1Glasgow University Department of Anaesthesia, Royal Infirmary, 816 Alexandra Parade, Glasgow G31 2ER, UK. 2University Department of Psychological Medicine, Division of Behavioral Sciences, Academic Centre, Gartnaval Royal Hospital, 1055 Great Western Road, Glasgow G12 0XH, UK 3Present address: Division of Ambulatory Anesthesia, Department of Anesthesiology, Duke University Medical Center, Box 3094, Durham, NC 27710, USA
Accepted for publication: March 30, 2000
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Abstract |
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Br J Anaesth 2000; 85: 396400
Keywords: recovery; anaesthetics i.v., propofol
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Introduction |
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Although it is possible to show that skills related to those required while driving are impaired, the criteria to be used in establishing whether a patient is fit to drive have not been established. Our previous study, which examined the relationship between blood alcohol concentration (BAC) and psychomotor effects,3 attempted to provide a standard by suggesting that the maximum permitted BACs for driving in three European countries (20, 50 and 80 mg 100 ml1) be used as reference points with which recovery from different anaesthetic techniques could be compared.
Propofol was chosen for this study because of its frequent use during day surgery. In this prospective cohort study, psychomotor function and memory were assessed in volunteers during recovery after a target-controlled infusion of propofol using the Diprifusor target-controlled infusion system. Assessments were made using tests of choice reaction time, tracking with secondary reaction time and within-list recognition.
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Methods |
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Subjects with a history of cardiac, pulmonary, neurological, hepatic or renal disease, psychiatric disorder or substance misuse were excluded. Subjects were instructed not to eat for 6 h or drink for 4 h before the study nor to drive a car or operate any machinery in the evening after the study. All subjects were driven home and were accompanied by a competent carer that evening.
Assessments took place during working hours in a laboratory in the Department of Anaesthesia, and lasted approximately 4 h. An intravenous cannula was inserted in the dorsum of the non-dominant hand. Full continuous monitoring, consisting of ECG, non-invasive arterial pressure and peripheral arterial oxygen saturation (SpO2), was instituted. The volunteers then repeated the psychomotor tests to reduce practice effects and to establish a performance baseline. All subjects received a target-controlled infusion of propofol using the Diprifusor target-controlled infusion system which was increased until the subjects were deeply sedated. A minimum score of 6 using the modified Steward coma scale4 (Table 1) was used as the criterion for the endpoint of sedation. The propofol concentration was then reduced to a target of 0.8 µg ml1, maintained at that concentration for 30 min, then reduced to targets of 0.4 and 0.2 µg ml1 for 30 min each. Supplemental oxygen was administered via nasal prongs if the oxygen saturation dropped below 96%. All three assessments of psychomotor performance were carried out three times each over the 30 min periods when the target blood propofol concentration was 0.8, 0.4 and 0.2 µg ml1. This gave a total of 10 assessments for each test, including baseline measurements. At the end of the study period the propofol infusion was discontinued and the patients recovered in the Department until fit enough to be escorted home.
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Primary tracking and secondary visual reaction time
Subjects operated a computer mouse to control an on-screen icon (a cross) with the task of maintaining it in contact with a target circle moving at varying velocity and direction across the screen of a visual display unit (VDU). The primary task score was the time spent on target (i.e. the proportion of the total time spent tracking during which the cross was kept in contact with the circle) expressed as the root mean square error (r.m.s.). While performing the tracking test, subjects had also to respond by pressing the keyboard spacebar when visual signals (star-shaped icons) appeared unpredictably in the periphery of the VDU. Secondary task performance was recorded in milliseconds.
Choice reaction time
Five numbered circles were displayed on the VDU, corresponding spatially to response keys 15 on the adjoining keyboard. During each trial, the subjects dominant hand rested on the keyboard spacebar. After a variable interval, one of the circles randomly changed colour, requiring the subject to lift their hand from the spacebar and press the appropriate response key. Reaction time was expressed in milliseconds and consisted of two components; decision time (time taken to lift hand from the spacebar) and movement time (time taken to move to the response key).
Within-list recognition
Subjects were presented with lists of 23 words, spoken at a rate of one word every 2 s. Seven of the words in each list were each repeated once at some point in the list; the subjects task was to detect these repetitions. By varying the number of words occurring between the repeated words, the detection task also becomes one of memory retention. Previous research has shown that the probability of detecting a repetition declines as the number of intervening words increases; this effect is particularly evident in sedated subjects, suggesting that encoding and retrieval processes are impaired. The number of words intervening between the repetitions of the target words was 0, 1, 2, 4, 8 or 16. The subjects responded to repeated words by raising their thumb. Two lists were presented in each test period.
Data were analysed with balanced analysis of variance using Minitab software version 10. Significant main effects were investigated further by pairwise comparisons of means using t-tests and Bonferroni corrections as appropriate. Within-list recognition data were analysed by non-parametric tests. All-alpha levels were two-tailed; those less than 0.05 were considered to indicate statistical significance.
