Physiologic Analysis and Clinical Performance of the Ventilatory Ratio in Acute Respiratory Distress Syndrome

Abstract

Rationale: Pulmonary dead space fraction (Vd/Vt) is an independent predictor of mortality in acute respiratory distress syndrome (ARDS). Yet, it is seldom used in practice. The ventilatory ratio is a simple bedside index that can be calculated using routinely measured respiratory variables and is a measure of impaired ventilation. Ventilatory ratio is defined as [minute ventilation (ml/min) × PaCO2 (mm Hg)]/(predicted body weight × 100 × 37.5).

Objectives: To determine the relation of ventilatory ratio with Vd/Vt in ARDS.

Methods: First, in a single-center, prospective observational study of ARDS, we tested the association of Vd/Vt with ventilatory ratio. With in-hospital mortality as the primary outcome and ventilator-free days as the secondary outcome, we tested the role of ventilatory ratio as an outcome predictor. The findings from this study were further verified in secondary analyses of two NHLBI ARDS Network randomized controlled trials.

Measurements and main results: Ventilatory ratio positively correlated with Vd/Vt. Ordinal groups of ventilatory ratio had significantly higher Vd/Vt. Ventilatory ratio was independently associated with increased risk of mortality after adjusting for PaO2/FiO2, and positive end-expiratory pressure (odds ratio, 1.51; P = 0.024) and after adjusting for Acute Physiologic Assessment and Chronic Health Evaluation II score (odds ratio, 1.59; P = 0.04). These findings were further replicated in secondary analyses of two separate NHLBI randomized controlled trials.

Conclusions: Ventilatory ratio correlates well with Vd/Vt in ARDS, and higher values at baseline are associated with increased risk of adverse outcomes. These results are promising for the use of ventilatory ratio as a simple bedside index of impaired ventilation in ARDS.

Keywords: ARDS; dead space; ventilatory ratio.

Figures

Figure 1.

(A) Correlation between physiologic dead space and ventilatory ratio in the primary dataset. (B) Correlation between CO2 production and ventilatory ratio in the primary dataset.

Figure 2.

Physiologic dead space in ordinal groups of ventilatory ratio in the primary dataset. Q1, 2.13. P value represents ANOVA test.

Figure 3.

Ventilatory ratio (VR) plotted against PaO2/FiO2 ratio in moderate and severe acute respiratory distress syndrome in the primary dataset. Percentages represent mortality rates in the population split into high-VR (VR ≥2) and low-VR (VR <2) groups. (A) Percentage mortality in high-VR group. (B) Percentage mortality rate in the low-VR group (horizontal dashed line = 2; P < 0.001).

Figure 4.

Probability of ventilator-free breathing to 28 days in acute respiratory distress syndrome (ARDS) population according to ventilatory ratio (VR) classification; patients with ARDS with low VR had a VR A) Data from the primary dataset. (B) Data from the FACTT dataset. P values represent Cox proportional hazards test comparing low-VR ARDS and high-VR ARDS. FACTT = Fluid and Catheter Treatment Trial.