On the role of the second-order derivative term in the calculation of convergent beam diffraction patterns.

The simulation of (scanning) transmission electron microscopy images and diffraction patterns is most often performed using the forward-scattering approximation where the second-order derivative term in z is assumed to be small with respect to the first-order derivative term in the modified Schrödinger equation. This assumption is very good at high incident electron energies, but breaks down at low energies. In order to study the differences between first- and second-order methods, convergent beam electron diffraction patterns were simulated for silicon at the [111] zone-axis orientation at 20 keV and compared using electron intensity difference maps and integrated intensity profiles. The geometrical differences in the calculated diffraction patterns could be explained by an Ewald surface analysis. Furthermore, it was found that solutions based on the second-order derivative equation contained small amplitude oscillations that need to be resolved in order to ensure numerical integration stability. This required the use of very small integration steps resulting in significantly increased computation time compared to the first-order differential equation solution. Lastly, the efficiency of the numerical integration technique is discussed.

Hypoxic vasoconstriction in pulmonary arterioles and venules.

Pulmonary microvessels (<70 microm) lack a complete muscular media. We tested the hypothesis that these thin-walled vessels do not participate in the hypoxic pressor response. Isolated canine lobes were pump perfused at precisely known microvascular pressures. A videomicroscope, coupled to a computerized image-enhancement system, permitted accurate diameter measurements of subpleural arterioles and venules, with each vessel serving as its own control. While vascular pressure was maintained constant throughout the protocol, hypoxia caused an average reduction of 25% of microvessel diameters. The constriction was reversed when nitric oxide was added to the hypoxic gas mixture. The nitric oxide reversal, combined with a lack of lobar blood flow redistribution as measured by fluorescent microspheres, shows that the constriction was active. This response suggests the unexpected potential for active intra-acinar ventilation-perfusion matching.

Transjugular intrahepatic portosystemic shunting (TIPS) for treatment of severe hepatic veno-occlusive disease.

Severe veno-occlusive disease (VOD) of the liver is a frequent cause of morbidity and mortality in patients undergoing transplantation. While surgical portosystemic shunts have been reported to be useful in the treatment of severe hepatic VOD with intractable ascites, few of these patients are surgical candidates. We report a case of severe VOD after autologous peripheral blood progenitor cell transplantation treated with transjugular intrahepatic portosystemic shunting (TIPS). This procedure resulted in marked improvement in the patient's ascites, coagulation status and urinary output. The safety and efficacy of this non-surgical approach for the treatment of patients with severe VOD requires prospective studies.

Measuring pulmonary microvessel diameters using video image analysis.

To directly determine the pressure-diameter relationship of individual pulmonary microvessels, it is necessary to measure the width of the column of blood in the vessel because microvascular walls are invisible when using intravital microscopy. To identify the margins of the blood column accurately, we developed a method for computer enhancement and measurement of vessel images. After recording microvessels on videotape, consecutive frames from the videotape were digitized by a computer. Pixels that changed from frame to frame (moving erythrocytes) were turned white, and unchanging pixels were turned black. In this way an image of the erythrocyte column with distinct edges was produced. The width of this column was measured with a heuristic technique involving interactions between the computer and the user. The measurements were reproducible and accurate. This technique has been used to measure microvascular diameters over a range of well-defined microvascular pressures and construct precise pressure-diameter curves.

Increased cerebral and decreased femoral artery blood flow velocities during direct laryngoscopy and tracheal intubation.

Direct laryngoscopy and tracheal intubation may be associated with increased heart rate, arterial blood pressure, cardiac index, and systemic vascular resistance. These responses have been attributed to sympathoadrenal stimulation. However, the studies measuring distribution of blood flow to various organs are limited. We prospectively evaluated blood flow velocities in the common carotid, middle cerebral, and femoral arteries before induction of anesthesia, after induction but before direct laryngoscopy, at the conclusion of direct laryngoscopy and tracheal intubation, and 3 min after tracheal intubation in 13 adult patients. Direct laryngoscopy and tracheal intubation produced increases in the heart rate (from 76 +/- 13 to 91 +/- 10 bpm; chi +/- SD), systolic blood pressure (from 168 +/- 20 to 206 +/- 21 mmHg), common carotid and middle cerebral blood flow velocities, but produced decreases in femoral artery blood flow velocities. Common carotid artery blood flow velocity increased from 49.4 +/- 12.5 cm/s to 65.2 +/- 20.7 cm/s (P < 0.05) at the conclusion of tracheal intubation. Middle cerebral artery blood flow velocity, which could be measured only in seven patients due to technical difficulties secondary to movement during laryngoscopy, increased from 62.4 +/- 20.5 cm/s to 78.0 +/- 27.7 cm/s (P < 0.05). In contrast, femoral artery blood flow velocity decreased from 107.6 +/- 37.8 cm/s to 76.8 +/- 28.6 cm/s (P < 0.05). These responses suggest that the hypertensive response due to direct laryngoscopy and tracheal intubation is associated with redistribution of blood flow in the body.

