Prognostic value of novel indices combining Shock Index, Reverse Shock Index, age, and oxygen saturation for predicting mortality in COVID-19 patients in Iran at emergency department triage: a cross-sectional study

Article information

Acute Crit Care. 2025;40(3):425-434

Publication date (electronic) : 2025 August 29

doi : https://doi.org/10.4266/acc.005040

Mehdi Torabi^,1, Atefe Noroozpour¹, Neda Naeemi Bafghi¹, Moghaddameh Mirzaee²

¹Department of Emergency Medicine, Kerman University of Medical Sciences, Kerman, Iran

²Health Research Center, Institute for Futures Studies in Health, Kerman University of Medical Sciences, Kerman, Iran

Corresponding author: Mehdi Torabi Department of Emergency Medicine, Kerman University of Medical Sciences, Gharani Ave, Kerman 7613747181, Iran Tel: +98-91-3199-2016, Fax: +98-343-2474-638 Email: mtorabi1390@yahoo.com

Received 2024 December 28; Revised 2025 June 18; Accepted 2025 July 4.

Abstract

INTRODUCTION

Coronavirus disease 2019 (COVID-19) is a viral disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that caused a high mortality in a pandemic. Meanwhile, emergency departments (EDs) were heavily involved in this outbreak. Therefore, rapid determination of the assignment of patients in the ED is of great importance [1,2]. Triage systems were implemented to prioritize patients based on the severity of their symptoms and the likelihood of requiring intensive care. This approach helped in managing the overwhelming number of patients and ensuring that those in critical condition received timely medical attention [3,4].

The role of vital signs in triage in determining the prognosis of critically ill patients referred to the ED is very important, although their accuracy in predicting the outcome of patients referred to the hospital can vary [5,6]. The utilization of these vital signs has led to the development of indices such as the Shock Index (SI), the reverse SI, and the age SI. The SI, defined as the ratio of heart rate to systolic blood pressure, the Reverse Shock Index (RSI), defined as the ratio of systolic blood pressure to heart rate, and the Age SI, defined as the SI multiplied by age, have been shown to be valuable predictors of mortality in patients [7-10]. These indices played a more significant key role in determining the prognosis of patients compared to vital signs. However, it seems that these indices may have more certainty in predicting the prognosis of patients along with other factors related to the disease. It has been seen that the SI, along with the Glasgow Coma Scale (GCS), can be very helpful in predicting the mortality of trauma patients, which is probably due to the role of GCS in these patients. Several studies were conducted in this field and confirmed that by adding GCS to these indices, the certainty of the predictive role of these indices can be increased [11,12].

Hypoxia, characterized by low oxygen saturation, is a critical factor in assessing the severity and prognosis of COVID-19 patients upon arrival at the ED triage. The virus primarily targets the respiratory system, leading to complications such as pneumonia and acute respiratory distress syndrome, which can result in significant hypoxemia [13]. Monitoring oxygen saturation is essential for identifying patients at risk of worsen outcomes and determining the need for interventions such as supplemental oxygen or mechanical ventilation. Studies have shown that early detection and management of hypoxia can improve patient outcomes by preventing the progression to more severe respiratory failure. Therefore, assessing oxygen saturation levels is a vital component of the triage process in emergency settings during the COVID-19 pandemic [14].

Although the prognostic role of these indices has been seen in COVID-19 patients, but considering the respiratory involvement in COVID-19 patients, it is possible that the value of the predictive role of these indices can be increased by using a combination of oxygen saturation in these indices. With these interpretations, we created several new indices using the SI and RSI along with oxygen saturation. Accordingly, the aim of the study was to investigate the prognostic value of the SI and RSI along with oxygen saturation for mortality prediction in COVID-19 patients presenting to the ED triage.

MATERIALS AND METHODS

The study was approved by the Research Ethics Committee of Kerman. University of Medical Sciences (IR.KMU.AH.REC.1401.270). The collection of data was conducted without the requirement for informed consent as the data collection forms were completed anonymously and the results were disseminated without any reference to the participants' identities.

Patient Selection

The study included all patients aged 18 years or older diagnosed with SARS-CoV-2, confirmed by a positive oropharyngeal or nasopharyngeal polymerase chain reaction test. Patients under the age of 18 with a negative test result, and pregnant patients and incomplete records were excluded from the study (Figure 1).

Figure 1.

Flowchart showing enrollment of patients.

