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Ventilatory ratio equation
- Ivan Hudecek (7 May 2009)
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Ivan Hudecek, Consultant Anaesthetist Manor Hospital,Walsal,WS2 9PS
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We are informed by Sinha et al(1) that their new ventilatory ratio (VR) has many "exciting potentials%. They however advance neither any compelling arguments nor any hard data as to its real usefulness. This is hardly surprising. Stripped to its bare minimum VR is no more than minute ventilation(VE)/kg multiplied by PaCO2. It makes little difference that in the case of Sinha et al it is divided by a constant (in this case 500)to get a "normal" value. Whether "normal" is 1 or 500 is unlikely to be of any significance. The authors show hardly any interest anyway in trying to find representative figures; instead they rely on half a century old nomogram and data from eight (!!!) anaesthetic charts. To give their equation a "deeper meaning" the authors resort to reverse engineering and inventing various novel "concepts" (accompanied by unconventional symbols).Thus we are presented with a multitude of meaningless calculations during which laws of mathematics and physiology are rewritten. View Image So we have finished where we have started ("VR"=CO2xVE/kg) but this simple truth does not stop the authors from implying that VR equation somehow quantifies alveolar ventilation and physiological dead space. Perhaps they should explain how this is possible without analysis of expired gases(2). The authors hint that VR may be "crucial" in the management of patients with permissive hypercapnia, but offer no suggestions as to how it would feature in say the ARDS network protocol(3). To conclude: VR only tells us how the PaCO2 x VE relationship varies from some imaginary normal. Why the BJA editors think that this common knowledge association somehow merits a verbose tractate is another matter. 1 P.Sinha. et al Ventilatory ratio:a simple bedside measure of ventilation Br J Anaesth 2009; 102: 692–7 2 Fletcher R. Deadspace:invasive or not? Br J Anaesth 2006; 96: 2–7 3 http://www.ardsnet.org/node/77791 Conflict of Interest:None declared |
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