). We have previously shown that near-infrared spectroscopy (NIRS) can be used clinically for monitoring cerebral blood flow autoregulation. The hypothesis of this study was that real-time autoregulation monitoring using NIRS-based methods is more accurate for delineating the MAP at the lower limit of autoregulation (LLA) during CPB than empiric determinations based on age, preoperative history, and preoperative blood pressure. METHODS: Two hundred thirty-two patients undergoing coronary artery bypass graft and/or valve surgery with CPB underwent transcranial Doppler monitoring of the middle cerebral arteries and NIRS monitoring. A continuous, moving Pearson correlation coefficient was calculated between MAP and cerebral blood flow velocity and between MAP and NIRS data to generate mean velocity index and cerebral oximeter index. When autoregulated, there is no correlation between cerebral blood flow and MAP (i.e., mean velocity and cerebral oximetry indices approach 0); when MAP is below the LLA, mean velocity and cerebral oximetry indices approach 1. The LLA was defined as the MAP at which mean velocity index increased with declining MAP to ≥ 0.4. Linear regression was performed to assess the relation between preoperative systolic blood pressure, MAP, MAP in 10% decrements from baseline, and average cerebral oximetry index with MAP at the LLA. RESULTS: The MAP at the LLA was 66 mm Hg (95% prediction interval, 43 to 90 mm Hg) for the 225 patients in which this limit was observed. There was no relationship between preoperative MAP and the LLA (P = 0.829) after adjusting for age, gender, prior stroke, diabetes, and hypertension, but a cerebral oximetry index value of >0.5 was associated with the LLA (P = 0.022). The LLA could be identified with cerebral oximetry index in 219 (94.4%) patients. The mean difference in the LLA for mean velocity index versus cerebral oximetry index was -0.2 ± 10.2 mm Hg (95% CI, -1.5 to 1.2 mm Hg). Preoperative systolic blood pressure was associated with a higher LLA (P = 0.046) but only for those with systolic blood pressure ≤ 160 mm Hg. CONCLUSIONS: There is a wide range of MAP at the LLA in patients during CPB, making estimation of this target difficult. Real-time monitoring of autoregulation with cerebral oximetry index may provide a more rational means for individualizing MAP during CPB.

The critically ill patient with ataxia-telangiectasia: a case series.

OBJECTIVE: To describe the presentation, clinical course, and outcomes of critically ill patients with ataxia-telangiectasia. DESIGN: Retrospective case series. SETTING: Adult and pediatric intensive care units at an urban tertiary academic center. PATIENTS: Seven consecutive patients with confirmed diagnosis of ataxia-telangiectasia had nine intensive care admissions between January 1995 and December 2009. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Mean age at time of admission 15.9 yrs (median, 13.9 yrs; range, 7.3-33.9 yrs). Mean duration of intensive care unit stay was 17 days (median, 9 days; range, 2-39 days). The most common admitting diagnosis was respiratory distress (six of seven patients). There was no difference in ventilator settings or duration of intensive care unit stay between survivors and nonsurvivors (p > .05). Forty-three percent (three of seven patients) survived to intensive care unit discharge with a 3-yr survival that was 14% (one of seven patients). CONCLUSIONS: Critically ill patients with ataxia-telangiectasia have complex, multisystem diseases. In this case series, the most common intensive care unit admission diagnosis was respiratory failure. Suspected or confirmed bacterial infections were prevalent. Neuropathologic autopsy findings were similar to those previously reported. Special considerations for the critical care of patients with ataxia-telangiectasia are discussed.

Cerebral blood flow autoregulation is preserved after continuous-flow left ventricular assist device implantation.

OBJECTIVE: To compare cerebral blood flow (CBF) autoregulation in patients undergoing continuous-flow left ventricular assist device (LVAD) implantation with that in patients undergoing coronary artery bypass grafting (CABG). DESIGN: Prospective, observational, controlled study. SETTING: Academic medical center. PARTICIPANTS: Fifteen patients undergoing LVAD insertion and 10 patients undergoing CABG. MEASUREMENTS AND MAIN RESULTS: Cerebral autoregulation was monitored with transcranial Doppler and near-infrared spectroscopy. A continuous Pearson correlation coefficient was calculated between mean arterial pressure (MAP) and CBF velocity and between MAP and near-infrared spectroscopic data, rendering the variables mean velocity index (Mx) and cerebral oximetry index (COx), respectively. Mx and COx approach 0 when autoregulation is intact (no correlation between CBF and MAP), but approach 1 when autoregulation is impaired. Mx was lower during and immediately after cardiopulmonary bypass in the LVAD group than in the CABG group, indicating better-preserved autoregulation. Based on COx monitoring, autoregulation tended to be better preserved in the LVAD group than in the CABG group immediately after surgery (p = 0.0906). On postoperative day 1, COx was lower in the LVAD group than in the CABG group, indicating preserved CBF autoregulation (p = 0.0410). Based on COx monitoring, 3 patients (30%) in the CABG group had abnormal autoregulation (COx ≥0.3) on the first postoperative day but no patient in the LVAD group had this abnormality (p = 0.037). CONCLUSIONS: These data suggest that CBF autoregulation is preserved during and immediately after surgery in patients undergoing LVAD insertion.

