Shock Index in COVID Era

Introduction

Shock, a clinical state of impaired oxygen delivery or utilization, is a major cause of mortality, morbidity, and increased resource utilization in the critically ill patient cohort.¹ Time-sensitive intervention and treatment as emphasized by the early researches mandate the need of an early identification strategy.², ³ In this context, an exhaustive list of clinical, hemodynamic, and laboratory parameters (Table 1) have been evaluated in various surgical and nonsurgical patient populations. However, a mere correction of the these parameters doesn't necessarily improve outcome in shock patients.⁴ Moreover, lack of specificity, invasive nature of the laboratory parameters, and need of additional monitoring equipments and expertise (such as ScvO₂ [central venous oxygen saturation], cardiac output, echocardiography, etc.) construct the ground to formulate other novel risk stratification parameters.

In conjunction to the aforementioned fact, a derived hemodynamic parameter, shock index (SI = heart rate [HR]/systolic blood pressure [SBP]) emerged as a simple, noninvasive, bedside, and objective parameter to predict the outcome and risk stratification in shock patients.

Types of SI

Originally SI was formulated to include both vascular and myocardial component (as reflected by SBP and HR, respectively) and a value more than the normal range (0.5–0.9) predicted poor tissue perfusion as evidenced by a positive correlation with serum lactate and mixed venous oxygen saturation.⁵ Subsequently, several modifications have been made to address the aforementioned issue as enlisted in Table 2. Additionally, respiratory rate and SpO₂ [peripheral blood oxygen saturation] have been introduced along with SI to incorporate the respiratory component (Table 2).

Where Can it Help (in COVID-19)?

Despite the continuous endeavor and ongoing researches, little is known about the novel coronavirus disease 2019 (COVID-19) and subsequent management of the patients requiring intensive care unit (ICU) admission. According to few studies, about 5 to 10% of the patients infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) require ICU admission.⁶ Shock (up to 67% of patients admitted in the ICU) has been implicated as the primary cause of death in 7% of COVID-19 cases and as a contributing factor in an additional 33%.⁷ All the four types of shock (hypovolemic, cardiogenic, distributive, and obstructive) have been observed in COVID-19 patients owing to the peculiar pathophysiology of the disease—hypovolemia occurring due to the associated fever, diarrhea, and fluid restrictive treatment protocol; systemic inflammation with circulating cytokine storm giving the etiology of septic/distributive shock; a direct injury to the myocardium due to myocarditis and pulmonary hypertension (caused by positive pressure ventilation, chronic hypoxia, pulmonary fibrosis) leading to right ventricular dilatation, all can attribute to the myocardial dysfunction leading to cardiogenic shock; and, finally, sudden pulmonary thromboembolism (owing to the inherent hypercoagulable disease pathology) and tension pneumothorax form the etiology of obstructive shock.⁸ Therefore, a close monitoring and early detection of the shock pathology are the cornerstone for a better outcome. On the other hand, a breach in the safety precautions adapted by the health care workers (HCW) in the process of patient care imposes a unique challenge in the ongoing COVID-19 pandemic. To address the this issue, the World Health Organization (https://www.who.int/publications-detail/clinical-management-of-severe-acuterespiratory-infection-when-novel-coronavirus-(ncov)-infectionis-suspected) and the National Institutes of Health (https://covid19treatmentguidelines.nih.gov/critical-care/hemodynamics/) released recommendations for the hemodynamic management of COVID-19 patients, ensuring minimum number of exposures for the HCWs without compromising patient safety. However, indication for endotracheal intubation in COVID-19 patients is controversial, albeit it carries the greatest risk of exposure (by aerosol generation). Previous researches recommend a lower threshold for endotracheal intubation (and mechanical ventilation) despite low arterial oxygen saturation.⁸ Compensation by the cardiovascular system may have a pivotal role as described by Tobin and colleagues.⁹ SI and its modifications might play a crucial role by indicating the compensatory cardiovascular reserve. On the other hand, being a noninvasive, continuous, and objective parameter, SI can be a valuable tool in the triage area (to determine the level of care) as well as in the inpatient management in titrating the treatment protocol without exhausting resources of the already overburdened health care system.

Encouraging Literatures

Limitations of SI

First, SI value doesn't change over a wide range of cardiovascular compensatory phase (increase in heart rate to compensate hypotension), particularly in the younger age group. Second, the mode of blood pressure recording (invasive vs. noninvasive) is not universal in the existing literature. Accordingly, the variations in systolic, diastolic, and mean blood pressure between noninvasive and invasive method would create ambiguity in calculating the SI value, specifically in the shock state.

Conclusion

SI, a noninvasive, simple, dynamic, objective, and parsimonious index, can appropriately predict the outcome in COVID-19 patients and upgrade the patient care in a timely manner.²² As it is aptly said, to be forewarned is to be forearmed and half the victory.