Resp Care
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Review Comparative Study
Alveolar mechanics in the acutely injured lung: role of alveolar instability in the pathogenesis of ventilator-induced lung injury.
With patients who have acute lung injury, respiratory function is routinely evaluated and the treatment may entail choices from various ventilatory strategies. The ventilatory strategies that have been used over the years are being replaced by newer protocols that represent improvements in patient treatment. However, the rationales for the various ventilatory strategies are largely empirical, because the physiology and mechanics of lung inflation are poorly understood. ⋯ We have researched alveolar histophysiology with animal experiments that combined a conventional histological approach with in vivo microscopy to assess alveolar dynamics during normal and disease-state ventilation. Our video and computer analyses document real-time changes of alveolar size and function, often in the same animal and in adjacent areas of the same lung. Our research indicates that, instead of supporting one theory of alveolar mechanics or another, the various behaviors reportedly exhibited by alveoli may be consistent and represent a continuum between normal alveolar function and the alveolar mechanics of acute lung injury.
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Calculation of total inspiratory resistance (Rtot) for patients on ventilatory support is typically based on measurement of airflow velocity and airway opening pressure during end-inspiratory occlusion by the inspiratory valve in the ventilator. Systematic error is introduced into Rtot measurements because the inspiratory valve closes over a period of time (not instantaneously, so gas continues to flow into the circuit while the valve is shutting) and because the circuit tubing is a distensible compartment between the occluding valve and the respiratory system. The Rtot-measurement error can be minimized with a rapidly-shutting occlusion valve positioned at the airway opening, or, alternatively, by mathematical correction that accounts for the valve-closure period and circuit tubing characteristics. ⋯ The Puritan Bennett 840 measures Rtot more accurately than the Puritan Bennett 7200. Our equations to mathematically correct Rtot measurements made with the PB7200 and PB840 are useful in settings where very accurate Rtot measurements are necessary.
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Comparative Study
Prognostic value of the pulmonary dead-space fraction during the first 6 days of acute respiratory distress syndrome.
The ratio of pulmonary dead space to tidal volume (VD/VT) in acute respiratory distress syndrome (ARDS) is reported to be between 0.35 and 0.55. However, VD/VT has seldom been measured with consideration to the evolving pathophysiology of ARDS. ⋯ In ARDS a sustained VD/VT elevation is characteristic of nonsurvivors, so dead-space measurements made beyond the first 24 hours may have prognostic value.
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Clinical practice often lags behind publication of evidence-based research and national consensus guidelines. ⋯ Web-based teaching tools are useful to educate intensive-care practitioners and to promote evidence-based practice.