Associated factors with umbilical arterial pH after cesarean delivery under spinal anesthesia: a retrospective cohort study

BACKGROUND

Maximum decrease of blood pressure and number of minutes of hypotension were independently associated with umbilical arterial pH. However, the impact of hypotension considering the duration of it on umbilical arterial pH is unknown.

METHODS

Pregnant women aged ≥ 20 years who delivered a baby at full-term via a cesarean delivery under a single-shot spinal anesthesia between January 2017 and March 2019 were included. The main outcome was to predict umbilical arterial pH, based on the value of the time integral of hypotension. Patient demographics, patient comorbidities, and intraoperative data, including the total dose of ephedrine and phenylephrine by fetal delivery and cumulative duration of maternal hypotension, were evaluated. Maternal hypotension was reflected as a decrease in systolic arterial pressure and mean arterial pressure to < 80% of baseline values. The systolic arterial pressure and mean arterial pressure were independently included in a multiple regression analysis along with all other explanatory factors to predict the umbilical arterial pH.

RESULTS

Of the 416 eligible patients, 381 were enrolled. When including the systolic arterial pressure or mean arterial pressure in the model, emergency cases, the total dose of ephedrine, hypertensive disorders of pregnancy, and systolic arterial pressure or mean arterial pressure values were found to be significant predictive factors of umbilical arterial pH.

CONCLUSION

Our results suggest that an elevated time integral of maternal hypotension may have a negative impact on umbilical arterial pH. Therefore, to minimize the risk of fetal acidosis, maternal hypotension should be prevented with the consideration of vasopressors selection.

Anesthesia-related care dissatisfaction: a cohort historical study to reveal related risks

BACKGROUND

Most previous reports have used questionnaires to investigate patient satisfaction regarding anesthesia-related care. We retrospectively investigated the dissatisfaction rate for anesthesia and the contributing factors for it using a questionnaire including anesthesia-related adverse events and a simplified patient satisfaction scale.

METHODS

This is a retrospective review of an institutional registry containing 21,606 anesthesia cases. We conducted multivariate logistic analysis in 9,429 patients using the incidence of dissatisfaction as a dependent variable and other covariates, including items of anesthesia registry and a postoperative questionnaire, as independent variables to investigate factors significantly associated with the risk of dissatisfaction with anesthesia.

RESULTS

In the study population, 549 patients rated the anesthesia service as dissatisfactory. Multivariate analysis identified the preoperative presence of coexisting disease [odds ratio (OR), 1.29; 95% confidence interval (CI), 1.05-1.59], combination of regional anesthesia (OR, 1.44; 95% CI, 1.10-1.88), self-reported awareness (OR, 1.99; 95% CI, 1.29-3.06), postoperative nausea and vomiting (PONV) (OR, 1.54; 95% CI, 1.25-1.90), occurrence of nightmares (OR, 1.96; 95% CI, 1.52-2.53), and the number of days taken to visit a postoperative anesthesia consultation clinic (OR, 1.01; 95% CI, 1.00-1.02) to be independently associated with dissatisfaction with anesthesia service.

CONCLUSIONS

Patients with coexisting disease, undergoing a combination of regional anesthesia, with self-reported awareness, experiencing PONV, suffering from nightmares, and who took longer to visit a postoperative anesthesia consultation clinic tended to rate our anesthesia service as dissatisfactory. Although the exact reasons for the factors contributing to dissatisfaction are unknown, this study suggests that there is room to improve our service.

Tetanic stimulation of the pudendal nerve prior to transcranial electrical stimulation augments the amplitude of motor evoked potentials during pediatric neurosurgery

OBJECTIVE

Reportedly, tetanic stimulation prior to transcranial electrical stimulation (TES) facilitates elicitation of motor evoked potentials (MEPs) by a mechanism involving increased corticomotoneuronal excitability in response to somatosensory input. However, the posttetanic MEP following stimulation of a pure sensory nerve has never been reported. Furthermore, no previous reports have described posttetanic MEPs in pediatric patients. The aim of this study was to investigate the efficacy of posttetanic MEPs in pediatric neurosurgery patients and to compare the effects on posttetanic MEP after tetanic stimulation of the sensory branch of the pudendal nerve versus the standard median and tibial nerves, which contain a mixture of sensory and motor fibers.

