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Anesth Analg 2001;93:954-959
© 2001 International Anesthesia Research Society

ANESTHETIC PHARMACOLOGY

Reexamined: The Recommended Endotracheal Intubating Dose for Nondepolarizing Neuromuscular Blockers of Rapid Onset

Aaron F. Kopman, MD*, Monika M. Klewicka, MS{dagger}, and George G. Neuman, MD{ddagger}

Departments of *Anesthesiology and {ddagger}Clinical Anesthesiology, New York Medical College, Valhalla, New York; and {dagger}Department of Anesthesiology, St. Vincent’s Hospital, New York, New York

Address correspondence and reprint requests to Aaron F. Kopman, MD, Department of Anesthesiology (Room NR 408), St. Vincent’s Hospital & Medical Center, 170 West 12th St., New York, NY 10011. Address e-mail to akopman{at}rcn.com


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
The results of any study of the relative importance of anesthetic depth versus intensity of neuromuscular block on conditions for endotracheal intubation can be manipulated by the investigator. Several independent factors, such as the depth of hypnosis induced, the interval between drug administration and laryngoscopy, the onset profile of the muscle relaxant, and the multiple of the 95% effective dose given, must be controlled. We attempted to design an induction sequence that provided good to excellent conditions for laryngoscopy and endotracheal intubation within 75–90 s of muscle relaxant administration with doses smaller than often suggested, while still administering only customary amounts of hypnotics and opioids. Alfentanil 12.5 µg/kg, propofol 2.0 mg/kg, and a test drug were administered rapidly. The test drugs were saline 0.05 mL/kg (n = 10), rapacuronium 1.0 or 1.2 mg/kg, or rocuronium 0.50 mg/kg (n = 30 each). Laryngoscopy was commenced 75 s after the test drug. Clinically acceptable conditions for intubation were achieved in all subjects after rocuronium or rapacuronium 1.2 mg/kg and in 28 of 30 patients after rapacuronium 1.0 mg/kg. In the Saline group, only 3 individuals achieved a good or excellent rating, and intubation was impossible in 2 of 10 individuals. For muscle relaxants of low potency, doses only 1.5 times the 95% effective dose can provide very satisfactory conditions for intubation if laryngoscopy is delayed to 75 s after drug administration.

IMPLICATIONS: The dose of muscle relaxant usually recommended for facilitating tracheal intubation approximates at least two times the drug’s effective dose (ED95). When the muscle relaxant in question has a rapid onset of action, this intubation dose may be decreased to 1.5 times the ED95.


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
Asubject of continuing interest to anesthesiologists is how best to create conditions conducive to ease of laryngoscopy and atraumatic placement of a tracheal tube. The topic still fascinates clinicians because we do not yet have a nondepolarizing muscle relaxant with an onset/offset profile similar to succinylcholine but without its many side effects.

Pino et al. (1) were unable to intubate the trachea of any of 10 subjects who received a placebo muscle relaxant. Kahwaji et al. (2) reported results only slightly better, with intubation scores of poor or impossible in 19 of 20 subjects when muscle relaxants were not administered. However, it is possible to devise protocols that do not use muscle relaxants and provide a very large percentage of good to excellent conditions for intubation within 90 s of induction of anesthesia (3,4). Consequently, the results of any study of the relative importance of anesthetic depth versus depth of neuromuscular block on conditions for intubation can be manipulated by the investigator. Several independent factors must be controlled.

  1. Depth of anesthesia induced: satisfactory tracheal intubation can be accomplished under deep anesthesia alone. However, increasing the dose of hypnotics and opioids may result in unacceptable hypotension and still not guarantee optimal conditions in all subjects in the absence of a muscle relaxant.
  2. The multiple of the 95% effective dose (ED95) administered: satisfactory conditions for endotracheal intubation cannot be ensured even after two times the ED95 dose of a neuromuscular blocker if the depth of anesthesia is not adequate (5). The administration of larger multiples of a relaxant’s ED95 provides increased reliability and speeds onset but may impose a price in increased duration of action and (for some drugs) undesirable side effects.
  3. The onset profile of the neuromuscular blocker chosen: since the introduction of atracurium and vecuronium into clinical practice, common usage has defined the "intubating dose" as a mass of drug equal to twice the ED95. After the administration of a drug of slow onset, such as cisatracurium (6), the correct dose for timely intubation may be three to four times the ED95 dose (7). However, muscle relaxants with very rapid onset profiles might provide clinically acceptable conditions for tracheal intubation at less than twice the ED95 dose.
  4. The interval between drug administration and laryngoscopy: ideally, relaxants and hypnotics or opioids should have similar peak onset times. The peak effects of propofol, alfentanil, and rocuronium or rapacuronium are not yet manifest at 60 s after administration. Thus, attempts to achieve intubation within 60 s must rely on doses of relaxant in excess of those required if the clinician is willing to accept a more leisurely intubation interval.

