title: Positive end-expiratory pressure increments during anesthesia in normal lung result in hysteresis and greater numbers of smaller aerated airspaces.
abstract: BACKGROUND
  Although it is recognized that pulmonary hysteresis can influence the effects of positive end-expiratory pressure (PEEP), the extent to which expansion of previously opened (vs. newly opening) peripheral airspaces contribute to increased lung volume is unknown.
  
  
  METHODS
  Following a recruitment maneuver, rats were ventilated with constant tidal volumes and imaged during ascending and descending ramps of PEEP.
  
  
  RESULTS
  The authors estimated peripheral airspace dimensions by measuring the apparent diffusion coefficient of He in 10 rats. In a separate group (n = 5) undergoing a similar protocol, the authors used computerized tomography to quantify lung volume. Hysteresis was confirmed by larger end-inspiratory lung volume (mean ± SD; all PEEP levels included): 8.4 ± 2.8 versus 6.8 ± 2.0 ml (P < 0.001) and dynamic compliance: 0.52 ± 0.12 versus 0.42 ± 0.09 ml/cm H2O (P < 0.001) during descending versus ascending PEEP ramps. Apparent diffusion coefficient increased with PEEP, but it was smaller during the descending versus ascending ramps for corresponding levels of PEEP: 0.168 ± 0.019 versus 0.183 ± 0.019 cm/s (P < 0.001). Apparent diffusion coefficient was smaller in the posterior versus anterior lung regions, but the effect of PEEP and hysteresis on apparent diffusion coefficient was greater in the posterior regions.
  
