Figure 2. These graphs adapted from Frizzola et al. 3 show that high flow therapy (HFT) provides a ventilation effect (impact on arterialCO2 tension) not seen with continuous positive airway pressure (CPAP). Moreover, the effect is more pronounced and occurs at lowerflows when the <strong>ca</strong>nnulae are fitted to allow a high degree of leakage around the nasal prongs.Figure 3. This graph adapted from Frizzola et al. 3 showsthe oxygenation relationship with HFT titration. Arterialoxygen tension rises with increased flow to a plateau, afterwhich more flow has no further effect. This inflexion pointis explained in the tracheal gas insufflation literature as thepoint where flow is adequate to purge all available deadspace.interface being easier to manage than a sealed CPAP system.These include patient tolerance, ease in nursing management,and accessibility for kangaroo <strong>ca</strong>re, as well as physiologicconcerns such as prone positioning to support spontaneousbreathing. 15,16 As we better define and optimize HFT as primarilya therapy to eliminate dead space, and understand the coincidingability to generate mild pressure and hydrate the air passages,HFT holds promise to emerge as a signifi<strong>ca</strong>nt advancement inneonatal respiratory support.HFT: A Unique Noninvasive Respiratory SupportModalityThe act of ventilation refers to the circulation of air so asto replace stale or noxious air with fresh air. In mammalianphysiology this process involves tidal volumes and lungcompliance be<strong>ca</strong>use of our anatomi<strong>ca</strong>l dead space. In otherwords, if we were to remove dead space entirely by putting ouralveolar surface on the outside of our body (eg gills on a fish),we would not need to have tidal volume excursions to exposethe alveolar surface to adequate V A in support of respiration.Obviously, this is not practi<strong>ca</strong>l for numerous reasons, includingthe need to condition gas before coming into contact with theblood, and our adaptation to use dead space for retaining CO2 asour innate pH buffering mechanism.Figure 4. This graph adapted from Frizzola et al. 3 showsthe end-distending pressure response to nasal prongswhich occlude the nares (low leak) versus prongs thatocclude no more than 50% of the nares (high leak). Ineach <strong>ca</strong>se pressure rises with increased flow, dissociatingthis relationship from the oxygenation curve (Figure 3).Referring back to Figure 2, note that the non-occlusiveprongs facilitated better ventilation, accomplishing optimaleffect at a lower flow, seen here where the extrapolationbars cross the x-axis. By following these bars to the y-axis,note the non-occlusive prongs accomplish this effi<strong>ca</strong>cywith signifi<strong>ca</strong>ntly less distending pressure.Nonetheless, by reducing dead space we <strong>ca</strong>n reduce the V Eneeded to accomplish adequate V A and therefore reduce workof breathing. Dead space elimination tactics have been used foryears in the form of tracheal gas insufflation 17,18 and transtrachealoxygen delivery. 19 In the last 10 or more years, advancementsin heated humidifi<strong>ca</strong>tion devices have made it possible toaccomplish ventilation by way of dead space elimination with anasal <strong>ca</strong>nnula.Translational research has shown that the primary mechanismof action for HFT is purging anatomi<strong>ca</strong>l dead space, thusachieving V A with lesser V E . A pivotal mechanistic study wasdone using neonatal piglets with a severe respiratory distressinduced by central venous oleic acid delivery. 3 In this model,three conditions were compared: HFT with a low leak aroundthe prongs (ie snug fit in the nares), HFT where no more than50% of the nares were occluded (ie non-occlusive prongs) andconventional mask CPAP. The low leak condition was created tomimic the situations where clinicians try to get a CPAP effect,whereas the ≤50% occlusion condition fits our recommendationfor the appli<strong>ca</strong>tion of HFT. Under these conditions, the model42 neonatal INTENSIVE CARE Vol. 26 No. 6 • <strong>October</strong> <strong>2013</strong>
evaluated titration of flow/CPAP pressure on CO2 removal,oxygenation and pressure development.As shown in Figure 2, under both HFT conditions, arterial CO2inversely correlated with flow rate wherein arterial CO2 tension(PaCO2) in these spontaneous breathers could be reduced backto pre-injury levels. More over, the PaCO2 in the
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