Anesthetic and perioperative risk in the patient with Ataxia-Telangiectasia.

OBJECTIVES/AIM: To report our relatively large experience with perioperative care for patients with Ataxia-Telangiectasia (A-T) and to identify the nature and frequency of complications. BACKGROUND: Ataxia-Telangiectasia is a rare autosomal recessive genetic disorder resulting in progressive multisystem degeneration and characteristic findings including complex neurodegeneration, immunodeficiency, increased risk of malignancy, and lung disease. Anecdotal reports have suggested high perioperative morbidity in patients with A-T, but few data exist. METHODS/MATERIALS: The Ataxia-Telangiectasia Clinical Center database was cross-referenced with operative records between 1995 and 2009 to identify patients with perioperative A-T, and medical records were reviewed for preoperative history, management techniques, and complications. RESULTS: Twenty-one patients with A-T underwent 34 anesthetics during the study period. The median age was 12.5 years (range 6-33 years). Common comorbidities included neurologic (100%), pulmonary (68%), immunologic (50%), oncologic (47%), and gastroenterologic (35%) disorders. Supplemental oxygen was required on postanesthesia care unit discharge for 24% of patients with a maximal duration of 24 h. Although mild postoperative hypothermia was relatively common (44% of anesthetics), there were no major complications, no unplanned admissions, and no mortality in this series. CONCLUSIONS: Although limited by its retrospective nature, this is the first series describing perioperative risk for patients with A-T. Our results indicate that general anesthesia, airway manipulation, and perioperative mechanical ventilation may be tolerated with only minor postoperative anesthetic concerns. Perioperative providers should be aware of the complex multisystem medical concerns that may arise in these patients.

Thrombotic disease in critically ill children.

OBJECTIVES: To summarize a) epidemiology of arterial and venous thromboembolism, pulmonary embolism, and deep venous thrombosis in children; b) the risk factors for thrombosis in the pediatric intensive care unit; c) diagnostic techniques for arterial/venous thromboembolism; and d) the current recommendations for management and prevention of thromboembolic disease in critically ill children. DATA SOURCE: Literature review, using National Library of Medicine PubMed and the following terms: arterial, venous thromboembolism; deep venous thrombosis; pulmonary embolism; thrombosis; as well as citations of interest from these articles. STUDY SELECTION: Both pediatric and adult literature addressing thrombotic disease were reviewed. DATA EXTRACTION AND SYNTHESIS: Articles were chosen for more extensive discussion when containing prospective studies, guidelines for practice, or data in critically ill patients. When data in children were unavailable, applicable data in adults were referenced. Due to the paucity of data in critically ill children, available adult and pediatric data were combined with institutional experience to provide suggestions for current practice and future inquiry. CONCLUSIONS: Increasing awareness regarding the recognition and current approaches to management and prevention of thromboembolic disease in children is needed among pediatric intensivists, so outcome of these life-threatening processes might be improved.

Critical evaluation of emergency stockpile ventilators in an in vitro model of pediatric lung injury.

OBJECTIVE: Modern health care systems may be inadequately prepared for mass casualty respiratory failure requiring mechanical ventilation. Current health policy has focused on the "stockpiling" of emergency ventilators, though little is known about the performance of these ventilators under conditions of respiratory failure in adults and children. In this study, we seek to compare emergency ventilator performance characteristics using a test lung simulating pediatric lung injury. DESIGN: Evaluation of ventilator performance using a test lung. SETTING: Laboratory. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Six transport/emergency ventilators capable of adult/child application were chosen on the basis of manufacturer specifications, Autovent 3000, Eagle Univent 754, EPV 100, LP-10, LTV 1200, and Parapac 200D. Manufacturer specifications for each ventilator were reviewed and compared with known standards for alarms and functionality for surge capacity ventilators. The delivered tidal volume, gas flow characteristics, and airway pressure waveforms were evaluated in vitro using a mechanical test lung to model pediatric lung injury and integrated software. Test lung and flow meter recordings were analyzed over a range of ventilator settings. Of the six ventilators assessed, only two had the minimum recommended alarm capability. Four of the six ventilators tested were capable of being set to deliver a tidal volume of less than 200 mL. The delivered tidal volume for all ventilators was within 8% of the nominal setting at a positive end expiratory pressure of zero but was reduced significantly with the addition of positive end expiratory pressure (range, ±10% to 30%; p < .01). All ventilators tested performed comparably at higher set tidal volumes; however, only three of the ventilators tested delivered a tidal volume across the range of ventilator settings that was comparable to that of a standard intensive care unit ventilator. CONCLUSIONS: Multiple ventilators are available for the provision of ventilation to children with respiratory failure in a mass casualty scenario. Few of these ventilators possess the minimum alarm functionality and consistently deliver the prescribed tidal volume that allows for safe and effective ventilation of critically ill pediatric patients. These findings will help clinicians understand the performance and limitations of available ventilators intended for use in children.

