Assessment of Airways Mechanical Properties by Forced Oscillatory Technique (FOT) and Laser Interferometry (LIR) During Anesthesia

• Change in respiratory system impedance [ Time Frame: baseline and 5 minutes of ventilation ] [ Designated as safety issue: No ]

  FOT measurements:

  Respiratory system impedance will be computed from the flow and pressure signals measured at the inlet of the tracheal tube. A composite waveform including 5, 11 and 19 Hz will be used as a stimulating signal 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.

  
  

• Change in chest wall displacement measured by LIR [ Time Frame: baseline and 5 minutes of ventilation ] [ Designated as safety issue: No ]

  Laser interferometry (LIR):

  The movement of 18 points of the chest wall will be measured by a self-mixing interferometer. The measurement points will be chosen along three vertical lines (the midline and the two parasternal lines) and will be equally distributed from the clavicles to the anterior superior iliac spines. The chest wall movement will be measured in response to two different stimulating waveforms: a composite waveform including 5, 11 and 19 Hz, a sinusoidal signal at 100 Hz.

  
  
• Change in oxygenation (paO2) [ Time Frame: baseline and 5 minutes of ventilation ] [ Designated as safety issue: No ]
  Arterial blood gas measurement (ABG)
  
  
• Change in Functional residual capacity (FRC) [ Time Frame: baseline and 5 minutes of ventilation ] [ Designated as safety issue: No ]
  

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)