A study of the behavior of the lungs in critically ill patients to optimise treatment

Having gained consent, patients will be sedated, or sedated and paralysed using muscle relaxants, to prevent spontaneous respiratory efforts. The patient’s initial ventilator settings- plateau pressure at end expiration (P plat), tidal volume (Vt), and positive end expiratory pressure (PEEP), will be recorded. All patients will be ventilated using volume controlled, synchronised intermittent mandatory ventilation (SIMV) while the study is being conducted. SIMV sets the tidal volume, with the airway pressure dependent on the patient’s condition. The tidal volume selected by the clinician treating the patient will not be changed for the duration of the measurements. However, if the patient is already on a pressure controlled mode of ventilation, the tidal volume is not directly set by the clinician. If a patient is on pressure controlled ventilation, they will be switched over to SIMV. The measured tidal volume of the pressure controlled mode will be then used to set the mandatory tidal volume of the SIMV mode. Patients who are spontaneous breathing will be trailed on SIMV and given additional sedation to synchronise their respiratory efforts with the ventilator. If necessary, a muscle relaxant may be added providing the patient is unaware of their surroundings. All patients in the study, while still mechanically ventilated will undergo a protocolised recruitment maneuver up to twice daily. Airway pressure and volume data from each maneuver will be collected. Heart rate, blood pressure and pulse oximetry will also be recorded to ensure there is no adverse impact on their circulation. During this maneuver, the airway pressure must be high enough to recruit most of the alveoli. However, if the peak airway pressure is too high, then there is a risk of over distending healthy alveoli and further injuring the lung. A target peak pressure of 45 cmH2O will be used. As the PEEP (Positive End Expiratory Pressure) is increased, the corresponding peak airway pressures also increase. However the increases in peak pressures are not predictable, and vary from patient to patient. To provide a safe trial for the patient, prior to the clinical protocol, a “practice protocol” will be performed to determine the maximum allowable PEEP (PEEPmax) that will produce a peak airway pressure of 45 cmH2O. Prior to the commencement of the protocol, a sample of blood will be taken to determine the arterial blood gas concentrations. This process is then repeated 30 minutes after the protocol is finished. The practice protocol will involve the clinician titrating PEEP until PEEPmax is achieved for the patient. Once PEEPmax is determined, then the clinician has an idea of the upper bound that the protocol can be carried out. Protocol 1. Pneumotachometer is attached to the ‘Y’ connector of the patient’s ventilation circuit. System is tested 2. Carry out PEEP titrations for practice protocol and determine PEEPmax 3. Apply 5 cm PEEP and carry out 5 PV loops with tidal volume of 500ml (or set to the patient’s previous setting, which ever is the lesser). This data will be captured. 4. This tidal volume will remain constant for the duration of the study. 5. Apply an additional 5 cm PEEP to the last PEEP used and carry out 5 PV loops. 6. Repeat step 5 until PEEPmax is achieved. 7. Trial Time (extra time required) = 11 – 17 minutes (Only PEEP titration), 41-47minutes (inclusion of arterial blood gas information). The observation will be carry out for 24 months.