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Measurement Issues

Tracheal Pressure

In clinical practice, PAO is measured in the ventilator or at the patient connection. This means that it is affected to a great extent by the endotracheal tube resistance, resulting in a peak inspiratory pressure (PIP) much greater than peak tracheal or peak alveolar pressure [54]. If an end-inspiratory occlusion technique is used, for example, the plateau pressure is the maximum pressure in the airway below the tube. Recording both PIP and plateau pressure gives useful information; for example, increased resistance caused by narrowing of the tube (secretions, kinking) will increase PIP and not affect plateau pressure [55], while the increased resistance is caused by the lungs of the patient below the tube, then both the PIP and the plateau pressure may increase.

By using measurements of flow, PAO, and knowing the resistance of the endotracheal tube, which is obtained from laboratory testing of clean tubes and connectors, tracheal pressure can be calculated continuously. This has been used in commercially available ventilators, and ventilator flow is increased to overcome the endotracheal tube resistance [56]. However, caution should be taken when using a predetermined value of endotracheal tube resistance. Tube resistance is determined on laboratory measurements with clean tubes, which is a rare case in the real intensive care unit environment. Gas humidification and position/angulation of the endotracheal have also significant effects on resistance [57].

Direct tracheal pressure measurements can be made by passing a catheter through the endotracheal tube in order to obtain tracheal pressure values where flow changes caused by the transition from trachea to endotracheal tube are minimized, typically 2 cm above the carina. When using side-hole catheters, the correct position is difficult to be verified. End-hole catheters, on the other hand, are less dependent on position, but measurements are affected by gas kinetic pressure. However, the kinetic energy of gas is so small that this effect on the total measured pressure is negligible.

The pressure catheter can be gas or fluid filled. Fluid-filled catheters, although requiring that the transducer is at the same level as the catheter tip to measure absolute pressure accurately, are less sensitive to secretions and occlusion [55].

Nevertheless, direct tracheal pressure measurements provide correct end- inspiratory and expiratory pressures, including intrinsic PEEP caused by ETT resistance, without stopping ventilation and irrespective of ventilatory mode.

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