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Background: Volatile anesthetics produce surgical immobility by suppressing the motor system. The anesthetic action site in the motor pathway is unclear. Anesthetic effects on the whole and the lower portion of motor pathway can be studied by measuring the motor evoked potentials (MEP) and the F wave. This study measured the effect of isoflurane on the MEP and the F wave., Methods: With institutional review board approval, we studied 12 adult patients with American Society of Anesthesiologists physical status I or II. After intubation, anesthesia was maintained with nitrous oxide/oxygen and propofol infusion. MEPs were elicited by transcranial electrical stimuli (train-of-five pulse; stimuli intensity 40-160 mA) through electrodes placed in the scalp at C3/C4 positions and recorded at the anterior tibialis muscle with an Axon Sentinel-4EP monitor. F waves were elicited by an electrode fixed over the posterior tibial nerve at the medial malleolus and recorded at the abductor hallucis muscle. After end-tidal concentration of isoflurane was maintained at 0.5% for 20 min, the MEP and F wave were measured again. MEP and F-wave changes before and after isoflurane were analyzed using paired Wilcoxon test with Bonferroni correction. The difference between the changes in MEP and F wave was analyzed using Friedman's test., Results: Motor evoked potential amplitudes (median, 205 microV; 25th-75th percentiles, 120-338 microV), F-wave amplitude (median, 100 microV; 25th-75th percentiles, 64.2-137.5 microV), and F-wave persistence (59 +/- 29%) were decreased to 0 microV (0-15 microV), 49 microV (12.4-99.6 microV), and 30 +/- 31%, respectively, by 0.5% isoflurane. MEP amplitude suppression was different from the changes in F-wave amplitude and persistence (P < 0.02)., Conclusions: Isoflurane 0.5% suppresses the motor pathway by decreasing both MEP and F wave. The MEP is suppressed more than the F wave.

Unlabelled: Increasing evidence indicates that the spinal cord is an important site of anesthetic action necessary for surgical immobility. Whether clinical hyper- or hypoventilation affects motor neuron excitability during general anesthesia is unknown. To clarify this issue, we studied seven adult ASA physical status I or II patients undergoing elective surgery. Spinal motor neuron excitability was determined by measuring the posterior tibial nerve H reflex and F wave. The baseline H reflex and F wave were recorded before anesthesia using electrodes placed over the soleus and abductor hallucis muscles. After inhaled induction, the end-tidal isoflurane concentration in O2 was maintained at 0.8%. Ventilation was controlled to maintain a steady-state ETCO2 of 25 +/- 1 and 45 +/- 1 mm Hg randomly for 20 min. Then the H-reflex and F wave were recorded. The difference in H reflex and F wave were analyzed using Student's paired t-test. The baseline H-reflex amplitude (6.8 +/- 2.7 mV) decreased to 4.0 +/- 2.0 mV (P < 0.01) at an ETCO2 of 25 mm Hg and to 2.0 +/- 2.2 mV (P < 0.01) at an ETCO2 of 45 mm Hg. The F-wave persistence (100%) decreased to 77% +/- 24% (P < 0.05) at an ETCO2 of 25 mm Hg and to 61% +/- 19% at an ETCO2 of 45 mm Hg (P < 0.01). Changing ETCO2 values affected H-reflex amplitude and F-wave persistence (P < 0.05), which suggests a change of spinal cord motor neuron excitability, which may affect surgical immobility., Implications: The spinal cord is important for preventing patient movement during surgery. The likelihood of movement may be predicted by measuring the spinal motor neuron excitability by using the H reflex and F wave. Our results show that intraoperative hyper- and hypoventilation can change the H reflex and F wave, which may affect the probability of patient movement during surgery.

Background: Recent evidence suggests that the spinal cord is an important site of anesthetic action that produces surgical immobility. Inhalation anesthetics depress the Hoffmann's reflex (H reflex) and F wave, indicating spinal motoneuron suppression. The aim of this study was to assess the correlation between isoflurane-induced immobility and H- and F-wave suppression., Methods: The baseline H reflex and F wave were measured before anesthesia in 15 adult patients. After induction, 1% end-tidal isoflurane was maintained for 20 min before the H and F waves were reelicited. Using an electric stimulus applied to the forearm and grading the response as movement or no movement, the authors increased or decreased the isoflurane concentration in 0.1% steps, depending on the movement responses. The H and F waves were recorded 20 min after each change of isoflurane concentration. The correlation between H- and F-wave suppression and surgical immobility was analyzed using a paired t test with Bonferroni correction., Results: H-reflex amplitude (2.74 +/- 1.63 mV) and F-wave persistence (70.69 +/- 26.19%) at the highest isoflurane concentration that allowed movement response to a stimulus are different (P < 0.01) from these (1.97 +/- 1.46 mV; 43.16 +/- 22.91%) at the lowest isoflurane concentration that suppressed response. At 0.8% isoflurane, the H-reflex amplitude was 3.69 +/- 1.83 mV with movement and 1.01 +/- 1.14 mV without movement (P < 0.01); F-wave amplitude was 0.29 +/- 0.15 mV with movement and 0.11 +/- 0.06 mV without movement (P < 0.01); F-wave persistence was 80 +/- 22.36% with movement and 34.9 +/- 25.75% without movement (P < 0.01)., Conclusions: The degree of H- and F-wave amplitude and F-wave persistence suppression correlates with movement response, suggesting that isoflurane-suppressive action in the spinal cord plays a significant role in producing surgical immobility.

