Excess Out-Of-Hospital Mortality and Declining Oxygen Saturation: The Sentinel Role of EMS Data in the COVID-19 Crisis in Tijuana, Mexico

Abstract

Objective: Emergency medical services (EMS) may serve as a key source of real-time data about the evolving health of COVID-19 affected populations, especially in low-and-middle-income countries (LMICs) with less rapid and reliable vital statistic registration systems. Although official COVID-19 statistics in Mexico report almost exclusively in-hospital mortality events, excess out-of-hospital mortality has been identified in other settings, including one EMS study in Italy that showed a 58% increase. EMS and hospital reports from several countries have suggested that silent hypoxemia--low oxygen saturation (SpO2) in the absence of dyspnea--is associated with COVID-19 outbreaks. It is unclear, however, how these phenomena can be generalized to LMICs. We assess how EMS data can be used in a sentinel capacity in Tijuana, a city on the Mexico-United States border with earlier exposure to COVID-19 than many LMIC settings.

Methods: We calculated numbers of weekly out-of-hospital deaths and respiratory cases seen by EMS in Tijuana, and estimate the difference between peak-epidemic rates (during April 14th-May 11th) and forecasted 2014-2019 trends. Results were compared with official COVID-19 statistics, stratified by neighborhood socioeconomic status (SES), and examined for changing demographic or clinical features, including mean (SpO2).

Results: An estimated 194.7 (95%CI: 135.5-253.9) excess out-of-hospital deaths events occurred, representing an increase of 145% (70%-338%) compared to forecasted trends. During the same window, only 8 COVID-19-positive, out-of-hospital deaths were reported in official statistics. This corresponded with a rise in respiratory cases of 274% (119%-1142%), and a drop in mean SpO2 to 77.7%, from 90.2% at baseline. The highest out-of-hospital death rates were observed in low-SES areas, although respiratory cases were more concentrated in high-SES areas.

Conclusions: EMS systems may play an important sentinel role in monitoring excess out-of-hospital mortality and other trends during the COVID-19 crisis in LMICs. Using EMS data, we observed increases in out-of-hospital deaths in Tijuana that were nearly threefold greater magnitude than increases reported using EMS data in Italy. Increased testing in out-of-hospital settings may be required to determine if excess mortality is being driven by COVID-19 infection, health system saturation, or patient avoidance of healthcare. We also found evidence of worsening rates of hypoxemia among respiratory patients seen by EMS, suggesting a rise in silent hypoxemia, which should be met with increased detection and clinical management efforts. Finally, we observed that social disparities in out-of-hospital death that warrant monitoring and amelioration.

Figure 1.. Weekly Case Breakdown by Triage Priority Code, 2019–2020

Part A shows only pre-hospital mortality cases and patients in critical condition who require urgent hospitalization. Part B shows the full distribution patients. Both parts refer to non-trauma patients, and include data from 2019 through April 2020. The vertical black line marks the week of March 31^st, when respiratory morbidity cases began to rise.

Figure 2.. Long-Run EMS-Documented Out-Of-Hospital Mortality and Respiratory Cases, 2014–2020

A) EMS-documented out-of-hospital mortality. B) EMS-documented respiratory cases. Parts A and B include expected values (black line) and 95% prediction intervals (grey band) based on model fit on data from 2014–2019, with forecasts through May of 2020. Both series exclude trauma-patients.

Figure 3.. EMS-Documented Out-Of-Hospital Mortality and Respiratory Cases Compared to Official COVID-19 Case and Mortality Numbers, March 17^th - May 11^th

A) EMS-documented out-of-hospital mortality, with the observed and expected number shown in text. B) EMS-documented respiratory cases, with the observed and expected number shown in text. C) Deaths among patients with confirmed COVID-19, according to official national government statistics, with the number reported as managed in the outpatient setting and the total reported in text. D) Number of patients with confirmed COVID-19, according to official national government statistics, with the number reported as managed in the outpatient setting and the total reported in text. Parts A-D refer to weekly totals. Parts A and B include expected values (black line) and 95% prediction intervals (grey band) based on forecasted trends from 2014–2019. Parts A and B exclude trauma patients.

Figure 4.. Trends in SpO2 and Percent Presenting Alert Among EMS-Documented Respiratory Cases

The distribution of SpO2 values over time is visualized weekly from March 31^st to May 11th, 2020 and compared to all data from 2019. Respiratory cases were divided into 5 quintiles of SpO2 values, and the median of each quartile is plotted. The color reflects the percent of individuals in each quartile that presented as alert, which is also plotted as text next to each point.

Figure 5.. Out-Of-Hospital Mortality and Respiratory Cases by Neighborhood and Neighborhood SES

The categorical socioeconomic status (SES) of each basic statistical unit (ageb) is mapped for Tijuana. Overlaid is the out-of-hospital mortality occurring during April 14^th to May 11^th (part A) and respiratory cases occurring during March 31^st to May 11^th (part B). The number of cases in each neighborhood (colonia) is shown as a point, with the size reflecting the magnitude. In the middle column, the points are organized by neighborhood SES. On the right, the number of cases is shown as a rate per 100,000 people, for each quintile of neighborhood SES.