Effect of remifentanil compared with fentanyl on intraocular pressure after succinylcholine and tracheal intubation{dagger}

H.-P. Ng1, F.-G. Chen1,*, S.-M. Yeong1, E. Wong2 and P. Chew2

Departments of 1Anaesthesia and 2Ophthalmology, National University Hospital, Singapore, 5 Lower Kent Ridge Road, Singapore 119074

{dagger}Presented in part at the International Anaesthesia Research Society, 2000.

Accepted for publication: July 3, 2000

Abstract

Rapid sequence induction using succinylcholine is associated with an increase in intraocular pressure (IOP). This may lead to loss of ocular contents in open globe injuries. No method has previously been shown to prevent this increase in IOP. We investigated whether remifentanil, an ultra-short-acting opioid, could attenuate this increase in IOP during rapid sequence induction of anaesthesia. Forty-five patients were randomized blindly to receive remifentanil 1 µg kg–1, fentanyl 2 µg kg–1 or placebo 1 min before thiopental, succinylcholine and tracheal intubation. IOP and haemodynamic variables were measured before, 1 min after the test solution, 30 s after thiopental, 30 s after succinylcholine, immediately after intubation and then every 3 min for 9 min. Remifentanil obtunded the increase in IOP after succinylcholine and intubation, so it could be suitable for use in open globe injuries.

Br J Anaesth 2000; 85: 785–7

Keywords: eye, intraocular pressure; Tonopen; neuromuscular block, succinylcholine; anaesthetics i.v., thiopental; analgesics opioid, remifentanil

Rapid sequence induction is an established technique in emergency anaesthesia to minimize the risk of pulmonary aspiration. Succinylcholine is the neuromuscular blocking agent of choice in this clinical situation because of its short onset time. It is, however, associated with an increase in intraocular pressure (IOP). Laryngoscopy and tracheal intubation further aggravate the increase in IOP.

Remifentanil is an ultra-short-acting mu-receptor agonist. It has a rapid onset of analgesia, with a peak effect 1 min after administration.1 Alexander, Hill and Lipham found that remifentanil 1 µg kg–1 given after induction with propofol 2 mg kg–1 prevented an increase in IOP after succinylcholine and tracheal intubation.2 The effect of remifentanil with thiopental on IOP has not been described. This study was undertaken to determine if a bolus of remifentanil given before induction of anaesthesia with thiopental can obtund the IOP effects and haemodynamic changes associated with rapid sequence induction.

Methods and results

After obtaining Hospital Ethics Committee approval and informed consent, we studied 45 unpremedicated (ASA I or II) patients scheduled for elective surgery. Patients with ocular, respiratory or cardiovascular disease were excluded. The patients were randomized into three groups using sealed envelopes. Group R received remifentanil 1 µg kg–1, group F received fentanyl 2 µg kg–1 and group P received normal saline as test solution. An applanation tonometer (Tonopen XL, Mentor O&O, Norwell, Massachusetts, USA) was used to measure the IOP in triplicate (mean value recorded) by one of the investigators (H.-P.N. or F.-G.C.) who was unaware of the test drug to be given.

Amethocaine 1% drops were instilled on the patient’s left eye and baseline IOP was measured (time I). After 3 min of preoxygenation, a bolus of test solution diluted to 10 ml was administered over 30 s. Measurement was repeated 1 min later (time II) and anaesthesia was then induced with i.v. thiopental 5 mg kg–1. At loss of eyelash reflex (time III), IOP was recorded and succinylcholine 2 mg kg–1 administered. Thirty seconds later (time IV), IOP was measured. Laryngoscopy and intubation were then performed and IOP was measured immediately and every 3 min for 9 min (times V–VIII). Anaesthesia was maintained with 1% isoflurane and 66% nitrous oxide in oxygen and ventilation adjusted to maintain normocarbia. At each IOP measurement, mean arterial pressure (MAP), heart rate, oxygen saturation, end-tidal PCO2, peak airway pressure and inspiratory concentration of isoflurane were recorded. Intravenous atracurium was given after time VI to maintain muscle relaxation. Surgery only began after the study was complete.

Physical characteristics (except for gender, which was analysed using the {chi}2 test), IOP, MAP and heart rate between groups were compared using one-way analysis of variance (ANOVA) with post hoc Bonferroni correction. Intra-group comparisons of IOP, MAP and heart rate differences from baseline were done using repeated measures ANOVA and post hoc Bonferroni correction. Results are expressed as mean (SD). A P value of <0.05 was considered significant.

There were no significant differences in physical characteristics (age, ASA status, gender or height) between groups except for weight between groups F and P (P=0.021). There were 10, 7 and 12 males in groups R, F and P, respectively. Mean (SD) ages were 31.8 (11.1), 35.5 (8.8) and 31.0 (10.1) yr; mean (SD) weights were 59.7 (11.1), 57.9 (7.6) and 69.1 (13.1) kg; and mean (SD) heights were 163.5 (7.0), 161.1 (7.4) and 169.0 (6.2) cm in groups R, F and P, respectively.

