The Effect of Sevoflurane and Desflurane on Upper Airway Reactivity

Background: Although bronchial reactivity can be assessed by changes in airway resistance, there is no well-accepted measure of upper airway reactivity during anesthesia. The authors used the stimulus of endotracheal tube cuff inflation and deflation to assess changes in airway reactivity in patients anesthetized with sevoflurane and desflurane.

Methods: Sixty-four patients classified as American Society of Anesthesiologists physical status I or II participated in this randomized, double-blind study. Patients were anesthetized with either sevoflurane or desflurane at 1.0 and 1.8 minimum alveolar concentration (MAC). The trachea was stimulated by inflating the endotracheal tube cuff. A blinded observer assessed the severity of patient response to the stimulus and changes in hemodynamic variables. The process was repeated at the second MAC treatment condition.

Results: At 1.0 MAC, patients anesthetized with desflurane had a more intense response and a greater likelihood of significant coughing and associated hemodynamic changes (both at P < 0.05). At 1.8 MAC, sevoflurane and desflurane both suppressed clinically significant responses to tracheal stimulation. Interrater reliability was excellent for this measure of upper airway reactivity (P < 0.001).     

Compound A Concentrations during Sevoflurane Anesthesia in Children

Background: Sevoflurane is a new inhalation agent that should be useful for pediatric anesthesia. Sevoflurane undergoes degradation in the presence of carbon dioxide absorbents; however, quantification of the major degradation product (compound A) has not been evaluated during pediatric anesthesia. This study evaluates sevoflurane degradation compound concentrations during sevoflurane anesthesia using a 2-1 fresh gas flow and a circle system with carbon dioxide absorber in children with normal renal and hepatic function.

Methods: The concentrations of compound A were evaluated during sevoflurane anesthesia in children using fresh soda lime as the carbon dioxide absorbent. Nineteen patients aged 3 months-7 yr were anesthetized with sevoflurane (2.8% mean end-tidal concentration) using a total fresh gas flow of 2 l in a circle absorption system. Inspiratory and expiratory limb circuit gas samples were obtained at hourly intervals, and the samples were analyzed using a gas chromatography-flame ionization detection technique. Carbon dioxide absorbent temperatures were measured in the soda lime during anesthesia. Blood samples were obtained before and after anesthesia for hepatic and renal function studies. Venous blood samples were obtained before anesthesia, at the end of anesthesia, and 2 h after anesthesia for plasma inorganic fluoride ion concentration.

Results: The maximum inspiratory concentration of compound A was 5.4 +/-4.4 ppm (mean+/-SD), and the corresponding expiratory concentration was 3.7+/-2.7 ppm (mean+/-SD). The maximum inspiratory compound A concentration in any patient was 15 ppm. Mean concentrations of compound A peaked at intubation and remained stable, declining slightly after 120 min of anesthesia. The duration of anesthesia was 240+/-139 min (mean+/-SD). Maximum soda lime temperature ranged between 23.1 degrees C and 40.9 degrees C. There was a positive correlation between maximum absorbent temperature and maximum compound A concentration (r2 = 0.58), as well as between the child's body surface area and maximum compound A concentration (r2 = 0.59). Peak plasma inorganic fluoride ion concentration was 21.5 +/-6.1 micro mol/l. There were no clinically significant changes in hepatic or renal function studies performed 24 h postanesthesia.     

Morphologic Changes in the Upper Airway of Children during Awakening from Propofol Administration

Background: The purpose of this study was to determine the morphologic changes that occur in the upper airway of children during awakening from propofol sedation.

Methods: Children undergoing magnetic resonance imaging of the head underwent additional scans of the upper airway during deep sedation with propofol; this was repeated on awakening. Axial views were obtained at the most posterior sites of the pharynx at the levels of the soft palate and tongue. Measurements were then obtained of the anterior-posterior (A-P) diameter, transverse diameter, and cross-sectional areas at these levels.

