Iatrogenic factors contributing to the worsening of gas exchange include high tidal volumes and therefore high airway pressures, positive end-expiratory pressure (PEEP), shear stress, high inspired oxygen concentrations, and nosocomial infection (Table 1).
Table 1 Effects of iatrogenic factors worsening gas exchange during mechanical ventilation
Previous ventilator strategies used in treating acute respiratory failure have included the following:
1. striving to achieve arterial blood gas partial pressures in the normal range by utilizing intermittent positive-pressure ventilation settings that are commonly used in the operating room (i.e. relatively large tidal volumes of 10 to 15 ml/kg, and respiratory rates of 10 to 12 breaths/min), thus generating high airway pressures as well as high shear stresses (see below);
3. increasing the fractional inspired oxygen (FiO 2);
4. breaching the integrity of the trachea by tracheal intubation, which is an infection risk. High peak airway pressure and high tidal volumes
If the intermittent positive pressure ventilator is set to deliver 12 breaths of 700 ml on a continuous mandatory ventilation mode, this means that the patient will receive 700-ml breaths, 12 times per minute, regardless of the pressure generated in the airways. This pressure will depend upon the degree of distensibility of the lung (compliance) and the resistance to flow in the airways. Where compliance is reduced, the pressure in the airway may become dangerously high, subjecting the patient not only to the risks of pneumothorax, pneumomediastinum, pneumopericardium, and bronchial rupture, but also to more subtle iatrogenic damage such as pulmonary interstitial emphysema, perivascular hemorrhage, and perialveolar hemorrhage ( Swami and. Ke.ogh 1.9.92).
In a non-homogeneous disease such as acute respiratory distress syndrome, there are areas of badly affected lung as well as areas of essentially normal tissue which have a normal compliance. These areas of normal lung will receive most of the set tidal volume intended for the whole lung and become overdistended. This volutrauma is probably more damaging than barotrauma (Dr§.yfuss...and.Saumon 1992).
In an attempt to limit the amount of volutrauma/barotrauma caused by positive-pressure ventilation, clinicians have instituted pressure-controlled ventilation where the upper pressure limit is restricted (e.g. 30-35 cmH2O).
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