Bispectral Index asymmetry and COMFORT score in paediatric intensive care patients.

BACKGROUND: The Bispectral Index (BIS) monitor has been suggested as a potential tool to measure depth of sedation in paediatric intensive care unit (PICU) patients. The primary aim of our observational study was to assess the difference in BIS values between the left and right sides of the brain. Secondary aims were to compare BIS and COMFORT score and to assess change in BIS with tracheal suctioning. METHODS: Nineteen ventilated and sedated PICU patients had paediatric BIS sensors applied to either side of their forehead. Each patient underwent physiotherapy involving tracheal suctioning. Their BIS data and corresponding COMFORT score, assessment as by their respective nurses, were recorded before, during, and after physiotherapy. RESULTS: Seven patients underwent more than one physiotherapy session; therefore, 28 sets of data were collected. The mean BIS difference values (and 95% CI) between left BIS and right BIS for pre-, during, and post-physiotherapy periods were 9.2 (5.9-12.5), 15.8 (11.9-19.7), and 7.5 (5.2-9.7), respectively. Correlation between mean BIS, left brain BIS, and right brain BIS to COMFORT score was highly significant (P<0.001 for all three) during the pre- and post-physiotherapy period, but less so during the stimulated physiotherapy period (P=0.044, P=0.014, and P=0.253, respectively). CONCLUSIONS: A discrepancy between left and right brain BIS exists, especially when the patient is stimulated. COMFORT score and BIS correlate well between light and moderate sedation.

Comparison of face masks in the bag-mask ventilation of a manikin.

BACKGROUND AND OBJECTIVE: We conducted a study investigating the effectiveness of four face mask designs in the bag-mask ventilation of a special manikin adapted to simulate a difficult airway. METHODS: Forty-eight anaesthetists volunteered to bag-mask ventilate the manikin for 3 min with four different face masks. The primary outcome of the study was to calculate mean percentage leak from the face masks over 3 min. Anaesthetists were also asked to rate the face masks using a visual analogue score. RESULTS: The single-use scented intersurgical face mask had the lowest mean leak (20%). This was significantly lower than the mean leak from the single-use, cushioned 7,000 series Air Safety Ltd. face mask (24%) and the reusable silicone Laerdal face mask (27%) but not significantly lower than the mean leak from the reusable anatomical intersurgical face mask (23%). CONCLUSIONS: There was a large variation in both performance and satisfaction between anaesthetists with each design. This highlights the importance of having a variety of face masks available for emergency use.

A comparison of 20 laryngoscope blades using an intubating manikin: visual analogue scores and forces exerted during laryngoscopy.

Fifty anaesthetists were recruited to use 20 different laryngoscope blades (one metal re-usable blade, five metal single-use blades and 14 plastic single-use blades, of which eight were bulb-type and 12 were fibreoptic-type) in a manikin to achieve a grade I Cormack and Lehane view. The anaesthetists were asked to provide visual analogue scores (VAS) for: ease of attachment of the blade to the handle; illumination; view of the larynx; and satisfaction for clinical use. The peak force applied and time to achieve the grade I Cormack and Lehane view were also measured. A cluster analysis method was used to group together blades with similar scores or measures. Ease of attachment, illumination, view, clinical use, force and duration were all significantly affected by the blade used (p < 0.0001 for all six). The mean peak force applied and mean duration for the 20 blades were 32-39 N and 4.4-9.5 s, respectively. All five metal single-use and four plastic single-use blades were always placed in the 'best' group in the cluster analysis. Two plastic blades provided a poor view and increased the duration of laryngoscopy.

Effect of catheter design on tracheal pressures during tracheal gas insufflation.

BACKGROUND AND OBJECTIVE: This study investigated the distribution of pressures within a model trachea, produced by five different tracheal gas insufflation devices. The aim was to suggest a suitable design of a tracheal gas insufflation device for clinical use. METHODS: Each device was tested using insufflation flow rates of 5 and 10 L min(-1). For each flow rate, the pressure within the tracheal model was measured at 33 fixed points. RESULTS: The Boussignac tracheal tube produced the most even pressure distribution, while a reverse-flow catheter produced pressure changes of the smallest magnitude. CONCLUSIONS: We suggest that catheters producing the lowest pressure changes are likely to be safer for clinical use.

Ventilatory assistance and respiratory muscle activity. 1: Interaction in healthy volunteers.

We have investigated the response of 12 normal, healthy subjects to resistance loading and ventilator assistance of spontaneous breathing. Three ventilators, the Hamilton Veolar, Engström Erica and Puritan Bennett 7200, were used to provide synchronized intermittent mandatory ventilation and two levels of pressure assistance. Total respiratory elastance and resistance were measured. The equivalent (negative) pressure of respiratory muscle activity (pmus) was then calculated from measurement of flow and pressure at the mouth. With ventilatory assistance, subjects maintained frequency, decreased inspiratory time and the magnitude of pmus, but increased tidal volume, thus not taking full advantage of ventilatory assistance. The waveform of pmus varied in detail within and between subjects and conditions, but the all-subject mean waveforms showed for all conditions a consistency of trajectory. Increasing the level of assistance decreased the duration and hence the (negative) peak value of pmus. The results suggest that some waveforms of flow or pressure from the ventilators may be more acceptable to patients than others, and that different patients may prefer different waveforms.