RESULTS

The patient demographic characteristics, vital signs, laboratory tests, and indices of 500 patients with confirmed cases of COVID-19 patients are shown in Table 1. The mortality rate in the hospital was documented to be 72 patients (14.4%). The findings were also reported separately for the survival and mortality groups. The mean age of the patients included in the study was 51±1 years, with a median age of 47 years. Furthermore, the distribution of patients across age groups revealed that 238 (47.6%) were male and 262 (52.4%) were female. A non-significant difference was observed between the two groups in terms of gender (P=0.14), while a significant difference was identified between the two groups in terms of age (P<0.0001). While the number of female participants exceeded that of males, the mortality rate was higher among males (55.6% vs. 44.4%). An examination of the vital signs revealed a statistically significant difference between the two groups with the exception of diastolic blood pressure (P<0.05). Furthermore, a statistically significant difference was observed in the LOS leading to death, with a longer duration observed in the deceased patients compared to those who survived (P<0.0001). As illustrated in Table 1, a statistically significant difference was observed in laboratory test results between the two patient groups. This discrepancy was evident in parameters such as WBC, hemoglobin, blood urea nitrogen (BUN), creatinine, aspartate aminotransferase (AST), alanine aminotransferase (ALT), bilirubin, blood glucose, lactate dehydrogenase (LDH), erythrocyte sedimentation rate (ESR) and D-dimer levels. The magnitude of this difference was statistically significant at the P<0.05 level. Subsequent to the extraction of indices from vital signs, a comparative analysis was conducted to ascertain the difference in these indices between survival and mortality patients. While we had hypothesized that this difference would be significant in terms of SI and RSI between the two groups of patients, this was not the case. However, when examining other indices, such as Age SI, SI/oxygen saturation (SpO₂), Age SI/SpO₂, RSI/Age, RSI×SpO₂, and RSI×SpO₂/Age, significant differences were observed between the two groups with patient mortality (P<0.0001).

Table 1.

Characteristics, vital signs, laboratory data, and indices in COVID-19 patients

To assess the strength of the association between variables and patient outcomes, i.e., mortality, logistic regression and OR analyses were employed. Univariate analysis revealed a significant relationship between the variables of age, systolic blood pressure, heart rate, respiratory rate, oxygen saturation, LOS, laboratory tests (WBC, hemoglobin, BUN, AST, bilirubin, blood glucose, LDH, ESR and D-dimer) and the indices (Age SI, SI/SpO₂, Age SI/SpO₂, RSI/Age, RSI, RSI×SpO₂, and RSI×SpO₂/Age) with mortality. Subsequently, the variables with P-values less than 0.2 were entered into the multivariate regression model and analyzed using the backward elimination method. This analysis revealed that only the variables of gender, oxygen saturation, hemoglobin, direct bilirubin, LDH, D-dimer, and the two indices (Age SI/SpO₂ and RSI×SpO₂/Age) exhibited a statistically significant relationship with the patients' outcomes (Table 2).

Table 2.

Univariate and multivariate regression analysis of variables according to their association with mortality in COVID-19 patients

Finally, the ROC curve for the two indices (Age SI/SpO₂ and RSI×SpO₂/Age) was used to determine their predictive role (Figures 2 and 3). In the context of mortality prediction, both Age SI/SpO₂ at a cutoff of 0.52 and RSI×SpO₂/Age at a cutoff of 1.9 were identified as effective predictors, exhibiting an AUC of 0.8 and achieving a sensitivity, specificity, and accuracy of over 70%. The OR of the Age SI /SpO₂ index was 8.23, indicating that for every unit increase from the cutoff of 0.52, the risk of mortality in patients increased by more than 8 times. The OR of RSI×SpO₂/Age at a cutoff of 1.9 was also reported to be 0.12, indicating that with a one-unit increase, the risk of mortality decreased by 88% (Table 3 and 4).

Figure 2.

Receiver operating characteristic (ROC) curve for Age SI/SpO₂ in predicting mortality in coronavirus disease 2019 (COVID-19). SI: Shock Index; SpO₂: oxygen saturation.

Figure 3.

Receiver operating characteristic (ROC) curve for RSI×SpO₂/Age in predicting mortality in coronavirus disease 2019 (COVID-19). RSI: Revere Shock Index; SpO₂: oxygen saturation.

Table 3.

Predictive value of Age SI/SpO₂ for mortality in COVID-19 patients (cutoff=0.52)

Table 4.

Predictive value of RSI×SpO₂/Age for mortality in COVID-19 patients (cutoff=1.90)

DISCUSSION

The prognostic significance of shock indices in critically ill patients is well established. In patients with SARS-CoV-2, given the involvement of the lung as a target organ, hypoxia, along with shock indices, especially the Age SI and the RSI/Age, can provide a more reliable index than other shock indices in predicting hospital mortality. SpO₂ (oxygen saturation) is a critical biomarker in COVID-19, as hypoxia is strongly associated with disease severity and mortality. Studies have shown that low SpO₂ levels, particularly in the context of “silent hypoxia,” are prevalent in COVID-19 patients and correlate with higher risks of intensive care unit (ICU) admission and in-hospital mortality. For instance, research has identified that SpO₂ levels dropping to 70%–80% in COVID-19 patients, even without overt respiratory distress, are linked to severe outcomes [16,17]. Also, hypoxia in patients with SARS-CoV-2 infection has been identified as a risk factor for progression to severe and critical stages of the disease [18]. The presence of silent hypoxia has been observed to exacerbate the risk of harm to these patients [19]. Furthermore, hypoxia has been identified as a pivotal risk factor in determining the prognosis of patients, with the capacity to predict hospital mortality in isolation [20]. Consequently, hypoxia is recognized as a significant risk factor for these patients, as demonstrated in this study.

