Department of Anaesthesia, University Childrens Hospital of Basel, Römergasse 8, CH-4058 Basel, Switzerland*Corresponding author
Accepted for publication: October 4, 2000
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Abstract |
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Br J Anaesth 2001; 86: 21722
Keywords: anaesthesia, general; airway; surgery, paediatric; anaesthetic techniques, inhalation; anaesthetics volatile, halothane
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Introduction |
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Patients and methods |
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Anaesthesia
Each patient was premedicated with midazolam 0.3 mg kg1 rectally 15 min before anaesthesia. Anaesthesia was induced with 3 vol% halothane via a face mask, with oxygen in 50% nitrous oxide from a circle system. Inspired halothane concentration was adjusted to give an end-tidal concentration of 1.0 vol%. Electrocardiogram, pulse oximetry and capnography with breathing frequency were recorded (Capnomac Ultima; Datex, Helsinki, Finland). The head position was standardized. The head was slightly extended using a special pillow, to obtain an angle of 110° between the horizontal plane of the operating table and a line connecting the lateral corner of the eye and the tragus of the ear, with neither the occiput of the head nor the shoulders raised above the operating table. We used this angle because a study of adults by Boidin7 and our own clinical observations in anaesthetized children (unpublished) suggest that this allows maximal widening of the upper airways.
Airway monitoring
We adapted a special airway endoscopy mask8 and a standardized fixation system (Secutape; TechniMed Ltd, Basel, Switzerland) to tailor the mask to each patient. A bronchofibrescope with an outer diameter of 3.5 mm (Olympus Optical, Volketswil, Switzerland) was inserted through the mask and one nostril into the nasopharynx, leaving the other nostril patent. The tongue and laryngeal structures were examined while the child was breathing spontaneously. The light source for the endoscopy was a xenon lamp (CLV-U40, Olympus Optical Co., Tokyo, Japan). For all measurements, the tip of the fibrescope was kept at the edge of the soft palate to give comparable views at baseline (chin unsupported) and during the subsequent manoeuvres. The manoeuvres were standardized and performed by the same investigator in all children to eliminate inter-investigator variability. A baseline measurement was made with the adapted facemask with the patients chin unsupported. Then chin lift was done with one hand without making the mandible protrude. The teeth were in light contact and the lips remained open, so the mouth was not completely closed. Then, in addition to chin lift, CPAP of 10 cm H2O was applied from the circle system to dilate or splint the upper airway. Jaw thrust was applied with both hands, displacing the jaw upwards and anteriorly (Esmarch manoeuvre), which allowed the mouth to remain open. This was done with maximal mandibular protrusion at zero end-expiratory airway pressure and then with CPAP of 10 cm H2O. After the measurements, the patients trachea was intubated for subsequent surgery.
Airway patency was assessed clinically as follows: stridor score 1, normal breathing sounds detected by auscultation over the trachea; 2, stridor over the trachea detected by stethoscope; 3, stridor detected without auscultation (audible); 4, no airway sound detectable over the trachea.
Video transformation and image analysis
Records were made for 1 min during each of the different airway manoeuvres on a Super VHS tape (SV-9500 MDP; Sony, Tokyo, Japan). The video sequences were transferred to a Macintosh computer using a frame grabber card (miroMotion DC 20; Miro Computer Products, Braunschweig, Germany). Video information (72 dots per inch, 25 frames per second) was transposed to a PICT format (Adobe Premiere 4.2.1; Adobe Systems Inc., San Jose, CA, USA) and analysed with an image analysis software package (Adobe Photoshop 4.0; Adobe Systems Inc.). The person who performed the image analysis was blinded as to a patients group. The images with the most narrowed and widened airway dimensions (corresponding to end-inspiration and end-expiration) were identified for the different conditions. The shortest distance between the tonsils (transverse dimension) and the distance between the tip of the epiglottis and the posterior pharyngeal wall (anteroposterior dimension) were measured. These are the most important pharyngeal airway distances during breathing in anaesthetized children.9
Statistical analysis
Airway dimensions were expressed as a percentage of distance from baseline, with the chin unsupported. Percentages instead of absolute values were used to reduce the problems of different distance and characteristics between subjects and radial distortion of images caused by the optical characteristics of the fibrescope.10 The different conditions were compared by repeated-measures analysis of variance. For post hoc comparisons, Tukeys test was applied and probability values calculated. Score values were analysed by means of the non-parametric Friedmans test for repeated-measures analysis. Spearmans rank correlation coefficient (rs) was applied to analyse possible relationships between variables. A P value of <0.05 was considered significant. For all calculations, Statistica/w 4.5 software (StatSoft, Tulsa, OK, USA) was used.
