Yet the indirect role of ventilation in oxygenation is illustrated by the ability to oxygenate by insufflation. Since then emphasis in practice has slowly changed and adequacy of oxygenation has replaced carbon dioxide as the principle focus for managing such patients. The initial primacy of carbon dioxide clearance in monitoring during mechanical ventilation was key. This led to the birth of positive pressure ventilation, respiratory physiological monitoring and with it intensive care medicine. The turning point was the realization that most deaths were due to inadequate ventilation. The importance of carbon dioxide clearance was first appreciated in the Danish polio epidemic when clinicians rapidly realised that existing methods such as negative pressure ventilation resulted in deaths despite adequate oxygenation. Such pathological processes are common in the critically ill and measurement of physiological deadspace should be a most valuable and intuitive tool to manage ventilation, yet it is seldom used. At the bedside this will manifest as altered carbon dioxide clearance. Whilst deadspace ventilation is probably of minimal consequence in normal conditions, its inherent clinical importance is that pathophysiological states of the lung result in its increase and hence a reduction in ventilatory efficiency. The remainder of the tidal volume is not involved in gas exchange and is conceptually known as the physiological deadspace. Ventilatory efficiency describes this volume as a proportion of the tidal volume. Finally we describe the indices currently available to track ventilatory efficiency at the bedside.Īlveolar ventilation is the amount of air per unit time that is involved in gas exchange. The potential reasons that have led to the variable being under-used are also examined. Described are the various clinical applications of deadspace measurements in the critically unwell. This review revisits the physiological concepts and methods of measuring deadspace. Thus making it more accessible and easy to monitor and study in large groups of patients, factors which have perhaps resulted in its under-utilisation in critical care. Several indices have been described that either predict deadspace or track ventilatory efficiency at the bedside. Recently though there has been a resurgence of interest in ventilatory efficiency. Deadspace and with it ventilatory efficiency has been largely forgotten. Yet indices of oxygenation seem to be the mainstay when instigating or fine-tuning ventilatory strategies. Since its first description by Bohr in the late 19th century to the current use of single-breath test for volumetric CO 2, our understanding of the physiological deadspace has vastly improved. Measuring deadspace ventilation should be the most reliable method of monitoring ventilatory efficiency in mechanically ventilated patients. Problems with ventilatory efficiency results in abnormal CO 2 clearance.
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