The use of electrical stimulation to monitor epidural needle advancement in a porcine model

Muscle twitches elicited with electrical stimulation (ES) during epidural insertion may indicate epidural needle location. We examined the potential application of ES at 5 mA as a continuous method of monitoring the response to epidural needle advancement in a porcine model. Five 20-kg pigs were used in this study. A needle with a stimulating current of 5 mA was inserted at 20 separate levels in each pig. The needle was advanced until a muscle twitch was observed without loss-of-resistance (LOR). The needle position was then assessed using LOR. At the end of the experiment, an autopsy was performed to assess the spinal cord for injury. A total of 100 needle insertions were performed in the 5 pigs. The threshold current in the epidural space was 3.6 +/- 0.6 mA. In 59 of the needle insertions, LOR was not obtained at the depth at which a muscle twitch was initially observed. However, after advancing these 59 needles another 1-2 mm, LOR was obtained. In the other 41 insertions, LOR was observed without further advancement of the needle. Autopsies indicated there were no dural punctures or spinal cord damage in any of the pigs. These observations suggest that ES can be used to signal that the epidural needle is in or approaching the epidural space. However, the high false positive predictive value (59%) makes it impractical and unreliable to detect the precise entry of a needle into the epidural space in pigs.     

Systematic evaluation of the highest current threshold for regional anaesthesia in a porcine model

Background: The purpose of this study was to determine systematically the highest minimal stimulation current threshold for regional anaesthesia in pigs. Methods: In an established pig model for regional anaesthesia, needle placements applying electric nerve stimulation were performed. The primary outcome was the frequency of close needle to nerve placements as assessed by resin injectates and subsequent anatomical evaluation. Following a statistical model (continual reassessment method), the applied output currents were selected to limit the necessary number of punctures, while providing guidance towards the highest output current range. Results: Altogether 186 punctures were performed in 11 pigs. Within the range of 0.3–1.4 mA, no distant needle to nerve placement was found. In the range of 1.5–4.1 mA, 43 distant needle to nerve placements occurred. The range of 1.2–1.4 mA was the highest interval that resulted in a close needle to nerve placement rate of ≥95%. Conclusions: In the range of 0.3–1.4 mA, all resin deposition was found to be adjacent to nerve epineurium. The application of minimal current intensities up to 1.4 mA does not obviously lead to a reduction of epineural injectate contacts in pigs. These findings suggest that stimulation current thresholds up to 1.4 mA result in equivalent needle tip localisation in pigs.     

An in vitro comparison of the electrical conducting properties of multiport versus single-port epidural catheters for the epidural stimulation test

Effective conduction of electricity through a catheter is essential for the success of the epidural stimulation test. In this in vitro study we examined the electrical conductivity of single and multiport epidural catheters (with and without embedded metal elements) after being primed with normal saline. Seven different types of 19-gauge catheters (n = 5), either single-port or multiport catheters, with or without embedded metal elements, were studied. The proximal end of each epidural catheter was connected to the cathode of a nerve stimulator via an electrode adapter. The catheter, primed with normal saline, was placed at the bottom of a syringe filled with 5 different volumes of saline (1, 2, 3, 4 and 5 mL) and attached to an electrode adapter. The voltage of the peripheral nerve stimulator was measured using an oscilloscope. The electrical resistance between the proximal and distal end of the catheter was calculated using Ohm's Law. In catheters without metal elements the electrical resistances were too high to be measured. In catheters that had metal elements, the mean electrical resistances of the same catheter design (single-port or multiport) were similar. However, the electrical resistances of the multiport metal reinforced epidural catheters were significantly lower (P < 0.05) than the single-port metal coil reinforced epidural catheters. The volume of saline in the syringe had no impact on the measured electrical resistances. This study suggests that multiport metal reinforced epidural catheters have low electrical resistances and, thus, are a reasonable alternative to single-port catheters for transmitting sufficient current for performing the epidural stimulation test. On the other hand, epidural catheters without metal elements (single-port or multiport) are not suitable for performing the stimulation test.     

Triggered electromyographic threshold for accuracy of thoracic pedicle screw placement in a porcine model.

