A pilot study to determine whether visually evoked hemodynamic responses are preserved in children during inhalational anesthesia.

BACKGROUND: Anesthetic depth is an important parameter to monitor during surgery, yet remains difficult to quantify, particularly in young children where developmental changes influence the electroencephalogram. A more fundamental physiological response to stimulation is the increase in cerebral blood flow secondary to increased metabolic demand, referred to as flow-metabolism coupling (FMC) and measurable using near-infrared spectroscopy (NIRS). Little is known about the effect of anesthesia on FMC; therefore, we studied visually evoked hemodynamic responses (VEHRs) using NIRS in children undergoing general anesthesia for minor surgical procedures. METHOD: We recruited 23 children (aged 2-5 years), undergoing surgery requiring general anesthesia. VEHRs were measured for 30 min using NIRS, including 5 min of baseline recording after anesthetic induction. Parameters recorded using NIRS included the concentrations of oxygenated (oxy), deoxygenated (deoxy), and total hemoglobin (Hb), which were separated into epochs for evoked response analysis after filtration of motion artifact and baseline drift. Goodness-of-fit statistics and classification rules were used to determine the existence of evoked responses, and a modified Gaussian equation was used to model each evoked response. RESULTS: Near-infrared spectroscopy data were recorded in 20/23 children, of whom nine showed a VEHR. No responses were seen in the baseline control data. When examining the relationship between VEHR and anesthetic agents, we noted that for 8/10 patients in whom preoperative or intraoperative fentanyl were administered and VEHRs were absent. CONCLUSION: We have shown that VEHRs can be detected using NIRS in some anesthetized children. Administration of fentanyl was associated with an absence of VEHRs. The mechanism underlying this association is unclear.

Using thermographic cameras to investigate eye temperature and clinical severity in depression.

Previous studies suggest that altered corneal temperature may be a feature of schizophrenia, but the association between major depressive disorder (MDD) and corneal temperature has yet to be assessed. The aim of this study is to investigate whether eye temperature is different among MDD patients than among healthy individuals. We used a thermographic camera to measure and compare the temperature profile across the corneas of 16 patients with MDD and 16 age- and sex-matched healthy subjects. We found that the average corneal temperature between the two groups did not differ statistically, although clinical severity correlated positively with right corneal temperature. Corneal temperature may be an indicator of clinical severity in psychiatric disorders, including depression.

Does Exposure to Diagnostic Ultrasound Modulate Human Nerve Responses to Magnetic Stimulation?

Ultrasound (US) at diagnostic frequency and power is known to alter nerve potentials; however, the precise mechanism of action is unknown. We investigated whether US alters resting nerve potential to lower the threshold for magnetic nerve stimulation. Seventeen healthy subjects were recruited. For each subject, a 1.5 MHz US imaging probe was placed onto the elbow with the beam directed at the ulnar nerve. The probe was coupled to the skin using standard acoustic coupling gel as would be done for a routine clinical US scan. Ulnar nerve stimulation was performed simultaneously with magnetic stimulation (MS). Successful magnetic stimulation of the ulnar nerve was confirmed with nerve potentials measured by electromyography. There was no significant change in electromyography signal when MS was performed during US exposure. US at the diagnostic frequency and power tested does not alter nerve thresholds with MS. Testing at other frequencies is required, however, before US is negated as a technique to modify MS thresholds.

Physiologically precise simulation of multiple lung gas exchange during anaesthesia by simultaneous gas infusion and extraction.

