| Part I The comparative analysis for 3-dimensional computed tomographic images of upper airway in three different head positions for tracheal intubationPurposeThis study evaluated the effect of different head positions of one volunteer on both the alignment of three upper airway axes(the axis of the mouth, the pharyngeal axis and the laryngeal axis) and the space for the procedure of tracheal intubation(the volume of oral and oropharyngeal cavity) by 3-dimensional computed tomographic imaging to build on anatomic evidence for optimizing tracheal intubation position.Material and MethodsOne ASA I female volunteer, 25 yr old, height 173 cm, weight 62 kg, BMI 20.7kg/m2, underwent CT scanning in three anatomic supine positions: neutral, head extension and sniffing. The three axes: the axis of the mouth(MA), the pharyngeal axis(PA), and the laryngeal axis(LA), were made on each scan to measure the various angles between these axes, which were marked as M-P, L-P and M-L. The reference lines for the occiput and the atlas were marked to measure the acute angle between them(marked as A-O). Three dimensional models of the upper airway were reconstructed from the CT scan images. The area of mouth and oropharynx were selected to calculate the volume of oral cavity(Vo) and oropharyngeal cavity(Vp).ResultsThe comparison for the total values of the three angles(L-P+M-P+M-L) was: neural > extension > sniffing position. The comparison for the L-P angle: neural ≈ sniffing < extension; The M-P angle: sniffing <extension <neural. The M-L angle: extension <sniffing <neural. The A-O angle: sniffing≈extension <neural. The comparison for the Vo and Vp was: neural < sniffing < extension. Vo and Vp in extension position was twice the volume in neural position and 4/3 times the volume in sniffing position.ConclusionThe sniffing position could achieve best alignment of the oral-pharyngeal-laryngeal axes, thus should be considered as the optimal position for direct laryngoscopic view for tracheal intubation. While the volume of oral and oropharyngeal cavity was largest when in extension position, which might provide more convenient space for the procedure of tracheal intubation.Part II Comparison of airflow changes due to head and neck position by computational fluid dynamicsPurposeThis study evaluates the influence on airway volume and airflow behavior by different head and neck positions in an awake volunteer with no risk of difficult airway.Material and methods3-D airway volume meshes were reconstructed from CT scans in three different positions(the neutral position, the extension position and the sniffing position) of a healthy adult volunteer with normal airway structure. We measured the airway volume and narrowest area of each airway model. CFD simulation was performed with Spalart-Allmaras model.ResultsThe airway volume and narrowest area was largest in sniffing position and least in neutral position. The pressure drop and velocity acceleration was more significant in neutral position than sniffing position or extension position at the same airflow. In sniffing position pressure differences decreased and velocity remained almost constant. There was reversed air flow near velopharynx which caused by air flow passed through oral passage in mouth closed ventilation and air flow passed through both oral passage and nasal passage in mouth opened ventilation. In neutral position, obvious decrease of pressure could be observed in nasopharynx in mouth closed ventilation and oropharynx in mouth opened ventilation.ConclusionBoth the sniffing position and extension position can increase airway volume while in sniffing position, there was less change in airflow velocity and pressure which indicated sniffing position as optimal for mask ventilation. Ventilation with mouth closed has got advantages over mouth opened for less turbulence in pharyngeal area and maintence of airway patency, especially in neutral position. |