METHODS

In 31 consecutive pediatric patients with a mean age of 6.0 ± 5.1 years who underwent lumbosacral surgery, MEPs were elicited by TES without tetanic stimulation (conventional MEPs [c-MEPs]) and following tetanic stimulation of the unilateral median and tibial nerves (mt-MEPs) and the sensory branch of the pudendal nerve (p-MEP). Compound muscle action potentials were elicited from abductor pollicis brevis (APB), gastrocnemius (Gc), tibialis anterior (TA), and adductor hallucis (AH) muscles. The success rate of monitoring each MEP and the increases in the ratios of mt-MEP and p-MEP to c-MEP were investigated.

RESULTS

The success rate of monitoring p-MEPs was higher than those of mt-MEPs and c-MEPs (87.5%, 72.6%, and 63.3%, respectively; p < 0.01, adjusted by Bonferroni correction). The mean increase in the ratio of p-MEP to c-MEP for all muscles was significantly higher than that of mt-MEP to c-MEP (3.64 ± 4.03 vs 1.98 ± 2.23, p < 0.01). Subanalysis of individual muscles demonstrated significant differences in the increases in the ratios between p-MEP and mt-MEP in the APB bilaterally, as well as ipsilateral Gc, contralateral TA, and bilateral AH muscles.

CONCLUSIONS

Tetanic stimulation prior to TES can augment the amplitude of MEPs during pediatric neurosurgery, the effect being larger with pudendal nerve stimulation than tetanic stimulation of the unilateral median and tibial nerves. TES elicitation of p-MEPs might be useful in pediatric patients in whom it is difficult to elicit c-MEPs.

Successful application of early tracheostomy in an intubated patient who suffered from irritative stimuli by an oral tracheal tube

We experienced a case in which simultaneous weaning from sedation and mechanical ventilation were difficult because of instability of tracheal tube fixation that was caused by size mismatch between the trachea and the tube and by severe tracheal deviation. Irritative stimuli caused by the oral tracheal tube prevented conversion from deep sedation to light or no sedation. In this case, very early tracheostomy, which provided better tube fixation and successfully reduced the irritative stimuli to the trachea, was effective to help achieve discontinuation of sedation and facilitated successful weaning from mechanical ventilation. Eventually, the tracheostomy tube was successfully removed immediately after discontinuation of mechanical ventilation.

Current Status of Clinical Engineer Anesthesia Assistants and Their Effect on Labor Task Shifting in Japan: A Prospective Observational Study in a Single Institute

Introduction:

Anesthesiologists are in short supply across the world, resulting in perpetually long working hours. To reduce the burden on anesthesiologists, tasks that can be performed by non-physicians must be shifted to other medical staff. In hospitals, clinical engineers can work as anesthesia assistants and perform some of the duties of anesthesiologists. This study aimed to evaluate the effect of task shift performed by clinical engineer anesthesia assistants (CEAAs).

Methods:

This was a 1-month prospective observational study that included 33 anesthesiologists (11 fellows and 22 certified anesthesiologists) and 11 CEAAs. The total activity and anesthesia times were extracted from the attendance record as indices of the anesthesiologists' work status. The CEAAs recorded the duration of work performed on behalf of the anesthesiologists as task shift time. The task shift rate was evaluated as follows: task shift time/(task shift time + total activity time) and task shift time/(task shift time) + (total anesthesia time).

Results:

The study period consisted of 19 weekdays. The average daily activity time of the anesthesiologists was 10.1 h, and the average anesthesia time was 8.5 h. The CEAAs performed a total of 546.8 h of task shift. The defined task shift rate was 20.1% when the total activity time was the denominator and 23.1% when the anesthesia time was the denominator.

Conclusions:

CEAAs might be effective in reducing the working hours of anesthesiologists through task shift. Their taking over a portion of the anesthesiologists' duties may allow the anesthesiologists to work more efficiently.

Translation and validation of the Japanese Version of the Quality of Recovery-15 Questionnaire

PURPOSE

Patient-reported outcomes after surgery and anaesthesia have recently attracted attention. A recent systematic review and a consensus guideline recommend that patients' recovery can be adequately measured using the quality of recovery-15 (QoR-15) during the perioperative period; however, the Japanese version of the QoR-15 (QoR-15J) is not available. We aimed to translate the QoR-15 into Japanese and assess its validity.