Results: Data were obtained on 16 children, aged 10 months to 7 yr. In both sedated and awakening states, most children had the smallest cross-sectional area of the pharynx at the level of the soft palate. During the sedated state, at the soft palate level, the transverse diameter was most narrow in 11 children, the A-P diameter was most narrow in 1 child, and they were equal in 2 children. During the sedated state, at the level of the tongue, the transverse diameter was most narrow in 9 children, the A-P diameter was most narrow in 5 children, and they were equal in 2 children. During awakening, at the soft palate level, the transverse diameter was most narrow in none of the children, the A-P diameter was most narrow in 13 children, and they were equal in 1 child. At the level of the tongue, the transverse diameter was most narrow in 4 children, and the A-P diameter was most narrow in 12 children. During awakening, the A-P diameter of the pharynx at the level of the soft palate decreased in 12 children, increased in 1 child, and remained the same in 1 child. (P < 0.001). The transverse diameter increased in 11 children, decreased in 1 child, and remained the same in 2 children (P = 0.001). The cross-sectional area at the level of the soft palate increased in 4 children, decreased in 8 children, and stayed the same in 2 children (P = 0.5). During awakening, the A-P diameter of the pharynx at the level of the tongue decreased in 11 children, increased in 4 children, and remained the same in 1 child. (P = 0.01). The transverse diameter increased in 11 children and decreased in 5 children (P = 0.07). The cross-sectional area at the level of the tongue increased in 7 children, decreased in 7 children, and stayed the same in 2 children (P = 0.9).     

Plasma Inorganic Fluoride Concentrations after Sevoflurane Anesthesia in Children

Background: Sevoflurane is degraded in vivo in adults yielding plasma concentrations of inorganic fluoride [Fluorine sup -] that, in some patients, approach or exceed the 50-micro Meter theoretical threshold for nephrotoxicity. To determine whether the plasma concentration of inorganic fluoride [Fluorine sup -] after 1-5 MAC *symbol* h sevoflurane approaches a similar concentration in children, the following study in 120 children scheduled for elective surgery was undertaken.

Methods: Children were randomly assigned to one of three treatment groups before induction of anesthesia: group 1 received sevoflurane in air/oxygen 30% (n = 40), group 2 received sevoflurane in 70% N2 O/30% O2 (n = 40), and group 3 received halothane in 70% N2 O/30% O sub 2 (n = 40). Mapleson D or F circuits with fresh gas flows between 3 and 6 l/min were used. Whole blood was collected at induction and termination of anesthesia and at 1, 4, 6, 12, and 18 or 24 h postoperatively for determination of the [Fluorine sup -]. Plasma urea and creatinine concentrations were determined at induction of anesthesia and 18 or 24 h postoperatively.

Results: The mean (+/-SD) duration of sevoflurane anesthesia, 2.7+/-1.6 MAC *symbol* h (range 1.1-8.9 MAC *symbol* h), was similar to that of halothane, 2.5+/-1.1 MAC *symbol* h. The peak [Fluorine sup -] after sevoflurane was recorded at 1 h after termination of the anesthetic in all but three children (whose peak values were recorded between 4 and 6 h postanesthesia). The mean peak [Fluorine sup -] after sevoflurane was 15.8+/-4.6 micro Meter. The [Fluorine sup -] decreased to < 6.2 micro Meter by 24 h postanesthesia. Both the peak [Fluorine sup -] (r2 = 0.50) and the area under the plasma concentration of inorganic fluoride-time curve (r2 = 0.57) increased in parallel with the MAC *symbol* h of sevoflurane. The peak [Fluorine sup -] after halothane, 2.0+/-1.2 micro Meter, was significantly less than that after sevoflurane (P < 0.0001) and did not correlate with the duration of halothane anesthesia (MAC *symbol* h; r2 = 0.007). Plasma urea concentrations decreased 24 h after surgery compared with preoperative values for both anesthetics (P < 0.01), whereas plasma creatinine concentrations did not change significantly with either anesthetic.     

Desflurane but Not Sevoflurane Impairs Airway and Respiratory Tissue Mechanics in Children with Susceptible Airways

Background: Although sevoflurane and desflurane exert bronchoactive effects, their impact on the airway and respiratory tissue mechanics have not been systematically compared in children, especially in those with airway susceptibility (AS). The aim of this study was to assess airway and respiratory tissue mechanics in children with and without AS during sevoflurane and desflurane anesthesia.

