The total work of breathing in the mechanically ventilated patient generally consists of the patient's own physiologic work of breathing, the disease process that the patient is suffering from, and also the resistance offered by the ventilator circuit and endotracheal tube (known as the "imposed work of breathing"). The imposed work of breathing is primarily determined by the inner diameter of the breathing circuit, and the endotracheal tube (the narrowest of all of the tubing) creates the most significant resistance to air flow. Poiseuille's equation for resistance to laminar flow dictates that resistance is inversely proportional to radius to the 4th power, so that small changes in radius have a 16-fold effect on resistance.
As the patient's secretions accumulate within the endotracheal tube, the effective lumen cross section is narrowed and the imposed work of breathing increases. Elevation in imposed work of breathing can be difficult to differentiate from elevation in work of breathing due to the disease process, and the patient's inability to wean from the ventilator may be erroneously attributed to worsening disease rather than an obstructed endotracheal tube.
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As an example, a 7 mm endotracheal tube with a measurable 1 mm circumferencial buildup of secretions experiences an effective lumen narrowed to 5 mm diameter. This decrease in diameter (radius) essentially doubles the imposed work of breathing, based on doubling of airway resistance. Poiseuille's equation for resistance to laminar flow, upon which estimates such as these are based, probably underestimates the net increase in resistance as it does not account for turbulent airflow caused by the irregular luminal topography on the secretion surface.
Work of breathing can increase insidiously during prolonged courses of mechnical ventilation due to gradual accumulation of secretions in the endotracheal tube, or suddenly and dramatically with resulting respiratory distress. Use of the CAM Rescue Cath™ can help reduce the imposed work of breathing attributable to the gradual accumulation of secretions, mucus and biofilm within the lumen of the endotracheal tube. In cases where the endotracheal tube becomes suddenly occluded by particularly thick, tenatious secretions that do not even allow passage of a suction catheter, the CAM Rescue Cath™ could reduce the need for emergency reintubation or tracheostomy.
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