Background: Recent evidence suggests that the spinal cord is an important site of anesthesia that is necessary for surgical immobility, but the specific effect of anesthetics within the spinal cord is unclear. This study assessed the effect of isoflurane and nitrous oxide on spinal motoneuron excitability by monitoring the H-reflex and the F wave., Methods: Eight adult patients, categorized as American Society of Anesthesiologists physical status 1 or 2, who were undergoing elective orthopaedic surgery were anesthetized with 0.6, 0.8, 1.0, and 1.2 times the estimated minimum alveolar concentration (MAC) of isoflurane. Nitrous oxide was added in graded concentrations of 30%, 50%, and 70%, whereas the isoflurane concentration was decreased to maintain a total MAC of 1. The H-reflex of the soleus muscle and the F wave of the abductor hallucis muscle were measured before anesthesia and 15 min after each change of anesthetic concentration. Four or more trials of the H-reflex and 18 trials of the F wave were recorded at each concentration of anesthesia. The effect of the anesthetics on the H-reflex and F wave was analyzed using. Dunnett's test., Results: H-reflex amplitude was decreased to 48.4 +/- 18.6% of preanesthesia level at 0.6 MAC isoflurane and to 33.8 +/- 19.1% when isoflurane concentration increased from 0.6 MAC to 1.2 MAC. F wave amplitude and persistence decreased to 52.2 +/- 33.6% and 44.4 +/- 26% of baseline at 0.6 MAC isoflurane, and to 33.8 +/- 26% and 21.7 +/- 22.8% at 1.2 MAC isoflurane. Isoflurane plus nitrous oxide (total 1 MAC) decreased H-reflex amplitude to 30.4-33.3% and decreased F wave persistence to 42.8-56.3% of baseline., Conclusions: Both isoflurane alone and isoflurane plus nitrous oxide decrease H-reflex and F-wave amplitude and F-wave persistence. These effects suggest that isoflurane and nitrous oxide decrease motoneuronal excitability in the human spinal cord. This may play an important role in producing surgical immobility.

Background: The minimum alveolar concentration (MAC) of volatile anesthetics is decreased during pregnancy, but MAC in the early postpartum period has not been reported. The aim of this study was to determine the MAC of isoflurane and to evaluate the relation between MAC and serum progesterone and beta-endorphin in patients after delivery., Methods: Eight patients undergoing elective bilateral tubal ligation during general anesthesia in the early postpartum period (< 12 h postpartum) and eight patients undergoing this procedure in the late postpartum period (12-25 h postpartum) were studied. Eight patients undergoing bilateral tubal ligation more than 6 weeks after delivery served as control subjects. Anesthesia was induced with propofol and maintained with isoflurane in oxygen to a steady end-tidal concentration of 0.8-1.0 vol% for 10 min. Reaction to a standardized electric stimulation applied to the forearm was graded as positive (gross or delayed movement) or negative. By using the bracketing technique, the concentration of isoflurane was increased or decreased by 0.1 vol%, depending on the positive or negative responses., Results: The MAC (mean +/- SD) in patients in the early postpartum period was significantly less (0.75 +/- 0.17 vol%) than that in control subjects (1.04 +/- 0.12 vol%; P < 0.01) and that in patients in the late postpartum period (0.95 +/- 0.2 vol%; P < 0.05). The difference in MAC values between late postpartum and control was not significant (P > 0.05). There was an inverse correlation between progesterone concentration postpartum and time after delivery (r = -0.527; P = 0.036), but P = 0.744). There was no correlation between plasma progesterone or beta-endorphin and MAC by multiple regression (r = 0.166; P = 0.950)., Conclusions: Isoflurane MAC remains 28% less than normal within the 1st 12 h postpartum and then returns to normal 12-25 h after delivery.