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Results |
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Psychomotor testing
Choice reaction time
Figure 1 illustrates the statistically significant (P<0.01) increase in choice reaction time as blood propofol concentration increased. Comparison of means by t-test showed that reaction times at all propofol concentrations were significantly longer than at baseline, and also significantly longer at 0.8 µg ml1 than at 0.4 µg ml1.
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Discussion |
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In contrast, within-list recognition did not discriminate significantly between propofol concentrations. The fact that memory performance at low concentrations of propofol was, on occasion, better than at baseline might suggest an unresolved practice effect. Equally, however, it might also reflect better concentration in the lightly sedated state because of reduced distractibility.
Common criticisms of psychomotor tests are that they often lack sensitivity and validity and that there are no well-defined gold-standard tests with normative databases to define criteria for impaired performance. Such criticisms are well founded but, none the less, the present results indicate that, even at low blood propofol concentrations, choice reaction time and a dual task are sensitive to quite subtle impairment. Furthermore, although such tests are artificial, they do reflect critical aspects of skills required when driving and are sensitive to sedative effects in the context of real-life environments requiring skilled performance.9
The question of a meaningful standard against which to compare performance impairment has been addressed by Tiplady10 and by our recent empirical work.3 Tiplady has noted that dose-related effects of alcohol upon psychomotor performance are reasonably well established and has described the consequent potential value of alcohol as a comparator to assess the effects of anaesthetic agents. He has also noted the virtue that BACs are set as arbitrary threshold standards for fitness versus non-fitness to drive. Using the present psychomotor tests, we have previously assessed impairment at BACs reflecting three current European drink-driving limits (20, 50 and 80 mg 100 ml1). The impairment found with propofol 0.2 µg ml1, the lowest dose used in the present study, was comparable to that measured at 20 mg 100 ml1 BAC in our previous experiment. The latter BAC is equivalent to the Swedish drink driving limit and marks evidence for impairment at a considerably lower BAC than the limit of 80 mg 100 ml1 set in the UK. On the basis of such experimental results, one might therefore generalize and conclude that affected individuals may be vulnerable to mishap in real-life situations. Tiplady, however, provides the important caveat that test performance cannot be validated in a strict sense against the likelihood of road accidents (or of accidents at home, for that matter) (reference 10, p. 33).
Given sufficiently sensitive testing conditions and appropriate tests, impairment of mental abilities can be demonstrated for four or more post-operative days.11 The question remains open as to whether driving after receiving an anaesthetic should be banned until psychomotor performance on all tests has returned to normal. It could be argued that a small degree of impairment, similar to that seen with a BAC of 20 mg 100 ml1 (or propofol 0.2 µg ml1 in volunteers), would be acceptable in drivers. The impairment in psychomotor performance found at this concentration must be compared with that measured in patients receiving, for example, tricyclic antidepressants, antihistamine agents for hay fever or analgesics for back pain. This is obviously an issue that will need full and widespread debate within the profession, but is of growing importance with an expanding and increasingly mobile day-case population.
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Footnotes |
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References |
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2 Kortilla K, Linnola M, Ertama P, Hakkinen A. Recovery and simulated driving after intravenous anaesthesia with thiopental, methohexital, propanidid or alphadione. Anesthesiology 1975; 43: 2919[ISI][Medline]
3 Grant S, Millar K, Kenny GNC. Blood alcohol concentration and psychomotor effects. Br J Anaesth 2000; 85: 4016.
4 Steward DJ. A simplified scoring system for the post-operative recovery room. Can Anaesth Soc J 1975; 22: 11113[ISI][Medline]
5 Hope AT, Woolman PS, Gray WM, Asbury AJ, Millar K. A system for psychomotor evaluation; design, implementation and practice effects in volunteers. Anaesthesia 1998; 53: 54550[ISI][Medline]
6 Millar K. Effects of anaesthetic and analgesic drugs. In: Smith AP, Jones DM, eds. Handbook of Human Performance. Academic Press, 1992; 33785
7 Moss E, Hindmarch I, Pan AJ, Edmondson RS. A comparison of recovery after halothane and alfentanil after minor surgery. Br J Anaesth 1987; 59: 97077[Abstract]
8 Munglani R, Andrade J, Sapsford D, Baddeley A, Jones JG. A measure of consciousness and memory during isoflurane administration: the coherent frequency. Br J Anaesth 1993; 71: 63341[Abstract]
9 Millar K, Finnigan F, Hammersley RH. Is residual impairment after alcohol an effect of repeated performance? Aviat Space Env Med 1999; 70: 12430[ISI][Medline]
10 Tiplady B. Alcohol as a comparator. In: Klepper ID, Sanders LD, Rosen M, eds. Ambulatory Anaesthesia and Sedation: Impairment and Recovery. London: Blackwell, 1991; 2637
11 Flatt JR, Birrell PC, Hobbes A. Effects of anesthesia on some aspects of mental functioning of surgical patients. Anesth Crit Care 1984; 12: 31524