University Department of Anaesthesia, Critical Care, and Pain Medicine, Royal Infirmary of Edinburgh, Lauriston Place, Edinburgh EH3 9YW, UK*Corresponding author
Accepted for publication: December 17, 2001
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Abstract |
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Methods. We studied 12 patients during anaesthesia with target-controlled infusions of propofol (plasma target concentration 46 µg ml1) and alfentanil (plasma target concentration 4060 ng ml1), having varicose vein surgery.
Results. After the initial skin incision, tidal volume increased promptly by 17 (4, 81)% (median, quartile values) (P<0.01). Respiratory frequency changed variably with no significant change overall [median change 2 (8, +50)%]. The duration of inspiration was virtually unaltered, and the duration of expiration decreased gradually by 5 (7, 32)%. Patients who showed more response also showed more change in tidal volume, so that there was a significant relationship between increased inspiratory flow rate and reduced expiratory time (P<0.05).
Conclusions. During opioid anaesthesia, the mechanism of ventilatory increase after stimulation involves changes in both drive and timing of breathing. This pattern of response does not resemble the changes seen during anaesthesia with potent volatile agents.
Br J Anaesth 2002; 88: 64952
Keywords: anaesthesia, general; anaesthetics i.v., propofol; analgesics opioid, alfentanil; ventilation, surgical stimulus
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Introduction |
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Methods |
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At the time of the first skin incision, we recorded the breaths taken during the next 20 s. No specific instructions were given to the surgical team about further manipulation. The tidal volume values from these breaths were expressed as fractions of the average of the three breaths before incision. We summarized these values and the timing of the breaths as median and quartile values. We compared the proportional changes in timing, tidal volume and drive using paired t-tests (Minitab version 12.1). Inspection of the changes suggested a relationship between the changes in drive and expiratory time which was confirmed by linear correlation.
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Results |
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After surgical skin incision over the saphenous vein junction, minute ventilation increased, with considerable individual variation in the response (Fig. 1). Median tidal volume increased by the second breath by 17% (Fig. 2). However, the range of change was large (between 4 and 81%; quartile values). The duration of inspiration was virtually unaltered, and the duration of expiration decreased more gradually by 5 (7 to 32)% (Fig. 2).
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Discussion |
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We found changes in both aspects of the control of breathing, often categorized as drive and timing. These features, which are the rate of increase in central inspiratory neural activity (indicated by the mean inspiratory flow rate, VT/TI) and the duration of the inspiratory and expiratory phase of the respiratory cycle, are considered to be controlled separately. Mean inspiratory flow rate is a good indication of central neural output, and timing indicates features such as bubopontine control and pulmonary and somatic afferent influences. Together, these measures provide a more fundamental indication of the control process than tidal volume and frequency.7
We found changes that differed from the respiratory response to a painful stimulus in a conscious subject. This is both an increase in respiratory drive, leading to greater mean inspiratory flow and larger tidal volumes, and also a proportionate reduction in the duration of both phases of the respiratory cycle (TI and TE), causing an increase in respiratory rate.1 There is generally a linear relationship between TI and TE, so that if TI is reduced, TE is reduced in proportion,8 and TI/Ttotal remains constant. A painful stimulus applied to patients anaesthetized with enflurane2 increases respiratory drive without a change in timing. However, other, less exact data report that a painful stimulus in patients anaesthetized with isoflurane9 increases minute ventilation by an increase in respiratory rate. The respiratory response to painful stimulation could differ according to the choice of anaesthetic.
In these studies, the anaesthetic was given by inhalation. The delivery of the anaesthetic and the depth of anaesthesia would be affected by the respiratory response to the initial painful stimulus. When using injected agents, as we did, the agents are delivered to the patient independently of the respiratory response. Our present study shows that, when a painful stimulus is applied to patients anaesthetized with propofol and alfentanil, in whom the duration of expiration is already increased, there is both an increase in respiratory drive and a reduction in expiratory duration. During anaesthesia, the effects of opioids are to reduce drive and increase duration of expiration, and both these effects are reduced by the stimulus. However, we did find great variation in the capacity of the stimulus to antagonize these effects, which may have been caused by different degrees of opioid activity in individual subjects.
Target-controlled infusion systems can generate blood concentrations greater than predicted6 and should be adjusted according to the clinical response.10 There may be a kinetic synergism between propofol and alfentanil, as propofol can inhibit the metabolism of alfentanil, and alfentanil can increase the plasma concentration of propofol.11 Propofol and alfentanil also interact pharmacodynamically. The opioid increases the effect of propofol, on consciousness and suppression of the lash reflex.12 Despite these potential effects, we established steady and generally acceptable anaesthesia with the agents before surgical stimulation. We conclude that, during opioid anaesthesia by propofol infusion, the mechanism of ventilatory increase after stimulation involves changes in both drive and timing of breathing.
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Acknowledgement |
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References |
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2
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