With these variables in mind, we thought it should be possible to design an induction sequence that provided good-to-excellent conditions for laryngoscopy and intubation within 75–90 s of relaxant administration by using doses that are smaller than often suggested, while still administering only usual and customary amounts of hypnotics and opioids.


   Methods
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 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
One-hundred ASA physical status I and II adult patients (ages 18 to 65 yr) undergoing elective surgical procedures were included in this study. All patients were free of neuromuscular disease and had a body mass index >=17.5 and <=27.5. The protocol was approved by our hospital’s Human Subject Review Committee, and informed consent was obtained.

After establishing IV access, midazolam (maximum 2 mg) was administered as deemed necessary. All patients were preoxygenated for 2 min before the induction of anesthesia. Anesthesia was induced with alfentanil 12.5 µg/kg, propofol 2.0 mg/kg IV, and (on the basis of random selection) one of four test drugs. All drugs were administered in rapid succession. Test drugs consisted of saline 0.05 mL/kg, rapacuronium 1.0 mg/kg, rapacuronium 1.2 mg/kg, or rocuronium 0.50 mg/kg. At 75 s after injection of the test drug was completed, the mask was removed from the face, and laryngoscopy commenced. Before placement of the tracheal tube, no other IV or inhaled anesthetics were administered. All attempts at intubation were performed by AFK, who was not informed which test drug was administered. Conditions for tracheal intubation were rated on the basis of the system developed by Agoston (8) and later incorporated into the consensus conference on Good Clinical Research Practice in Pharmacodynamic Studies of Neuromuscular Blocking Agents (9) (Table 1). If tracheal intubation was not accomplished within 45 s from the start of laryngoscopy, intubating conditions were deemed to be unacceptable. At 2–3 min after the administration of the test drug, the indirectly elicited tactile train-of-four count at the adductor pollicis muscle was evaluated.


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  		Table 1.  Scoring Conditions for Tracheal Intubation
 
After 10 patients had been recruited into the Saline group, it became obvious that the induction sequence in this group produced conditions for intubation that were clinically unacceptable in the majority of patients. As a consequence, no further patients were added to this group. The other three groups consisted of 30 subjects each.

The distribution of clinically acceptable and good and excellent conditions in the three groups receiving a muscle relaxant was compared by {chi}2 test with a two-way contingency table (StatView 4.5; Abacus Concepts, Berkeley, CA). Observed differences were considered significant if P < 0.05.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
One-hundred subjects were enrolled in this investigation, and there were no statistically significant demographic differences among the four groups studied. After an induction sequence that consisted of alfentanil 12.5 µg/kg and propofol 2.0 mg/kg, clinically acceptable conditions for intubation were achieved in only 3 of 10 subjects in the absence of a muscle relaxant.

When rapacuronium 1.0 mg/kg was added to this sequence, acceptable conditions were achieved in 28 of 30 patients, but less than excellent conditions were present in 11 individuals. In two individuals, conditions were rated as poor. When the dose of rapacuronium was increased to 1.2 mg/kg, all subjects had clinically acceptable conditions for intubation, and in 25 of 30 subjects, the conditions were rated as excellent. After rocuronium 0.50 mg/kg, 25 intubations were rated as excellent and 5 were rated as good. Although the 1.0 mg/kg rapacuronium group had less frequency of excellent ratings than the other two relaxant groups, our study did not have sufficient power to conclude that this difference was statistically significant (Table 2).


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  		Table 2.  Intubation Scores
 
In the majority of subjects, the tracheal tube was in place with cuff inflated within 90 s of relaxant administration. This interval never exceeded 2 min, even in subjects who presented anatomic impediments to intubation. Finally, even when excellent conditions for intubation were recorded, twitch response at the thumb was generally still quite brisk at 75–90 s after muscle relaxant administration. It was also usually still possible to elicit some indirectly evoked response at the adductor pollicis at 2–3 min after the bolus in most individuals.


   Discussion
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 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
Doses equivalent to or more than twice the ED95 are generally considered to be the proper intubation dose for such commonly used muscle relaxants as vecuronium and atracurium. Larger doses are usually recommended when intubation must be accomplished in <90 seconds with these drugs. Our hypothesis when planning this study was that when using drugs of low potency and fast onset, such as rapacuronium and rocuronium, satisfactory conditions for routine tracheal intubation might be achieved with lower multiples of the ED95 than are customary with more potent muscle relaxants. We believed that 1.5 times the ED95 dose was a reasonable reference point, because this would translate into a dose of rocuronium of approximately 0.50 mg/kg (10), not greatly different from the generally accepted intubation dose of 0.60 mg/kg for that drug (11–13). In addition, for most nondepolarizing muscle relaxants, it seems that the SD about the ED95 approximates 20%–25% of the mean value (coefficient of variation of 20%–25%) (10,14–16). Therefore, 1.5 times the ED95 dose will produce a 95% block or greater in 98% of the population. In a recent study from this department, we reported an ED95 value for rapacuronium of 0.75 mg/kg (14). Thus, rapacuronium 1.0 and 1.2 mg/kg represent 1.3 and 1.6 times the ED95 doses, respectively. Our present data confirm that for these drugs of low potency and fast onset, a dose approximating 1.5 times the ED95 represents a reasonable intubation dose in situations in which a 75- to 90-second time to intubation is acceptable. These results were obtained by using an anesthetic induction sequence that we consider conservative and reasonable.

