A 65-year-old patient presented with recurrent cardiac decompensation 12 years after aortic prosthesis replacement and expanded polytetrafluoroethylene (ePTFE) membrane pericardial substitution. Diagnosis of pericardial constriction was difficult. Only one cardiac imaging method, radionuclide ventriculography, was helpful. Upon re-operation, an epicardial fibrous strap which restricted right ventricle (RV) diastolic expansion was found between the anterior free wall and diaphragmatic portion of the RV. Clinical status dramatically improved after surgical removal of this bridle, as did ventricular filling curves in radionuclide imaging. This case shows that delayed cardiac constriction is possible after ePTFE pericardial substitution, especially if the membrane is applied to both anterior and diaphragmatic aspects of the heart.
Lung mechanics was studied in six paralyzed tracheotomized rabbits ventilated with a specially devised computer-controlled ventilator. The target flow waveform contained noninteger multiple frequencies ranging from 0.83 to 6-10 Hz and met a neither-sum-nor-difference criterion to minimize the effects of nonlinearity (B. Suki and K. Lutchen. IEEE Trans. Biomed. Eng. 39: 1142-1151, 1992). The actual flow, however, contained harmonics of the two lowest frequencies. Measurements were performed at mean airway pressure (Paw) levels of 8 and 12 hPa and during histamine-induced bronchoconstriction. Smooth impedance curves were observed in unchallenged rabbits at low mean Paw levels. In contrast, unrealistic impedance fluctuations, suggestive of cross talk from the unwanted frequency components in the flow input, were seen at high mean Paw levels and during acute bronchoconstriction. Model analysis was performed by using the actual flow signal as an input to various nonlinear models. The impedance fluctuations observed at high mean Paw levels were well simulated by a model featuring a volume-dependent elastance, and those observed after histamine were almost perfectly reproduced by a model where resistance increased with the reciprocal of lung volume. We conclude that impedance data biased by cross talk may provide useful information on the presence and nature of respiratory system nonlinearities.
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