Methods: Respiratory system impedance was measured in healthy control children (group C, n = 20) and in those with AS (group AS, n = 20). Respiratory system impedance was determined during propofol anesthesia and during inhalation of sevoflurane and desflurane 1 minimum alveolar concentration in random order. Airway resistance, tissue damping, and elastance were determined from the respiratory system impedance spectra by model fitting.

Results: Children in group AS exhibited significantly higher respiratory impedance parameters compared with those in group C. Sevoflurane slightly decreased airway resistance (-7.0 +/- 1.5% vs. -4.8 +/- 2.4% in groups C and AS, respectively) in both groups. In contrast, desflurane caused elevations in airway resistance and tissue mechanical parameters, with markedly enhanced airway narrowing in children with AS (18.2 +/- 2.8% vs. 53.9 +/- 5%; P < 0.001 for airway resistance in groups C and AS, respectively). Neither the order of drug administration nor the time after the establishment of their steady state concentrations affected these findings.     

Collapsibility of the Upper Airway at Different Concentrations of Propofol Anesthesia

Background: This study investigated the effect of varying concentrations of propofol on upper airway collapsibility and the mechanisms responsible for it.

Methods: Upper airway collapsibility was determined from pressure-flow relations at three concentrations of propofol anesthesia (effect site concentration = 2.5, 4.0, and 6.0 [mu]g/ml) in 12 subjects spontaneously breathing on continuous positive airway pressure. At each level of anesthesia, mask pressure was transiently reduced from a pressure sufficient to abolish inspiratory flow limitation (maintenance pressure = 12 +/- 1 cm H2O) to pressures resulting in variable degrees of flow limitation. The relation between mask pressure and maximal inspiratory flow was determined, and the critical pressure at which the airway occluded was recorded. Electromyographic activity of the genioglossus muscle (EMGgg) was obtained via intramuscular electrodes in 8 subjects.

Results: With increasing depth of anesthesia, (1) critical closing pressure progressively increased (-0.3 +/- 3.5, 0.5 +/- 3.7, and 1.4 +/- 3.5 cm H2O at propofol concentrations of 2.5, 4.0, and 6.0 [mu]g/ml respectively; P < 0.05 between each level), indicating a more collapsible upper airway; (2) inspiratory flow at the maintenance pressure significantly decreased; and (3) respiration-related phasic changes in EMGgg at the maintenance pressure decreased from 7.3 +/- 9.9% of maximum at 2.5 [mu]g/ml to 0.8 +/- 0.5% of maximum at 6.0 [mu]g/ml, whereas tonic EMGgg was unchanged. Relative to the levels of phasic and tonic EMGgg at the maintenance pressure immediately before a decrease in mask pressure, tonic activity tended to increase over the course of five flow-limited breaths at a propofol concentration of 2.5 [mu]g/ml but not at propofol concentrations of 4.0 and 6.0 [mu]g/ml, whereas phasic EMGgg was unchanged.     

Serum Fluoride Concentrations,Biochemical and Histopathological Changes Associated with Prolonged Sevoflurane Anaesthesia in Horses*

The volatile anaesthetic sevoflurane is degraded to fluoride (F⁻) and a vinyl ether (Compound A), which have the potential to harm kidney and liver. Whether renal and hepatic injuries can occur in horses is unknown. Cardiopulmonary, biochemical and histopathological changes were studied in six healthy thoroughbred horses undergoing 18 h of low‐flow sevoflurane anaesthesia. Serum F⁻ concentrations were measured and clinical laboratory tests performed to assess hepatic and renal function before and during anaesthesia. Necropsy specimens of kidney and liver were harvested for microscopic examination and compared to pre‐experimental needle biopsies. Cardiopulmonary parameters were maintained at clinically acceptable levels throughout anaesthesia. Immediately after initiation of sevoflurane inhalation, serum F⁻ levels began to rise, reaching an ongoing 38–45 μmol l⁻¹ plateau at 8 h of anaesthesia. Serum biochemical analysis revealed only mild increases in glucose and creatinine kinase and a decrease in total calcium. Beyond 10 h of anaesthesia mild, time‐related changes in urine included increased volume, glucosuria and enzymuria. Histological examination revealed mild microscopic changes in the kidney involving mainly the distal tubule, but no remarkable alterations in liver tissue. These results indicate that horses can be maintained in a systemically healthy state during unusually prolonged sevoflurane anaesthesia with minimal risk of hepatocellular damage from this anaesthetic. Furthermore, changes in renal function and morphology observed after sevoflurane inhalation are judged minimal and appear to be clinically irrelevant; they may be the result of anaesthetic duration, physiological stressors, sevoflurane (or its degradation products) or other unknown factors associated with these animals and study conditions.     