Direct measurement of pulmonary microvascular distensibility.

Pulmonary vascular distensibility has an important influence on pulmonary hemodynamics. Although many measurements of distensibility have been made on large pulmonary vessels, there is less information on microvascular distensibility. We have measured the distensibility of the smallest (< 70-microns-diam) precapillary arterioles and postcapillary venules. Isolated dog lobes, at 2.5 cmH2O transpulmonary pressure, were perfused at low flows, which caused the arteriovenous pressure gradient to be very small and thereby permitted accurate estimation of microvascular pressure. As microvascular pressure was systematically varied between 0 and 30 mmHg, subpleural microvascular diameters were determined from computer-enhanced images obtained by videomicroscopy. Arteriolar and venular distensibilities were not different from each other. The microvascular pressure-diameter relationship was alinear with distensibility coefficients of 1-3% mmHg-1, values that are of the same order of magnitude as previously measured distensibilities of 100- to 1,000-microns-diam canine pulmonary vessels.

Evaluation of a new fluid warmer effective at low to moderate flow rates.

BACKGROUND: The tendency of intravenous fluid exiting the heat exchanger of a fluid warmer to cool to room temperature increases as the rate of infusion slows and the length of tubing between the heat exchanger and the patient increases. Thus, slow to moderate flow rates result in the delivery of fluid near room temperature despite the use of a fluid warmer. The volumes infused even at low flow rates may be large relative to the size of infants and children and may result in a significant decrease in patient temperature. METHODS: A new warmer (Hotline, Level 1 Technologies) that actively heats the fluid in the delivery tubing was evaluated and compared to two different conventional dry-wall warmers: the model DW1000A (Baxter Health Care) and the FloTem IIe (DataChem). Cold blood (4-10 degrees C) and room temperature saline (22 degrees C) were pumped through the warmers and the delivered temperature was measured as the flow rate was varied from 50 to 12,000 ml/h. RESULTS: The Hotline was more effective than the Baxter or the FloTem IIe at flow rates between 50 and 6,000 ml/h for saline and at flow rates between 50 and 3,000 ml/h for blood. Insulating the tubing beyond the heat exchangers of the conventional warmers improved their performance, but the delivered temperatures were still less than those of the Hotline at low flow rates. CONCLUSIONS: The Hotline is more effective than conventional warmers at slow flow rates, and may be useful for preventing hypothermia when large volumes of fluid relative to patient size are infused at slow rates.

Early extubation of the trachea after repair of secundum-type atrial septal defects in children.

To investigate the role of anaesthetic management in early extubation of the trachea in children after closure of a secundum-type atrial septal defect (ASD II), a retrospective chart review for a two-year period was performed. We identified 36 children who underwent surgical repair of an isolated ASD II. In 19 children (53%) the tracheas were extubated in the operating room immediately after surgery and in 17 patients (47%) the tracheas remained intubated and the lungs were ventilated in the Intensive Care Unit. There was no difference in age (69.5 +/- 33.8 vs 72.9 +/- 45.0 mo) or weight (19.5 +/- 8.1 versus 20.5 +/- 12.7 kg) between the two groups (mean +/- SD). Children in the extubated group had a shorter duration of cardiopulmonary bypass (43.4 +/- 7.8 min) than those remaining intubated (31.7 +/- 12.7 min) (P < 0.05). The children whose tracheas were extubated early received a lower perioperative fentanyl dose (5.9 +/- 6.4 micrograms.kg-1) than those remaining intubated (35.1 +/- 8.5 micrograms.kg-1). Those children in the extubated group had a lower hourly requirement for morphine by infusion (13.6 +/- 5.7 vs 18.2 +/- 5.4 micrograms.kg-1.hr-1) and a shorter stay (20.5 +/- 3.7 versus 29.0 +/- 11.2 hr) in the Intensive Care Unit. Re-intubation of the trachea was not required in any of the children and no deaths occurred. Early extubation after ASD II repair is safe and, given the results of this study, may offer certain advantages over prolonged intubation and ventilation in these children.

Perioperative fluid and transfusion management.

Optimal perioperative fluid management in pediatric patients entails a knowledge of the effects of preoperative fasting, perioperative third space losses, and hemorrhage on the patient's fluid compartments. We explain which of the various available intravenous fluids should be used to correct various fluid and electrolyte losses that may occur. The authors also review techniques for limiting homologous transfusion requirements and discuss certain complications associated with blood transfusion.