METHODS

After translating into Japanese, 205 patients who underwent various types of surgery under general anaesthesia were enrolled in the study. QoR-15J was evaluated before surgery and 24 h and 48 h after surgery. Additionally, 30 patients answered the QoR-15J at 25 h after surgery. We assessed the feasibility, reliability, validity, and responsiveness of the QoR-15J.

RESULTS

One patient who did not undergo surgery and two patients who were discharged within 24 h were excluded. One hundred and eighty-seven patients answered the QoR-15J at 24 h after surgery (completion rate, 92.6%) and 183 patients with complete data were included in the final analysis. The Cronbach's alpha coefficient for internal reliability and Spearman rank correlation coefficient for test-retest reliability were 0.84 and 0.91, respectively. The Cohen effect size for all items was 1.42.

CONCLUSIONS

QoR-15 was translated into Japanese and the Japanese version's validity was assessed in patients undergoing various types of surgery under general anaesthesia. Our results suggest that QoR-15J is feasible, reliable, valid, and responsive.

Tetanic stimulation of the peripheral nerve augments motor evoked potentials by re-exciting spinal anterior horn cells

Tetanic stimulation of the peripheral nerve, immediately prior to conducting transcranial electrical stimulation motor evoked potential (TES-MEP), increases MEP amplitudes in both innervated and uninnervated muscles by the stimulated peripheral nerve; this is known as the remote augmentation of MEPs. Nevertheless, the mechanisms underlying the remote augmentation of MEPs remain unclear. Although one hypothesis was that remote augmentation of MEPs results from increased motoneuronal excitability at the spinal cord level, the effect of spinal anterior horn cells has not yet been investigated. We aimed to investigate the effect of tetanic stimulation of the peripheral nerve on spinal cord anterior horn cells by analyzing the F-wave. We included 34 patients who underwent elective spinal surgeries and compared the changes in F-waves and TES-MEPs pre- and post-tetanic stimulation of the median nerve. F-wave analyses were recorded by stimulating the median and tibial nerves. TES-MEPs and F-wave analyses were compared between baseline and post-tetanic stimulation time periods using Wilcoxon signed-rank tests. A significant augmentation of MEPs, independent of the level corresponding to the median nerve, was demonstrated. Furthermore, F-wave persistence was significantly increased not only in the median nerve but also in the tibial nerve after tetanic stimulation of the median nerve. The increased F-wave persistence indicates an increase of re-excited motor units in spinal anterior horn cells. These results confirm the hypothesis that tetanic stimulation of the peripheral nerve may cause remote augmentation of MEPs, primarily by increasing the excitability of the anterior horn cells.

Life-threatening airway obstruction caused by angioedema in a morbidly obese postoperative patient: a case report

Background

We report a case of a morbidly obese patient who developed life-threatening airway obstruction due to angioedema.

Case presentation

A 50-year-old Japanese morbidly obese female was treated with enalapril for 10 years, with no history of angioedema. After 3 h of completion of breast cancer resection under general anesthesia with tracheal intubation, she developed airway obstruction and respiratory arrest. Her oral cavity was occupied with a swollen tongue. It was extremely difficult to determine the airway anatomical orientation although tracheal intubation was attempted using a videolaryngoscope. At this time, she probably started gasping respiration, which generated a faint bubble and revealed a possible airway. Her airway was established using a tracheal tube without confirming the glottis or the vocal cord.

Conclusions

Angioedema induced by angiotensin-converting enzyme (ACE) inhibitors is rare; however, once it occurs, it can be potentially life threatening, especially for patients with possible difficult airway. Considering the risk–benefit ratio, we must be careful in administering ACE inhibitor therapy in morbidly obese patients.