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Results |
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Discussion |
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Effect of anaesthesia
During anaesthesia, a collapsible segment in the upper airway may narrow or close during inspiration.11 In a study of thoracoabdominal motion in children, clinically significant upper airway obstruction was found at 2 MAC (minimal alveolar concentration) sevoflurane.12 We identified airway collapse during late inspiration, which is similar to reports of collapse during sleep.13 Severe inspiratory collapse may be associated with a marked decrease in intraluminal pressure.13 The distal pharynx is sucked in and may even obstruct. The lateral walls of the pharynx have a complex architecture, with a number of muscles that have different biomechanical relationships with each other and with other pharyngeal structures.14 15 In addition to the depression of the activity of upper airway muscles with halothane,16 other factors, such as the thickness of the lateral pharyngeal wall, may play a critical role.15 17 Large tonsils also seem to increase airway collapsibility during inhalational anaesthesia. Our study also supports previous findings that the position of the epiglottis in relation to the posterior pharyngeal wall affects airway patency.18 However, lateral narrowing and posterior displacement of the epiglottis cannot be assessed clinically, for example by airway sounds.
Effect of chin lift (without protrusion of the mandible)
We found that chin lift did not improve the patency of the airway. Upper airway narrowing during inspiration results from an imbalance between inspiratory muscle activity and the negative intraluminal pressure generated during inspiration. Halothane anaesthesia affects phasic activity of inspiratory muscles in a dose-dependent manner.16 Both anaesthetic agent and the chin-lift manoeuvre affect upper airway muscle tension. The action of negative intraluminal pressure is no longer balanced by the action of the upper airway dilator muscles19 and severe collapse, with or without complete obstruction, may occur. Lifting the chin could increase pharyngeal compliance so that the tonsils are sucked in without counterbalance from muscle activity. During propofol anaesthesia, chin lift alone could preserve airway patency;18 however, these children had normal tonsils and this study used magnetic resonance imaging, which did not follow dynamic airway changes.
Effect of jaw thrust (mouth open with maximal mandibular protrusion)
Compared with chin lift, jaw thrust has the advantage that the tension from the tongue and suprahyoid muscles is greater, thus pulling the hyoid ventrally against the root of the tongue and actively widening the pharynx. In addition, the mouth is opened and breathing becomes easier than during chin lift.20 However, there is no correlation between the degree of the mandibular protrusion and the widening of the pharynx in adults.21 Mouth opening without mandibular protrusion increases upper airway collapsibility during sleep.22 We found that jaw thrust manoeuvres, which may cause post-operative discomfort, worsened airway calibre during inspiration, although impairment of airway patency occurred in fewer patients during jaw thrust (six patients) compared with chin lift (10 patients).
Effect of additional CPAP
Continuous positive airway pressure may have several effects, including interactions between changes in chest wall stability, pulmonary mechanics, lung volume and respiratory muscle dynamics.23 24 Continuous positive airway pressure increases airway volume and airway area within the retropalatal and retroglossal regions and increases lateral dimensions more than anteriorposterior dimensions.15 In our study, CPAP restored airway patency in children with large tonsils during chin lift and jaw thrust by dilating or splinting the upper airway.
In conclusion, in spontaneously breathing children with adenotonsillar hyperplasia, chin lift plus CPAP is recommended; jaw thrust plus CPAP is no better and may cause post-operative discomfort. Although little is known about the biomechanics of the upper airway and how the various soft tissues interact mechanically to control the dimensions of the upper airway, the degree of stridor may indicate the efficacy of airway manoeuvres.
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Acknowledgements |
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References |
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