The Shock Index revisited – a fast guide to transfusion requirement? A retrospective analysis on 21,853 patients derived from the TraumaRegister DGU®

Uncontrolled hemorrhage is still one of the leading causes of early death in multiply injured trauma patients [1, 2]. The initial assessment and management of hypovolemic shock therefore remains one of the key aspects in trauma resuscitation. For the initial evaluation of circulatory depletion, the American College of Surgeons has defined in its training program Advanced Trauma Life Support (ATLS) four classes of hypovolemic shock. This classification is based upon an estimated percentage blood loss. Furthermore, ATLS suggests corresponding vital signs such as the heart rate, systolic blood pressure and the mental status to allocate each patient to the respective shock class [3]. However, the clinical validity of the ATLS classification of hypovolemic shock has been questioned recently by data from the TARN registry and the TraumaRegister DGU® [4–6]. As an alternative, we have hypothesized that a classification based on the physiological parameter base deficit (BD) would be more appropriate to differentiate the presence and extent of hypovolemic shock in trauma patients. Recently, we introduced and characterized four categories of worsening BD emphasizing that this BD-based classification correlated with higher transfusion requirements, mortality as well as injury severity and discriminated patients at risk for early blood transfusions and death more appropriately than the current ATLS classification of hypovolemic shock [7].

Although point-of-care testing (POCT) can provide BD within minutes, not every emergency department (ED) is equipped with this technology. In the absence of POCT, we alternatively suggest the assessment of the Shock Index (SI). The SI, defined by the ratio of heart rate (HR) to systolic blood pressure (SBP), was first introduced by Allgöwer and Burri in 1967 [8, 9]. Although the HR and SBP alone have been shown unreliable in determining the presence of hypovolemic shock [4–6, 10–13], their ratio as reflected by the SI has been previously emphasized to serve as a capable measure for hemodynamic instability [14–18] and to risk-stratify patients for transfusion requirements and outcomes [14, 15, 17]. Subsequently, the SI was suggested as a useful clinical indicator for acute hypovolemia, especially in patients who present with HR and SBP within normal ranges [19, 20].

The aims of this study were to characterize four groups of worsening SI based upon a large cohort of multiply injured patients, to report transfusion requirements and outcomes within these four groups, and to compare this SI-based classification in its ability to risk-stratify patients according to their need for early blood product transfusion with our recently introduced BD-based classification of hypovolemic shock.

Over the past years, several approaches to assess the extent of hypovolemic shock during early trauma resuscitation have been proposed. Using a combination of vital signs (for example, HR, SBP and mental status) and urinary output, the ATLS classification of hypovolemic shock became one of the most promoted classifications over the last decade. However, the importance and reliability of vital signs to determine the presence and extent of hypovolemic shock have been questioned [4–6, 13]. We recently introduced and characterized four classes based on BD representing a modified classification of hypovolemic shock [7]. Although POCT can provide BD within minutes after the patient’s ED admission, not every ED is equipped with this technology. However, one key element of ATLS is its universal and worldwide application, independent of venues and time points of trauma care. In the present analysis, we suggest the SI as an easy accessible and clinical relevant tool to assess the presence of hypovolemic shock in the absence of POCT technology. The four strata of SI discriminated transfusion requirements and MT rates equally with our recently introduced BD-based classification of hypovolemic shock.

There is a growing body of evidence that the ratio of HR to SBP, as reflected by the SI, is the most promising vital sign to detect acute hypovolemia and circulatory failure. A linear relationship between hemorrhage and increasing SI has been described in patients suffering from gastrointestinal bleeding as well as open wounds, and the SI has been shown to correlate best with the quantity of intraperitoneal hemorrhage in ruptured ectopic pregnancies [8, 28, 29]. Clinically, Rady and coworkers have reported in 275 patients who presented to the ED for urgent medical care that SI >0.9 was associated with the need for immediate treatment and admission of the patient [20]. While HR and SBP still presented within normal limits, these patients displayed reduced central venous oxygen saturation and lactate acidosis, all indicators for the presence of hypovolemic shock [20, 30–32]. When healthy blood donors were subjected to a defined blood loss of 450 ml, the SI substantially increased whereas the HR and SBP, taken as separate values, still remained within normal ranges [19]. Due to a negative relationship between SI and left ventricular stroke volume as well as cardiac output, the SI can be used clinically as a fast and noninvasive tool to assess cardiac function during acute hypovolemia [18, 31].

In the present study, an increasing SI category reflected injury severity by increasing the ISS as well as higher percentages of thoracic, abdominal and pelvic injuries. Simultaneously, increasing SI paralleled incidences of multiple organ failure and sepsis, all important factors influencing mortality and outcome of trauma patients. In 2,445 trauma patients treated in an urban level I center, SI >0.9 was associated with an increased overall mortality (12.8% vs. 6.1% in patients with SI <0.9). A fivefold increase in mortality was observed in patients with an increase in SI ≥0.3 between the scene of the accident and ED arrival [14]. Zarzaur and colleagues demonstrated that the SI was also a significantly better predictor for 48-hour mortality compared with the SBP and HR. In the overall trauma population as well as in the subgroup of patients aged <55 years, the SI correlated best with transfusion of ≥4 blood units within the first 48 hours after hospital admission [15, 16].

