Pressure support compensation for inspiratory work due to endotracheal tubes and demand continuous positive airway pressure.

We evaluated the use of pressure support to compensate for the added inspiratory work of breathing due to the resistances of endotracheal tubes and a ventilator demand-valve system for continuous positive airway pressure (CPAP). A mechanical model was used to simulate spontaneous breathing at five respiratory rates through 7-mm, 8-mm, and 9-mm endotracheal tubes with and without a ventilator demand CPAP circuit. Added work was measured as the integral of the product of airway pressure and volume during inspiration. Additional work was a function of the tube's size, and each 1-mm decrease in the tube's diameter resulted in a 67 to 100 percent increase in work. Adding the ventilator CPAP circuit further increased work and was responsible for 30 to 50 percent of the total work resulting from a tube and CPAP circuit together. Pressure support was added to a level at which net work on the airway was zero, and a relationship between mean inspiratory flow (VT/TI) and the optimal level of pressure support was established for each endotracheal tube. The inspiratory work of breathing was then measured in normal subjects breathing with and without each endotracheal tube plus the demand CPAP circuit. Work per liter of minute ventilation due to the endotracheal tube and CPAP circuit was increased from 54 to 240 percent over levels measured while breathing through an open airway. For each endotracheal tube and VT/TI, a level of pressure support (range, 2 to 20 cm H2O) was found which eliminated added work in the spontaneously breathing subject. This level correlated well with that predicted from the data derived using the mechanical model. We conclude that when adjusting for an endotracheal tube's diameter and VT/TI, pressure support can be used to compensate for the added inspiratory work due to artificial airway resistances.

Comparison of standard weaning parameters and the mechanical work of breathing in mechanically ventilated patients.

Standard bedside criteria of respiratory mechanical capability and the mechanical work of spontaneous breathing were measured in 17 mechanically ventilated patients. Eleven patients were extubated within 24 hs of study and required only a brief period of mechanical ventilation (group 1). Group 2 consisted of six patients requiring more prolonged ventilator support. Group 1 patients met three of four bedside criteria; seven patients met all four. Five of six patients in group 2 also satisfied three of four standard criteria while ventilator-dependent, whereas only two patients satisfied all four when successfully weaning. As group 2 patients progressed from unsuccessful to successful weaning there was no consistent improvement in bedside criteria; however, measures of work did significantly improve. Hence, satisfaction of bedside mechanical weaning criteria is associated with weaning success in patients requiring brief mechanical ventilation. In patients requiring prolonged ventilation, work may be a better indicator of successful weaning.

Evaluation of whole blood theophylline enzyme immunochromatography assay.

A new whole blood enzyme immunochromatographic (EIC) theophylline assay was evaluated in 18 low (25 to 37 percent) and 15 high hematocrit (49 to 56 percent) samples. A good correlation was observed between EIC and fluorescence polarization methods for plasma samples (r = 0.95). However, comparison of results between EIC whole blood and plasma values demonstrates a significant proportional bias that is inversely related to the sample's hematocrit. The EIC method for whole blood samples may substantially underestimate theophylline levels in polycythemic patients with theophylline values near or above the toxic range and underestimate levels in those with anemia, if a correction is not made for the sample's hematocrit. A correction formula to approximate plasma theophylline concentrations from whole blood measurements is described.

Quantitative computed tomography evaluation of benign solitary pulmonary nodules.

A retrospective evaluation of stable solitary pulmonary nodules was completed in 14 patients using a GE 8800 CT scanner and the Computerized Imaging Reference Systems, Inc., computed tomography chest phantom. Patients with a stable nodule for at least 24 months and no evidence of calcification within the nodule by plain radiography were selected for the study. Using the phantom, a quantitative assessment of nodule density was done in each patient. Of 14 nodules, five were categorized as benign. Linear conventional tomography was performed on these five patients, and only one had evidence of benign calcifications. All patients lived in an area endemic for Coccidioides immitis and six of 14 had culture, serologic, or skin test evidence of previous infection with C. immitis. Two of these six patients had nodules that were characterized as benign using the computed tomography chest phantom. This study indicates that the computed tomography chest phantom is useful in identifying benign nodules in patients living in an area endemic for C. immitis pulmonary infections.