Effects of sustained pressure application on compliance and blood gases in healthy porcine lungs.
BACKGROUND
Short periods of sustained increase in airway pressures (Press(up)) are believed to re-open lung areas that collapsed upon induction of anaesthesia. Recruitment of alveolar surface is usually assessed in terms of changes in the pressure-volume (PV) curve. The purpose of this study was to analyse PV-curves before and after a Press(up) and to ascertain whether such changes are compatible with the concept of recruitment of lung volume.


METHODS
During ketamine anaesthesia, 12 healthy piglets were subjected to a Press(up) with end-expiratory pressure (PEEP) of 12 cmH2O and end-inspiratory pressure of 40 cmH2O. Before and after Press(up), PV-curves were obtained from a slow insufflation of 630 ml at zero PEEP (ZEEP).


RESULTS
Compliance was non-linear both before and after Press(up) increasing up to 300 ml and sharply decreasing thereafter. After Press(up), the entire compliance curve was shifted to a higher absolute level. Up to 100 ml and a pressure level corresponding to the lower inflection point on the PV-curve (LIP), compliance was higher before Press(up). No effects on blood gases could be observed.


CONCLUSIONS
If the similar shape of the compliance curve corresponds to a similar chain of re-opening and overdistension events, this would imply that all volume gained by Press(up) is lost within 10 min, without explaining the higher absolute compliance following Press(up). We speculate that a) re-opening of rapidly collapsing small airways determines the initial compliance increase; b) the lower compliance after Press(up) until LIP indicates reduced intratidal re-opening of lung regions; and c) changes in bronchomotor tone induced by Press(up) raise the absolute compliance, with a similar scenario of alveolar and small airway recruitment now taking place but at different degrees of airway stiffness.
Markström
A
A
Department of Surgical Sciences/Section of Anaesthesiology and Intensive Care, University Hospital, Uppsala, Sweden. agneta.markstrom@ane.ds.sll.se
Hedlund
A
A
Sjöstrand
U
U
Nordgren
A
A
Lichtwarck-Aschoff
M
M
D000403:Airway Resistance; D000758:Anesthesia; D000778:Anesthetics, Dissociative; D000818:Animals; D005260:Female; D006439:Hemodynamics; D008168:Lung / Q000502:physiology*; D008170:Lung Compliance; D008297:Male; D011175:Positive-Pressure Respiration; D011312:Pressure; D016684:Prone Position; D011659:Pulmonary Gas Exchange; D015656:Respiratory Mechanics; D013552:Swine; D013990:Tidal Volume
D000778:Anesthetics, Dissociative