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Biomedical subjects

P C Beatty

Publications and source records attributed to P C Beatty.

At least 19 recordsLinked to original sources
PubMed1996

The performance of a variable-flow indirect calorimeter.

Indirect calorimetry estimates energy expenditure from measurement of respiratory gas exchange volumes. This paper considers the design and evaluation of an indirect calorimeter, the Europa GEM, suitable for use in nutritional research. The calorimeter is of the ventilated hood, flow-through type and is intended for use with spontaneously breathing patients. Our aim was to develop an accurate, flexible instrument with a high level of automation. Performance was assessed in a laboratory simulation using reference gas injections (n = 24) producing a mean error of 0.3 +/- 2% in oxygen consumption (VO2), 1.8 +/- 1% in carbon dioxide production (VCO2) and 1.4 +/- 1.5% in respiratory quotient (RQ). In order to investigate the effect of FeCO2 on error multiplication a further subdivision (n = 8) of tests at FeCO2 = 0.5%, 0.75% and 1% was made by modulating the air flow through the hood. However, the predicted increase in system accuracy with increasing FeCO2 was not apparent in practice.

Calorimetry, Indirect↗
PubMed1994

Recovery after oral surgery with halothane, enflurane, isoflurane or propofol anaesthesia.

We have compared the recovery characteristics of four different techniques for maintenance of anaesthesia in 99 day-case patients admitted for oral surgery. All patients received propofol for induction of anaesthesia followed by halothane, enflurane, isoflurane or propofol infusion for maintenance of anaesthesia. Each patient was subjected to a battery of psychometric tests which included Spielberger state, trait, mood stress and mood arousal questionnaires, Maddox-Wing test and five-choice serial reaction time. All tests were performed before operation and at 0.5, 1, 2, 4, 24 and 48 h after operation. Performance in the reaction time test decreased significantly in the immediate postoperative period, returning almost to preoperative values by 4 h. However, only those patients who received enflurane or propofol had returned to their performance level before surgery by 4 h, although all four groups had achieved this target by 24 h. There was a further improvement in performance at 48 h. Anxiety and stress were high before surgery and decreased rapidly in the postoperative period. The Maddox-Wing test demonstrated a significant impairment in performance in the first 1 h after surgery, which returned to normal by discharge at 4 h. There were no significant differences between the four groups in these latter tests.

Adult↗
PubMed1994

Efficiency of the Carden "Ventmasta" in A and D modes during controlled ventilation in children.

We have determined the efficiencies of the enclosed Mapleson A and Mapleson D modes of the Carden "Ventmasta" ventilator during controlled ventilation in 19 anaesthetized children. In addition, we determined the suitability for the A mode of the fresh gas formula, VF = 0.6 x weight0.5. Efficiency was assessed in terms of the fraction of fresh gas delivered to the alveoli. When the minute volume to fresh gas flow ratio exceeded 1.5, fractional delivery of fresh gas was 23% greater in the A mode than in the D mode (0.74 vs 0.60) (P < 0.0001). Under the same conditions, mean end-tidal carbon dioxide concentration in 27 children undergoing ventilation in the A mode with VF = 0.6 x weight0.5 was 4.6% (range 3.5-5.4%). We conclude that the Carden system is up to 23% more efficient in the A mode than in the D mode, and that under the conditions of this study, normocapnia or mild hypocapnia was produced accurately using the formula VF = 0.6 x weight0.5.

Adolescent↗
PubMed1994

Determination of the onset of rebreathing in an enclosed afferent reservoir breathing system in anaesthetized, spontaneously breathing adults: a comparison of three methods.

We used three methods to determine the onset of rebreathing in the Ohmeda enclosed afferent reservoir breathing system and compared the results with the previously published rebreathing characteristics of this system. Of the methods studied, expiratory limb capnography proved unsuitable for determining the onset of rebreathing in this system. Inspiratory limb capnography and minimum inspired carbon dioxide at the mouth did enable the onset of rebreathing to be determined. However the fresh gas flow:minute volume ratio at which rebreathing occurred as determined by these criteria was less than that determined by the Kain and Nunn criteria and thus offer no clinical advantage over the latter.