Effect of Lateral Positioning on Upper Airway Size and Morphology in Sedated Children

Background: Lateral positioning decreases upper airway obstruction in paralyzed, anesthetized adults and in individuals with sleep apnea during sleep. The authors hypothesized that lateral positioning increases upper airway cross-sectional area and total upper airway volume when compared with the supine position in sedated, spontaneously breathing children.

Methods: Children aged 2-12 yr requiring magnetic resonance imaging examination of the head or neck region using deep sedation with propofol were studied. Exclusion criteria included any type of anatomical or neurologic entity that could influence upper airway shape or size. T1 axial scans of the upper airway were obtained in the supine and lateral positions, with the head and neck axes maintained neutral. Using software based on fuzzy connectedness segmentation (3D-VIEWNIX; Medical Imaging Processing Group, University of Pennsylvania, Philadelphia, PA), the magnetic resonance images were processed and segmented to render a three-dimensional reconstruction of the upper airway. Total airway volumes and cross-sectional areas were computed between the nasal vomer and the vocal cords. Two-way paired t tests were used to compare airway sizes between supine and lateral positions.

Results: Sixteen of 17 children analyzed had increases in upper airway total volume. The total airway volume (mean +/- SD) was 6.0 +/- 2.9 ml3 in the supine position and 8.7 +/- 2.5 ml3 in the lateral position (P < 0.001). All noncartilaginous areas of the upper airway increased in area in the lateral compared with the supine position. The region between the tip of the epiglottis and vocal cords demonstrated the greatest relative percent change.     

Changes in Electroencephalogram and Autonomic Cardiovascular Activity during Induction of Anesthesia with Sevoflurane Compared with Halothane in Children

Background: This study was design to assess clinical agitation, electroencephalogram (EEG) and autonomic cardiovascular activity changes in children during induction of anesthesia with sevoflurane compared with halothane using noninvasive recording of EEG, heart rate, and finger blood pressure.

Methods: Children aged 2-12 yr premedicated with midazolam were randomly assigned to one of three induction techniques: 7% sevoflurane in 100% O2 (group SevoRAPID); 2%, 4%, 6%, and 7% sevoflurane in 100% O2 (group SevoINCR); or 1%, 2%, 3%, and 3.5% halothane in 50% N2O-50% O2 (group HaloN2O). An additional group of children who received 7% sevoflurane in 50% N2O-50% O2 (group SevoN2O) was enrolled after completion of the study. Induction was videotaped. EEG, heart rate, and finger blood pressure were continuously recorded during induction until 5 min after tracheal intubation and analyzed in frequency domain using spectral analysis.

Results: Agitation was more frequent when anesthesia was induced with 100% O2 compared to the mixture of oxygen and nitrous oxide. No seizures were recorded in any group. In the four groups, induction of anesthesia was associated with an increase in EEG total spectral power and a shift toward the low-frequency bands. Sharp slow waves were present on EEG tracings of the three sevoflurane groups, whereas slow waves and fast rhythms (spindles) were observed in the halothane group. Sevoflurane induced a greater withdrawal of parasympathetic activity than halothane and a transient relative increase in sympathetic vascular tone at loss of eyelash reflex.     

