Succinylcholine (SCH) has been used for more than 50 years to facilitate tracheal intubation. Its rapid onset and short duration of action make SCH an excellent choice for morbidly obese patients because hemoglobin desaturation occurs rapidly after apnea, and intubation of the trachea must be accomplished quickly. Because the appropriate intubating dose of SCH in obese patients is unknown, we studied patients undergoing bariatric operations to determine their responses to different dosing regimens of SCH.
After obtaining approval from the Stanford University Medical Center IRB and with informed written patient consent, we studied 45 morbidly obese (body mass index (BMI) >40 kg/m2) adults scheduled for elective laparoscopic gastric bypass surgery. All patients were ASA physical status II and III, were free from neuromuscular disease, and had normal hepatic and renal function. Patients with severe gastroesophageal reflex disease and those with an airway that suggested problematic tracheal intubation (Mallampati score III or IV and a large neck) (1) were excluded.
In the operating room, patients received midazolam, 2 mg IV. Oxygen was administered by face mask until complete denitrogenation of the lungs was demonstrated by an end-tidal oxygen concentration >90% for 3 min. General anesthesia was then induced with fentanyl IV, 3 μg/kg lean body weight (LBW), and propofol IV, 2.5 mg/kg LBW, and the lungs were ventilated via bag and mask with 100% oxygen.
An arm was immobilized, but free movement of the thumb was allowed. An accelerometer was taped to that thumb, and the response to ulnar nerve stimulation of the adductor pollicis muscle at the wrist was recorded using the TOF-Watch SX® Acceleromyograph (Organon Teknika B.V.; Boxtel, The Netherlands). A 5-s, 50-Hz supramaximal tetanic stimulus was administered, and the TOF-Watch SX® was calibrated. For baseline stabilization, single stimuli were then administered at 10-s (0.1 Hz) intervals for 3 min followed by a second calibration and continuation of the 0.1-Hz single twitch stimulation. Thereafter, a single bolus dose of SCH was administered over 5 s in a rapid-flowing IV line in the arm opposite the study arm.
In a randomized, double-blind fashion, patients were assigned to one of three study groups. Patients in Group I received SCH 1 mg/kg ideal body weight (IBW), those in Group II received 1 mg/kg LBW, and those in Group III received 1 mg/kg total body weight (TBW). IBW was calculated using the formula: IBW = 22 × H2, where H is the height of the patient in meters (2). For morbidly obese patients, LBW was estimated as 130% IBW (3). All doses of SCH in each study group were administered in identical filled 20 mL syringes.
After SCH administration, patients' lungs were ventilated by mask with 100% oxygen, and additional propofol was given as needed. The trachea was intubated when after two consecutive stimuli, no further decrease in twitch height was observed. Laryngoscopy conditions were scored based on the guidelines of the Consensus Conference on Good Clinical Research Practice in Pharmacodynamic Studies of Neuromuscular Blocking Agents (4). Conditions were rated as (a) excellent, if laryngoscopy was easy with a relaxed jaw and no resistance to the laryngoscopy blade, if the vocal cords were abducted and did not move, if there was no airway reaction, and if the patient did not move his or her limbs during endotracheal intubation; (b) good, if the jaw was not fully relaxed and if there was slight resistance to the laryngoscopy blade, if the vocal cords were not fully abducted or moving, if there was diaphragmatic movement (cough, hiccups, or breathing) <10 s, or if there was slight limb movement during tracheal intubation; and (c) poor, if there was poor jaw relaxation or if there was active resistance of the patient to laryngoscopy, if the vocal cords were closed, if there was diaphragmatic movement for more than 10 s, or if there was vigorous limb movement during tracheal intubation.
After tracheal intubation, general anesthesia was maintained with 50% nitrous oxide and isoflurane (0.8–1.2% end-tidal concentration) in oxygen. The recovery from neuromuscular block was recorded for 20 min, during which time the patient received no additional muscle relaxants. As recommended in the guidelines of the Copenhagen consensus conference (4), all twitch height data recorded during recovery from neuromuscular block were normalized to the final twitch height value. All data were transferred to a laptop computer using the TOF-Watch SX® software.
The incidence and degree of fasciculation after SCH administration were recorded. Muscle fasciculation was scored as absent (no fasciculation), mild (fibrillations), or gross (fasciculations). Each patient was interviewed on postoperative Day 1 to determine the incidence of myalgias. Patients were asked a standardized set of questions, the third of which was “Do you have discomfort anywhere other than your incision sites?” (5) When the answer was yes, the location, duration, and degree of pain was recorded.
Data are reported as mean ± sd or the incidence of observations. Between-group differences were assessed for significance by one-way analysis of variance followed by Tukey's test. Fisher's exact test was used to compare intubation condition scores in each group. A value of P < 0.05 was regarded as statistically significant. A power analysis indicated that studying three groups of 15 patients would result in a 90% probability to detect a 2-min difference in recovery times assuming an sd of 90 s.
The three study groups were comparable with respect to age, BMI, and gender (Table 1). The onset times and recovery from neuromuscular blockade are shown in Table 2 and Figure 1.
There was no difference in the onset time of maximum neuromuscular blockade among groups. Maximum block was significantly less in Group I. The recovery intervals were significantly shorter in Groups I and II. Table 3 shows the intubating condition scores. In one third of the patients in Group I, intubating conditions were rated poor. In contrast, none of the patients in Group III had poor intubating conditions. After SCH administration, five patients had no muscle fasciculation, 16 had mild muscle fibrillations, and 24 had gross fasciculation, but there were no differences in the incidence or degree of fasciculation among the three study groups (Table 2).