Adult↗
PubMed1993

Fresh gas requirements of an enclosed afferent reservoir breathing system in anaesthetized, spontaneously breathing adults.

Using two methods of determining the onset of rebreathing, we have determined the minimum fresh gas flow rate (VF) of the Ohmeda enclosed afferent reservoir breathing system (EAR) in anaesthetized, spontaneously breathing adults. Rebreathing as defined by the Kain and Nunn criteria did not occur when the VF/VE ratio was greater than 0.70. A mathematical model was used to calculate the degree of rebreathing at each VF. From this model, rebreathing did not occur when VF was 0.86 VE or more and this value of VF/VE is considered appropriate to eliminate rebreathing in clinical practice.

Aged↗
PubMed1993

Measurement of breath-by-breath gas exchange during general anaesthesia.

We describe a single flow transducer breath-by-breath gas exchange measurement system suitable for use during general anaesthesia. The system uses a Fleisch No. 2 pneumotachograph, a mass spectrometer and a microcomputer to give real-time continuous measurements. Correction for apparent gas exchange attributable to changes in gas stored in the lung (functional residual capacity) is available. The correction assumes no gas exchange of either nitrogen or argon for air-breathing subjects or argon only during anaesthesia, while the inspired concentrations are maintained at atmospheric values. The method has been tested against Douglas bag measurement and compared with results from conventional systems used by other authors. The system measurements show broad agreement with Douglas bag measurements, although the limits of agreement are wide for air-breathing volunteers. The system between-breath variation was typical of breath-by-breath methods in other areas of medical research.

Anesthesia, General↗
PubMed1992

Measurement of breath-by-breath gas exchange during general anaesthesia using a single pneumotachograph.

A breath-by-breath gas exchange measurement system using a single pneumotachograph suitable for use during general anaesthesia is described. The system's accuracy has been assessed by a combination of error sensitivity analysis, laboratory testing of the component measurements used to calculate gas exchange and measurements on volunteers and patients. The system is shown to have a mean accuracy of +/- 2.6 ml breath-1 for VCO2 measurements, +/- 7.12 ml breath-1 for VO2 and +/- 5.55 ml breath-1 for VN2O measurement. The application of a lung gas stores correction using argon improved between breath variability by 50%.

Algorithms↗
PubMed1992

Fresh gas requirements of an enclosed afferent reservoir breathing system during controlled ventilation in children.

An enclosed afferent reservoir breathing system (EAR) designed by Ohmeda was evaluated during anaesthesia with controlled ventilation in 104 healthy children. Carbon dioxide production and arterial carbon dioxide tension were measured in 12 children in order to determine the proportion of fresh gas (VF) involved in gas exchange. When the ratio of minute volume ventilation to fresh gas flow (VE:VF) exceeded 1.5, fractional utilization of fresh gas with the EAR was 0.92. This value and values of carbon dioxide production obtained from 43 children were used to derive a simple formula relating fresh gas flow requirements to body weight. The formula, VF = 0.6 x weight 0.5, was assessed in 49 children weighing 10-70 kg. The mean end-tidal partial pressure of carbon dioxide in these patients was 4.5 kPa (range 3.8-5.2 kPa). We conclude that the EAR has an efficiency of 92% in the use of fresh gas during controlled ventilation in healthy children, provided the VE:VF ratio is greater than 1.5. Under these conditions, normocapnia to mild hypocapnia was produced accurately using the formula VF = 0.6 x weight 0.5.

Adolescent↗
PubMed1992

Fresh gas requirements of an enclosed afferent reservoir breathing system in anaesthetized, spontaneously ventilating children.