STUDY DESIGN: A porcine model of thoracic pedicle screw insertion was used to determine the effect of screw position on triggered electromyographic response. OBJECTIVE: To develop a model of intraoperative detection of misplaced thoracic pedicle screws. SUMMARY OF BACKGROUND DATA: Triggered electromyographic stimulation has been a valuable aid in determining appropriate placement of lumbar pedicle screws. The use of pedicle screws is increasing in the thoracic spine. Misplaced thoracic pedicle screws may have significant implications if the spinal cord is injured. This study was an attempt to determine whether the established lumbar model can be used for thoracic pedicle screws. METHODS: Five 120- to 150-lb domestic pigs had 85 pedicle screws placed bilaterally in the thoracic spine at each level from T6 to T15. Screws were inserted entirely in the pedicle (Group A). After removal of the medial pedicle wall, the screws were reinserted in the pedicle with no neural contact (Group B). The screws were then placed with purposeful contact with the neural elements (Group C). The screws were stimulated, eliciting an electromyographic response in the intercostal muscles for each instrumented level. The type of response noted was classified as either primary (response from appropriate nerve root), secondary (response at different root) or no response (response at different root, no response at appropriate root). RESULTS: Two hundred fifty responses were recorded. A primary response was noted in 72% of recordings. There was a relatively consistent decrease in the triggered electromyographic response from Group A (mean 4.15 +/- 1.80 mA) to Group C (mean 3.02 +/- 2.53 mA) screws (P = 0.0003). There was little difference in the response obtained from Group A to Group B (mean 4.37 +/- 2.48 mA) screws (P > 0.05). When a primary response was recorded, the mean threshold electromyographic response recorded was significantly lower than recordings with secondary and no response recordings (P < 0.05). CONCLUSION: Even though there was a consistent decrease between the A and C screws that was more definitively separated when a primary response was elicited, it was not possible to determine a cutoff trigger electromyographic level that would consistently differentiate intraosseous from epidural pedicle screw placement. Furthermore, this method could not differentiate screws clearly in the pedicle from screws with medial pedicle wall breakthrough. A more direct method of spinal cord monitoring must be established to provide the surgeon with early warning of the potential of neural injury in the placement of thoracic pedicle screws.     

Electrical stimulation of the epidural space using a catheter with a removable stylet

BACKGROUND AND OBJECTIVES: Electrical stimulation can be used to verify the location of epidural catheters. With the traditional technique, the latter must be primed with saline to allow for electrical conduction: any air lock will, thus, hinder the flow of current. Therefore, we set out to explore an alternative mode of stimulation by use of a catheter containing a removable stylet. This study examines the reliability of this new technique. METHODS: In 71 patients undergoing surgery, a thoracic epidural catheter was inserted preoperatively. Loss of resistance was used to identify the epidural space. The TheraCath was introduced 5 cm into the space and connected to a neurostimulator via a 2-headed alligator clip. The intensity, pulse width, and level of myotomal contractions were recorded upon stimulation of the epidural space. A bolus of lidocaine was then injected and the patient assessed for sensory block to ice. RESULTS: The failure rate in proper epidural catheter placement was 8.5%. Epidural stimulation yielded a mean threshold of 1.90 +/- 1.80 nanocoulombs. A total of 37 catheters produced a unilateral muscular response; however, block to ice was bilateral. When compared with the ice test, the sensitivity, specificity, positive predictive value, and negative predictive value of epidural stimulation with the TheraCath were 92%, 83%, 98%, and 50%, respectively. CONCLUSION: We conclude that the TheraCath, because of the removable stylet, provides effective electrostimulation of the epidural space. Its use was simple and devoid of complications. Nonetheless, further studies are required before implementing its routine use in clinical settings.     

The threshold current in the intrathecal space to elicit motor response is lower and does not overlap that in the epidural space: a porcine model

Purpose

Using electrical epidural stimulation, a current of 1 to 10 mA is required to confirm the presence of the tip of an epidural catheter in the epidural space. The purpose of this study was to examine the hypothesis that the threshold current required to elicit a motor response in the intrathecal space is significantly lower than that in the epidural space in a porcine model.

Methods

Four 20-kg pigs were used in this experiment. Eighteen gauge, insulated, Tuohy needles were advanced into the epidural space using the loss of resistance technique at five different spinal levels in each pig. When the epidural space was entered, an electrical current was applied to the needle and increased progressively until a motor response was elicited. The needle was then further advanced until cerebrospinal fluid (CSF) was observed or until the needle had been advanced a maximum of 1 cm. At this point, the current was reapplied and increased until motor activity was evident.

Results

A total of 20 needles were inserted in four pigs. The current required to produce a motor response in the epidural space was 3.45 ± 0.73 mA (mean ± SD). The current required to produce a motor response in the intrathecal space (entry confirmed by the presence of CSF) was 0.38 ± 0.19 mA (mean ± SD). Two needles were advanced 1 cm without obtaining CSF but the current thresholds were similar to those obtained when CSF was evident (0.4 mA and 0.3 mA, respectively).

