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Official Journal of the Italian Society of Anesthesiology, Analgesia, Resuscitation and Intensive Care
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Minerva Anestesiologica 2001 April;67(4):238-47

Copyright © 2009 EDIZIONI MINERVA MEDICA

language: English

Pathophysiology of prone positioning in the healthy lung and in ALI/ARDS

Pelosi P., Caironi P., Taccone P., Brazzi L.

Università degli Studi - Milano Ospedale Maggiore Policlinico IRCCS - Milano (Italy) Istituto di Anestesia e Rianimazione Università dell’Insubria - Varese * Dipartimento di Scienze Cliniche e Biologiche


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­Prone posi­tion was initial­ly intro­duced in ­healthy anes­the­tized and par­a­lyzed sub­jects for sur­gi­cal spe­cif­ic rea­sons. ­Then, it was ­used dur­ing ­acute res­pir­a­to­ry fail­ure to ­improve gas ­exchange. The inter­est on ­prone posi­tion dur­ing ALI/­ARDS pro­gres­sive­ly ­increased, ­even if the mech­a­nisms lead­ing to a res­pir­a­to­ry improve­ment are not yet com­plete­ly under­stood. In nor­mal sub­jects, dur­ing anes­the­sia and par­al­y­sis, ­prone posi­tion deter­mines a ­more homo­ge­ne­ous dis­tri­bu­tion of the grav­i­ta­tion­al gra­di­ent of alveo­lar infla­tion, a ven­ti­la­tion dis­trib­ut­ed ­towards the non depen­dent ­lung ­regions and a ­reverse of the grav­i­ta­tion­al dis­tri­bu­tion of region­al per­fu­sion, ­even if fac­tors oth­er ­than grav­ity are ­involved. More­over, ­prone posi­tion caus­es, ­both in ­healthy sub­ject and in ­obese ­patients, an improve­ment in oxy­gen­a­tion and in func­tion­al resid­u­al capac­ity with­out affect­ing res­pir­a­to­ry ­system, ­lung and ­chest ­wall com­pli­ance. In ALI/­ARDS ­patients, ­prone posi­tion ­lead to a ­reverse of the alveo­lar infla­tion and ven­ti­la­tion dis­tri­bu­tion, due to the ­reverse of hydro­stat­ic pres­sure over­ly­ing ­lung paren­chy­ma, the ­reverse of ­heart ­weight, and the chang­es in ­chest ­wall ­shape and mechan­i­cal prop­er­ties. Lit­tle ­data are avail­able for the mod­ifi­ca­tions in region­al ­lung per­fu­sion. The pos­sible mech­a­nisms ­involved in oxy­gen­a­tion improve­ment dur­ing ­prone posi­tion in ALI/­ARDS ­patients are: 1) ­increased ­lung vol­umes; 2) redis­tri­bu­tion of ­lung per­fu­sion; 3) recruit­ment of dor­sal spac­es ­with ­more homo­ge­ne­ous ven­ti­la­tion and per­fu­sion dis­tri­bu­tion. ­From a clin­i­cal ­point of ­view, ­prone posi­tion ­seems to be a ­very prom­is­ing treat­ment for ALI/­ARDS, ­even if its use is not yet a stan­dard clin­i­cal prac­tice. We ­have recent­ly fin­ished a ran­dom­ized-con­trolled ­trial in ­order to inves­ti­gate the clin­i­cal ­impact of ­this pro­ce­dure. In the pre­lim­i­nary ­phase of the ­study per­formed in 35 Ital­ian Inten­sive ­Care ­Units, we stud­ied, ­from 1996 to 1998, 73 ­patients ­with a PaO2/FiO2 of 123±42 and a ­SAPS (Sim­pli­fied ­Acute Phys­iol­o­gy ­Score) of 38±11. ­After the ­first ­hour of ­prone posi­tion­ing, the PaO2/FiO2 ­ratio of 76% of the ­patients had ­increased by ­more ­than 20 ­mmHg (respond­er) ­with a ­mean ­increase of 78±53 ­mmHg. The pro­por­tion of respond­ers ­increased to 85% ­after 6 ­hours of ­prone posi­tion­ing. The inci­dence of maneu­ver-relat­ed com­pli­ca­tions and ­severe and ­life-threat­en­ing com­pli­ca­tions was extreme­ly ­rare. The over­all mor­tal­ity at ICU dis­charge was 51% and the ICU ­stay was sim­i­lar in sur­vi­vors and non sur­vi­vors (17.8±11.6 vs 17.8±11.4 ­days).

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