Diagnostic test of the desaturation index and the calculated desaturation index to identify acute lung injury and/or acute respiratory distress syndrome at the bedside

Introduction

We have previously shown that the desaturation index (DI) and the calculated desaturation index (Dic) have a statistical correlation with several indices of gas exchange and with lung infiltrates on the thorax X-ray. However, the utility of the DI and Dic as a diagnostic tool has never been tested. The aim of this study is therefore to determine the capability of the DI and the Dic: to identify acute lung injury (ALI) and/or acute respiratory distress syndrome (ARDS) at the bedside.

Patients and methods

Patients with and without ALI and/or acute respiratory failure (ARDS) were followed during the period of mechanical ventilation. Every morning, blood gases analysis and thorax X-rays were taken and definitions for ALI/ARDS were completed according to the American–European Consensus Conference. We registered the positive end expiratory pressure, inspired fraction of oxygen (FiO₂) and arterial saturation by pulse oximetry (SpO₂) to calculate the Dic. Then, in order to determine the DI, all patients were subjected to breathe 100% oxygen, with a gradual reduction of the FiO₂ to 21%, in steps of 20% lasting 8 min each, and at each step the SpO₂ was registered (we stop the maneover after reaching 85% of the SpO₂). Finally, we calculated the DI according our equation that has been described previously. Two or more quadrants with infiltrates on the thorax X-ray and PaO₂/FiO₂ ratio (adjusted to high altitude) lower than 221 mmHg were considered positive for ALI/ARDS. The sensitivity and specificity were calculated according to conventional equations.

Results

During the study period, 198 ALI/ARDS definitions were completed. Table 1 presents the sensitivity and specificity tests for the DI and the Dic. Results are shown as the percentage and confidence interval.

Table 1

Conclusion

We conclude that the DI and the Dic, which are noninvasive indices, have an acceptable sensitivity and specificity to identify ALI/ARDS patients at the bedside.