View on laryngoscopy was considered adequate in all patients. In Groups I and III, the tracheas of 13 of 15 patients were intubated on the first attempt, and the tracheas of the remaining two patients were successfully intubated on the second attempt. The tracheas of all patients in Group II were intubated on the first attempt. There were no differences in the incidence of nonincisional pain on the first postoperative day (Table 4).
It is important to establish a safe, secure airway in the morbidly obese patient undergoing general anesthesia, because hemoglobin can desaturate rapidly once the patient is paralyzed for intubation (6). SCH, with its rapid onset and relatively short duration of action, is an excellent choice for tracheal intubation in obese patients.
For average-weight adults, the usual dose of SCH, 1.0 mg/kg, is given on the basis of TBW. Several studies have demonstrated that, after a rapid-sequence general anesthetic induction, satisfactory intubating conditions can be achieved with as little as SCH 0.3 mg/kg TBW, and that for 95% of average-weight patients, intubating conditions are satisfactory with SCH 0.5–0.6 mg/kg TBW (7–8). In average-weight patients, TBW approximates IBW.
In morbidly obese patients, TBW is much heavier than IBW. Because the level of plasma pseudocholinesterase activity (9) and the volume of extracellular fluid (10)determine the duration of action of SCH, and both of these factors are increased in obesity, it is postulated that morbidly obese patients may have larger absolute SCH dose requirements than average-weight patients. A study of moderately obese patients (BMI >30 kg/m2) found a positive correlation between pseudocholinesterase activity and BMI (9). When the dose of SCH was based on TBW, a similar duration of action, as measured by twitch depression, was found in all weight groups. The authors concluded that SCH should be administered based on actual weight or TBW for all patients, including obese patients. The potency of SCH in obese adolescents (BMI >30 kg/m2) was also found to be similar to that in nonobese control subjects when dosed based on TBW (11). However, despite the increasing numbers of bariatric and other operations now being performed on morbidly obese patients, SCH dosing studies have not been performed on this patient population.
Our study found almost identical onset time to maximum block whether SCH was given based on IBW, LBW, or TBW. Maximum twitch depression of the adductor pollicis was achieved in all study groups in approximately 90 s. However, maximum block was only 93% of control in Group I, compared with 99% in Group II and 100% in Group III.
Although all tracheas were intubated successfully on the first or second attempt, intubating conditions were not similar among groups. Conditions were considered good to excellent for all patients only in Group III. Conditions were poor in 5 of 15 (33%) patients when SCH was given based on IBW (Group I) and 4 of 15 (27%) when based on IBW (Group II).
Time to recovery of twitch response also differed among groups. In Group I, recovery to 50% of twitch height occurred in 5 min, compared with almost 7 min for Group II and 8.5 min in Group III. Benumof et al. (12) suggested that if problems were encountered during intubation and subsequent mask ventilation, the duration of action of 1 mg/kg TBW SCH would prevent spontaneous return of ventilation before hemoglobin desaturation occurred. Two subsequent studies of average-weight adults did demonstrate that the recovery of twitch after SCH 1 mg/kg TBW was prolonged enough to allow some patients to experience arterial hemoglobin desaturation, even after complete administration of oxygen (13–14).
Based on these studies, it was suggested that because smaller doses are associated with faster twitch recovery, SCH (0.5 mg/kg TBW) would provide adequate conditions while allowing for a more rapid return of spontaneous breathing and airway reflexes (15). Although reducing the dose of SCH does shorten the duration of action, as measured at the adductor pollicis, the extent of depression of the diaphragm, laryngeal adductors, and upper airway muscles is unknown. Naguib et al. (16) subsequently demonstrated that, even with SCH 0.56 mg/kg TBW, almost two thirds of apneic average-weight patients will experience hemoglobin desaturation before spontaneous diaphragmatic movement and breathing resumes.
Benumof et al. (12) defined the time to 50% twitch recovery as the time to functional recovery with regard to arterial hemoglobin desaturation risk. They believed that this degree of recovery should allow adequate spontaneous ventilation if the patient has a patent airway. Average-weight patients are usually easier to ventilate via mask than are apneic morbidly obese patients. After complete denitrogenation, the hemoglobin of an apneic morbidly obese patient will desaturate to hypoxemic levels in <3–4 min, whereas an average-weight patient has 8–10 min before hypoxemia develops (6). Because recovery to 50% twitch height in our small-dose SCH group did not occur for 5 min, a SCH dose based on IBW will provide less-than-optimal intubating conditions without protecting the morbidly obese patient from hypoxemia if difficulty is encountered with laryngoscopy and mask ventilation.
Despite its advantages, there continues to be reluctance to use SCH because of its association with postoperative myalgias (17). Although it is not possible to separate true SCH myalgias from muscle pain or discomfort which are a result of other factors, (5) the incidence of postoperative nonincisional pain in our study was almost nonexistent, and no patient in any group experienced moderate or severe nonincisional pain.
In summary, for morbidly obese patients successful tracheal intubation can be achieved when 1.0 mg/kg SCH is administered based on IBW, LBW or TBW. None of these dosing regimens will provide both adequate intubating conditions and a safe (short) duration of apnea. For complete neuromuscular paralysis and predictable laryngoscopy conditions, a larger SCH dose (1 mg/kg TBW) is recommended.
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