Relationship of inspiratory and expiratory times to upper airway resistance during pulsatile needle cricothyrotomy ventilation with generic delivery circuit  M.W. Lim, S.W. Benham  British Journal of Anaesthesia  Volume 104, Issue 1, Pages 98-107 (January 2010) DOI: 10.1093/bja/aep341 Copyright © 2010 The Author(s) Terms and Conditions

Fig 1 The generic cannula cricothyrotomy circuit12–14 on which our models are based consists of a 14 G i.v. cannula functioning as transtracheal catheter, a three-way stopcock as standardized alternate expiratory resistance, green oxygen tubing, and wall oxygen flowmeter. These components, which are readily available in acute clinical environments, are serially connected. British Journal of Anaesthesia 2010 104, 98-107DOI: (10.1093/bja/aep341) Copyright © 2010 The Author(s) Terms and Conditions

Fig 2 Electrical analogue of the generic cannula cricothyrotomy circuit shown in Figure 1. The elastance of the respiratory component is the reciprocal of the respiratory compliance. The respiratory and upper airway resistances are linear resistors. The potential difference across the transtracheal cannula varies as a cubic function of the through flow. The potential difference across the alternate expiratory resistor varies as a quadratic function of the through flow. The two main junctions are TAP (between the transtracheal cannula resistance, the alternate expiratory resistance, and the fresh gas port) and RUT (between the respiratory resistance, the upper airway resistance, and the transtracheal cannula resistance). The circuits are closed through earth. The alternate expiratory pathway is drawn in dashed lines. This pathway is opened during expiration but closed (not available) during inspiration when the expiratory port of the stopcock is occluded. British Journal of Anaesthesia 2010 104, 98-107DOI: (10.1093/bja/aep341) Copyright © 2010 The Author(s) Terms and Conditions

Fig 3 Experimental set up with the generic cannula cricothyrotomy circuit inserted into the simulated airway. The lung is simulated by a partially water-filled U-tube manometer, the respiratory compliance of which depends on the tilt of the table. The upper airway resistance is simulated by a number of 20 G i.v. cannulae. British Journal of Anaesthesia 2010 104, 98-107DOI: (10.1093/bja/aep341) Copyright © 2010 The Author(s) Terms and Conditions

Fig 4 The calculated inspiratory (light), expiratory (dark) and total respiratory (solid line) times are plotted against the upper airway resistance on semi-logarithmic axes. The fresh gas flow is set at 9 litre min−1, the tidal volume at 100 ml (expiratory time at 95% emptying), the respiratory compliance at 20 ml cm H2O−1, and the respiratory resistance at 6 cm H2O litre−1 s. The inspiratory graph is a hyperbolic curve and the expiratory graph a sigmoid curve. The total respiratory time is shortest where the inspiratory and expiratory graphs intersect. This theoretically occurs at upper airway resistance ∼3.9×10−2 cm H2O litre−1 s, which is around 50 times the resistance of the normal unobstructed human airway.18 19 Note that experimental figures indicate a higher upper airway resistance value is more accurate because of unmodelled mechanical factors. This upper airway resistance may thus be considered ‘optimal’ for NBC ventilation. This pattern is similar to graphs at other fresh gas flows (not shown). British Journal of Anaesthesia 2010 104, 98-107DOI: (10.1093/bja/aep341) Copyright © 2010 The Author(s) Terms and Conditions