UNLABELLED: A lung gas exchange simulator was tested which produces simultaneous uptake and/or elimination of multiple gases by an artificial test lung with physiologically realistic gas expired and exhaust gas flows, using a combination of infusion of diluting/enriching gases into the lung with lung gas extraction. A deterministic algorithm is incorporated which calculates required gas infusion and extraction flow rates for any set of possible target gas exchange values with any given set of fresh gas flows and concentrations. Six different scenarios were simulated, comprising a range of gas exchange values for each gas species which lie within a physiologically realistic range for anaesthetized patients. For each of these experiments the system was tested for 15 consecutive measurements over 25 min by measurement of gas exchange in the system using the Haldane transformation. RESULTS: the mean bias and standard error of the mean bias (SE, in parentheses) relative to the target value was: +0.001 (0.002) l min(-1) for O(2) uptake, -0.002 (0.005) l min(-1) for CO(2) production, -0.001 (0.002) l min(-1) for uptake of nitrous oxide and +0.3 (0.1) ml min(-1) for uptake of a volatile anaesthetic agent (isoflurane). The confidence limits of the mean bias were within 5% of the target value for all gases and scenarios with the exception of those where a low uptake of anaesthetic gas was specified. The confidence limits of the mean bias for the lower uptakes of isoflurane were within 10% of the target value for these scenarios and within 15% for the low uptake of N(2)O. Good accuracy and precision of this approach to lung gas exchange simulation were demonstrated, resulting in a versatile simulator.

A new method for measurement of gas exchange during anaesthesia using an extractable marker gas.

A new method for the measurement of pulmonary gas exchange during inhalational anaesthesia is described which measures fresh gas and exhaust gas flows using carbon dioxide as an extractable marker gas. The theoretical precision of the method was compared by Monte Carlo modelling with other approaches which use marker gas dilution. A system was constructed for automated measurement of uptake of oxygen, nitrous oxide, volatile anaesthetic agent and elimination of carbon dioxide by an anaesthetized patient. The accuracy and precision of the method was tested in vitro on a lung gas exchange simulator, by comparison with simultaneous measurements made using nitrogen as marker gas and the Haldane transformation. Good agreement was obtained for measurement of simulated uptake or elimination of all gases studied over a physiologically realistic range of values. Mean bias for oxygen and nitrous oxide uptake was 0.003 l min(-1), for isoflurane 0.0001 l min(-1) and for carbon dioxide 0.001 l min(-1). Limits of agreement lay within 10% of the mean uptake rate for nitrous oxide, within 5% for oxygen and isoflurane and within 1% for carbon dioxide. The extractable marker gas method allows accurate and continuous measurement of gas exchange in an anaesthetic breathing system with any inspired gas mixture.

Molecular imprinted polymeric porous layers in open tubular capillaries for chiral separations.

A new method has been developed for the preparation of molecular imprinted polymers as porous layers in open tubular (MIP-PLOT) capillary column formats for use in chiral separations by capillary liquid chromatography. The synthesis was based on 'in-capillary' ultraviolet (UV) initiated polymerization using light emitting diodes (LEDs) in conjunction with the continuous delivery of the pre-polymerization reagents into the polymerization zone of the capillary using an automated capillary delivery device. The relationships between exposure times, UV-light intensity and polymer layer thickness have been determined, as well as the effects of reagent delivery rate and multiple LED exposures on the layer thickness for various compositions of pre-polymerization mixtures. The polymer surface morphology was investigated by scanning electron microscopy (SEM). The non-steroidal anti-inflammatory drug S-ketoprofen was used as the template for the preparation of the MIP imprinted PLOT coatings. The separation performance with the ketoprofen racemate was investigated by capillary liquid chromatography. In contrast to alternative methods, which require the use of expensive chiral selectors, the described MIP PLOT stationary phases used non-chiral polymer precursors to create enantioselective nano-cavities through molecular self-assembly processes. The described fabrication methods provide a new avenue to tailor-make chiral MIP-PLOT capillary columns for the separation of chiral compounds present in complex or racemic analyte mixtures of chemical and biological origin.

Non-invasive automated measurement of cardiac output during stable cardiac surgery using a fully integrated differential CO(2) Fick method.

