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Biomedical subjects

C P Heneghan

Publications and source records attributed to C P Heneghan.

At least 19 recordsLinked to original sources
PubMed1988

Effect of surgical stimulation on the auditory evoked response.

Previous studies have shown a dose-related effect of a number of general anaesthetic agents on the early cortical waves in the auditory evoked response (AER). In this study the effect of surgical stimulation on these waves was examined in 11 patients anaesthetized with thiopentone, nitrous oxide and halothane and paralysed with pancuronium. The inspired nitrous oxide concentration and end-tidal halothane concentration were held constant at 70% and 0.3%, respectively, and baseline AER recordings were made. Following surgical stimulation there was a progressive and significant increase in the amplitude of waves Nb and Pb/Pc. Unambiguous autonomic responses were seen in three patients, but these were not significantly correlated with changes in the AER. We conclude from this, and previous studies, that the amplitude of cortical waves in the AER are sensitive not only to anaesthetic concentration but also to surgical stimulation. The AER may, therefore, provide a useful index of depth of anaesthesia, that is the balance between the effects of surgical stimulation and anaesthetic depression on central nervous system activity.

Adult↗
PubMed1987

Effect of isoflurane on the auditory evoked response in man.

We have examined the effects of isoflurane (0.6-2.9% end-tidal) on the auditory evoked response (AER) in six patients before elective surgery. Isoflurane produced significant dose-related changes in the AER: reductions in amplitude and increases in latency of the cortical waves Pa and Nb, and increases in the latency of the brainstem waves III and V. When isoflurane was compared with halothane and enflurane using an MAC-based comparison, we found no differences in the effect of the three agents on the amplitude of the early cortical waves, although the latencies showed significant differences. The consistent dose-related effect on the amplitudes of the cortical waves implies that the AER could be a promising index of the depth of anaesthesia.

Adolescent↗
PubMed1987

Comparison of the respiratory depressant effects of halothane and isoflurane in routine surgery.

Respiratory depression has been studied in 21 patients anaesthetized with either halothane and nitrous oxide, or isoflurane and nitrous oxide while undergoing routine surgery. Spontaneous ventilation was preserved and the end-tidal concentrations of the two volatile anaesthetics were at approximately equal MAC-multiples. The minute volume of pulmonary ventilation was not significantly different for the two anaesthetics, but tidal volume was significantly greater and frequency significantly lower with isoflurane. Arterial and end-tidal PCO2 were consistently lower in the patients receiving isoflurane, the difference being significant in many of the sub-groups. There was no evidence of a progressive increase in PCO2 with either anaesthetic during surgery lasting up to 225 min. Mean values for PCO2, however, tended to be well above the normal reference range, with mean arterial PCO2 in the range 7.2-8.6 kPa.

Adult↗
PubMed1986

Effect of isoflurane on bronchomotor tone in man.

Twenty patients were anaesthetized with thiopentone, the trachea intubated and the lungs ventilated with 70% nitrous oxide and oxygen. Normocapnia was maintained and, following control measurements of the specific conductance of the lower airways (s.Glaw), either 1.7% isoflurane or 1.3% halothane was added to the inspired gas mixture, 10 patients receiving each drug. s.Glaw was measured repeatedly during the next 30 min. There was a tendency for s.Glaw to increase--indicating a reduction in bronchomotor tone--during the administration of isoflurane, the effect approaching statistical significance. The administration of halothane was associated with a significant increase in s.Glaw. There was a statistically significant increase in the expiratory reserve volume, and a decrease in mean respiratory resistance over the tidal range in both groups. These results indicate that isoflurane does not cause an increase in bronchomotor tone, and may have a tendency to decrease it. This suggests that the previously reported increase of respiratory resistance during isoflurane anaesthesia resulted from a reduction in lung volume, rather than a change in bronchomotor tone.

Adolescent↗
PubMed1986

Selective effect of althesin on the auditory evoked response in man.

The auditory evoked response (AER) has been studied in six patients before the induction of general anaesthesia, during anaesthesia with nitrous oxide in oxygen and mechanical ventilation, then with a stepwise increasing rate of infusion of Althesin ranging from 18 micrograms kg-1 min 1 to 90 micrograms kg-1 min-1. The sections of the AER examined in this study were the brainstem waves I, III and V and the early cortical waves Pa and Nb. There were dose-related changes in latency and amplitude of waves Pa and Nb, latency increasing and amplitude diminishing with increasing concentrations of alphaxalone as measured in venous blood. In contrast, there were no changes in either latency or amplitude of the brainstem waves. This is consistent with other work suggesting that Althesin has little effect on neuronal function below the level of the superior colliculus.

Adolescent↗
PubMed1985

Pulmonary gas exchange and diaphragmatic position. Effect of tonic phrenic stimulation compared with that of increased airway pressure.