Preoperative underweight is associated with adverse postoperative events in patients undergoing microvascular reconstruction surgery for oral and maxillofacial cancer

We aimed to describe the prevalence of postoperative complications and evaluate its relationship with underweight, obesity, preoperative nutritional status, and systemic inflammation status in patients undergoing microvascular reconstruction for oral and maxillofacial cancer. Patients who were ≥20 years old and underwent microvascular reconstruction surgery between January 2009 to June 2019 were investigated. Patient demographics including body mass index, prognostic nutritional status, and neutrophil-lymphocyte ratio were collected. Logistic regression analysis was applied to evaluate these impacts on postoperative complications. A postoperative complication was defined as a Clavien-Dindo classification more than or equal to II. Of the 145 patients included in the analysis, 83 patients (57.2%) experienced postoperative complications, belonging to a Clavien-Dindo classification Ⅱ (n=71), Ⅲb (n=11), and Ⅳa (n=1). Multiple logistic regression revealed that a body mass index less than 18.5 kg/m² (odds ratio 6.19, 95% confidential interval 1.34-28.6, P=0.02) was related to postoperative complications. Another multiple logistic regression model including all explanatory factors found that underweight (P=0.03) was related to postoperative complications. This retrospective study showed that preoperative underweight was associated with postoperative complications as evaluated by the Clavien-Dindo classification.

Preoperative serum biomarkers in the prediction of postoperative delirium following abdominal surgery

AIM

Elderly patients undergoing abdominal surgery have a high risk of developing postoperative delirium. This study aimed to explore risk factors for postoperative delirium by focusing on preoperative hematologic markers, and to develop a model that would be useful in predicting the occurrence of postoperative delirium.

METHODS

This retrospective study included patients aged ≥65 years who underwent elective abdominal surgery under general anesthesia from January 2017 to May 2019. Patients' demographics, hematologic data and intraoperative data were recorded. Neutrophil-to-lymphocyte ratio, platelet-to-lymphocyte ratio, platelet-to-white blood cell ratio and prognostic nutritional index were calculated. We evaluated risk factors of postoperative delirium assessed using a chart-based method for identification of delirium and developed a predictive model using preoperative and intraoperative data.

RESULTS

Of 931 eligible patients, 833 with a mean age of 73.9 years were included in the analysis. The rate of incidence of postoperative delirium was 7.3%. Age, duration of surgery and preoperative platelet-to-lymphocyte ratio or platelet-to-white blood cell ratio were statistically significant risk factors. The predictive model, which included age, duration of surgery and platelet-to-lymphocyte ratio, had the largest area under the curve of 0.77.

CONCLUSIONS

Age, duration of surgery and preoperative platelet-to-white blood cell ratio or platelet-to-lymphocyte ratio are associated with the development of postoperative delirium. A predictive model, which includes age, duration of surgery and preoperative platelet-to-lymphocyte ratio, is useful in predicting the development of postoperative delirium after elective abdominal surgery. Geriatr Gerontol Int 2020; 20: 1208-1212.

Cerebrospinal fluid drainage to prevent postoperative spinal cord injury in thoracic aortic repair

BACKGROUND

Cerebrospinal fluid drainage (CSFD) is recommended as a spinal cord protective strategy in open and endovascular thoracic aortic repair. Although small studies support the use of CSFD, systematic reviews have not suggested definite conclusion and a large-scale study is needed. Therefore, we reviewed medical records of patients who had undergone descending and thoracoabdominal aortic repair (both open and endovascular repair) at multiple institutions to assess the association between CSFD and postoperative motor deficits.

METHODS

Patients included in this study underwent descending or thoracoabdominal aortic repair between 2000 and 2013 at 12 hospitals belonging to the Japanese Association of Spinal Cord Protection in Aortic Surgery. We conducted a retrospective study to investigate whether motor-evoked potential monitoring is effective in reducing motor deficits in thoracic aortic aneurysm repair. We use the same dataset to examine whether CSFD reduces motor deficits after propensity score matching.

RESULTS

We reviewed data from 1214 patients [open surgery, 601 (49.5%); endovascular repair, 613 (50.5%)]. CSFD was performed in 417 patients and not performed in the remaining 797 patients. Postoperative motor deficits were observed in 75 (6.2%) patients at discharge. After propensity score matching (n = 700), mixed-effects logistic regression performed revealed that CSFD is associated with postoperative motor deficits at discharge [adjusted odds ratio (OR), 3.87; 95% confidence interval (CI), 2.30-6.51].

CONCLUSION

CSFD may not be effective for postoperative motor deficits at discharge.

Anesthetic management using a combination of anterior quadratus lumborum block and erector spinae plane block for robot-assisted partial nephrectomy: two case reports

BACKGROUND

There has been increasing attention regarding quadratus lumborum block (QLB) and erector spinae plane block (ESPB) as effective truncal blocks. There have been reports of combined QLB and ESPB usage in hip surgery resulting in a symbiotic increase in effectiveness. However, there have been no reports regarding robot-assisted partial nephrectomy (RAPN), which requires multiple port holes ranging from near the xiphoid process to below the umbilicus. We hypothesized that the combined use of QLB and ESPB was an option for anesthesia and analgesia during RAPN.