OBJECTIVES: To re-evaluate the accuracy and precision of a non-invasive method for measurement of cardiac output based on the differential CO(2) Fick approach using an automated change in respiratory rate delivered by a ventilator under control by a prototype measurement system. METHODS: Twenty-four patients during coronary artery bypass surgery, pre- and postcardiopulmonary bypass were recruited. After routine cannulation including pulmonary artery catheter, relaxant general anesthesia was induced. After hemodynamic and ventilatory stability were achieved, simultaneous paired measurements were made by the differential Fick method and by bolus thermodilution. Measurements were generated by inducing a change in respiratory rate by the ventilator under computer control. In Group 1, this involved an increase in respiratory rate from 8 to 12 breaths/min. In Group 2, this involved a decrease from 12 to 6 breaths/min. RESULTS: Nineteen measurements were made in each Group, 12 pre-CPB and 7 post-CPB. In Group 1 mean bias was -0.06 l/min, with a precision of agreement of 0.87 l/min, r = 0.91. In Group 2 (excluding one outlier) mean bias was -0.07 l/min, with a precision of 1.12 l/min, r = 0.71. CONCLUSIONS: Acceptable agreement with thermo- dilution during surgery was found, particularly where the ventilatory change involved an increase in respiratory rate from a lower baseline. This approach has potential to be readily integrated into modern anesthesia delivery platforms, allowing routine non-invasive cardiac output measurement.

Noninvasive, automated and continuous cardiac output monitoring by pulmonary capnodynamics: breath-by-breath comparison with ultrasonic flow probe.

BACKGROUND: Cardiac output monitoring is most important where cardiovascular stability is potentially threatened, such as during major surgery and in critically ill patients. However, continuous monitoring of cardiac output is still not performed routinely during anesthesia and critical care, because of invasiveness, expense, and inaccuracy of available technologies. METHODS: A technique termed the capnodynamic method was tested for breath-to-breath measurement of pulmonary blood flow from lung carbon dioxide mass balance, using measured carbon dioxide elimination and end-tidal concentration. A prototype measurement system was constructed for a feasibility study in six anesthetized sheep. Large and rapid fluctuations in cardiac output were generated by repeated dobutamine and esmolol challenge. Measurements were compared with an indwelling ultrasonic flow probe placed on the ascending aorta or pulmonary artery. RESULTS: Cardiac output measured by the flow probe varied between zero and 8.67 l/min, with a mean of 3.50 l/min. Overall mean bias [SD of the difference] between the methods (capnodynamic - flow probe) was -0.25 [0.94] l/min, r = 0.79 (P < 0.001). During periods of stability in cardiac output of 5 min or more, mean bias was -0.20 [0.55] l/min. The method successfully indicated two cardiac arrest events, which were induced in one of the animals. CONCLUSIONS: The method satisfactorily tracked wide fluctuations in cardiac output in real time. The capnodynamic method may have potential for continuous noninvasive cardiac output monitoring in patients undergoing anesthesia for major surgery, and in critical care, on a routine basis.

Continuous measurement of cardiac output by inert gas throughflow: comparison with thermodilution.

OBJECTIVE: The throughflow method is a new technique for continuous and minimally invasive measurement of cardiac output by the Fick principle, which uses ventilation of the 2 lungs with unequal inspired gas concentrations by means of a double-lumen endobronchial tube. It exploits steady-state gas exchange and thus permits rapid repetition of measurement. DESIGN: Comparison of paired measurements by the throughflow method using N(2)O exchange with bolus thermodilution. SETTING: Departments of anesthesiology in 2 university teaching hospitals. PARTICIPANTS: Nine patients undergoing cardiac surgery in the precardiopulmonary bypass period. INTERVENTIONS: Patients intubated with a double-lumen endobronchial tube were ventilated with 45% nitrous oxide (N(2)O) to the left lung (zero to the right lung). Arterial blood gas samples were taken to measure alveolar deadspace to allow correction for the alveolar-arterial N(2)O difference and to correct for the presence of unmeasured shunt perfusion. MEASUREMENTS AND MAIN RESULTS: Throughflow measurements correlated with thermodilution (r = 0.719, p < 0.05) with a mean bias of -0.208 L/min (-5.2%). The standard error of the bias was 0.060 L/min, with 95% confidence limits for the bias of -0.088 L/min and -0.328 L/min. The limits of agreement between the 2 methods were +0.960 L/min and -1.376 L/min. CONCLUSIONS: The throughflow method showed good agreement with thermodilution. It permits continuous cardiac output measurement without the need for sampling of mixed venous blood, using techniques of lung isolation, which are readily available in clinical anesthetic practice.