Konrad Maurer

Spinal opioid receptor‐sensitive muscle afferents contribute to the fatigue‐induced increase in intracortical inhibition in healthy humans

We investigated the influence of spinal opioid receptor‐sensitive muscle afferents on cortical changes following fatiguing unilateral knee‐extensor exercise. On separate days, seven subjects performed an identical five sets of intermittent isometric right‐quadriceps contractions, each consisting of eight submaximal contractions [63 ± 7% maximal voluntary contraction (MVC)] and one MVC. The exercise was performed following either lumbar interspinous saline injection or lumbar intrathecal fentanyl injection blocking the central projection of spinal opioid receptor‐sensitive lower limb muscle afferents. To quantify exercise‐induced peripheral fatigue, quadriceps twitch force (Qtw,pot) was assessed via supramaximal magnetic femoral nerve stimulation before and after exercise. Motor evoked potentials and cortical silent periods (CSPs) were evaluated via transcranial magnetic stimulation of the motor cortex during a 3% MVC pre‐activation period immediately following exercise. End‐exercise quadriceps fatigue was significant and similar in both conditions (ΔQtw,pot−35 and −39% for placebo and fentanyl, respectively; P= 0.38). Immediately following exercise on both days, motor evoked potentials were similar to those obtained prior to exercise. Compared with pre‐exercise baseline, CSP in the placebo trial was 21 ± 5% longer postexercise (P < 0.01). In contrast, CSP following the fentanyl trial was not significantly prolonged compared with the pre‐exercise baseline (6 ± 4%). Our findings suggest that the central effects of spinal opioid receptor‐sensitive muscle afferents might facilitate the fatigue‐induced increase in CSP. Furthermore, since the CSP is thought to reflect inhibitory intracortical interneuron activity, which may contribute to central fatigue, our findings imply that spinal opioid receptor‐sensitive muscle afferents might influence central fatigue by facilitating intracortical inhibition.

Preexisting Subclinical Neuropathy as a Risk Factor for Nerve Injury after Continuous Ropivacaine Administration through a Femoral Nerve Catheter

NERVE injury is a well-recognized complication of peripheral nerve blocks. Capdevila et al. reported an incidence of 0.21% in a large prospective multicenter study including more than 1,400 patients with perineural catheters. Beside factors related to the regional anesthesia technique, other causes such as concomitant patient disease, inadequate positioning or manipulation during surgery, and the neurotoxicity of local anesthetics have been implicated. We report on a patient who developed a femoral neuropathy after continuous femoral block. The only risk factor found in this patient was a postoperatively discovered, preexisting subclinical polyneuropathy.

Continuous spinal anesthesia/analgesia vs. single‐shot spinal anesthesia with patient‐controlled analgesia for elective hip arthroplasty

Background:  In total hip replacement surgery several anesthesiological techniques can be used. In this study we compared continuous spinal anesthesia (CSA) and postoperative analgesia vs. single‐shot spinal anesthesia (SPA) and postoperative patient‐controlled intravenous analgesia with morphine (SPA).

Bilateral continuous interscalene block of brachial plexus for analgesia after bilateral shoulder arthroplasty.

CONTINUOUS interscalene block of the brachial plexus is a common technique for analgesia after total shoulder arthroplasty because it provides good postoperative analgesia. Paresis of the ipsilateral hemidiaphragm is a well-described side effect of this technique. 1 Bilateral interscalene block is generally considered an absolute contraindication because total paresis of the diaphragm could lead to respiratory insufficiency. We report a case of bilateral continuous interscalene block of the brachial plexus after bilateral total shoulder arthroplasty.

Impact of Restrictive Intravenous Fluid Replacement and Combined Epidural Analgesia on Perioperative Volume Balance and Renal Function Within a Fast Track Program