CASE PRESENTATION

Anterior QLB and ESPB were applied to two patients undergoing scheduled RAPN. With intravenous patient-controlled analgesia, the post-surgery numerical rating scale scores were < 3/10 at rest and < 5/10 upon movement, throughout the perioperative time.

CONCLUSIONS

The combination of QLB and ESPB could be an option for the postoperative analgesia in RAPN.

Impact of intraoperative hypotension and blood loss on acute kidney injury after pancreas surgery

Purpose

This study aimed to investigate factors associated with postoperative Acute Kidney Injury (AKI) focusing on intraoperative hypotension and blood loss volume.

Methods

This was a retrospective cohort study of patients undergoing pancreas surgery between January 2013 and December 2018. The primary outcome was AKI within 7 days after surgery and the secondary outcome was the length of hospital stay. Multivariate analysis was used to determine explanatory factors associated with AKI; the interaction between the integrated value of hypotension and blood loss volume was evaluated. The differences in length of hospital stay were compared using the Mann-WhitneyU-test.

Results

Of 274 patients, 22 patients had experienced AKI. The cube root of the area under intraoperative mean arterial pressure of < 65 mmHg (Odds Ratio = 1.21; 95% Confidence Interval 1.01–1.45; p =  0.038) and blood loss volume of > 500 mL (Odds Ratio = 3.81; 95% Confidence Interval 1.51–9.58; p =  0.005) were independently associated with acute kidney injury. The interaction between mean arterial hypotension and the blood loss volume in relation to acute kidney injury indicated that the model was significant (p <  0.0001) with an interaction effect (p =  0.0003). AKI was not significantly related with the length of hospital stay (19 vs. 28 days, p =  0.09).

Conclusion

The area under intraoperative hypotension and blood loss volume of > 500 mL was associated with postoperative AKI. However, if the mean arterial pressure is maintained even in patients with large blood loss volume, the risk of developing postoperative AKI is comparable with that in patients with small blood loss volume.

Ocular blood flow by laser speckle flowgraphy to detect cerebral ischemia during carotid endarterectomy

Laser speckle flowgraphy (LSFG) is a noninvasive technique that can measure relative blood flow velocity in the optic fundus contributed by the ophthalmic artery, the main first branch originating from the internal carotid artery (ICA). The aim of this study was to assess the feasibility of ocular blood flow measurement by LSFG to detect ischemic stress due to carotid clamping during carotid endarterectomy (CEA). Nineteen patients undergoing CEA with ocular blood flow measurement by LSFG and intraoperative monitoring (IOM) were prospectively enrolled between August 2016 and March 2019. The mean blur rate (MBR) of ocular blood flow by LSFG, representing relative blood flow of the branch of the retinal artery originating from the optic nerve head, was compared between before and after carotid clamping during CEA. The correlation between the reduction ratio of MBR and the regional saturation oxygen (rSO₂) index by near infrared spectroscopy was investigated. Ocular blood flow measurement by LSFG could not be performed in one patient with a severe cataract. In the other 18 patients, LSFG could be performed in all 106 sessions during surgery. The MBR reduction ratio between before and after carotid clamping ranged from - 12 to 100%. The MBR reduction ratio was positively correlated with the rSO₂ index (r = 0.694, 95% confidence interval: 0.336-0.877, p = 0.001). The MBR reduction ratio of ocular blood flow by LSFG after carotid clamping was significantly correlated with the rSO₂ index. The ocular blood flow by LSFG could be considered an adjunct modality for evaluating cerebral ischemic tolerance during CEA.

Perianesthetic death: a 10-year retrospective observational study in a Japanese university hospital

BACKGROUND

Studies reporting on perianesthetic death and anesthesia-related death are limited. The present study aimed to assess the incidence of perianesthetic death and its relation to anesthesia and to describe the patient characteristics and main events leading to death in cases of anesthesia-related death and anesthesia-contributory death.

