Assessment of Airways Mechanical Properties by FOT and LIR During Anesthesia
Condition: Anesthesia
Study type: Observational

Summary: 
      The aims of the present study are:

        1. To evaluate the effect of the induction of anaesthesia and paralysis in terms of changes
           in oscillatory mechanics parameters

        2. To evaluate the mechanical properties of the respiratory system in terms of input and
           transfer oscillatory impedance in response to PEEP changes
    

Detailed description: 
      It has been recently shown that respiratory system reactance (Xrs) obtained by the forced
      oscillation technique (FOT) at 5 Hz is more reliable than dynamic compliance for assessing
      lung collapse and the effects of lung RMs in a porcine ALI model ( Ref.1,2).

      Specifically, Xrs (and its derived variable CX5, the oscillatory compliance at 5 Hz)
      identifies the minimum positive end-expiratory pressure (PEEP) level required to maintain
      lung recruitment with high sensitivity and specificity. Moreover, it has been recently
      demonstrated that Xrs may be used to identify the lowest level of PEEP able to prevent
      atelectasis and that PEEP setting strategy based on maximizing Xrs is able to limit lung
      injury compared to oxygenation-based approach in a porcine lavage model of lung injury. (
      Ref.3)

      Recently, at the biomedical engineering department of Politecnico di Milano measurements of
      chest wall displacement have been successfully performed by means of an optical sensor
      realized using a laser self-mixing interferometer (LIR). The advantage of this approach is
      that it is contact-less, that by deflecting the laser been it is possible to scan any region
      of the chest wall surface and that it allows to measure also low-frequency vibrations.

      Protocol

        1. The baseline pulmonary function will be assessed by spirometry before surgery.

        2. Just before surgery measurements of oscillatory mechanics will be performed at the
           following stages:

             -  awake patient

             -  spontaneous ventilation through the laryngeal mask after induction of anesthesia

             -  pressure controlled ventilation (PCV) after the administration of neuromuscular
                blocking agent and intubation and PEEP 0 cmH2O

             -  PCV with PEEP 5 cmH2O

             -  Recruitment maneuver (RM) (peak pressure 30 cmH2O and PEEP 15 cmH2O) for 2 minutes

             -  PCV with PEEP 5 cmH2O after RM

      At each stage the following measurements will be performed:

        -  Input impedance at 5-11-19 Hz by FOT

        -  the movements of the chest wall assessed by laser interferometry (LIR) applying two
           different stimulating waveforms: 5-11-19 Hz and 100 Hz

        -  functional residual capacity (FRC)

        -  arterial blood gas measurement (ABG)
    

Intervention name(s): Forced oscillatory technique (FOT) and laser interferometry (LIR).
Intervention type(s): Procedure
Intervention description(s): FOT: The stimulating signal is generated by an A/D-D/A board and amplified by a power amplifier that drives a loudspeaker the output of which is connected to inspiratory line of the ventilator. Pressure and flow are measured at the inlet of the endotracheal tube by a piezoresistive pressure transducer and a mesh-type heated pneumotachograph coupled with a differential pressure transducer.
LIR: The scanner unit is made by three laser interferometers, the beams of which are deflected on the chest wall surface by a mirror moved by a stepper motor triggered on the respiratory pattern of patient.
Time perspective: Prospective
Sampling method: Probability Sample
Study population: 
        Patients ASA classification I-II, scheduled for elective surgery requiring general
        anesthesia
      
Keywords: Forced oscillatory technique; Laser interferometry; Intravenous anesthesia; Mechanical ventilation; Pressure controlled ventilation; Airways mechanical properties; Respiratory system reactance; Oscillatory mechanics; Atelectasis