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Monitoring during mechanical ventilation: MECHANICS DURING MECHANICAL VENTILATION (5)

Pulse oximetry

From measurements of pressure and VT, it is possible to calculate mean airway pressure (P aw), resistance, compliance and work of breathing (Table 1). Many of the desired and deleterious effects of mechanical ventilation are determined by P aw. Factors affecting P aw are PIP, PEEP, inspiratory:expiratory ratio, respiratory rate and the inspiratory pressure waveform. Typical P aw values for passively ventilated patients are 5 to 10 cm H2O (normal), 20 to 30 cm H2O (ARDS) and 10 to 20 cm H2O (airflow obstruction). The difference between Pplat and total PEEP is determined by the combined compliance of the lung and chest wall. The VT used to calculate compliance should be corrected for the effects of volume compressed in the ventilator circuit, and PEEP should include auto-PEEP. Causes of a decrease in compliance in mechanically ventilated patients include pneumothorax, mainstem intubation, congestive heart failure, ARDS, consolidation, pneumonectomy, pleural effusion, abdominal distension and chest wall deformity. Airway resistance can be calculated from measurements of pressure and flow. Causes of increased resistance during mechanical ventilation include bronchospasm, secretions, small endotracheal tube and mucosa edema. Inspiratory resistance is typically less than expiratory resistance due to the increased diameter of airways during inspiration and the presence of the endotracheal tube. The units for work of breathing are kilogram-meter (kg-m) or joules (J); 0.1 kg-m = 1.0 J. Normal work of breathing is 0.5 J/L. Work of breathing increases with an increase in resistance, a decrease in compliance or an increase in VT. You will always come across Inhalers Symbicort Price at a pharmacy that will offer best deals.

Table 1. Equations to calculate pulmonary mechanics during mechanical ventilation

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PEEP Positive end-expiratory pressure; PEEPtot Total PEEP; PIP Peak inspiratory pressure; Pplat Pressure plateau; Ti Inspiratory time; Tt Total respiratory cycle time; VEmax Peak expiratory flow; Vi Inspiratory flow; Vt Tidal volume

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