Three-Dimensional Changes in the Upper Airway During Neuromuscular Stimulation of Laryngeal Muscles

During swallowing, airway protection depends upon adequate glottal closure and laryngeal elevation to prevent the entry of substances into the airway. Three-dimensional changes in the upper airway during laryngeal muscle stimulation in a canine model were quantified in animals implanted with Peterson type stimulating electrodes in the inferior and superior portions of the thyroarytenoid muscle, together with a reference electrode. Computer tomography scanning was performed on an IMATRON scanner with a 3 mm slice thickness advanced at overlapping 1 mm increments. Stimulation of the thyroarytenoid muscle produced adductions of the vocal fold towards the midline and changes in the supraglottic region as well as the glottis; the glottic wall was compressed medially above and below the glottis. These results suggest that chronic neuromuscular stimulation can effect glottic protection by reducing the glottal opening and may be beneficial for patients with central control disorders affecting airway protection during swallowing.     

Effect of Increasing Depth of Propofol Anesthesia on Upper Airway Configuration in Children

Background: The upper airway tends to be obstructed during anesthesia in spontaneously breathing patients. The purpose of the current study was to determine the effect of increasing depth of propofol anesthesia on airway size and configuration in children.

Methods: Magnetic resonance images of the upper airway were obtained in 15 children, aged 2-6 yr. Cross-sectional area, anteroposterior dimension, and transverse dimension were measured at the level of the soft palate, dorsum of the tongue, and tip of the epiglottis. Images were obtained during infusion of propofol at a rate of 50-80 [mu]g [middle dot] kg-1 [middle dot] min-1 and after increasing the depth of anesthesia by administering a bolus dose of propofol and increasing the infusion rate to 240 [mu]g [middle dot] kg-1 [middle dot] min-1.

Results: Overall, the cross-sectional area of the entire pharyngeal airway decreased with increasing depth of anesthesia. The reduction in cross-sectional area was greatest at the level of the epiglottis (24.5 mm2, 95% confidence interval = 16.9-32.2 mm2; P < 0.0001), intermediate at the level of the tongue (19.3 mm2, 95% confidence interval = 9.2-29.3 mm2; P < 0.0001), and least at the level of the soft palate (12.6 mm2, 95% confidence interval = 2.7-22.6 mm2; P < 0.005) in expiration and resulted predominantly from a reduction in anteroposterior dimension. The airway cross-sectional area decreased further in inspiration at the level of the epiglottis. The narrowest portion of the airway resided at the level of the soft palate or epiglottis in the majority of children.     

Effects of General Anesthesia and Paralysis on Upper Airway Changes Due to Head Position in Humans

Background: In supine patients with their heads in flexion, general anesthesia causes posterior displacement of upper airway structures that is associated with airway obstruction, and extension of the head helps restore patency. However, the independent effects of head position, general anesthesia, and muscle paralysis on upper airway structures are not known.

Methods: Lateral radiographs of the neck were taken in supine patients with the head in flexion and extension, during consciousness, and after induction of general anesthesia and muscle paralysis. The following measurements were made: distances from the horizontal plane to the epiglottis, the hyoid, and the thyroid cartilage to detect anteroposterior displacements; distances from the transverse plane to the hyoid and the thyroid cartilage to detect cephalocaudad displacements; and widths of the oropharynx, the laryngeal vestibule, and the laryngeal sinus.

Results: With the head in flexion, anesthesia and paralysis compared with the conscious state caused posterior displacement of the epiglottis, narrowing of the oropharynx, and widening of the laryngeal vestibule. With the head in extension, anesthesia and paralysis compared with the conscious state caused anterior displacements of the epiglottis, the hyoid, and the thyroid cartilage, narrowing of the oropharynx, and widening of the laryngeal vestibule and the laryngeal sinus.     

Clinical Characteristics of Sevoflurane in Children: A Comparison with Halothane

Background: For pediatric patients, sevoflurane may be an alternative to halothane, the anesthetic agent used most commonly for inhalational induction. The induction, maintenance, and emergence characteristics were studied in 120 unpremedicated children 1-12 yr of age randomly assigned to receive one of three anesthesia regimens: sevoflurane with oxygen (group S), sevoflurane with nitrous oxide and oxygen (group SN), or halothane with nitrous oxide and oxygen (group HN).

