Relationship of inspiratory and expiratory times to upper airway resistance during pulsatile needle cricothyrotomy ventilation with generic delivery circuit.

BACKGROUND

Narrow-bore cricothyrotomy retains a clinical role, due to the availability of its component equipment in acute clinical environments, ease of assembly, and operator preference. However, due to infrequent use, there is a need to model this for research and teaching. We present mathematical and laboratory models.

METHODS

Using electrical analogy, we mathematically modelled a generic cannula cricothyrotomy circuit, relating inspiratory and expiratory times to the upper airway resistance (R(u)). We constructed a laboratory model to support our mathematical model. The simulated lung is a smooth-bore tube on a tilting table. The upper airway is simulated by 20 G cannulae. Inspiratory and expiratory times for the water meniscus to travel a preset distance (corresponding to tidal volume) were measured and plotted against the number of cannula.

RESULTS

From the mathematical model, inspiratory time increases hyperbolically with decreasing R(u), such that there is a minimum R(u) beyond which most of the fresh gas flow leaks out without inflating the chest. Conversely, as R(u) increases, inspiratory time decreases to a plateau. Expiratory time is limited by respiratory factors at low R(u) and by the resistance of the transtracheal expiratory pathway at high R(u), producing a sigmoid-shaped expiratory curve. The experimental results seem consistent with these predictions, although direct theory-experiment mapping is problematic because of the difficulty in assigning a single value to the dynamically changing upper airway resistance.

CONCLUSIONS

We can exploit the contrasting changes in inspiratory and expiratory times with the upper airway resistance to optimize conditions for emergent cannula cricothyrotomy ventilation.