Conclusion

The threshold current of an insulated needle required to elicit a motor response in the intrathecal space, was significantly (P < 0.01) lower than that in the epidural space in a porcine model.     

Estimation of the potency of ORG 9426 using two different modes of nerve stimulation

The potency of ORG 9426, a new nondepolarising muscle relaxant, has been estimated using two different modes of nerve stimulation in patients anaesthetised with thiopentone, nitrous oxide-oxygen and intravenous fentanyl. The force of contraction of adductor pollicis was measured following a single twitch (ST) at 0.1 Hz or a train-of-four (TOF) mode of stimulation at 2 Hz every ten seconds. Dose-response curves were constructed using a single-dose method. The ED50, ED90 and ED95 were 147,272 and 305 micrograms.kg-1 respectively using the ST mode and 125,230 and 257 micrograms.kg-1 using the TOF mode of stimulation. The ED50S were not significantly different but the differences between ED90S and ED95S were significant (P less than 0.05) indicating greater sensitivity of the neuromuscular junction using TOF stimulation. The results of this study suggest that the information obtained by single-twitch stimulation is not the same as that obtained from the first response of the TOF stimulation, suggesting apparently increased sensitivity (and apparently greater potency) with the TOF mode of stimulation. Org 9426 appears to be a drug with relatively low potency.     

不同比例缩小体积肝移植的大动物研究

目的研究不同比例缩小体积肝移植的结果,确定猪能耐受的最小体积的肝移植。方法远系繁殖猪70头分为原位全肝移植作为对照组和3组不同比例缩小体积原位肝移植(按照缩小体积的移植肝占受体切除肝脏重量的百分比;1组:60%;2组:30%;3组:20%);实验采用原位经典肝移植方式(静脉转流)。术后第3天和第5天取肝标本。结果对照组和3组不同比例缩小体积肝移植的移植物与受体肝脏重量百分比(GIWRW)分别为87·4%±8·3%、59·9%±5·2%、33·6%±4·9%和22·1%±3·4%;移植物与受体体重百分比(GIWBW)分别2·4%±0·4%、1·43%±0·17%、0·81%±0·09%和0·53%±0·06%。对照组、1组和2组存活率达100%;3组存活率仅为53%。4个组的动物处死后肝脏移植物重量均有显著的增加。结论安全的缩小体积肝移植,应以移植物与受体肝脏重量百分比不小于33%,同时移植肝与受体重量百分比不小于0·8%为限。     

Vasopressor response in a porcine model of hypothermic cardiac arrest is improved with active compression-decompression cardiopulmonary resuscitation using the inspiratory impedance threshold valve

During normothermic cardiac arrest, a combination of active compression-decompression (ACD) cardiopulmonary resuscitation (CPR) with the inspiratory threshold valve (ITV) significantly improves vital organ blood flow, but this technique has not been studied during hypothermic cardiac arrest. Accordingly, we evaluated the hemodynamic effects of ACD + ITV CPR before, and after, the administration of vasopressin in a porcine model of hypothermic cardiac arrest. Pigs were surface-cooled until their body core temperature was 26 degrees C. After 10 min of untreated ventricular fibrillation, 14 animals were randomly assigned to either ACD CPR with the ITV (n = 7) or to standard (STD) CPR (n = 7). After 8 min of CPR, all animals received 0.4 U/kg vasopressin IV, and CPR was maintained for an additional 10 min in each group; defibrillation was attempted after 28 min of cardiac arrest, including 18 min of CPR. Before the administration of vasopressin, mean +/- SEM common carotid blood flow was significantly higher in the ACD + ITV group compared with STD CPR (67 +/- 13 versus 26 +/- 5 mL/min, respectively; P < 0.025). After vasopressin was given at minute 8 during CPR, mean +/- SEM coronary perfusion pressure was significantly higher in the ACD + ITV group, but did not increase in the STD group (29 +/- 3 versus 15 +/- 2 mm Hg, and 25 +/- 1 versus 14 +/- 1 mm Hg at minute 12 and 18, respectively; P < 0.001); mean +/- SEM common carotid blood flow remained higher at respective time points (33 +/- 8 versus 10 +/- 3 mL/min, and 31 +/- 7 versus 7 +/- 3 mL/min, respectively; P < 0.01). Without active rewarming, spontaneous circulation was restored and maintained for 1 h in three of seven animals in the ACD + ITV group versus none of seven animals in the STD CPR group (not significant). During hypothermic cardiac arrest, ACD CPR with the ITV improved common carotid blood flow compared with STD CPR alone. Moreover, after the administration of vasopressin, coronary perfusion pressure was significantly higher during ACD + ITV CPR, but not during STD CPR. IMPLICATIONS: New strategies are needed to improve the efficiency of cardiopulmonary resuscitation (CPR) in hypothermic cardiac arrest. Active compression-decompression CPR with the inspiratory threshold valve improved carotid blood flow (and coronary perfusion pressure with vasopressin) compared with standard CPR.     