Cerebral hemodynamics in neonates and infants undergoing cardiopulmonary bypass and profound hypothermic circulatory arrest: assessment by transcranial Doppler sonography.

Profound hypothermic circulatory arrest (PHCA) is followed by a transient period of increased intracranial pressure and a longer period of neurophysiologic dysfunction. To investigate the effect of cardiopulmonary bypass (CPB) with PHCA on cerebral hemodynamics, we used transcranial Doppler sonography to measure cerebral blood flow velocity in 10 neonates and infants before and after PHCA. Cerebral blood flow velocity was compared before and after PHCA during normothermic cardiopulmonary bypass at the same mean arterial pressure, central venous pressure, hematocrit, and arterial carbon dioxide tension. Cerebral blood flow velocity decreased exponentially with decreasing nasopharyngeal temperature before PHCA (P less than 0.05) and remained decreased after PHCA during normothermic CPB, compared with values for normothermic CPB before PHCA (P less than 0.005). During normothermic CPB after PHCA, the modified cerebral vascular resistance (mm Hg.cm.s-1) was increased above values for normothermic CPB before PHCA (P less than 0.05). The results of this study suggest that the observed increase in intracranial pressure during PHCA is not caused by increased cerebral perfusion, but rather that cerebral perfusion is reduced in response to a decreased demand for cerebral metabolic oxygen.

Anterior fontanel pressure and visual evoked potentials in neonates and infants undergoing profound hypothermic circulatory arrest.

To determine the effects of cardiopulmonary bypass with profound hypothermic circulatory arrest (PHCA) on anterior fontanel pressure (AFP) and visual evoked potentials (VEPs), 21 neonates and infants undergoing cardiopulmonary bypass (CPB) with PHCA for surgical correction of congenital heart defects were studied. Mean (+/- SD) minimum nasopharyngeal, esophageal, and rectal temperatures of 16.4 +/- 2.2, 11.2 +/- 2.7, and 17.7 +/- 1.9 degrees C, respectively, were achieved for a mean duration of PHCA of 51.6 +/- 18.7 min. AFP increased significantly above pre-CPB values for the first 21.7 +/- 8.1 min of rewarming. The duration of this increase in AFP was related logarithmically and directly to the product of the nasopharyngeal temperature (NPT) at the end of PHCA and the duration of PHCA (r2 = 0.82, P less than 0.0001). Nineteen of these patients had simultaneous monitoring of VEPs. The latency of both the N70 and P100 components of the VEPs increased as temperature decreased. The cerebral perfusion pressure was linearly and inversely related to the AFP (r2 = 0.72, P less than 0.01). The VEPs disappeared as a nasopharyngeal temperature (NPT) of 18.9 +/- 2.8 degrees C and reappeared after 21.9 +/- 8.8 min post-PHCA at an NPT of 32.8 +/- 1.4 degrees C. There was no significant difference between duration of increased AFP (20.9 +/- 8.1 min) and the duration of absence of VEPs during the post-PHCA period. The duration of increased AFP correlated linearly and directly with the duration of absence of VEPs (r2 = 0.84, P less than 0.005). These data demonstrate that transient neurophysiologic dysfunction occurs after PHCA. This dysfunction is related to the duration of elevation of the AFP and cannot be explained solely by a temperature effect.

Accuracy of end-tidal PCO2 measurements using a sidestream capnometer in infants and children ventilated with the Sechrist infant ventilator.

To determine the accuracy of end-tidal PCO2 (PETCO2) measurements analyzed with a sidestream capnometer in infants and children whose lungs were ventilated with a Sechrist infant ventilator and an Ayre's t-piece, we compared PETCO2 measurements obtained from the proximal (PETCO2-p) and distal (PETCO2-d) ends of the tracheal tube to arterial PCO2 (PaCO2) in 37 healthy infants and children between 1.3 and 24.5 kg. Both PETCO2-p and PETCO2-d accurately approximated PaCO2, however, the mean (+/- SD) arterial to end-tidal PCO2 difference (delta(a-ET)PCO2) was significantly greater with proximal (1.27 +/- 1.54 mmHg) than with distal sampling (0.64 +/- 1.64 mmHg) (P less than 0.01). In the subgroup of patients who weighted less than 12 kg, the delta(a-ET)PCO2 using proximal gas sampling (1.94 +/- 1.29 mmHg) was also significantly greater than it was using distal sampling (0.74 +/- 1.31 mmHg) (P less than 0.001). We conclude that although statistically different, both proximal and distal estimates of PETCO2 provide acceptable estimates of PaCO2 in healthy infants and children who are ventilated with a Sechrist infant ventilator and an Ayre's t-piece system.