Methods: Anesthetic was administered (via a Mapleson D, F or Bain circuit) beginning with face mask application in incremental doses to deliver maximum inspired concentrations of 4.5% halothane or 7% sevoflurane. End-tidal concentrations of anesthetic agents and vocal cord position were noted at the time of intubation. Elapsed time intervals from face mask application to loss of the eyelash reflex, intubation, surgical incision, and discontinuation of the anesthetic were measured. Heart rate, systolic, diastolic, and mean blood pressures, and end-tidal anesthetic concentrations were measured at fixed intervals. Anesthetic MAC-hour durations were calculated. The end-tidal concentration of anesthetic was adjusted to 1 MAC (0.9% halothane, 2.5% sevoflurane) for at least the last 10 min of surgery. Intervals from discontinuation of anesthetic to hip flexion or bucking, extubation, administration of first postoperative analgesic, and attaining discharge criteria from recovery room were measured. Venous blood was sampled at anesthetic induction, at the end of anesthesia, and 1, 4, 6, 12, and 18-24 h after discontinuation of the anesthetic for determination of plasma inorganic fluoride content.

Results: Induction of anesthesia was satisfactory in groups SN and HN. Induction in group S was associated with a significantly greater incidence of excitement (35%) than in the other groups (5%), resulting in a longer time to intubation. The end-tidal minimum alveolar concentration multiple of potent inhalational anesthetic at the time of intubation was significantly greater in patients receiving halothane than in patients receiving sevoflurane. Induction time, vocal cord position at intubation, time to incision, duration of anesthesia, and MAC-hour duration were similar in the three groups. During emergence, the time to hip flexion was similar among the three groups, whereas the time to extubation, time to first analgesic, and time to attaining discharge criteria were significantly greater in group HN than in groups S and SN. Mean heart rate and systolic blood pressure decreased during induction in group HN but not in groups S and SN. The maximum serum fluoride concentration among all patients was 28 micro Meter.     

Epinephrine: Systemic Effects and Varying Concentrations in Local Anesthesia

The range of vasoconstrictors available for use with local anesthetics in dentistry has been reviewed with emphasis on epinephrine and its physiological effects. All of the vasoconstrictors reviewed provide satisfactory results in dental anesthetic solutions when administered in appropriate concentrations and volumes. Possible drug interactions of concern to dentists include the use of vasoconstrictors with inhalational anesthetics, tricyclic antidepressants, beta blockers and, possibly, phenothiazines. Data reviewed indicates that the amounts of epinephrine used in dentistry can result in significant elevations in circulating levels of ephinephrine and concomitant physiologic changes. Evidence reviewed suggests that 1:200,000 epinephrine concentration results in optional duration and depth of local anesthesia. With the potential for adverse effects from epinephrine concentrations that are needlessly increased, it appears that in most clinical situations a 1:200,000 concentration of epinephrine can be used in an efficacious manner.     

Laryngeal Mask Airway Is Associated with an Increased Incidence of Adverse Respiratory Events in Children with Recent Upper Respiratory Tract Infections

Background: The laryngeal mask airway (LMA) has been advocated as an alternative technique to tracheal intubation for airway management of children with recent upper respiratory tract infections (URIs). The authors determined the occurrence of adverse respiratory events and identified the associated risk factors to assess the safety of LMA in children.

Methods: During a period of 5 months, parents of children scheduled to undergo general anesthesia with an LMA were asked to fill out a questionnaire regarding their child's medical history and potential symptoms of URI. In addition, all episodes of adverse respiratory events in the perioperative period (laryngospasm, bronchospasm, coughing, airway obstruction, and oxygen desaturation) as well as details of anesthesia management were recorded.

Results: Among the 831 children included in the study, 27% presented with a history of a recent URI within the last 2 weeks before anesthesia. The presence of a recent URI doubled the incidence of laryngospasm (odds ratio, 2.6; 95% confidence interval, 1.3-5.0), coughing (odds ratio, 2.7; 95% confidence interval, 1.7-4.3), and oxygen desaturation (odds ratio, 1.9; 95% confidence interval, 1.2-2.8). This incidence was even higher in young children; in those undergoing ear, nose, and throat surgery; and when there were multiple attempts to insert the LMA.