In vitro performance comparison of the Sensormedics 3100A and B high-frequency oscillatory ventilators.

OBJECTIVE: The Sensormedics 3100A and 3100B are widely used to provide high-frequency oscillatory ventilation in clinical practice. Infants and children <35 kg are typically oscillated with the 3100A and >35 kg with the 3100B. This study compares the effect of ventilator and patient parameters on delivered tidal volume during high-frequency oscillatory ventilation of a test lung with these devices. DESIGN: Laboratory-based study. SUBJECTS: Test lung and Sensormedics 3100A and 3100B high-frequency oscillators. INTERVENTIONS: A previously validated hot-wire flowmeter (Florian) was placed in series with either a 3100A (n = 3) or 3100B (n = 3) ventilator and a Michigan test lung. Tidal volumes were measured over a range of mean airway pressure, inspiratory:expiratory ratio, frequency, pressure amplitude, and endotracheal tube internal diameter. MEASUREMENTS AND MAIN RESULTS: The 3100A and 3100B delivered similar tidal volumes across a range of ventilator parameters for an inspiratory:expiratory ratio of 1:1, differing by <10%. However, at an inspiratory:expiratory ratio of 1:2, there was a statistically significant decrease in tidal volume for the 3100B compared with the 3100A at lower frequencies, which was partially mitigated by increasing pressure amplitude. The difference in the generated pressure and flow waveforms may account for the observed tidal volume differences between the high-frequency oscillatory ventilation models. Delivered tidal volume was highly dependent on endotracheal tube size. CONCLUSIONS: Multiple variables contribute to the delivered tidal volume during high-frequency oscillatory ventilation, including ventilator model selection and endotracheal tube size. It is possible that real-time, clinical monitoring of delivered tidal volume during high-frequency oscillatory ventilation would allow better titration and maximize performance of these ventilators in caring for critically ill patients.

Real-time continuous monitoring of cerebral blood flow autoregulation using near-infrared spectroscopy in patients undergoing cardiopulmonary bypass.

BACKGROUND AND PURPOSE: Individualizing mean arterial blood pressure targets to a patient's cerebral blood flow autoregulatory range might prevent brain ischemia for patients undergoing cardiopulmonary bypass (CPB). This study compares the accuracy of real-time cerebral blood flow autoregulation monitoring using near-infrared spectroscopy with that of transcranial Doppler. METHODS: Sixty adult patients undergoing CPB had transcranial Doppler monitoring of middle cerebral artery blood flow velocity and near-infrared spectroscopy monitoring. The mean velocity index (Mx) was calculated as a moving, linear correlation coefficient between slow waves of middle cerebral artery blood flow velocity and mean arterial blood pressure. The cerebral oximetry index was calculated as a similar coefficient between slow waves of cerebral oximetry and mean arterial blood pressure. When cerebral blood flow is autoregulated, Mx and cerebral oximetry index vary around zero. Loss of autoregulation results in progressively more positive Mx and cerebral oximetry index. RESULTS: Mx and cerebral oximetry index showed significant correlation (r=0.55, P<0.0001) and good agreement (bias, 0.08+/-0.18, 95% limits of agreement: -0.27 to 0.43) during CPB. Autoregulation was disturbed in this cohort during CPB (average Mx 0.38, 95% CI 0.34 to 0.43). The lower cerebral blood flow autoregulatory threshold (defined as incremental increase in Mx >0.45) during CPB ranged from 45 to 80 mm Hg. CONCLUSIONS: Cerebral blood flow autoregulation can be monitored continuously with near-infrared spectroscopy in adult patients undergoing CPB. Real-time autoregulation monitoring may have a role in preventing injurious hypotension during CPB. Clinical Trials Registration- at www.clinicaltrials.gov (NCT00769691).