Sigmoid colonic reflectance pulse oximetry and tonometry in a porcine experimental hypoperfusion shock model

BACKGROUND: The objective of the present study was to determine if colonic endoluminal mucosal oxygen saturation, as determined by reflectance pulse oximetry, is an appropriate method to monitor colonic ischemia produced by a gradual partial constriction of the aorta in a porcine experimental model. METHODS: Piglets were anesthetized and mechanically ventilated. A Nellcor RS-10 reflectance pulse oximeter probe was attached to a Foley balloon catheter and passed transanally to record the oxygen saturation of the sigmoid colonic mucosa. For comparison, a tonometry catheter was introduced into the sigmoid colon. Hypoperfusion shock was induced by partial gradual constriction of the aorta. The same measurements but no intervention was performed in the control group. RESULTS: During one-third reduction of the aortic blood flow, the plethysmographic signal disappeared suddenly and the sigmoid colonic mucosa oxygen saturation (ScO2) measurements showed zero in seven out of the eight pigs. During two-thirds flow reduction, no plethysmographic signals were observed in any of the piglets. During the reperfusion period, ScO2 measurements with a good plethysmographic signal were obtained immediately after complete release of the aortic constriction in all pigs. Significantly decreased intramucosal pH and increased difference between the arterial and regional PCO2 (PCO2 gap) were observed during two-thirds blood flow reduction. CONCLUSION: In contrast to the results of previous investigations, the sigmoid colonic pulse oximetry was not useful because the method proved to be overly sensitive and non-linear in relation to regional blood flow.     

The placement of the epidural catheter at the predicted site by electrical stimulation test

More accurate segmental and sagittal positioning of the epidural catheter tip is required for the success of continuous epidural analgesia, spinal cord monitoring, and percutaneous epidural spinal cord stimulation. We examined the usefulness of an electrical stimulation test for verifying the proper placement of the epidural catheter tip at the predicted site in the posterior epidural space by using a locally developed epidural catheter with electrodes at its tip. The test included the observation of segmental bilateral muscle twitches and the patient's report of feeling in the region stimulated by moving the epidural catheter electrode back and forth and changing the direction of the bevel of the Tuohy needle. The success rate of midline placement at the required spinal segment was significantly more frequent (99%; P < 0.001) in the group (n = 289) receiving the electrical stimulation test compared with the group (n = 277) not receiving the test (success rate 57%). The results indicate the usefulness of this method. We concluded that the electrical stimulation test is effective for verifying the proper placement of the catheter electrode tip. IMPLICATIONS: Ideally the epidural catheter tip should be positioned in the posterior epidural space near the midline. We concluded that the electrical stimulation test is effective for verifying the proper placement of the catheter electrode tip.     

Determination of epidural catheter placement using nerve stimulation in obstetric patients

Background and Objectives. Peripheral nerve and spinal cord stimulation techniques have been used for many years. However, electrical stimulation methods rarely have been used to confirm epidural catheter placement. This study examines the practicality of this technique to confirm epidural catheter placement in obstetric patients. Methods. Thirty-nine obstetric patients in labor were studied. An electrocardiography (ECG) adapter (Arrow-Johans) was attached to the proximal end of the epidural catheter (19-gauge Arrow Flextip plus). Then, the lowest milliamperage (1–10 mA) necessary to produce a motor response (truncal or limb movement) was applied using this setup. A positive motor response indicated that the catheter was in the epidural space. This test was performed initially after catheter insertion and repeatedly after incremental local anesthetic doses. Results. The sensitivity and specificity of the new test was 100% and 100%, respectively, with 38 true positive tests and 1 true negative test. The threshold current required to produce a positive test increased after incremental doses of local anesthetic. A case of intravascular epidural catheter migration was detected using this new test. In this case, the patient's increased threshold milliamperage returned to the baseline value within 1 hour, despite having received local anesthetic. Intravascular epidural catheter migration was subsequently confirmed by a positive epinephrine test dose. Thus, this test appears to be a potentially useful method to detect intravascular catheter placement. Conclusion. This study demonstrates that this test may have a role in improving the success rate of epidural anesthesia.