Ventilatory assistance and respiratory muscle activity. 2: Simulation with an adaptive active ("aa" or "a-squared") model lung.

The aim of this study was to develop a lung model which adapted its active simulation of spontaneous breathing to the ventilatory assistance it received--an "aa" or "a-squared" lung model. The active element required was the waveform of negative pressure (pmus), which is equivalent to respiratory muscle activity. This had been determined previously in 12 healthy volunteers and comprised a contraction phase, relaxation phase and expiratory pause. Ventilatory assistance had shortened the contraction and relaxation phases without changing their shape, and lengthened the pause phase to compensate. In this study, the contraction and relaxation phases could be adequately represented by two quadratic equations, in addition to a third to provide a smooth transition. Therefore, the adaptive element required was the prediction of the duration of the contraction phase. The best predictive variables were flow at the end of contraction or peak mouth pressure. Determination of either of these allowed adjustment of the "standard" waveform to the level of assistance produced by an "average" ventilator, in a manner that matched the mean response of 12 healthy conscious subjects.

A method of testing the compliance of 15 and 22 mm conical connectors on heat and moisture exchangers to ISO 5356-1:1987.

Heat and moisture exchangers (HMEs) are used to warm and humidify the gas delivered to patients during anaesthesia and in intensive care. Typically, HMEs are connected between the tracheal tube or catheter mount and the breathing system. This point of connection has been identified as having a high risk of disconnection. To reduce the possibility of disconnection, the International Standard for HMEs specifies that the connectors on an HME shall comply with ISO 5356-1: 1987, the International Standard for cones and sockets. A simple method is described to test whether or not the connectors comply with this standard. This method is used when evaluating HMEs for the Medical Devices Agency (MDA). Results are quoted from the MDA series Evaluation. The majority of connectors tested were found to comply with the standard. The test method is also suitable for testing conical connectors on other devices.

A novel method of measuring the concentration of anaesthetic vapours using a dew-point hygrometer.

The Antoine equation relates the saturated vapour pressure of a volatile substance, such as an anaesthetic agent, to the temperature. The measurement of the 'dew-point' of a dry gas mixture containing a volatile anaesthetic agent by a dew-point hygrometer permits the determination of the partial pressure of the anaesthetic agent. The accuracy of this technique is limited only by the accuracy of the Antoine coefficients and of the temperature measurement. Comparing measurements by the dew-point method with measurements by refractometry showed systematic discrepancies up to 0.2% and random discrepancies with SDS up to 0.07% concentration in the 1% to 5% range for three volatile anaesthetics. The systematic discrepancies may be due to errors in available data for the vapour pressures and/or the refractive indices of the anaesthetics.

A model lung with direct representation of respiratory muscle activity.

We describe the theory, construction and testing of an active model lung. This is designed so that when it is connected to a ventilator, the interaction between spontaneous ventilatory activity in the patient and the pressures and flows generated by the ventilator are reproduced faithfully. A waveform of negative pressure, pmus, equivalent to the action of the respiratory muscles, is applied indirectly to the compliance unit of a conventional model lung. It is shown that, when the compliance and resistance of a subject have been measured, the waveform of pmus can be determined. This was undertaken in three volunteers breathing under various conditions, including connection to a ventilator operating in different modes. When these conditions were reproduced with the lung model replacing, but matched to, each subject, the original waveforms of flow and pressure at the mouth were reproduced.

Excessive work of breathing during intermittent mandatory ventilation.

Difficulties were experienced in weaning a patient from a ventilator by means of intermittent mandatory ventilation (IMV). The difficulty was overcome by installing an alternative IMV system (Hudson "disposable IMV valve") through which the patient drew her spontaneous breaths. Laboratory measurements showed that the resistance of the ventilator breathing system was much higher than that of the alternative system, mainly as a result of the resistance of the humidifier. It was calculated from measurements with a preset pattern of simulated breathing that the extra, external, work of breathing through the ventilator breathing system was approximately 1.5 times the normal internal mechanical work of breathing for a normal patient; with the alternative system, the extra work was only 0.5 times the normal. It is stressed that the breathing systems of IMV ventilators should be judged with the same rigour as other systems through which the patient is required to breathe spontaneously. It is recommended that manufacturers should provide the necessary information.

The Afya anaesthesia system. A laboratory study.

The design of and the results of laboratory performance tests on the Afya anaesthesia system are described. The system has a low inspiratory and expiratory resistance and the concentrations of ether delivered ranged from 75 to 130% of the vaporizer setting. The error was maximum at vaporizer settings of between 15 to 20%. The system is suitable for spontaneous and controlled ventilation of the lungs in both adults and infants.