METHODS

We conducted a retrospective chart review of patients in whom anesthesia procedures were performed by anesthesiologists at a Japanese tertiary hospital between January 2008 and December 2017. Perianesthetic death was defined as death occurring within 48 h of an anesthetic, and it was divided into the following three categories: anesthesia-related death, anesthesia-contributory death, and nonanesthesia-related death. Patient demographics and perioperative factors were analyzed in cases of anesthesia-related death and anesthesia-contributory death.

RESULTS

Among 46,378 patients who underwent anesthetics, 41 experienced perianesthetic death, with an incidence of 8.8/10,000 anesthetics (95% confidence interval [CI], 6.1-11.6). No patient experienced anesthesia-related death, whereas 10 experienced anesthesia-contributory death, with an incidence of 2.1/10,000 (95% CI, 0.69-3.6), and 31 experienced nonanesthesia-related death, with an incidence of 6.8/10,000 (95% CI, 4.2-9.1). The events leading to anesthesia-contributory death were hypovolemia, myocardial infarction, arrhythmia, and respiratory failure, and they occurred during anesthesia maintenance in 5 patients and after surgery in 5 patients.

CONCLUSIONS

The incidence of perianesthetic death was 8.8/10,000 anesthetics; however, anesthesia-related death was not detected. Ten patients experienced anesthesia-contributory death, and hypovolemia during or after surgery was most frequently associated with anesthesia-contributory death.

A case of subcutaneous emphysema/mediastinal emphysema during the use of humidified high-flow nasal cannula

Background

Heated, humidified, high-flow nasal cannula (HHFNC) oxygen therapy allows optimal humidification of inspired gas at high flows and creates a distending pressure similar to nasal continuous positive airway pressure [1]. It has been safely used in adults with moderate hypoxemia with few complications [2, 3]. Hereby, we report serious complications occurred during HHFNC oxygen therapy.

Case presentation

A 53-year-old female with hemophagocytic lymphohistiocytosis (HLH) was admitted to the intensive care unit because of respiratory failure. After weaning from mechanical ventilation which lasted for 2 weeks, HHFNC therapy at 40 L/min with an FiO2 of 0.5 was started for hypoxemia. Four days later, dyspnea and hypoxemia occurred and chest X-ray and CT scan revealed localized pneumothorax, subcutaneous emphysema, and massive pneumomediastinum. After cessation of HHFNC, respiratory condition improved.

Conclusion

Subcutaneous emphysema, pneumothorax, and pneumomediastinum should be notified as a serious complication during HHFNC therapy.

Feasibility of adjunct facial motor evoked potential monitoring to reduce the number of false-positive results during cervical spine surgery

OBJECTIVE

False-positive intraoperative muscle motor evoked potential (mMEP) monitoring results due to systemic effects of anesthetics and physiological changes continue to be a challenging issue. Although control MEPs recorded from the unaffected side are useful for identifying a true-positive signal, there are no muscles on the upper or lower extremities to induce control MEPs in cervical spine surgery. Therefore, this study was conducted to clarify if additional MEPs derived from facial muscles can feasibly serve as controls to reduce false-positive mMEP monitoring results in cervical spine surgery.

METHODS

Patients who underwent cervical spine surgery at the authors' institution who did not experience postoperative neurological deterioration were retrospectively studied. mMEPs were induced with transcranial supramaximal stimulation. Facial MEPs (fMEPs) were subsequently induced with suprathreshold stimulation. The mMEP and subsequently recorded fMEP waveforms were paired during each moment during surgery. The initial pair was regarded as the baseline. A significant decline in mMEP and fMEP amplitude was defined as > 80% and > 50% decline compared with baseline, respectively. All mMEP alarms were considered false positives. Based on 2 different alarm criteria, either mMEP alone or both mMEP and fMEP, rates of false-positive mMEP monitoring results were calculated.

RESULTS

Twenty-three patients were included in this study, corresponding to 102 pairs of mMEPs and fMEPs. This included 23 initial and 79 subsequent pairs. Based on the alarm criterion of mMEP alone, 17 false-positive results (21.5%) were observed. Based on the alarm criterion of both mMEP and fMEP, 5 false-positive results (6.3%) were observed, which was significantly different compared to mMEP alone (difference 15.2%; 95% CI 7.2%-23.1%; p < 0.01).

CONCLUSIONS

fMEPs might be used as controls to reduce false-positive mMEP monitoring results in cervical spine surgery.