BACKGROUND AND OBJECTIVE Key factors of Fast Track (FT) programs are fluid restriction and epidural analgesia (EDA). We aimed to challenge the preconception that the combination of fluid restriction and EDA might induce hypotension and renal dysfunction. METHODS A recent randomized trial (NCT00556790) showed reduced complications after colectomy in FT patients compared with standard care (SC). Patients with an effective EDA were compared with regard to hemodynamics and renal function. RESULTS 61/76 FT patients and 59/75 patients in the SC group had an effective EDA. Both groups were comparable regarding demographics and surgery-related characteristics. FT patients received significantly less i.v. fluids intraoperatively (1900 mL [range 1100-4100] versus 2900 mL [1600-5900], P < 0.0001) and postoperatively (700 mL [400-1500] versus 2300 mL [1800-3800], P < 0.0001). Intraoperatively, 30 FT compared with 19 SC patients needed colloids or vasopressors, but this was statistically not significant (P = 0.066). Postoperative requirements were low in both groups (3 versus 5 patients; P = 0.487). Pre- and postoperative values for creatinine, hematocrit, sodium, and potassium were similar, and no patient developed renal dysfunction in either group. Only one of 82 patients having an EDA without a bladder catheter had urinary retention. Overall, FT patients had fewer postoperative complications (6 versus 20 patients; P = 0.002) and a shorter median hospital stay (5 [2-30] versus 9 d [6-30]; P< 0.0001) compared with the SC group. CONCLUSIONS Fluid restriction and EDA in FT programs are not associated with clinically relevant hemodynamic instability or renal dysfunction.

Interscalene and infraclavicular block for bilateral distal radius fracture.

UNLABELLED Brachial plexus blockade is a suitable technique for surgery of the forearm, because it provides good intraoperative anesthesia as well as prolonged postoperative analgesia when long-acting local anesthetics are used. However, simultaneous blockade of both upper extremities has rarely been performed (1), because local anesthetic toxicity caused by the amount of drug needed to achieve an efficient block on both sides may be a problem. We report a case of successful bilateral brachial plexus block with ropivacaine in a patient with bilateral distal radius fracture, with each fracture requiring an open osteosynthesis. IMPLICATIONS This case report presents the performance of a simultaneous blockade of both upper extremities in a patient who sustained a bilateral distal radius fracture. The patient was known to be difficult to intubate and to have a severe hypersensitivity to opioids.

Prevalence of medication overuse headache in an interdisciplinary pain clinic

BackgroundMedication overuse headache (MOH) has been recognized as an important problem in headache patients although the pathophysiological mechanisms remain unclear. The diagnosis of MOH is based on clinical characteristics defined by the International Headache Society. The aim was the evaluation of the diagnostic criteria of MOH in a mixed population of chronic pain patients to gain information about the prevalence and possible associations with MOH.MethodsData of all patients referred to the interdisciplinary pain clinic at the University Hospital of Zurich between September 2005 and December 2007 were retrospectively analyzed. Demographic data (age, sex, history of migration), as well as data about duration of pain disease, category of pain disease (neurological, psychiatric, rheumatologic, other), use of medication, history of trauma, and comorbidity of depression and anxiety have been collected.ResultsTotally 178 of 187 consecutive chronic pain patients were included in the study. A total of 138 patients (78%) used analgesics on 15 or more days per month. Chronic headache was more prevalent among patients with analgesic overuse (39.8%) than without analgesic overuse (18%). The prevalence of MOH was 29%. The odds ratio (OR) for a patient with medication overuse to have chronic headache was 13.1 if he had a history of primary headache, compared to a patient without a primary headache syndrome. Furthermore, history of headache (OR 2.5, CI [1.13;5.44]), history of migration (OR 2.9, CI [1.31;6.32]) and comorbid depression (OR 3.5, CI [1.46;8.52]) were associated with overuse of acute medication, in general.ConclusionsPrimary headaches have a high risk for chronification in patients overusing analgesics for other pain disorders. Whereas history of headache, history of migration and comorbidity of depression are independentely associated with analgesic overuse in this group of patients.

Protons regulate the excitability properties of rat myelinated sensory axons in vitro through block of persistent sodium currents.

Little information is available on the pH sensitivity of the excitability properties of mammalian axons. Computer‐assisted threshold tracking in humans has helped to define clinically relevant changes of nerve excitability in response to hyperventilation and ischaemia, but in vivo studies cannot directly differentiate between the impact of pH and other secondary factors. In this investigation, we applied an excitability testing protocol to a rat saphenous skin nerve in vitro preparation. Changes in extracellular pH were induced by altering pCO2 in the perfusate, and excitability properties of large myelinated fibres were measured in the pH range from 6.9 to 8.1. The main effect of protons on nerve excitability was a near linear increase in threshold which was accompanied by a decrease in strength‐duration time constant reflecting mainly a decrease in persistent sodium current. In the recovery cycle, late subexcitability following 7 conditioning stimuli was substantially reduced at acid pH, indicating a block of slow but not of fast potassium channels. Changes in threshold electrotonus were complex, reflecting the combined effects of pH on multiple channel types. These results provide the first systematic data on pH sensitivity of mammalian nerve excitability properties, and may help in the interpretation of abnormal clinical excitability measurements.