The ability to facilitate intubation with <=1.5 times the ED95 doses of nondepolarizing muscle relaxants offers the clinician several potential benefits. First, clinical duration is shortened. Antagonism of residual effect can be achieved sooner, and the drug may be used more conveniently in brief surgical procedures (17). Second, economic benefits may accrue from adopting a smaller initial dose. This is particularly true for rapacuronium. The commonly recommended intubation dose for this drug is 1.5 mg/kg (18). Because rapacuronium is available only in 100-mg vials, the clinician who wants to administer this dose has to open two vials for any patient whose weight exceeds 70 kg, thus doubling the acquisition cost of the drug. Finally, the potential for any dose-related drug-induced side effects is reduced.

In this study we did not attempt to collect onset data because we agree with Agoston (8) that "onset time of neuromuscular block at the adductor pollicis should not be considered as a meaningful, quantifiable endpoint defining optimal intubating conditions. Consequently, its meticulous recording in all studies is probably obsolete." If time is taken to properly stabilize neuromuscular monitors after the induction of anesthesia, the resulting protocol may have little relevance to actual clinical practice. For example, Pino et al. (1) could not intubate any of 10 patients who received a placebo relaxant after a fentanyl (2 µg/kg) and propofol (2 mg/kg) induction, and they reported that rocuronium 0.45 mg/kg (a dose essentially the same as the one we used) produced unacceptable conditions in two thirds of subjects. Their results are quite different from those we report in this study. Why this marked discrepancy in outcome? Pino et al. (1) took several minutes to stabilize their mechanomyographic monitoring system before administering the test drug and performing laryngoscopy. We believe that this delay (a variation from normal clinical practice) was of sufficient duration to allow the plasma level of propofol to decrease significantly and thus adversely affect conditions for intubation. Our results confirm the findings of other investigators that propofol 2.0–2.5 mg/kg plus alfentanil 10–15 µg/kg by itself does not reliably achieve clinically acceptable conditions for tracheal intubation (19).

There is ample evidence that onset of block at the adductor pollicis lags considerably behind the neuromuscular effects seen at muscles that have greater relevance to ease of intubation, such as the laryngeal adductors, diaphragm, and masseter (20–22). Our results corroborate these observations. Rocuronium 0.5 mg/kg or rapacuronium 1.0–1.2 mg/kg did not routinely abolish evoked responses at the adductor pollicis in our patients. This observation may seem odd. If the cited ED95 for rocuronium at the adductor pollicis approximates 0.33 mg/kg, why does an initial bolus of 0.50 mg/kg not abolish evoked responses at the thumb? Probably because the dose-response relationships of muscle relaxants are almost never measured in the first five minutes after the induction of anesthesia. If drift in the neuromuscular response to indirect stimulation is to be avoided, a period of baseline stabilization is required for both mechanomyographic and electromyographic recordings. Marked peripheral vasodilatation is usually observed during this time period. Hand skin temperature may increase by 5°C in the first few minutes of general anesthesia (23,24). Presumably this increase in hand temperature reflects an increase in muscle perfusion, as well as skin blood flow. If this is so, then drug delivery to the muscles of the hand will be enhanced and should result in larger peak drug levels at the myoneural junction. As a consequence, a dose that produces 95% twitch depression at the adductor pollicis after 15 minutes of general anesthesia may result in a lesser degree of block (at the hand) when administered immediately after the induction of anesthesia. Plaud and Donati (25) have demonstrated exactly this phenomenon with mivacurium. Whatever the mechanism, the clinical message is clear. The indirectly evoked muscular response at the hand is not a very useful measure for evaluating readiness for tracheal intubation. A brisk (but diminished) response of the adductor pollicis does not preclude excellent conditions for laryngoscopy.

