Methodology & Protocols

The goal of this research was to recruit up to 10 MV patients receiving treatment in the Christchurch Hospital ICU. All patients enrolled in the study underwent a recruitment manoeuvre over different levels of PEEP. If eligible, patients were then transported to the CT scanner for four lung CT’s. The following sections discuss the inclusion and exclusion criteria for patients, and specific protocols used to adjust PEEP.

5.3.1 PATIENT SELECTION CRITERIA

1. Patients on MV therapy.

81 3. Patients were included in trial if PF ratio was between 150 and 300 mmHg on the day of consent. Patients who have a PF ratio, partial pressure of arterial oxygen (PaO2) divided by the fraction of inspired oxygen (FiO2), between 200 – 300 mmHg are defined as ALI. PF ratios below 200 mmHg are defined as ARDS. Patients with a PF ratio less than 150 mmHg were deemed too unwell for inclusion.

EXCLUSION CRITERIA

1. Patients who were likely to be discontinued from MV therapy in 24 hours.

2. Patients who were moribund and/or not expected to survive for greater than 72 hours

3. Patients who were not disorientated in time, place or person. These patients were excluded as they would have required additional sedation, which may have prolonged their length of stay.

4. Patients with asthma were excluded.

5. Patients who had significant brain injury. The application of high airway pressures can increase blood pressure in the brain that could have proved harmful.

6. Women less than 45 years were excluded for the CT part of the study to mitigate any added risk of breast cancer

7. Patients who had already received 20 mSv through medical radiation in the last twelve months or would receive a cumulative radiation dose of ≥ 20 mSv (over a 12 month period) as a result of the additional study scans, were excluded from the CT section of the study.

8. Patients who had already received 100 mSv through medical radiation in the last five years or would receive a cumulative radiation dose of ≥ 100 mSv (over a 5 year period) as a result of the additional study scans, were excluded from the CT section of the study.

5.3.2 TRIAL SETUP

Because patients were sedated, consent was obtained on behalf of the patient by immediate family members only. If patients recovered later, then they would provide final consent on the use of the data. Consented patients were sedated, or sedated and paralysed using muscle relaxants, to prevent spontaneous respiratory efforts. The patient’s initial ventilator

settings, plateau pressure at end expiration, tidal volume, and initial PEEP setting were all recorded.

All patients were ventilated using volume controlled mode, synchronised intermittent mandatory ventilation (SIMV) while the study was conducted. SIMV sets the tidal volume, with the airway pressure dependent on the patient’s condition. The tidal volume was selected by the clinician treating the patient and was not changed for the duration of the measurements. However, if the patient was already on a volume controlled mode, then tidal volume was not directly set by the clinician. If a patient was on pressure controlled ventilation, they were switched to SIMV. The measured tidal volume of the pressure controlled mode was then used to set the mandatory tidal volume of the SIMV mode. Patients who were spontaneously breathing were put on SIMV mode and given additional sedation to synchronise their respiratory efforts with the ventilator. In addition, muscle relaxants were used if the patient was unaware of their surroundings. Patients were sedated with propofol and rocuronium bromide (ROC) was the preferred relaxant.

All patients in the study underwent a protocolised recruitment manoeuvre with the airway pressure and the volume data collected. Heart rate, blood pressure and body temperature were also recorded. Before the recruitment manoeuvre and 30 minutes post manoeuvre, an arterial blood gas was taken to measure the PF ratio. This measurement was used to evaluate any changes on blood oxygenation.

Patients were ventilated using a Puritan Bennett PB840 ventilator (Covidien, Boulder, CO, USA) in the Department of Intensive Care, Christchurch Hospital, New Zealand. A Hamilton Medical flow sensor (Hamilton Medical, Switzerland) was attached to the y- piece of the tubing and connected to a calibrated pneumotachometer. The pneumotachometer was used to obtain the pressure and flow measurements, and could capture the volume changes due to PEEP. A standard Dell™ (Dell, Austin, TX, USA) laptop was used in conjunction with the software Labview Signal Express (National Instruments, Austin, TX, USA) to obtain the raw measurements and store it in a text file. Raw data was then processed with the aid of Matlab 2009 (The Mathworks, Natick, Massachusetts, USA) for use with the model.

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5.3.3 STUDY PROTOCOLS

The protocol used in this study was based on the work by Gattinoni et al [Gattinoni et al., 2006], but was modified to satisfy the objectives of this study. The protocol involved a PEEP trial, with PEEP incremented in steps of 5 cmH2O and peak airway pressure limited to 45 cmH2O. Two individual protocols were developed for the recruitment manoeuvre without and with the added CT scans. In addition, an expiratory and inspiratory hold was performed to measured auto-PEEP and inspiratory resistance respectively. Once the maximum PEEP was achieved, PEEP was decremented in steps of 5 cmH2O until PEEP was back to the initial setting.