  
  CONCLUSIONS
  The authors' study results suggest that in healthy lungs, larger lung volumes due to hysteresis are associated with smaller individual airspaces. This may be explained by opening of previously nonaerated peripheral airspaces rather than expansion of those already aerated. Setting PEEP on a descending ramp may minimize distension of individual airspaces.
authors.affiliation: * Assistant Professor, Department of Anesthesiology and Critical Care, † Research Specialist, ‡ Project Manager, § Student, ‖ Research Assistant Professor, # Technical Director, ‡‡ Professor, Department of Radiology, †† Professor, Department of Anesthesiology and Critical Care and Stavropoulos Sepsis Research Program, University of Pennsylvania, Philadelphia, Pennsylvania. ** Professor, Departments of Critical Care Medicine and Anesthesia, Hospital for Sick Children, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Cereda
authors.forename: Yi
authors.initials: Y
authors.lastname: Xin
authors.forename: Kiarash
authors.initials: K
authors.lastname: Emami
authors.forename: Jessie
authors.initials: J
authors.lastname: Huang
authors.forename: Jennia
authors.initials: J
authors.lastname: Rajaei
authors.forename: Harrilla
authors.initials: H
authors.lastname: Profka
authors.forename: Biao
authors.initials: B
authors.lastname: Han
authors.forename: Puttisarn
authors.initials: P
authors.lastname: Mongkolwisetwara
authors.forename: Stephen
authors.initials: S
authors.lastname: Kadlecek
authors.forename: Nicholas N
authors.initials: NN
authors.lastname: Kuzma
authors.forename: Stephen
authors.initials: S
authors.lastname: Pickup
authors.forename: Brian P
authors.initials: BP
authors.lastname: Kavanagh
authors.forename: Clifford S
authors.initials: CS
authors.lastname: Deutschman
authors.forename: Rahim R
authors.initials: RR
authors.lastname: Rizi
mesh_terms: D000758:Anesthesia / Q000706:statistics & numerical data*; D000818:Animals; D007090:Image Interpretation, Computer-Assisted; D008168:Lung / Q000033:anatomy & histology* / Q000502:physiology*; D008176:Lung Volume Measurements; D008279:Magnetic Resonance Imaging; D008297:Male; D011175:Positive-Pressure Respiration / Q000009:adverse effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D014057:Tomography, X-Ray Computed
references.citation: J Appl Physiol (1985). 1990 Jun;68(6):2280-6
references.pmid: 2384408
references.citation: J Appl Physiol (1985). 2012 Dec 15;113(12):1945-7
references.pmid: 22797308
references.citation: Ann Biomed Eng. 2011 Mar;39(3):1112-24
references.pmid: 21132371
references.citation: J Clin Invest. 1997 Mar 1;99(5):944-52
references.pmid: 9062352
references.citation: Histol Histopathol. 2002 Apr;17(2):383-92
references.pmid: 11962742
references.citation: Thorax. 1986 Aug;41(8):620-8
references.pmid: 3787544
references.citation: J Appl Physiol (1985). 1987 Apr;62(4):1480-7
references.pmid: 3597219
references.citation: Histol Histopathol. 2004 Jan;19(1):159-66
references.pmid: 14702184
references.citation: J Appl Physiol (1985). 2012 Jan;112(1):135-48
references.pmid: 21940853
references.citation: Am J Respir Crit Care Med. 2001 Nov 1;164(9):1701-11
references.pmid: 11719313
references.citation: J Appl Physiol (1985). 2003 Nov;95(5):1926-36
references.pmid: 12871961
references.citation: Am J Respir Crit Care Med. 2001 Jan;163(1):69-78
references.pmid: 11208628
references.citation: Br J Anaesth. 1996 Jun;76(6):760-6
references.pmid: 8679345
references.citation: Am J Respir Cell Mol Biol. 2008 May;38(5):572-8
references.pmid: 18096874
references.citation: Crit Care. 2006;10(4):R122
references.pmid: 16925814
references.citation: J Appl Physiol (1985). 2007 Jan;102(1):477-84
references.pmid: 16873601
references.citation: Magn Reson Med. 2006 Dec;56(6):1293-300
references.pmid: 17058206
references.citation: J Appl Physiol. 1962 May;17:391-7
references.pmid: 13917520
references.citation: J Appl Physiol (1985). 2012 Aug 15;113(4):666-76
references.pmid: 22678969
references.citation: BMJ. 1990 Jan 27;300(6719):230-5
references.pmid: 2106931
references.citation: N Engl J Med. 2000 May 4;342(18):1301-8
references.pmid: 10793162
references.citation: Crit Care Med. 2013 Feb;41(2):527-35
references.pmid: 23263577
references.citation: JAMA. 2010 Mar 3;303(9):865-73
references.pmid: 20197533
references.citation: Respir Physiol. 1995 Dec;102(2-3):205-15
references.pmid: 8904012
references.citation: J Appl Physiol (1985). 2011 Feb;110(2):499-511
references.pmid: 21127207
references.citation: Anaesthesia. 2012 Sep;67(9):999-1008
references.pmid: 22708696
references.citation: Am J Respir Crit Care Med. 2003 Jun 15;167(12):1633-40
references.pmid: 12663325
references.citation: Acad Radiol. 2004 Oct;11(10):1171-9
references.pmid: 15530811
references.citation: J Appl Physiol (1985). 2012 Mar;112(6):937-43
references.pmid: 22096115
references.citation: Am J Respir Crit Care Med. 1998 Jan;157(1):294-323
references.pmid: 9445314
references.citation: J Appl Physiol Respir Environ Exerc Physiol. 1979 Nov;47(5):1002-10
references.pmid: 511700
references.citation: Anesthesiology. 1994 Apr;80(4):751-60
references.pmid: 8024128
references.citation: J Appl Physiol Respir Environ Exerc Physiol. 1983 Oct;55(4):1321-32
references.pmid: 6629967
references.citation: Am J Respir Crit Care Med. 1999 Nov 1;160(5):1697-1702
references.pmid: 10556143
references.citation: J Appl Physiol Respir Environ Exerc Physiol. 1983 Apr;54(4):1048-56
references.pmid: 6853280
references.citation: Respir Physiol. 1972 Jun;15(2):190-213
references.pmid: 5042167
references.citation: J Appl Physiol. 1970 Oct;29(4):422-31
references.pmid: 4990020
references.citation: Am J Respir Crit Care Med. 1994 May;149(5):1327-34
references.pmid: 8173774
references.citation: Crit Care Med. 2004 Sep;32(9):1817-24
references.pmid: 15343007
references.citation: Am J Respir Crit Care Med. 2010 Feb 15;181(4):394-418
references.pmid: 20130146
references.citation: Anesthesiology. 2013 Jun;118(6):1307-21
references.pmid: 23542800
references.citation: Am J Respir Crit Care Med. 1998 Oct;158(4):1230-4
references.pmid: 9769286
references.citation: Am J Respir Crit Care Med. 2006 Aug 1;174(3):279-89
references.pmid: 16675780