Natural Breathing

During natural breathing the static pressure at each point of measurement within the upper airway has a form similar to that of the breath flow wave. Positive pressures were measured during expiration and conversely negative pressure during inspiration. Overall the pressure wave appears in phase with the flow wave as illustrated by the results from a single measurement position in Figure 27; however, throughout the transition from expiration to inspiration the pressure lags the flow wave, in particular during the steep acceleration to peak inspiration. This is also shown in Figure 27 where it can be seen that the inspiration portion of the breath cycle is shorter than expiration and achieves higher peak flow rates, thus the steeper curve.

Figure 27: Transient static pressure at the anterior nasopharynx and scaled flow rate during natural breathing showing pressure lagging flow at the drop to peak inspiration. Flow rate scaled by a factor of

0.479 Pa/LPM.

In general the absolute magnitudes of peak expiration and inspiration pressures increase as the measurement points move towards the lungs, though some local decreases in magnitude are also seen. This result is indicative of the airway resistance that opposes the flow and causes pressure loss between the lungs and the external nares. The presence of airway resistance contributes to the work of breathing, with greater effort required to breathe for greater levels of resistance.

The transient static pressures at the superior nasopharynx follow a profile very similar in form to the flow rate curve with very close agreement between the ratios of peak inspiratory to expiratory values. This is illustrated in Figure 28 in which the flow rate curve has again been normalised by the peak inspiratory pressure for ease of comparison. This indicates that the pressure drop across the nasal cavity between the external nares and the point of measurement is related to the flow rate by a similar factor for both expiration and inspiration. Specifically, this implies that the flow resistance within the nasal cavities is similar in both flow directions, agreeing with the steady-stated in vitro findings of Croce et al. (2006). As previously mentioned, in vivo research has shown that in live subjects an increase in resistance is seen during inspiration due to the compliance of the nasal vestibule and later the nasopharynx (Shepard and Burger, 1990). This highlights a limitation of the use of rigid airway models as deformation of the airway is prevented and this change in resistance during inspiration is not replicated in the results.

At the superior trachea the flow rate and pressure ratios of peak inspiratory to expiratory values do not show the same agreement seen at the superior nasopharynx. This is shown in Figure 29 where the flow rate curve has been normalised by the peak inspiratory pressure as before. From this result it can be deduced that there is a slightly greater resistance to flow across the entire upper airway during inspiration than during expiration.

Between the superior nasopharynx and the trachea there is a large pressure drop during inspiration as illustrated by the plot of the peak static pressures at all measurement points in Figure 30. This drop begins as the cross-section of the airway decreases toward the minimum cross section at the velopharynx. Past the velopharynx a large irregular cavity appears on the anterior wall formed by the uvula, tongue and epiglottis in superior to inferior order. Another cavity is formed at the inferior region of the laryngopharynx below the anterior opening to the larynx. The superior of these cavities causes an expansion of the pharynx cross-section in both expiration and inspiration directions, however only expiration flow experiences a slight pressure recovery at this cross-section widening, whilst a slight drop in pressure is seen for the inspiration flow. Inspiration pressures have an overall downward trend along the flow path while in comparison expiration shows large drops in pressure with frequent recoveries.

Figure 28: Transient static pressures at the superior nasopharynx during a natural breathing cycle showing close agreement in peak magnitudes to the normalised flow rate. Flow rate scaled by a factor of

0.432 Pa/LPM.

Figure 29: Transient static pressures at the perimeter of the superior trachea during natural breathing in comparison to the flow rate scaled by a factor of 1.130 Pa/LPM.

Figure 30: Expiration and inspiration peak static gauge pressures at every pressure tap location with anatomic regions labelled.

In the same region between the superior nasopharynx and the trachea the pressure drop experienced during expiration is significantly lower relative to the flow rates achieved. While the nasopharynx to laryngopharynx region causes large losses during peak inspiration, the losses during peak expiration are not equivalent as shown previously in Figure 30. Research by Stringer et al. (2010) which simulated steady state flow within the same airway showed that during peak expiration the flow is directed from the anterior opening of the larynx into the pharynx, illustrated in Figure 26 (a), in a superior-posterior direction as given in Figure 31 (b). The flow path is jet-like within the pharynx and has little interaction with the two large cavities, unlike the flow path during peak inspiration in Figure 31 (a) which shows streamlines more evenly spread across the pharynx cross section and interaction with both cavities. This difference in the flow distribution across the pharynx gives some explanation as to the differences seen in the pressure losses. As the flow passes from the velopharynx into the inferior nasopharynx during expiration there is a small recovery of pressure due to the smooth expansion. This is shown in Figure 32 by the increase in the expiration curve peak between the velopharynx and inferior nasopharynx.

(a) (b)

Figure 31: CFD results of steady state flows through airway 06B showing streamlines in m/s for (a) peak inspiration flow of 22 m/s and (b) peak expiration flow of 32 m/s (adapted from Stringer et al. (2010)).

Figure 32: Transient static pressures at the posterior velopharynx and posterior-inferior nasopharynx showing small recovery of pressure during expiration.