Detection, quantification and differentiation of upper and lower airway obstruction during mechanical ventilation

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During mechanical ventilation, increased upper airway resistance may be due to a partial obstruction of the artificial airway or a large airway abnormality. Increased lower airway resistance is usually due to acute or chronic airways disease. Either of these problems can lead to increased work of breathing, uncoordinated ventilator support, respiratory distress and difficulty weaning. Recognition of these problems can lead to important interventions such as bronchoscopy, artificial airway change or reposition, bronchodilator therapy and change of ventilation strategy that can improve patient distress and safety. Quantification of these variables can allow assessment of severity and the response to intervention. Our hypotheses are that 1. the most valuable variable for quantifying reduced expiratory airflow will be the passive expiratory volume expired during the first second of expiration PEV1 expressed as a percentage of the Vt (PEV1/Vt ratio). This is similar to the well recognised standard of FEV1/VC ratio from spirometric quantification of (lower) airflow obstruction. 2. the most valuable variable for quantifying increased inspiratory resistance during VCV with decelerating inspiratory flow or during PCV will be the difference between the Paw and calculated Palv during early inspiration divided by the inspiratory flow at that time. 3. Upper airway obstruction will be distinguished from and lower airway obstruction by a higher inspiratory resistance relative expiratory resistance We propose to examine these hypotheses in patients with normal lungs with added inspiratory resistances and in patients with lower airway obstruction (COAD or asthma) to show whether measurements of flow and volume are independent of or can account for ventilator pattern (flow rate, flow pattern, tidal volume) and can recognise and quantify both increasing resistances in large (upper) airways and lower airway obstruction to distinguish between them. Methods Two patient groups will be selected based on the following lung pathology 1. Normal Lungs: 20 mechanically ventilated patients with no significant underlying lung disease and no significant lung injury will be selected. Each will be studied to provide a normal values of the resistance measurements. Then each will be given a series of 3 upper airway resistances for a period of less than a minute to mimic large airway obstruction (similar to a partially blocked endo-tracheal or tracheostomy tube). This will be achieved by placing a 10 cm sterilized flexible tube in the endo-tracheal or tracheostomy tube to simulate a residual diameter equivalent to an ETT size (internal diameter) 6, 4.5 and 3 mm. This is little different to and carries no more risk than routine patient suctioning where the artificial airway is partly occluded to a similar degree for a short time 2. Lower airway obstruction (COPD, Asthma): 20 mechanically ventilated patients with known obstructive airways disease (lower airway obstruction) Each patient group will be placed in controlled mechanical ventilation and given the same series of ventilatory patterns to determine the reliability of the measurements of inspiratory and expiratory resistance measurements under different ventilatory conditions

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