We are not the first authors to suggest that even modest doses of muscle relaxant may greatly facilitate tracheal intubation. Barclay et al. (26) found that rocuronium 0.3 mg/kg combined with propofol 2.5 mg/kg and alfentanil 10 µg/kg provided a large proportion of optimal intubation conditions at 120 seconds. However, because these authors also started a propofol infusion and administered N2O immediately after the rocuronium bolus, many clinicians may not want to adopt this protocol. Prien et al. (27) also concluded that an ED90 dose of rocuronium will rapidly provide good or excellent intubation conditions in most cases after either alfentanil 20 µg/kg and propofol 2.0–2.5 mg/kg or fentanyl 3 µg/kg and thiopental 4–6 mg/kg. Unfortunately, these results are difficult to interpret for other reasons: after the induction of anesthesia, the authors took more than five minutes to stabilize their neuromuscular monitor. They did not attempt intubation until maximal block at the adductor pollicis was achieved, and their ED90 dose of rocuronium achieved 100% block at the adductor pollicis in almost all subjects.

We are aware of only one other study in which evaluation of intubating conditions after doses of <1.5 mg/kg of rapacuronium was performed. Kahwaji et al. (2) reported clinically acceptable conditions in only 60% of adults (less than 65 years old) after a dose of rapacuronium 1.0 mg/kg. We do not believe that their results refute our observations. First, their protocol mandated laryngoscopy at 60 seconds after a fentanyl-thiopental induction, a sequence rather different from the one we used. Of greater importance, 32% of their patients had poor or worse conditions for intubation after rapacuronium 1.5 mg/kg, and even 2.5 mg/kg produced excellent conditions in only 77% of their patients. These results are simply not in agreement with the observations of most other investigators (18,28,29).

A recent report by Gaspar et al. (30) is of interest because their induction sequence was rather similar to ours. Anesthesia was induced with alfentanil 15 µg/kg and propofol 2 mg/kg. One minute after the induction, patients received a bolus of rapacuronium 1.5 mg/kg. Laryngoscopy was commenced 60 seconds later. By using the same scoring technique that we used, they rated conditions good or excellent in only 13 of 19 attempts (68%). In contrast, we report figures of >90% and 100% after 1.0 and 1.2 mg/kg of rapacuronium, respectively, although our total induction sequence was shorter by 45 seconds. Can a delay of only 15 seconds (laryngoscopy at 75 vs 60 seconds) improve conditions for intubation sufficiently to explain these differences between our results and those of Gaspar et al.? We believe so. Sixty seconds after rapacuronium administration, <70% of the drug’s peak neuromuscular effects are manifest at the adductor pollicis. This increases to 85%, 92%, and 95% at 70, 80, and 90 seconds, respectively (10). It may be argued that the active state of the adductor pollicis has little to do with the ease of intubation. Onset of block at the diaphragm, masseter, and adductors of the vocal cords is clearly more relevant. The only available relevant evidence comes from a single study by Debaene et al. (31). Peak block at the vocal cords occurred at 66 seconds (with an SD of ± 19 seconds) after a dose of 1.5 mg/kg. Thus, at 60 to 75 seconds, a small but substantial number of individuals will still not have achieved maximal block at the larynx.

The significance of proper timing is illustrated when comparing intubation studies performed without muscle relaxants. Hovorka et al. (32) concluded that, after the induction of anesthesia with propofol 2.5 mg/kg plus alfentanil 30 µg/kg and lidocaine 1.5 mg/kg, endotracheal intubation is not recommended without the use of muscle relaxants. However, these authors commenced laryngoscopy only 45 seconds after the induction sequence was complete. In contrast, Davidson and Gillespie (19) reported acceptable conditions for intubation in 93% of subjects with a similar dose of propofol and only alfentanil 20 µg/kg plus lidocaine 1.0 mg/kg, as long as laryngoscopy was delayed until after the eyelash reflex had been abolished (the exact time sequence was not specified).

In situations in which rapid sequence intubation is strongly indicated or in which any somatic reaction to tracheal intubation must be avoided, we do not suggest that the doses of muscle relaxants used in this study are sufficient to guarantee successful intubation within 60 seconds or absolute patient immobility. Similarly, we do not want to extrapolate our results to other anesthetic induction sequences. Nevertheless, when rapid-onset muscle relaxants are administered, we do not believe that it is necessary to exceed intubation doses larger than 1.5 times the ED95 for routine tracheal intubation. Especially where the anticipated duration of surgery is uncertain or likely to be brief, these reduced doses may offer the anesthesiologist practical clinical advantages.

Shortly after the completion of this study, rapacuronium was voluntarily withdrawn from the US market by its manufacturer. It is unclear at this time (March 2001) whether this change in the drug’s status is only temporary. Although in a sense our observations have been superseded by outside events, we believe the principles established remain valid. They apply not only to rocuronium and rapacuronium, but also to other rapid-onset drugs yet to be developed.


   Acknowledgments
 
Supported, in part, by an unrestricted grant from Organon, Inc., of West Orange, NJ.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

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Accepted for publication May 23, 2001.




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Lippincott, Williams & Wilkins Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins and Stanford University Libraries' HighWire Press®. Copyright 2001 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999 HighWire Press