The values of heart rate, MAP and IOP are shown in Figure 1. No significant differences in heart rate among the three groups were recorded at any time. When comparing MAP between groups, significant differences were seen at times V–VII. The MAP of group R was significantly lower than that of group P at times V (P=0.018) and VI (P=0.004). Similarly, MAP in group F at times VI (P=0.024) and VII (P=0.006) was significantly lower than that in group P.



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Fig 1 Multiple comparisons of HR, MAP, and IOP at times I to VIII. {dagger}P<0.05 compared with group P, *P<0.05 compared with baseline values within group (n=15 in each group). I=baseline, II=after test solution, III=after thiopental, IV=after suxamethonium, V=immediately after intubation, VI=3 min after intubation, VII=6 min after intubation, VIII=9 min after intubation.

 
There were no significant differences in baseline IOP between the three groups. After giving the test solution, there was a significant decrease, compared with baseline, in IOP in group R (P=0.011) but not in group P or F. Thiopental decreased IOP significantly compared with baseline in all groups (point III in Figure 1) (P<0.001). Succinylcholine and intubation increased IOP in all groups. However, IOP in group R after intubation was not significantly different from that at baseline (point V in Figure 1), unlike that in the other groups (P<0.001 in group P; P=0.046 in group F).

There were significant differences in IOP between the three groups at times V–VII. The IOP of group R was lower than that of group P at times V (P=0.027), VI (P=0.02) and VII (P=0.015). When comparing differences from baseline IOP between the three groups, the difference in group R is lower than that at group P at times II and IV–VII and that of group F is significantly lower than that of group P at times V and VI (P<0.05).

Discussion

Succinylcholine is used during rapid sequence induction despite its effects on IOP because its speed of onset and effect allow intubation in 30–60 s. Its short half-life also allows fast recovery of muscle power if the airway conditions are difficult.

Rocuronium, a new non-depolarizing neuromuscular blocking drug with a short onset time, has been found not to increase IOP.3 Unfortunately, it does not provide as rapid and predictable intubating conditions as succinylcholine unless doses of three times the ED95 are used.4 Its long duration of action may then cause problems in the presence of an unexpectedly difficult airway.

In our study, fentanyl 2 µg kg–1 failed to attenuate the increase in IOP. Sweeney and colleagues found that fentanyl 2.5 µg kg–1 or alfentanil 10 µg kg–1 attenuated the increase in IOP associated with succinylcholine and intubation.5 In their study, the larynx and tracheal were sprayed with lidocaine 1.5 mg kg–1 to reduce the deleterious effects of intubation. This is not usual practice in an emergency because attenuation of the gag reflex potentially increases the risk of aspiration. Our protocol attempts to simulate as closely as possible a rapid sequence induction technique.

Remifentanil is a new short-acting narcotic with a rapid termination of effect and a half-life of 8–10 min. It provides a profound, yet brief, period of analgesia of almost immediate onset. The main untoward effects of remifentanil are respiratory depression, bradycardia and muscle rigidity. We did not observe any of these effects despite giving the bolus of remifentanil over 30 s. There was also no complaint of nausea or difficulty in breathing.

In our study, remifentanil obtunded the increases in IOP associated with succinylcholine and intubation. This may be a result of the short onset time of remifentanil, producing a maximum decrease in IOP by the time of administration of succinylcholine and intubation. We administered succinylcholine at doses of 2 mg kg–1 and none of our patients coughed or gagged during intubation. In the study by Alexander and colleagues,2 succinylcholine 1 mg kg–1 was used. Four patients either coughed or gagged and had to be excluded, possibly because of the low dose of succinylcholine used.

We chose thiopental as an induction agent instead of propofol, as it is more widely used for rapid sequence induction. Propofol decreases IOP more than thiopental, but causes significantly more hypotension.6 Its combination with remifentanil may not be appropriate in elderly or hypovolaemic patients.

This study has limitations. The gold standard for measurement of IOP is the Goldman Tonometer, but this is impractical during rapid sequence induction of anaesthesia. The Tonopen XL is an electronic applanation tonometer which uses a micro strain gauge transducer. Previous studies had shown that IOP measurements made with the Tonopen were sufficiently close to those made with the Goldman tonometer and can be considered clinically accurate.7

The second limitation of this study was that it was conducted in patients with normal eyes rather than on patients with open globe injuries. The sequence of IOP changes in patients with open globe injuries may not be similar to those in patients with normal eyes. In an open globe, IOP is atmospheric and any increase in pressure results in further loss of ocular contents rather than an increase in IOP.

The third limitation of this study is that we used remifentanil at doses of 1 µg kg–1; a dose of 2 µg kg–1 would be equipotent with the dose of fentanyl used (2 µg kg–1). We were concerned about the side-effects of giving a bolus of remifentanil of >1 µg kg–1 over 30 s. The effect of remifentanil on IOP may be more pronounced had we given doses equipotent to that of fentanyl.

In conclusion, we found that remifentanil 1 µg kg–1 obtunded the increase in IOP associated with succinylcholine and tracheal intubation without any unwanted haemodynamic effects.

Footnotes

* Corresponding author Back

References

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