Paul Junor

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.

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.

Development andin vitro validation of a device for measuring non-shunt cardiac output by nitrous oxide throughflow

A system has been developed for measuring non-shunt cardiac output by the throughflow technique, using nitrous oxide in patients undergoing general anaesthesia. The throughflow measurement technique is a non-invasive method based on inert gas throughflow theory. In vitro validation of the measurement system was performed using a lung gas exchange simulator. The accuracy and precision of the throughflow measurement system was assessed by comparing measured and target values for five simulated values of non-shunt cardiac output, from 2.88 to 9.86 l min−1. This showed an overall mean bias of −0.03l min−1 (range −0.00 to −0.10 l min−1), with a mean coefficient of variation of the difference of 1.39% (1.20–1.93%). These results indicate that the measurement system is suitable for monitoring the non-shunt cardiac output in patients undergoing general anaesthesia using nitrous oxide throughflow.

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

Abstract. 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.

A comparison of reading and listening performance: computer‐controlled speech and printed text presentations

Print-handicapped people enrolled at any level of the education system are expected to acquire the same information as their peers who are readers of printed text. Research which compares performance rates of reading print with auding of materials read onto tape generally reports substantially higher performance by readers of print. The different strategies employed by readers and listeners are said to be responsible for the variation in performance. This article describes the development of one method and the associated instrumentation which permits a more exact comparison of reading and listening performance. It ensures that important factors such as word rate, timing, inter-word pauses, and overall presentation duration are identical for visual or audio presentations. The inclusion of these factors enables a more direct comparison of reading performance with listening performance at a range of presentation speeds, including accelerated presentations from time-compressed tapes. An investigation which employed this technique for comparing reading and listening performance has provided new findings on how readers and listeners operate. This should provide a more complete understanding of the benefits and limitations of accelerated reading and listening techniques in the educational context.

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

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.

Identifying ovine transcranial acoustic windows

Transcranial Doppler ultrasound (TCD) is used to measure cerebral blood flow velocity in patients with cerebrovascular pathology. The task of locating an adequate acoustic window, through which TCD may be performed, is frequently time consuming or impossible for patients with thick or dense temporal bones. We examined the possibility of utilising B-Mode ultrasound images to guide detection of acoustic windows prior to TCD. B-Mode images of the temporal bone were acquired on deceased sheep shortly after death. We found that on selected sheep, the inner table of the temporal bone was visible on B-Mode imaging. We suggest that visibility of the inner table of the temporal bone on B-Mode imaging is associated with a high probability of successful TCD at that location.

Ultrasound detection of the skull-brain interface: A phantom study

Transcranial Sonography is a clinical procedure for monitoring cerebral blood flow in patients suspected of suffering from severe blood flow disruption in the brain. The Transcranial Doppler (TCD) clinicians are currently only guided by their experience and the temporal bone surface to position the Doppler probe on a suitable site for TCD. This site is known as an acoustic window. Neither experience nor the bone surface can sufficiently improve TCD procedure time for new patients. A suitable acoustic window for transcranial Doppler ultrasound can be difficult to find, and in some patients, no such acoustic window is possible. It is hypothesized that the visibility of the temporal bone in an acousto-spectroscopic image suggests an appropriate transcranial Doppler ultrasound acoustic window. The aim of this research is to investigate the ultrasonic detection of such a pathological acoustic window. It was anticipated that an acoustic window would best be detected by its Acousto-spectroscopic properties in an attenuation-velocity image. These images contrast hard tissue from soft tissue based on their acousto-spectroscopic properties. Identifying where the temporal bone is the thinnest could potentially indicate the best location for performing a Transcranial Doppler procedure. Experimental investigations on a specially designed layered transcranial phantom were carried out to detect the region where the temporal bone is the thinnest and hence suggest an adequate acoustic window. It was shown that a plausible transcranial acoustic window is visible in an ultrasound image, a previously unreported and very promising result. Attenuation-velocity images showed greater boundary contrast between hard and soft tissue. However, hard and soft tissue regions appear similar in comparison to B-Mode images. It was determined that nonuniform scattering from inhomogeneous life-like tissues causes attenuation-velocity images to display less media contrast such as bone versus soft tissue contrast. This research is the first to report that B-Mode images are currently the best method for visually detecting a plausible potential acoustic window for Transcranial Doppler.

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.