Age is a well-established risk factor for COVID-19 mortality, with older patients exhibiting higher rates of severe outcomes due to comorbidities and reduced physiological reserve In-hospital mortality increased markedly with increasing age. A sample of adults hospitalized with laboratory-confirmed COVID-19 with older age were associated with higher risks of ICU admission and in-hospital mortality [21,22]. This study showed that in patients with SARS-CoV-2, hypoxia combined with increasing Age, SI and RSI plays a significant role in ruling out hospital mortality.

The assessment of mortality risk in hospitalized patients with COVD-19 is imperative for the timely administration of medical interventions. The development of rapid and accurate triage tools can facilitate the prediction of high-risk patient conditions with greater efficiency and precision. Concomitantly, the role of shock indices in determining the prognosis of patients with COVD-19 may prove beneficial. In a study, Avci and Doğanay [23] compared the SI with other indices in terms of hospital mortality of patients with COVD-19 and concluded that the SI has more value in determining patient mortality than other indices such as the Age SI, although they also confirmed the prognostic role of other indices. However, a contradictory study by Rensen et al. [24] failed to demonstrate the usefulness of the SI in predicting hospital mortality or identifying high-risk patients in a cohort study conducted on patients with confirmed cases of the disease. In a meta-analysis by Alsagaff et al. [25], the role of the SI in predicting hospital mortality was considered effective, although the recommendation was to conduct larger studies. The prognostic significance of hypoxia in patients with SARS-CoV-2 infection underscores the potential value of integrating oxygenation metrics with other indices in prognostication. Ates et al. [26] investigated the prognostic role of Respiratory Rate Oxygenation along with other shock indices in predicting mortality in patients with COVD-19. The study noted that this index, in contrast to the SI, possesses a more substantial prognostic significance. Additionally, the study underscored the pivotal role of age in this context. Consequently, the integration of the SI with oxygen saturation and age may enhance the diagnostic efficacy for critical illnesses in patients [26]. In support of this assertion, Hsieh et al. [27] in their study found that the role of Age SI/oxygen saturation and Age SI was more effective than SI alone in predicting hospital mortality and the need for ICU admission in patients with confirmed cases of severe acute respiratory syndrome caused by the novel coronavirus. In a separate study by Oh and Lee [28], the prognostic significance of shock indices in patients with SARS-CoV-2 was examined. It was observed that the combination of SI/oxygen saturation and Age SI exhibited a more robust predictive capability for hospital mortality compared to SI alone In contrast, this study suggests that, given the high negative predictive value, Age SI/SpO₂ may be more useful for ruling out mortality risk rather than definitively identifying high-risk patients.

The RSI is another index whose prognostic role in predicting mortality in patients with SARS-CoV-2 infection may be effective. This index, which is used to assess hemodynamics in patients, can enhance the accuracy of predicting the condition of critically ill patients when combined with other factors, such as the GCS. In their study on patients with COVD-19, Wu et al. [29] investigated the prognostic role of the RSI in combination with GCS in these patients. Their findings indicated that a straightforward calculation method can be employed to reliably predict patient hospitalization and mortality. In a separate study, Matsuda et al. [30] reported that the prognostic role of this index in sepsis conditions is superior to that of the SI. However, this index appears to be more advantageous in trauma conditions [31-33]. Given the predominant role of hypoxia in patients with SARS caused by SARS-CoV-2 compared to the GCS, we investigated the prognostic role of oxygen saturation along with the RSI and age in these patients. In this study, the relatively low positive predictive value (PPV; approximately 30%) RSI×SpO₂/Age indicates a high rate of false positives, suggesting these indices are better suited for ruling out mortality risk than definitively identifying high-risk patients.

It is imperative to acknowledge the limitations inherent to this study. The retrospective and single-center nature of the study were unavoidable limitations. Additionally, the exclusion of patients under the age of 18, and pregnant patients from the study constitutes a significant limitation. Additionally, challenges arose during the review of archival files, largely due to deficiencies inherent in the documentation. The low PPV (approximately 30%) of the indices limits their specificity for identifying high-risk patients, potentially leading to over-triage in busy ED settings. This underscores the need for integration with other diagnostic tools and validation in diverse cohorts to refine thresholds and improve predictive accuracy.

The utilization of vital signs-derived indices within the ED triage setting may prove efficacious in predicting mortality among patients diagnosed with SARS-CoV-2. The combination of SI or RSI along with hypoxia, and age have been identified as potentially more significant role in ruling out hospital mortality in patients with this condition than vital signs alone, given the established role of hypoxia as a major risk factor in such cases. Future studies should validate the prognostic utility of Age SI/SpO₂ and RSI×SpO₂/Age in multicenter cohorts to ensure generalizability across diverse populations.

KEY MESSAGES

Article information Continued