title: Use of an inspiratory impedance threshold valve during chest compressions without assisted ventilation may result in hypoxaemia.
abstract: BACKGROUND
  Although the concept of intermittent airway occlusion with the inspiratory impedance threshold valve (ITV) is a well-recognised strategy for improving efficiency of cardiopulmonary resuscitation (CPR), little is known about possible pulmonary side effects.
  
  
  METHODS
  After a baseline chest CT-scan, 24 pigs with beating hearts undergoing apnoeic oxygenation received an injection of a contrast medium and were then assigned randomly to either active compression-decompression CPR with ITV (ACD ITV CPR), ACD CPR alone, or standard-CPR with ITV (standard-ITV CPR), or standard-CPR alone. After a maximum of 5 min of chest compressions or if oxygen saturation dropped below 70%, the experiment was stopped, haemodynamic variables and blood gas values were measured, and another CT-scan was performed; all animals underwent a 30 min recovery-period and a third subsequent CT-scan.
  
  
  RESULTS
  At baseline arterial oxygen saturation by pulse oxymetry was 99% in all four groups; in both the ACD ITV CPR and the standard-ITV CPR groups, arterial oxygen saturation dropped below 70% within 126+/-9s, whereas chest compressions in all ACD CPR and standard-CPR pigs were performed over 5 min (P<0.001). Before stopping chest compressions arterial oxygen pressure decreased in the ACD ITV CPR group from 426+/-96 to 42+/-8 mmHg while it decreased in the ACD CPR group only from 415+/-116 to 197+/-127 mmHg (P<0.001 between groups); in the standard-ITV CPR group arterial oxygen partial pressure decreased from 427+/-109 to 34+/-5 mmHg while oxygen partial pressure decreased only from 467+/-44 to 144+/-98 mmHg in the standard-CPR group (P<0.004 between groups). After the second CT scan arterial oxygen partial pressure decreased further to 19+/-2 mmHg in the ACD ITV CPR versus 210+/-41 mmHg in the ACD CPR group; to 20+/-2 mmHg in the standard-ITV CPR versus 148+/-33 mmHg in the standard-CPR group. Lung-density values (Hounsfield units) were significantly higher in the ACD ITV CPR versus ACD CPR group (-134+/-54 versus -330+/-77) and standard-ITV CPR versus standard-CPR group (-98+/-50 versus -387+/-42). After a 30 min recovery-period, there were no significant differences in arterial oxygen partial pressure (ACD ITV CPR 275+/-110 mmHg versus ACD CPR 379+/-111 mmHg and standard-ITV CPR 265+/-138 mmHg versus standard CPR 367+/-55 mmHg). Furthermore, there were no differences in lung density values between groups after 30 min of recovery.
  
  
  CONCLUSIONS
  In this animal model with a beating heart, intermittent airway obstruction through an ITV combined with apnoeic oxygenation and without active ventilation resulted in hypoxaemia due to transiently impaired lung function.
authors.affiliation: Department of Anaesthesiology and Critical Care Medicine, Innsbruck Medical University, Anichstrasse 35, 6020 Innsbruck, Austria. holger.herff@uibk.ac.at
authors.forename: Holger
authors.initials: H
authors.lastname: Herff
authors.forename: Claus
authors.initials: C
authors.lastname: Raedler
authors.forename: Rolf
authors.initials: R
authors.lastname: Zander
authors.forename: Volker
authors.initials: V
authors.lastname: Wenzel
authors.forename: Christian A
authors.initials: CA
authors.lastname: Schmittinger
authors.forename: Erich
authors.initials: E
authors.lastname: Brenner
authors.forename: Michael
authors.initials: M
authors.lastname: Rieger
authors.forename: Karl H
authors.initials: KH
authors.lastname: Lindner
mesh_terms: D000403:Airway Resistance; D000818:Animals; D016887:Cardiopulmonary Resuscitation / Q000009:adverse effects* / Q000295:instrumentation*; D004195:Disease Models, Animal; D004867:Equipment Design; D005260:Female; D006323:Heart Arrest / Q000628:therapy*; D000860:Hypoxia / Q000000981:diagnostic imaging / Q000209:etiology*; D008297:Male; D013552:Swine; D014057:Tomography, X-Ray Computed; D016896:Treatment Outcome