The present study emphasizes the role of the SI by demonstrating that the index may discriminate the presence of hypovolemic shock upon ED admission with respect to the need for hemostatic resuscitation and transfusion requirements. To our knowledge, no gold standard has yet been introduced to assess hypovolemic shock including blood loss in trauma, which triggers therapeutic measures and interventions. There is thus currently no option to test our novel approach against a gold standard. The authors have therefore decided to compare this novel approach with our recently introduced and characterized BD-based classification of hypovolemic shock. The assessment of massive transfusion rates and the percentage of patients who received ≥1 blood product, all surrogates for the presence of hypovolemia, clearly demonstrate that the SI can be used equally with our recently suggested BD-based classification of hypovolemic shock in the absence of POCT or laboratory support [7]. Especially, MT rates >30% in group III patients and >50% in group IV patients emphasize this equality. The proposed classification based on the SI may therefore be used as a reasonable clinical alternative if laboratory testing such as POCT is not available.

These results reported here are substantiated by Vandromme and colleagues, who demonstrated that patients with prehospital SI >0.9 have a 1.5-fold increased risk for MT. A further increase of SI >1.3 was even associated with a MT rate of 20% [17]. When the SI at ED admission was assessed, patients with SI between 0.7 and 0.9 had a twofold increased risk for MT and as the SI reached 1.3 or above a 20-fold increase in risk was observed [17].

However, there are some differences between our study and the work by Vandromme and colleagues. When patients were stratified by prehospital SI, a mean ISS of 12.7 for all patients was reported. Even in patients with the highest SI category the mean ISS was 20.3. One should note that in our analysis the mean ISS was already 19.3 in group I patients increasing to 37.3 in group IV patients, and therefore the present study comprises a much more severely injured cohort of patients than all other previous reports. When the frequencies of packed red blood cells as well as massive transfusions were assessed, Vandromme and colleagues had focused on the amounts of blood products administered within the first 24 hours after hospital admission. In contrast, by analyzing the initial phase from ED admission to ICU admission, our study focused primarily on the first hours because this is clinically the most important phase to detect transfusion requirements and to activate blood bank resources and massive transfusion protocols. By inclusion criteria, Vandromme and colleagues had excluded patients with SBP <90 mmHg from their study, which most probably led to elimination of patients with higher transfusion requirements a priori. However, the current definition of hypotension as SBP <90 mmHg and a cutoff point for trauma team activation has been questioned. Both in blunt and penetrating trauma and also in the prehospital and in-hospital phase of care, SBP <110 mmHg was already associated with a significant increase in mortality [33–36]. In contrast, there was no restriction on SBP in our study because we intended to propose a classification that can be adopted easily and quickly to all trauma patients upon ED arrival.

In summary, the authors suggest assessing trauma patients in the ED based on the SI if laboratory or POCT devices are not available. Vandromme and colleagues have already proposed that calculation of the SI in the prehospital setting may facilitate the early identification of a relatively high risk for MT and therefore may be incorporated into prehospital triage protocols [17]. Following the ATLS paradigm ‘keep algorithms simple’, the SI may serve as a principle trigger for action in the ED. For group I and group II patients, a careful observation as well as blood typing should be sufficient, unless clinical circumstances dictate otherwise. In group III patients, preparation for transfusion with blood typing and cross-match should be initiated. In group IV patients, where MT rates were >60%, the trauma leader should definitely be prepared for a MT; for example, by activation of a MT protocol and corresponding logistics.

The findings of the present study should be considered in terms of their limitations and strengths. Although, there were only small differences between the vital signs at ED admission and at scene, calculation of the SI at ED admission might be influenced by the prehospital care (for example, the administration of intravenous fluids and/or the use of vasopressors). Furthermore, our trauma registry does not contain information about prior medication such as the use of beta-blockers or antihypertensive agents. Pain and anxiety might also have an influence on SBP and HR and therefore on the SI. Furthermore, the TraumaRegister DGU® comprises, by strict inclusion criteria, only severely injured trauma patients. This may have introduced a selection bias and possibly influenced the accuracy of this classification. Lastly, most patients included in the TraumaRegister DGU® sustained blunt trauma. However, the cardiovascular responses in patients with blunt trauma have been speculated to perhaps differ from those with penetrating injuries. As the percentage of penetrating trauma patients was only marginal in the present analysis, the utility of the four groups of SI was not tested sufficiently in this subgroup. The population of patients seen in this setting is therefore possibly not generalizable. A further validation on penetrating injuries together with a prospective randomized controlled trial to assess the accuracy of our suggested classification is needed.

The SI upon ED arrival may be considered a clinical indicator of hypovolemic shock with respect to transfusion requirements, hemostatic resuscitation and mortality. The four SI groups have been shown to equal our recently suggested BD-based classification. In daily clinical practice, the SI may be used to assess the presence of hypovolemic shock if laboratory or POCT technology is not available.