Editorial 3

Editorial: Airway pressure and xenon anaesthesia

Since the rediscovery of xenon as an anaes- 1/40 (3 mm small bronchi length as opposed thetic in the 1990’s, there has been some to 120 mm length), and Re to around concern about the effects of xenon’s high 1/100, according to data from Lumb and Tsu- density and viscosity as compared to air, oxy- da (4,5). Thus, the denominator of the equa- gen and . Several investigators tion will not change while the numerator is observed higher driving pressures at their decreased by a factor of about 400. It is ob- ventilators, necessary to generate standard vious that this change will virtually eliminate ventilation patterns (1). In animal studies it the influence of density. was shown that the physical properties of Although Re will be up to 4 times higher xenon indeed could explain these elevated for a high xenon concentration as compared pressures and that the greatest pressure loss to air, as it also depends on density, this will within the system occurred over the endotra- not be important for pressure distribution, cheal tube (2,3). Katz and colleagues present with Re in a range of less than 1 in small a combined study of simulation and experi- bronchi. Accordingly, a hypothetical pressure mental measurements to demonstrate the ef- loss across the trachea of as high as 30 hPa fects of ventilation with xenon-oxygen mix- would translate to less than 0.1 in the small tures on external airway and intrapulmonary bronchi, regardless of the gas mixture insuf- pressure distribution under adult human con- flated. Keeping this in mind, it is not surpris- ditions. They have developed an interesting ing that the simulation yields a pressure of model and – again – clearly demonstrate that less than 1 hPa from bronchial generation 5 the most important pressure drop is found on, regardless of flow pattern and xenon con- over the external tubing, including the endo- centration, as demonstrated in figures 1 to 3 tracheal tube. Within the lung, xenon-con- of the Katz paper. taining gas will most likely not result in signif- The authors probably correctly state that icantly higher pressures than any air-oxygen there is really no more pressure difference mixtures. Although their modelling is very so- from generation 15 on. This means no pres- phisticated, there may be an easier way to sure-driven mass flow but only convection look at the problem: When substituting the and diffusion which depend on viscosity and law of Hagen and Poiseuille for laminar flow diffusion properties. As xenon’s kinematic with some of the human airway dimensions viscosity is less than 30% of that of air and as provided by Nunn’s Applied Respiratory oxygen (which are almost identical), and its Physiology (7th edition, 2010 (4)), it becomes diffusion coefficient in air is around 40% of evident that already at the level of the small that of oxygen, a difference in mass transport bronchi (generation 12) gas density becomes between xenon-containing gas and oxygen in negligible for pressure loss: air is likely to occur within the terminal air- ways. However, at that point the picture be- ρ 2 P1 – P2 = (64/Re) x (l/d) x ( /2) x v , comes very complex and the forces and pat- terns directing flow are not completely un- with Re as the Reynolds number, l as length derstood yet (5-7). In any case, pressure obvi- and d as diameter, ρ as density, v as the ously does not determine mass transport in mean velocity. When d is increased from the small airways anymore, and thus pressure 18 mm (trachea) to about 2000 mm (2000 distribution is only relevant within larger air- small bronchi of 1 mm diameter each), the ways. pressure difference becomes less than 1/400 Much more than inspiration, it would because v2 will decrease to about 1/10, l to have been interesting to model expiration ac- 4 Editorial cordingly because this is where a difference xenon anaesthesia. Appl Cardiopulm Patho- physiol 2009; 13: 208-11 between a high-density gas and air/oxygen 4. Lumb AB. Functional anatomy of the respira- may become clinically relevant: as the au- tory tract. In: Horne T, ed. Nunn’s Applied thors show in their figure 7C, the pressure – Respiratory Physiology. Edinburgh: Churchill which is probably distributed uniformly Livingstone, 2011: 13-26 throughout the lung at the beginning of expi- 5. Tsuda A, Henry FS, Butler JP. Gas and aerosol ration – would be about 6 instead of 3 hPa mixing in the acinus. Respir Physiol Neurobi- when a xenon-containing mixture is used. ol 2008; 163: 139-49 Still, it would be interesting if uniformity of 6. Kleinstreuer C, Zhang Z, Li Z. Modeling air- pressure distribution will hold true also for flow and particle transport/deposition in pul- xenon as the authors suggest in their figure. monary airways. Respir Physiol Neurobiol In any case, a pressure of 6 hPa is in the 2008; 163: 128-38 7. Kumar H, Tawhai MH, Hoffman EA, Lin CL. range of therapeutically administered PEEP The effects of geometry on airflow in the aci- and could be causing gas trapping and even- nar region of the human lung. J Biomech tually over-insufflation due to extended expi- 2009; 42: 1635-42 ration time. On the other hand, just like delib- 8. Scorsone D, Bartolini S, Saporiti R, Braido F, erately applied PEEP this may have Baroffio M, Pellegrino R, Brusasco V, Crimi E. favourable effects on alveolar Does a low-density gas mixture or oxygen by preventing airway closure during anaes- supplementation improve exercise training in thesia. COPD? Chest 2010; 138: 1133-9 Moreover, gas distribution in the pul- 9. Maggiore SM, Richard JC, Abroug F, Diehl JL, monary acinus which does not depend on Antonelli M, Sauder P, Mancebo J, Ferrer M, pressure as stated above, may be influenced Lellouche F, Lecourt L, Beduneau G, Brochard L. A multicenter, randomized trial by adding a compound with different diffu- of noninvasive ventilation with helium-oxy- sivity and viscosity by changing the small lo- gen mixture in exacerbations of chronic ob- cal concentration gradients driving gas ex- structive lung disease. Crit Care Med 2010; change. Another noble gas which has in fact 38: 145-51 been investigated extensively in this regard is helium, with much lower density and inter- mediate diffusivity when compared to xenon and air. However the clinical effects aside from lowered large airway pressure in high- frequency ventilation appear quite limited (8,9).

References 1. Rueckoldt H, Vangerow B, Marx G, Haubitz B, Meyer MC, Piepenbrock S, Leuwer M. Xenon inhalation increases airway pressure in ventilated patients. Acta Anaesthesiol Scand 1999; 43: 1060-4 Correspondence address 2. Baumert JH, Reyle-Hahn M, Hecker K, Ten- Jan-H. Baumert, M.D., Ph.D., DEAA brinck R, Kuhlen R, Rossaint R. Increased air- Dept. of Anesthesiology way resistance during xenon anaesthesia in UMC St Radboud pigs is attributed to physical properties of the Geert Grooteplein 10 gas. Br J Anaesth 2002; 88: 540-5 NL-6500 HB Nijmegen 3. Schmidt M, Marx T, Papp-Jambor C, Reinelt The Netherlands H, Schirmer U. Airway pressures during [email protected]