the cannula makes use of the natural funneling effect of the lips and mouth opening to produce a superior signal fig4 illustrates the inspiratory airflow 10 the surrounding air is funneled into the oral opening 11 between the lips 12 13 and into the upper airway the velocity of the airflow is somewhat higher over the surface of the lips than out in the middle of the oral opening when the oral opening is large the airflow velocity is reduced when the oral opening is small the airflow velocity is increased the scoops and baffles of existing art are positioned in the middle of the opening where the velocity is always at its lowest the inlet or opening of the oral prong 9 is positioned along the surface of the upper lip 12 in the center of the mouth where the velocity is greatest this placement helps to reduce the effect of varying sizes of the oral opening fig5 shows the placement of the oral prong 9 of the cannula the oral prong of the cannula is trimmed along a trim line 15 where the upper and lower lips 12 13 meet with one another this positions the inlet opening of the oral prong 9 of the cannula at the point where the flow velocity pressure is greatest if the oral prong 9 is a little long the amplitude of the inspiratory signal is not affected but it does reduce patient comfort because the oral prong is in the mouth if the oral prong is trimmed too short the signal amplitude may be reduced fig6 illustrates the trim zone 16 of the best results the trim angle is not particularly critical although a trim perpendicular to the.