We have determined the minimum fresh gas flow rate (VF) for use with the Ohmeda enclosed afferent reservoir breathing system (EAR) in 10 anaesthetized children breathing spontaneously. First, we determined the VF required to prevent rebreathing as detected by increased total ventilation (VE) and end-tidal carbon dioxide partial pressure. Second, we used a mathematical model to calculate the degree of rebreathing occurring at each VF. A VF equal to the predicted alveolar ventilation was sufficient to prevent clinically detectable rebreathing in all patients. From the model, no rebreathing occurred when VF/VE was 0.78 or more. We have shown previously that the EAR functions efficiently during controlled ventilation with a VF = 0.6 x weight 0.5. As this VF is slightly greater than the predicted alveolar ventilation, we suggest that the EAR may be used with a VF = 0.6 x weight 0.5 regardless of the mode of ventilation.

Adolescent↗
PubMed1992

Physical characteristics of an enclosed afferent reservoir breathing system.

We have assessed the characteristics of the Ohmeda Enclosed Afferent Reservoir Breathing System (EAR) using simulated spontaneous ventilation and controlled ventilation. The additional work of breathing through the system was measured and shown to be comparable to that of a modified Mapleson D breathing system (Bain) for fresh gas flows producing similar end-tidal carbon dioxide concentrations. It was shown under conditions of simulated controlled ventilation that end-tidal gas concentration was relatively insensitive to variations in inspired to expired ratio (I: E), tidal volume (VT) and deadspace (VD). Measurement of the volume of carbon dioxide rebreathed using simulated spontaneous ventilation led to the prediction that rebreathing of carbon dioxide would begin to occur in the EAR when fresh gas flow to total ventilation ratio (VF: VE) was approximately 0.87. However, comparison of the results of model lung tests and clinical data suggests that great caution should be taken in extrapolating such results into clinical advice.

Anesthesia, Closed-Circuit↗
PubMed1992

Fractional delivery of fresh gas: a new index of the efficiency of semi-closed breathing systems.

In earlier clinical studies, we have found a significant difference in the fractional utilization of fresh gas (FU) when using an enclosed afferent reservoir breathing system (EAR) for adult patients compared with children. This difference was explained by a large arterial to end-tidal carbon dioxide difference in the adults, reflecting a larger alveolar deadspace. In the present study, a new index of breathing system efficiency, fractional delivery of fresh gas (Fd) is proposed, which is independent of alveolar deadspace. In order to demonstrate this, values of Fd were calculated for the EAR during controlled ventilation of adults, children and a lung model. There were no significant differences between the groups. A maximum efficiency of 0.94 for the EAR was close to the theoretical limit of 1.0 predicted by the results at minute volume ventilation to fresh gas flow ratio (VE:VF) values greater than 2.0. For adult patients, the values of Fd were shown to be significantly greater than the values of FU at the same VE:VF ratio (Fd = 0.91, FU = 0.72 at VE:VF = 2.0 (P < 0.05)).

Adolescent↗
PubMed1992

The additional work of breathing through Portex Polar 'Blue-Line' pre-formed paediatric tracheal tubes.

The work of breathing through north- and south-facing Portex Polar 'Blue-Line' paediatric tracheal tubes of sizes 3.0-7.0 mm ID has been measured using sinusoidal flow at equivalent ventilatory rates of 10-50 breaths min-1 with tidal volumes of 10-500 ml. North-facing tubes are designed to sit with the connection on the forehead after intubation, whilst south-facing ones are designed so that the connection sits on the chin of the patient. It was found that the total work of breathing through north-facing tubes is approximately 8% higher than the total work of breathing through south-facing tubes of the same size, irrespective of tidal volume or respiration rate. The total work of breathing was dependent on total tube length but independent of tube design. The endotracheal connectors themselves were found to contribute a significant proportion of the total work of breathing but there was no significant difference between the inspiratory and expiratory performance of the tubes.

Anesthesiology↗