Assessment of Airways Mechanical Properties by FOT and LIR During Anesthesia

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

• The baseline pulmonary function will be assessed by spirometry before surgery.
• 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)

Inclusion criteria

• age >18 years
• ASA I-II, scheduled for elective surgery requiring general anesthesia
• signed informed consent

Exclusion criteria

• patient refusal
• BMI > 35
• co-existing respiratory disease (COPD, asthma, restrictive lung disease)
• pregnancy