The influence of diaphragmatic position on abnormal gas exchange has been examined to investigate the theory that the impairment in gas exchange in anaesthetized man is caused by disturbance of diaphragmatic mechanics, resulting in abnormalities of dependent lung ventilation. A gas exchange abnormality, probably caused by airway closure in the dependent regions of the lung, was induced in anaesthetized rabbits by reducing lung volume to residual volume and allowing passive re-expansion. The effects on gas exchange of increases in lung volume produced by two methods--the application of positive end-expiratory pressure (PEEP) and phrenic nerve stimulation (PNS)--were compared. Both methods were adjusted to give the same increase in lung volume. PNS was found to produce greater caudal movement of the diaphragm than PEEP, particularly in the dependent regions. PNS also improved gas exchange significantly more than PEEP. These findings support the theory that alterations in diaphragmatic mechanics during anaesthesia contribute to the gas exchange impairment in man.

Animals↗
PubMed1985

Effect of etomidate on the auditory evoked response in man.

The effect of etomidate on the auditory evoked response was examined in a double-blind study carried out before the start of surgery. Fourteen patients were anaesthetized with 70% nitrous oxide and oxygen after induction with thiopentone. Ventilation was controlled. Seven of the patients received a continuous infusion of etomidate, increasing in five equal steps from 0.01 mg kg-1 min-1 to 0.05 mg kg-1 min-1 over a period of 50 min. The other seven received similarly an equivalent volume of saline. The patients given etomidate were easily distinguishable from those given saline, solely on the basis of changes in the early cortical peaks Pa and Nb. In the etomidate group the latencies of these peaks increased and their amplitudes decreased. These changes were linearly related to serum etomidate concentration. There was no effect of etomidate on the brainstem response.

Adolescent↗
PubMed1984

Effects of halothane or enflurane with controlled ventilation on auditory evoked potentials.

The effects of increasing concentrations of halothane and enflurane on selected components of the auditory evoked response were studied in 12 patients; six received halothane and six enflurane. After the induction of anaesthesia with thiopentone, anaesthesia was maintained with 70% nitrous oxide in oxygen. Ventilation was controlled. The inspired concentration of the inhalation agent was increased incrementally, halothane in steps of 0.5% up to 2.5%, and enflurane in steps of 1% up to 5%. With both agents, linear dose-related increases were seen in the latencies of waves III, V, Pa and Nb and the interpeak intervals I-V and III-V, with decreases in the amplitudes of Pa and Nb. In five of the patients the inhalation agent was discontinued at the end of the test period, resulting in reversal of the changes in some or all of these waves. End-tidal carbon dioxide tension was controlled and variations of temperature and arterial pressure were insufficient to produce the observed changes. The results show that halothane and enflurane delay neural transmission along the brainstem and cortical sections of the auditory pathway and that the effects of these agents are approximately related to their known anaesthetic potencies.

Adolescent↗
PubMed1984

Changes in lung volume and (PAO2-PaO2) during anaesthesia.

The theory, that the decrease in lung volume which occurs after induction of general anaesthesia is the cause of the higher (PAO2-PaO2) during anaesthesia, was examined in 18 patients. Lung volume was increased during anaesthesia by changing the posture of the patient to 30 degrees head-up, but there was no improvement in (PAO2-PaO2). There were no correlations between change in (PAO2-PaO2) and in cardiac output between different postures. It is concluded that gas exchange is independent of change in lung volume during anaesthesia, at least over the range examined. It remains possible that restoration of lung volume to pre-anaesthetic values may not restore regional distribution of ventilation to pre-anaesthetic values.

Adolescent↗
PubMed1981

Respiratory mass spectrometry during general anaesthesia.

The performance of a respiratory mass spectrometer, modified commercially for use during general anaesthesia, has been evaluated. Stability and linearity proved satisfactory for monitoring respiratory gases and accurate measurement was possible when the inspired concentration of all gases was less than 80%. The system has been installed centrally in a three-room operating theatre suite and the selection of long sampling probes which permit an adequate response time for all gases is discussed.

Anesthesia, General↗
PubMed1981

Non-invasive measurement of cardiac output during anaesthesia. An evaluation of the soluble gas uptake method.

A technique for measuring cardiac output which depends on the uptake of an inert soluble gas from the lungs has been evaluated during anesthesia. A respiratory mass spectrometer has been used to follow the concentrations of argon and freon-22 during passive rebreathing in anaesthetized patients before cardiopulmonary bypass. Values for cardiac output obtained with this technique are reproducible, but lower than those recorded using the direct Fick technique before and after the rebreathing manoeuvre. A reduction in cardiac output caused by vigorous rebreathing is the most likely explanation for the discrepancy and, although serial measurements of oxygen consumption may permit application of a correction factor, a method of measurement which causes significant haemodynamic disturbance cannot be recommended for widespread use.

Adult↗