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Crackle-sound recording to monitor airway closure and recruitment in ventilated pigs

¹ Dept of Medical Informatics and Engineering, and ⁴ Dept of Anaesthesiology, University of Szeged, Szeged, Hungary, ² Division of Anaesthesiological Investigations, University of Geneva, and ³ Paediatric Anaesthesia Unit, Geneva Children's Hospital, Geneva, Switzerland.

CORRESPONDENCE: Z. Hantos, Dept of Medical Informatics and Engineering, University of Szeged, Korányi fasor 9, H-6720 Szeged, Hungary. Fax: 36 62544566. E-mail: hantos{at}dmi.u-szeged.hu

Keywords: Airway reopening, airway resistance, atelectasis, forced oscillations, pulmonary elastance

Received: September 7, 2005
Accepted November 8, 2005

It was hypothesised that the recruitment of atelectatic lung areas is signified by changes in the airway and tissue mechanics, and by the appearance of crackle activity attributed to the sudden reopening of collapsed airways. The authors also assumed that the acoustic activity is an earlier indicator of lung recruitment than the change in the overall mechanical state of the lungs.

Six thoracotomised and mechanically ventilated mini-pigs were studied. Low-frequency pulmonary impedance was measured at end-expiratory pauses at transpulmonary pressures of 4 and 1 hPa to estimate airway resistance (R_aw) and the coefficient of lung tissue elastance (H), and tracheal sounds were recorded during subsequent slow inflations to 30 hPa, in the control state and following increasing doses of i.v. methacholine (Mch).

R_aw and H were higher at baseline and increased more in response to Mch at 1 hPa than at 4 hPa. The crackles detected during the subsequent inflations were concentrated around and associated with the development of the lower knee of the pressure–volume curve. The number of crackles increased faster following the Mch doses and reached statistical significance earlier than R_aw and H.

Crackle recording during mechanical ventilation can be employed as a simple method with which to monitor lung recruitment–derecruitment.

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