SpO2/FiO2 ratio: a prognostic marker for influenza patients

INTRODUCTION

With the start of year 2015, India faced epidemic of H1N1 Influenza. Number of critically ill patients was admitted in hospitals over a short period of time. In such situation it is very important to predict which patients in the ICU will require critical care resources, such as mechanical ventilators and what will be the prognosis of a patient. Most of the patients of influenza die due to ARDS. According to New Berlin definition ¹, PO₂/FiO₂ ratio is required to categories patients. Arterial blood gas analysis for each patient may prove impractical with a large number of patients and constrained resources. So in this study we tried to find out the utility of SpO₂ /FiO₂ ratio for monitoring and predicting the outcome of H1N1 positive patients of category C admitted in Government Medical College, Nagpur, Maharashtra, India from 14^th January’ 2015 to 15^th April’2015.

METHODS

This study is a hospital based observational study done at Government Medical College, Nagpur, Maharashtra, India from 14^th January to 15^th April 2015. In this study, 156 H1N1 positive patients of category C ² were included. Inclusion criteria were H1N1 positive patients of category C of age more than 12 years. Continuous monitoring of SpO₂ was done. At the same time record of Fio₂ requirement of patients was kept. SpO₂ was measured with pulse oximetry. We calculated SpO₂ /FiO₂ on the day of admission, on day3, day 5, day 7, day 10 and day of discharge or death. Patients were monitored till discharge or death. While calculating SpO₂ we avoided confounding factors like abnormal haemoglobin, peripheral circulatory failure, nail polish etc.

RESULTS

156 patients were included. 96 were female and 60 were male patients. The mean age of patients was 42.5 ± 14.75 years. Oxygen supplementation as required by the patients is shown in Table 1.

SpO₂ was normal in 30 patients on admission despite their chest signs and they did not require oxygen supplementation. All of them survived. Oxygen by Reservoir Face Mask was required for 49 patients for maintenance of their SPO₂ above 90% and all these patients survived and were discharged. 23 patients were managed with Non-invasive positive pressure ventilation and all survived. 54 patients required invasive ventilation. Only 2 out of 54 patients survived.

SpO₂/FiO₂ ratio was calculated on day one and values were used to divide patients into 5 groups as per Modified Sequential Organ Failure Assessment (MSOFA) Score³. In each group, percentage of survivors and non survivors was calculated and then compared with other groups. Results are shown in Table 2.

Mean SpO₂/FiO₂ ratio on admission was 393.5±100.23 in Survivors as compared to 277.88±105.46 in Non-Survivors. Out of 70 patients with SpO₂/FiO₂ >400, 2 patients (2.8%) died, whereas out of 14 patients with SpO₂/FiO₂ <150, 10 patients (71.42%) died. Increase in mortality is seen as the SpO₂/FiO₂ ratio dropped. Inverse relationship was seen between SpO₂/ FiO₂ ratio and mortality (p < 0.0001). Mortality was significantly high in patients with SpO₂/FiO₂ ratio < 150.

Continuous monitoring was done with SpO₂. SpO₂/FiO₂ ratio was calculated on Day 3, day 5, day 7, day 10 and day of discharge or death and tried to find out their relationship with mortality. Observations are shown in Table 3.

SpO2/FiO₂ ratio progressively dropped in patients who died. SpO2/FiO₂ ratio gradually improved in patients who survived (Figure 1). Above results showed that SpO₂/FiO₂ ratio is a reliable marker to monitor and to predict outcome of H1N1 positive patients.

DISCUSSION

It is very important to know the respiratory status of a case of H1N1 influenza. The SOFA ⁴ score is a validated measure of organ failure over time and a predictor of mortality in critically ill patients. But repeated arterial blood gas analysis may prove impractical. Hence this study was undertaken to find out effectiveness of SpO₂/FiO₂ ratio in monitoring of patients and predicting their outcome.

Pulse oximetry is the most commonly utilized technique to monitor oxygenation which is non-invasive and safe. In this method arterial haemoglobin O₂ saturation is measured by differentiating oxyhemoglobin form deoxygenated haemoglobin using their respective light absorption at wave lengths of 660 nm (red) and 940 nm (infra-red).⁵ It is a very easy method to know the status of patients.

Various studies are done in past which have compared SpO₂/FiO₂ ratio with PO₂/FiO₂ ratio to know their relationship. In the study done by Todd W. Rice et al, it was found that An SpO₂/FiO₂ ratio of 235 corresponded with a PO₂/FiO₂ ratio of 200, while SpO₂/FiO₂ ratio of 315 corresponded with a PO₂/FiO₂ ratio of 300.⁶ Thus continuously monitored SpO₂/FiO₂ ratio can be used as a surrogate for the PO₂/FiO₂ ratio. The use of the SpO₂/FiO₂ ratio may better facilitate the screening and rapid identification of clinical status of patients and can be used to predict prognosis while avoiding repeated arterial punctures and reducing the cost of blood gas determinations.

In our study, we found significant relationship between SpO₂/FiO₂ ratio and mortality. Hence we came to the conclusion that instead of doing arterial blood gas analysis every time we can easily monitor H1N1 positive patients with SpO₂/FiO₂ ratio. It can also help to take important decisions regarding ventilatory support. This will decrease the burden over the Health care system during H1N1 epidemics. Thus SpO₂/FiO₂ ratio on day 1 as well as serial monitoring can be used as a prognostic marker in H1N1 positive patients.

ACKNOWLEDGEMENTS

I would like to thank Dr. A. Niswade (Dean, GMC, Nagpur) for permitting us to publish this case report. I would like to thank to Dr. More (statistician) for helping us to calculate statistics.

Funding: No funding sources

Conflict of interest: None declared

Ethical approval: The study was approved by the Institutional Ethics Committee

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