Ulnar nerve pressure: influence of arm position and relationship to somatosensory evoked potentials.

BACKGROUND: Although the ulnar nerve is the most frequent site of perioperative neuropathy, the mechanism remains undefined. The ulnar nerve appears particularly susceptible to external pressure as it courses through the superficial condylar groove at the elbow, rendering it vulnerable to direct compression and ischemia However, there is disagreement among major anesthesia textbooks regarding optimal positioning of the arm during anesthesia. METHODS: To determine which arm position (supination, neutral orientation, or pronation) minimizes external pressure applied to the ulnar nerve, we studied 50 awake, normal volunteers using a computerized pressure sensing mat. An additional group of 15 subjects was tested on an operating table with their arm in 30 degrees, 60 degrees, and 90 degrees of abduction, as well as in supination, neutral orientation, and pronation. To determine the onset of clinical paresthesia compared to the onset and severity of somatosensory evoked potential (SSEP) electrophysiologic changes, we studied a separate group of 16 male volunteers while applying intentional pressure directly to the ulnar nerve. Data are presented as mean (median; range). RESULTS: Supination minimizes direct pressure over the ulnar nerve at the elbow (2 mmHg [0; 0-23]; n = 50), compared with both neutral forearm orientation (69 mmHg [22; 0-220]; P < 0.0001), as well as pronation (95 mmHg [61; 0-220]; P < 0.0001). Neutral forearm orientation also results in significantly less pressure over the ulnar nerve compared to pronation (P < or = 0.04). The estimated contact area of the ulnar nerve with the weight-bearing surface was significantly (P < 0.0001) smaller in the supine position (2.2 cm2 [0.5; 0-9]; n = 50) compared with both neutral orientation (5.5 cm2 [5.0; 0-13]) and pronation (5.8 cm2 [6; 0-12]). With the forearm in neutral orientation, ulnar nerve pressure decreased significantly (P < or = 0.01; n = 15) as the arm was abducted at the shoulder from 0 degrees to 90 degrees. In the 16 male subjects tested, notable alterations in ulnar nerve SSEP signals (decrease > or = 20% in N9-N9' amplitude) were detected in 15 of 16 awake males during application of intentional pressure to the ulnar nerve. However, eight of these subjects did not perceive a paresthesia, even as SSEP waveform amplitudes were decreasing 23-72%. Two of these eight subjects manifested severe decreases in SSEP amplitude (> or = 60%). CONCLUSIONS: Extrapolating these results to the clinical setting, the supinated arm position is likely to minimize pressure over the ulnar nerve. With the forearm in neutral orientation, pressure over the ulnar nerve decreases as the arm is abducted between 30 degrees and 90 degrees. In addition, up to one half of male patients may fail to perceive or experience clinical symptoms of ulnar nerve compression sufficient to elicit SSEP changes.

Tracheal intubation of healthy pediatric patients without muscle relaxant: a survey of technique utilization and perceptions of safety.

UNLABELLED: We conducted a survey of Society for Pediatric Anesthesia anesthesiologists practicing within the United States to determine the frequency of tracheal intubation of healthy infants and children using an inhaled anesthetic without muscle relaxation (IAWMR). We also examined reasons for the use of this technique. Of all responders who listed their most often used technique for tracheal intubation of healthy infants and children, IAWMR was chosen over intubation with a muscle relaxant by 38.1% and 43.6%, respectively. Anesthesiologists who most often used IAWMR for tracheal intubation of healthy infants and children had over twice the odds (odds ratio [OR] 2.30 for infants, 95% confidence interval [CI] 1.18-4.50; P = 0.015) of classifying their own practice as nonacademic, and one-third the odds (OR 0.34 for infants, 95% CI 0.17-0.68; P = 0.002) of conducting more than half of their cases in a supervisory role. Anesthesiologists who use IAWMR to tracheally intubate healthy pediatric patients most commonly selected as their reasons the lack of need for a muscle relaxant and the desire to avoid both succinylcholine and the excessive duration of nondepolarizing muscle relaxants. IMPLICATIONS: Inhaled anesthetic without muscle relaxation is the most often used method of intubation for more than one third of Society for Pediatric Anesthesia anesthesiologists when tracheally intubating healthy, fasted pediatric patients undergoing elective procedures. The frequency of this practice seems to be highest in nonacademic practices.

Rocuronium versus succinylcholine for rapid-sequence induction using a variation of the timing principle.

STUDY OBJECTIVE: To determine if, using a variation of the "timing" principle, 0.6 mg/kg of rocuronium can achieve an onset time and intubating conditions similar to those achieved with succinylcholine. STUDY DESIGN: Prospective, randomized, double-blind clinical comparison. SETTING: Operating room in a university medical center. PATIENTS: 42 ASA physical status I and II patients undergoing general anesthesia for elective surgery. INTERVENTIONS: All patients were fitted with a Grass FT-10 force transducer attached to the thumb. Supramaximal stimulation was applied to the ulnar nerve with a variable current peripheral nerve stimulator. 22 patients (succinylcholine group) received a placebo bolus injection followed 20 seconds later by thiopental 4 to 5 mg/kg and succinylcholine 1 mg/kg; 20 additional patients (rocuronium group) received a bolus dose of rocuronium 0.6 mg/kg followed 20 seconds later by thiopental 4 to 5 mg/kg and a placebo bolus injection. MEASUREMENTS AND MAIN RESULTS: We measured the onset time from administration of the muscle relaxant to 95% twitch reduction and assessed the quality of intubating conditions 60 seconds after the induction of anesthesia. There was a significant difference in the mean onset time of rocuronium (72 sec) versus succinylcholine (42 sec, p < 0.0001). However, there was no significant difference in intubating conditions 60 seconds after administration of thiopental. CONCLUSION: Rocuronium given 20 seconds prior to thiopental provides intubating conditions equivalent to thiopental-succinylcholine for rapid-sequence inductions, circumventing rocuronium's longer onset time to 95% neuromuscular blockade.

