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Open Access Highly Accessed Research

Analysis of ventilatory ratio as a novel method to monitor ventilatory adequacy at the bedside

Pratik Sinha1*, Nicholas J Fauvel1, Pradeep Singh2 and Neil Soni1

Author Affiliations

1 Magill Department of Anaesthesia, Intensive Care Medicine and Pain Management, Chelsea and Westminster Hospital, 369 Fulham Road, London SW10 9NH, UK

2 Department of Mathematics, Southeast Missouri State University, One University Plaza, Cape Girardeau, MO 63701, USA

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Critical Care 2013, 17:R34  doi:10.1186/cc12541

Published: 27 February 2013

Abstract

Introduction

Due to complexities in its measurement, adequacy of ventilation is seldom used to categorize disease severity and guide ventilatory strategies. Ventilatory ratio (VR) is a novel index to monitor ventilatory adequacy at the bedside. <a onClick="popup('http://ccforum.com/content/17/1/R34/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://ccforum.com/content/17/1/R34/mathml/M1">View MathML</a>. <a onClick="popup('http://ccforum.com/content/17/1/R34/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://ccforum.com/content/17/1/R34/mathml/M2">View MathML</a> is 100 mL.Kg-1.min-1 and <a onClick="popup('http://ccforum.com/content/17/1/R34/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://ccforum.com/content/17/1/R34/mathml/M3">View MathML</a> is 5 kPa. Physiological analysis shows that VR is influenced by dead space (VD/VT) and CO2 production <a onClick="popup('http://ccforum.com/content/17/1/R34/mathml/M4','MathML',630,470);return false;" target="_blank" href="http://ccforum.com/content/17/1/R34/mathml/M4">View MathML</a>. Two studies were conducted to explore the physiological properties of VR and assess its use in clinical practice.

Methods

Both studies were conducted in adult mechanically ventilated ICU patients. In Study 1, volumetric capnography was used to estimate daily VD/VT and measure <a onClick="popup('http://ccforum.com/content/17/1/R34/mathml/M5','MathML',630,470);return false;" target="_blank" href="http://ccforum.com/content/17/1/R34/mathml/M5">View MathML</a> in 48 patients. Simultaneously, ventilatory ratio was calculated using arterial blood gas measurements alongside respiratory and ventilatory variables. This data was used to explore the physiological properties of VR. In Study 2, 224 ventilated patients had daily VR and other respiratory variables, baseline characteristics, and outcome recorded. The database was used to examine the prognostic value of VR.

Results

Study 1 showed that there was significant positive correlation between VR and VD/VT (modified r = 0.71) and <a onClick="popup('http://ccforum.com/content/17/1/R34/mathml/M5','MathML',630,470);return false;" target="_blank" href="http://ccforum.com/content/17/1/R34/mathml/M5">View MathML</a> (r = 0.14). The correlation between VR and VD/VT was stronger in mandatory ventilation compared to spontaneous ventilation. Linear regression analysis showed that VD/VT had a greater influence on VR than <a onClick="popup('http://ccforum.com/content/17/1/R34/mathml/M5','MathML',630,470);return false;" target="_blank" href="http://ccforum.com/content/17/1/R34/mathml/M5">View MathML</a> (standardized regression coefficient 1/1-VD/VT: 0.78, <a onClick="popup('http://ccforum.com/content/17/1/R34/mathml/M5','MathML',630,470);return false;" target="_blank" href="http://ccforum.com/content/17/1/R34/mathml/M5">View MathML</a>: 0.44). Study 2 showed that VR was significantly higher in non-survivors compared to survivors (1.55 vs. 1.32; P < 0.01). Univariate logistic regression showed that higher VR was associated with mortality (OR 2.3, P < 0.01), this remained the case after adjusting for confounding variables (OR 2.34, P = 0.04).

Conclusions

VR is an easy to calculate bedside index of ventilatory adequacy and appears to yield clinically useful information.