Brief Report
Shock index in diagnosing early acute hypovolemia

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Abstract

Objective

The aim of this study was to determine the hemodynamic response and calculated shock index (SI = heart rate [HR]/systolic blood pressure [SBP]) in early acute blood loss.

Methods

This was a prospective observational study that enrolled healthy blood donors. Patients were excluded if not eligible for blood donation. Baseline vital signs were obtained, 450 mL of blood was removed over 20 minutes, and vital signs were repeated immediately postdonation while lying and after 1 and 5 minutes of standing. Difference was tested using a paired t test with P < .01 set for significance.

Results

Forty-six patients were enrolled; means for each time interval are shown below with 95% confidence intervals.

Conclusions

A significant elevation in mean SI was observed in healthy volunteers after standing for 1 and 5 minutes. Although significant changes in HR and SBP were observed, these indices were still within “normal” limits. The SI may be more useful in early hemorrhage than either the HR or SBP alone.

Introduction

The initial triage and evaluation of patients with possible hypovolemia are guided by the interplay of the patient's symptoms and the clinician's intuition. Clinical assessment must take into consideration the patient's presenting vital signs. Abnormal vital signs should raise a warning flag in the clinician's mind when dealing with the acutely ill or injured patient. It is well known that both intrinsic and extrinsic variables can result in abnormal vital signs. Any deviation from the norm should be satisfactorily explained in the process of a patient's evaluation.

The shock index (SI) is a simple calculation of heart rate (HR) divided by systolic blood pressure (SBP). The SI is normally 0.5 to 0.7 and has been shown to be elevated in the setting of acute hypovolemia and left ventricular dysfunction [1], [2], [3].

It has been shown that early hypovolemia (acute blood loss of 450 mL) is insufficient to produce changes in orthostatic HR or SBP [4]. Acute blood loss of less than 450 mL in healthy individuals rarely produces abnormal vital signs (HR >100 beats per minute [bpm], SBP <100 mm Hg). We hypothesized that although HR and SBP are rarely abnormal in early hemorrhage, perhaps using the 2 measures simultaneously in the form of the SI would provide a more useful indicator of early blood loss.

Our study used a cohort of healthy blood donors as a model of simulated early acute hypovolemia. Our objective was to determine if a controlled blood loss of 450 mL, in otherwise healthy individuals, was sufficient to cause an alteration in either HR, SBP, or calculated SI.

Section snippets

Methods

The patients in this study were selected from a pool of healthy blood donors. Voluntary donors who presented at a hospital-based blood donation drive were approached for enrollment in the study. Patients were excluded from the study for reasons that would preclude blood donation: temperature greater than 100.4°F, hemoglobin <11 mg/dL, age <17 years, weight <110 lb, or pregnancy. To minimize variables that could affect HR and blood pressure, patients actively using any prescription medications

Results

A total of 46 patients consented to participation in this study after being prescreened for their suitability as a blood donor. The study population was 65% female, with a mean age of 46 (range 17-66) years. Results for the HR, SBP, DBP, and SI are shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4 respectively.

Empty CellHR (bpm)SBP (mm Hg)DBP (mm Hg)SI
Before donation74 (71, 77)23 (118, 127)77 (75, 80)0.61 (0.58, 0.64)
After (lying)74 (71, 78)117 (112, 122)76 (73, 78)0.65 (0.61, 0.70)
After (standing 1 min)85 (80, 90)

Discussion

The HR is notoriously labile in response to a variety of internal and external stimuli, but the presence of tachycardia is generally regarded as an early warning sign in the setting of hypovolemia [5]. Our data would suggest that, although there is a statistically significant elevation in HR after acute blood loss, the mean for this elevation still falls within the upper limits of normal (<100 bpm). In the clinical setting, there is no knowledge of prehemorrhage baseline HR, so we often depend

Limitations

To conduct this study, we made 2 assumptions, first, that our patients were euvolemic before blood donation. It would have made our study more rigorous from a scientific point of view if we had been able to establish the volume status of our study subjects before donation. Second, we assumed that postural changes in a euvolemic patient would not significantly affect HR, SBP, or SI. It has been demonstrated that postural changes do not significantly alter vital signs in healthy volunteers [4],

Conclusions

Using a model of acute uncompensated blood loss of 450 mL, there was a statistically significant elevation in HR and SI and a concurrent decrease in SBP after hemorrhage. The observed change in HR and SBP was of questionable clinical utility because the observed values did not exceed the accepted norm for these variables. The SI was elevated beyond the upper limit of normal after only 450-mL blood loss and may be more useful clinically than either the HR or SBP in acute hemorrhage.

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