Tidal volume calculator
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CO 2 production ( )ĬO 2 production is a measure of metabolic activity. In clinical practice, it is anticipated that variation in alveolar ventilation is greater than therefore, changes in VR would foremost represent ventilatory efficiency. Ravenscraft and colleagues 9 have demonstrated that changes in ventilatory efficiency had a greater impact on ‘excess’ minute ventilation than changes in in critically unwell mechanically ventilated patients. Limited data are available on the respective contributions of ventilatory efficiency and CO 2 production on changes in minute ventilation and Pa co 2. In equation ( 13), if the ratio remains constant, that is, an individual in a steady state of CO 2 production, then any changes in VR would directly represent changing ventilatory efficiency or, stated otherwise, a change in the physiological deadspace ventilation. Figure 1 shows the hyperbolic relationship of minute ventilation and Pa co 2, for given values of VR.ĭiscussion Factors influencing VR Ventilatory efficiency As the ratio is dependent on minute ventilation and Pa co 2, any alterations in ventilatory settings that result in a change in VR would either be due to changes in alveolar ventilation or a significant change in the CO 2 production. Similarly, VR would have a linear relationship to ventilatory frequency and tidal volume, provided the other variable remains constant. Provided the other variable remains constant, VR has a linear relationship with both Pa co 2 and . Conversely a decreasing VR represents decreasing carbon dioxide production, increasing ventilatory efficiency, or both. When considering dynamic changes, an increasing VR represents increasing carbon dioxide production, decreasing ventilatory efficiency, or both. Where predicted values match actual values, as in normal individuals, the range of VR will be distributed around unity. Inspection of equation ( 13) shows that VR is governed by carbon dioxide production and ventilatory efficiency in a logically intuitive way. For the purposes of analysis, we have used a single value of VR per patient, this was the highest recorded VR value.Ĭo-variate analysis was carried out using the Mann–Whitney U-test. For ICU patients, VR was calculated twice a day during the course of ICU admission, and for perioperative patients, a single VR value was calculated. Ninety-two of the patients were admitted to the ICU and eight patients were perioperative patients. In order to derive an impression of the range of values of VR, a retrospective analysis of intensive care unit (ICU) and anaesthetic charts was carried out to calculate the VR in 100 mechanically ventilated patients. Outlined below is the theoretical description of VR, an index we believe in time will be shown to have a wide range of clinical applications. We present the physiological analysis of VR, followed by a description of the calculation and rationale of the predicted values. We use the ratio of the product of measured and Pa co 2 to predicted values of the same parameters to derive a novel index called ventilatory ratio (VR).
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The ideal index reflecting CO 2 elimination would need to be simple to use and easily repeatable. Especially in an era where permissive hypercapnia is widely practiced, 5 the development of such an index becomes evermore crucial.Ĭlinical problems with CO 2 elimination will be manifest as an elevation in Pa co 2, a requirement for increased minute ventilation, or a combination of both. 1–4 Although attention is paid to minute ventilation, ventilatory frequency, tidal volumes, and Pa co 2, there is no common unifying index that can be easily used to assess the efficacy of CO 2 elimination at the bedside. Measurements and indices of oxygenation, such as Pa o 2, Sp o 2, and Pa o 2/ F i o 2 or a–a (alveolar–arterial) gradients are frequently utilized to adjust ventilatory settings and aid in clinical decision-making. Although carbon dioxide measurements are used to guide ventilatory adequacy, most ventilatory strategies are aimed primarily at adequate oxygenation. Over the last five decades, emphasis in mechanical ventilation has increasingly focused on improving oxygenation, while avoiding iatrogenic complications.