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of long thin spikes of smoke, and showed that importance of frequency with their conclusion that the “quicker the puff, the thinner and sharper the spike”. Because the velocity and concentration profiles of oxygen are not flat (with higher concentrations of oxygen at the core of the flow profile, fresh gas can penetrate the alveoli despite very low tidal volumes). Taylor showed that this flow profile contributes to longitudinal mixing of gas, which can be described as a process of diffusion that increases with average velocity and tube diameter, exceeding the molecular diffusion by many fold (5). Consequently, this mechanism of gas mixing appears particularly important in the large airways. At the same time as fresh gas is propelled forward, there is also lateral (radial) mixing of gas with movement of particles from the centre to the periphery of the gas flow, resulting in enhanced dispersion of gas. Finally, together with the accelerated projection of fresh gas into the lung, co-axial flow results in a slower moving peripheral expiratory gas flow, effectively pushed out of the lung by the force of the incoming gas. 4.2 Asymmetric flow profiles Gas mixing is further enhanced by differences in flow profiles between inspiration and expiration. Whereas the inspiratory flow profiles are sharp and pointed, expiratory flow profiles are more blunt in appearance. Figure 4-1: Augmented longitudinal flow and enhanced dispersion: boundary. Flow profile and boundary surface between two gases with different flows: a) low flow with flat profile; b) high flow with tapering, peaked flow profile. In high flow, fresh gas (O2 ) diffuses radially (laterally) permitting gas exchange at the boundary surface. D-8051-2016 02 02 02 02 a b D-8052-2016 Figure 4-2: Asymmetric inspiratory and expiratory flow profiles. The peaked inspiratory flow profile penetrates the broader expiratory flow profile, permitting net movement of fresh gas distally, and expired CO2 moves towards the airway opening via lateral pathways. CO2 O2 12 I 4 Gas mixing during high-frequency oscillations I I 13 Pressure amplitude and respiratory mechanics 4.3 Branching angle asymmetries Secondary gas movements occur at airway bends and bifurcations creating turbulent eddies that enhance radial gas mixing (movement of gas particles from the centre of flow to the stationary boundary layer along the wall) at the expense of longitudinal gas transport. Turbulences occurring at asymmetric airway branches induce pendelluft (see section 4.5), potentially further enhancing gas mixing. : Branching angle asymmetries. Gas flow profiles and boundary surfaces deform at airway bifurcations, together with generation of secondary eddying gas movements to enhance gas mixing.D-8053-2016 4.4 Direct alveolar ventilation Proximal alveoli (i.e. alveoli in close proximity to the central airways) continue to receive ventilation directly via bulk convective gas flow. Consideration of these airways is important in selecting frequency of ventilation, as lower frequencies require higher tidal volumes for effective gas exchange, which may be injurious to proximal alveoli. 4.5 Intra-alveolar pendelluft Some degree of regional inhomogeneity is present even in the healthy lung. Intra- and inter-regional differences in compliance and resistance result in ventilation of neighbouring alveolar units out of phase: each alveolar or acinar compartment will fill and empty at different time constants. The resulting asynchronous behaviour results in intra-alveolar pendelluft – a form of mutual gas exchange in the peripheral gas exchanging compartments (6). Even small fresh-gas volumes are able to reach a large number of alveoli. 4.6 Cardiogenic mixing Low-frequency cardiogenic mixing additionally augments gas mixing in the lung zones adjacent to the heart. Cardiogenic mixing may account for up to half of the oxygen uptake in the presence of totally apnoeic respiration (7). 4.7 Molecular diffusion The same early observations by Henderson and colleagues in 1915 on the behaviour of tobacco smoke in a glass tube contributing to the concept of augmented longitudinal gas transport, also demonstrated the rapidity with which the boundary layer of a gas disappears when flow stops – a process referred to as molecular