Fluoroscopy-assisted intubation of a child with an unstable subluxation of C1/C2.

Patients presenting with unstable cervical spine injuries are at risk for additional neurological injury as a consequence of airway manipulation. Techniques of awake intubation may not always be desirable or practical, particularly in the pediatric patient. We describe the use of fluoroscopy during the induction of anesthesia and intubation of a child with an unstable C1/C2 spinal subluxation. Fluoroscopy is readily available and noninvasive. This technique allows for rapid establishment and maintenance of optimal head and neck positioning during induction of general anesthesia and performance of laryngoscopy and tracheal intubation.

Migration of a ventriculoperitoneal shunt into the pulmonary artery.

Ventriculoperitoneal (VP) shunts have been reported to migrate into a number of unusual locations within the abdomen, chest, and pelvis. We report a case in which a documented, correctly placed VP shunt subsequently migrated to an intravascular location. This intravascular migration led to malposition of the shunt tubing within the pulmonary artery. Attempts to remove the catheter via a postauricular incision were complicated by arrhythmias induced by traction on the shunt tubing. Eventual removal of the shunt was accomplished in stages, through the use of an intravascular, radiographically guided snare, introduced through the femoral vein.

Effect of local anesthetic concentration on capillary blood flow in human skin.

OBJECTIVES: The objective of this study was to evaluate and compare the effect of lidocaine, mepivacaine, and bupivacaine on capillary blood flow in humans over therapeutic and subtherapeutic concentrations. METHODS: The effect of each treatment in eight unmedicated male volunteers was measured in a randomized, controlled, double-blind comparison. Each participant received subcutaneous injections (total, 14), at separate sites on the abdomen, consisting of 0.2 ml lidocaine (0.05%, 0.5%, 1%, and 2%), mepivacaine (0.05%, 0.5%, 1%, and 2%), bupivacaine (0.025%, 0.25%, 0.5%, and 0.75%), saline, or saline with epinephrine (5 micrograms/ml), and at an additional site a needle stick was performed and no injection made. Cutaneous blood flow was measured with a laser Doppler capillary perfusion monitor before and for 60 minutes after these interventions. RESULTS: The maximum increase in cutaneous blood flow was 277 +/- 141% to 511 +/- 136% (mean +/- SE) after lidocaine, 124 +/- 110% to 316 +/- 155% after mepivacaine, and 242 +/- 193% to 725 +/- 198% after bupivacaine. The increase in blood flow depended on local anesthetic concentration: low concentrations induced minimal changes, whereas higher concentrations caused great increases in cutaneous blood flow. Injection of saline or needle stick alone increased cutaneous blood flow 285 +/- 237% and 260 +/- 121%, respectively. CONCLUSIONS: Our findings indicate that the trauma of needle stick or saline injection produces a significant increase in cutaneous capillary blood flow. Injection of clinically useful concentrations of bupivacaine and lidocaine produced even greater increases in capillary blood flow, indicating a vasodilatory effect. Injection of the lowest concentrations of lidocaine and bupivacaine caused flow to increase to a magnitude similar to that after injection of saline. In contrast, clinically useful concentrations of mepivacaine do not increase capillary blood flow to a greater extent than saline, and lower concentrations tend to blunt the increase in blood flow, indicating a mild vasoconstrictor effect.

Effect of midazolam and diazepam premedication on central nervous system and cardiovascular toxicity of bupivacaine in pigs.

To determine the effect of benzodiazepine premedication on central nervous system and cardiovascular effects of bupivacaine, the authors administered toxic doses of bupivacaine to awake spontaneously breathing pigs after intravenous premedication with midazolam (0.06 mg/kg), diazepam (0.15 mg/kg), or saline. Five minutes after administration of one of these solutions, they began an infusion of bupivacaine at 2 mg.kg-1.min-1. The bupivacaine infusion was continued until cardiovascular collapse. They then attempted to resuscitate the animals via open chest cardiac massage and a standard resuscitation protocol. Premedication with midazolam or diazepam significantly delayed the onset of ventricular dysrhythmias (P less than 0.05), decreased the incidence of seizures (P less than 0.05), and prevented the increase in blood pressure and heart rate following bupivacaine infusion (P less than 0.05). Benzodiazepine premedication did not affect the dose of bupivacaine or the blood concentration required to produce cardiovascular collapse. The ability to resuscitate animals premedicated with midazolam did not differ from control; however, significantly fewer animals premedicated with diazepam were resuscitated (P less than 0.05). A clinically relevant observation was that almost all animals premedicated with a benzodiazepine progressed directly to cardiovascular collapse without first manifesting seizures.