Kirk P. Andriano

Automated measurement of functional residual capacity by sulfur hexafluoride washout

We have constructed a computerized, totally automated system for measuring functional residual capacity (FRC) during mechanical ventilation, at any positive end-expiratory pressure (PEEP) and fraction of inspired oxygen. This system uses washout of a small amount (0.5 to 1.0%) of an insoluble, nontoxic tracer gas, sulfur hexafluoride, to measure FRC. It requires no modification of the ventilator and only minimal changes in the breathing circuit; it can be programmed to make measurements routinely without manual intervention.The system was evaluated with three tests. (1) The prototype sulfur hexafluoride analyzer characteristic curve was determined, and the analyzer was evaluated to determine carbon dioxide interference. (2) A comparison with nitrogen washout FRC measurements was made in an extensive bench test with a Plexiglas lung model. The bench test was designed to determine the effects of changing gas composition and minute volume. (3) A study was done in six healthy dogs to determine reproducibility of the FRC measurements at four PEEP levels (0, 5, 10, and 15 cm H2O: two repetitions in each animal).The sulfur hexafluoride analyzer was well characterized by an exponential equation with a multiple r2 = 0.996. The analyzer was not affected by the presence of carbon dioxide (pairedt test,t19 = 1.23,P > 0.10). The bench test indicated that FRC (measured) = 0.969 × FRC (true) − 5.3 ml. (Multiple r2 = 0.979.) This was significantly better than the nitrogen washout system, whose regression equation was also a function of minute volume. In the six animals studied, increasing PEEP always increased FRC and did not significantly alter reproducibility of the FRC measurement (P > 0.1).This automated sulfur hexafluoride washout system should make routine FRC measurements both relatively simple and possible without altering normal ventilatory therapy.

Computer-controlled optimization of positive end-expiratory pressure.

Positive end-expiratory pressure (PEEP) is a standard treatment for patients with refractory hypoxemia due to an acute restrictive pathology. The therapeutic range of PEEP can be quite narrow. PEEP therapy has been optimized using invasive variables such as oxygen transport and pulmonary shunt, and noninvasive variables such as compliance; however, the measurements are complex. We constructed a computerized PEEP-optimization system consisting of a Siemens 900C ventilator, Siemens prototype sulfur hexafluoride analyzer, Siemens 940 lung mechanics analyzer, and a DEC 11/ 23 microcomputer. The user may choose from three different noninvasive PEEP titration algorithms: maximizing static total respiratory system compliance (CTR), maximizing functional residual capacity(FRC)-based compliance (CFRC), and normalizing FRC. The device was tested in six dogs with pulmonary injury induced by oleic acid. The system was constrained to 3-cm H2O PEEP steps at 20-min intervals. The algorithm normalizing FRC reached optimal PEEP levels in 40 min, with a mean difference from the desired FRC of 15 ± 48 (SEM) ml. This corresponds to a mean percent error of 1.0% ± 2.63%. The CFRC and CTR algorithms reached optimal PEEP levels in 60 and 40 min, respectively, and maintained a maximal compliance for 85% of the time. This system provides fully automated noninvasive PEEP titration and is flexible enough to incorporate easily any other PEEP titration algorithms. It should improve patient care by guaranteeing that PEEP therapy is truly optimized throughout the patients recovery.

Variability of the respiratory response to diazepam

The authors investigated the respiratory effects of diazepam in 24 healthy volunteers using a modified Read® rebreathing circuit. Resting end-tidal CO2 (PETCO2) and the slopes of the ventilatory (&OV0312;E/PETCO2) and occlusion pressure (P0.1/PETCO2) response to CO2 were measured just prior to and 5, 20, 40, and 60 min after diazepam, 0.1 mg/kg iv. The slope of &OV0312;E/PETCO2 for all 24 subjects analyzed as a single group was never significantly depressed. The slope of P0.1/PETCO2 for all 24 subjects analyzed as a single group was significantly depressed only at 5 min after diazepam. The resting PETCO2, however, had small but statistically significant increases throughout the 1 h of study. Group or cluster analysis of the slope of P0.1/PETCO2 clearly divided subjects into one group of five subjects, whose P0.1/PETCO2 slope was significantly and consistently augmented for 1 h and a second group of 19 subjects whose P0.1/PETCO2 slope was always less than control for the entire hour. Diazepam may, through effects on pulmonary mechanics and/or the central nervous system, sometimes enhance respiratory responses to CO2 rebreathing. Failure to select for such group effects when studying drug effects by CO2 rebreathing may obscure the severity and duration of respiratory depression that occurs in the majority of individuals. Resting PETCO2 indicated consistent depression of resting minute ventilation by diazepam and may be a more appropriate or sensitive measure of mild or subtle drug-induced respiratory effects.

Technical note: biomechanical analysis of two absorbable fracture fixation pins after long-term canine implantation.

The design requirements for bioabsorbable fracture fixation devices for specific applications are as yet unknown. Therefore, a range of initial mechanical properties and degradation kinetics may provide developers with additional choices for the design of absorbable fracture fixation devices. This study evaluated the changes in push-out strength, polymer mechanical properties, and bone mechanical properties of self-reinforced poly(glycolide) (SR-PGA) and poly(ortho ester) (POE) fracture fixation pins implanted into the canine femoral canal for 18 months. Mechanical testing indicated that SR-PGA pins had degraded to a pasty consistency by 3 months, showing complete loss of all mechanical properties. Meanwhile, POE pins showed a simultaneous linear decrease in both compressive strength and stiffness to almost zero by the end of the study period, suggesting that these devices were undergoing surface erosion. However, changes in specimen diameter, which would support this mechanism, were not apparent. The decrease in polymer density after 12 months suggests that there was an increase in bulk erosion for POE devices. This was further supported by the observation of internal polymer resorption noticed in specimen cross-sections after 18 months. This observation appears to be related to the method of polymer processing; hot-compression molding of fine powdered polymer. The appearance of grain boundaries would provide a path for water to penetrate into the bulk polymer and cause autocatalysis in the interior of the implant.

Transcutaneous PO2 poorly estimates arterial PO2 in adults during anesthesia.

We compared values of PaO2 and transcutaneous PO2 in 21 adult patients during anesthesia. In 282 simultaneous determinations during anesthetic periods of 3–10 h, transcutaneous PO2 was a poor predictor of absolute PaO2 and changes in PaO2. Trancutaneous PO2 monitoring in adults during anesthesia is of unproven value.