Sensitivities of rat primary sensory afferent nerves to magnesium: implications for differential nerve blocks.

Context Contrasting findings have been published regarding the role of magnesium sulphate used as an additive to local anaesthetics in peripheral nerve blocks. Objective To clarify the effect of magnesium sulphate on nerve excitability. Setting C and A&bgr; compound action potentials were recorded extracellularly in vitro in saphenous nerves from adult rats. Animals Saphenous nerves (n = 30) from male Wistar rats (n = 19), 12 to 16 weeks old. Intervention Primary sensory afferents were tested with a computerised threshold tracking program (QTRAC) with a supramaximal 1 ms current pulse either alone or after 300 ms of conditioning polarising ramp currents in the presence and absence of 10 mmol l−1 magnesium sulphate, 80 &mgr;mol l−1 lidocaine and a combination of both. Main outcome measures Changes in current thresholds to elicit compound action potential amplitudes of 40% of the maximal response. Results Magnesium sulphate increased excitability thresholds to a greater extent in A&bgr; fibres than in C fibres. It enhanced the effects of lidocaine in both A&bgr; fibres [mixture 0.470 mA (SD 0.105) versus lidocaine 0.358 mA (SD 0.080), P < 0.001] and C fibres [mixture 2.531 mA (SD 0.752) versus lidocaine 2.385 mA (SD 0.656), P = 0.008]. Preconditioning experiments also showed that magnesium sulphate had an enhancing effect with lidocaine in A&bgr; fibres [mixture 0.620 mA (SD 0.281) versus lidocaine 0.543 mA (SD 0.315), P = 0.005], but not in C fibres [mixture 2.412 mA (SD 0.641), lidocaine 2.461 mA (SD 0.693), P = 0.17]. Conclusion These results suggest that the binding of magnesium ions depends on both the type and conformational state of voltage-gated sodium channels. They also may help to explain the conflicting reports regarding the clinical effects of magnesium sulphate as an additive to lidocaine in peripheral nerve blocks.

Continuous elastomeric pump-based ropivacaine wound instillation after open abdominal aortic surgery: how reliable is the technique?

INTRODUCTION We aimed at quantifying the impact of continuous wound infusion with ropivacaine 0.33% on morphine administration and subjective pain relief in patients after open abdominal aortic repair in a double-blind, placebo-controlled study. METHODS Before closing the abdominal wound, 2 multihole ON-Q® Soaker Catheters™ (I-Flow Corporation, Lake Forest, California, USA) were placed pre-peritoneally in opposite directions. Either ropivacaine 0.33% or saline 0.9% was delivered by an elastomeric pump at a rate of 2 mL/h for 72 hours in each of the catheters. Postoperative pain and morphine administration were assessed using the numerical rating scale (NRS) in 4-hour intervals. Total plasma concentrations of ropivacaine, unbound ropivacaine, and α1-acid glycoprotein (AAG) were measured daily. Mean arterial pressure, pulse rate, oxygen saturation, total amount of morphine administration, ventilation time, and length of stay in the intensive care unit (ICU) were recorded. At the end of the study period, the wound site and the condition of the catheters were assessed. RESULTS The study was terminated prematurely due to a malfunction of the elastomeric balloon pump resulting in toxic serum levels of total ropivacaine in 2 patients (11.4 μmol/L and 10.0 μmol/L, respectively) on the second postoperative day. Six patients had been allocated to the ropivacaine group, and 9 patients had been allocated to the control group. Demographic and surgical data were similar in both groups. During the first 3 postoperative days, no difference between the ropivacaine and the control group was found in NRS (P = .15, P = .46, and P = .88, respectively) and morphine administration (P = .48). Concentrations of unbound serum ropivacaine (0.11 ± 0.08 μmol/L) were below toxic level in all patients. CONCLUSION Continuous wound infusion of ropivacaine 0.33% 2 mL/h using an elastomeric system was not reliable and did not improve postoperative pain control in patients after open abdominal aortic surgery.