The Shock Index in Veterinary Patients

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Shock Index = Heart Rate divided by Systolic Arterial Blood Pressure

Healthy human adults: 0.5-0.7

Shock, Heart Rate and Blood Pressure

Shock = significant reduction in systemic tissue oxygen delivery
Most often due to systemic reduction in tissue blood flow or systemic hypoperfusion
Most commonly four types of hypoperfusion-related shock identified: hypovolaemic, distributive, cardiogenic and obstructive; more than one type may exist concurrently.

Uncomplicated hypovolaemic shock in dogs:

Low effective circulating volume triggers sympathetic nervous system-driven compensatory mechanisms: increased heart rate (positive chronotropy), increased force of cardiac contraction (positive inotropy), peripheral vasoconstriction.
Aim to improve effectiveness of circulation and hence oxygen delivery to key organs and tissues


1. Heart rate during hypovolaemia potentially also influenced by other factors, e.g. pain
2. Findings may be less predictable in other types of shock
3. ‘Shocky’ cats are typically relatively bradycardic regardless of predominant type of shock – hence shock index cannot be used.

Systolic arterial blood pressure:

Blood pressure is not the same thing as blood flow/perfusion; used as a surrogate for perfusion
Systemic hypotension may be late in onset during worsening hypovolaemia; physical perfusion parameters may suggest hypoperfusion despite measured normotension.
Treat blood pressure as an adjunct to perfusion assessment that is supplementary to but not more important than or does not replace cardiovascular exam
May also be affected by other influences, e.g. pain

The Shock Index

Can it help us in the earlier detection and/or treatment of dogs in shock?
Can it allow us to suspect occult hypoperfusion?

Some questions about the shock index:

  • What is it meant to do? How is it meant to help us in our clinical practice? Is it being suggested as something that helps us to improve our management in a way that has an impact on the patients in terms of their progression and outcomes? Could it help us with prognostication?
  • Is it something that can be used for all types of shock, across all disorders that might lead to shock, or do we need to be more granular than that?  
  • What is the evidence around the clinical use of the shock index?
  • What are the implications of using the shock index? Is it quick and easy to do? Does it require any equipment or additional training or resources? And does it end up costing the pet’s carers any extra money? 

“Detecting dogs that are in the late compensatory or early decompensatory stage of hypovolaemic shock may not be particularly challenging but we know that the sooner we can pick up on changes in perfusion that have triggered compensatory responses the sooner we can intervene and reverse the situation…At least in theory this improves patient-centred outcomes. Especially bearing in mind…the potential pitfalls with heart rate changes and with blood pressure then maybe combining these two parameters into one index allows us to smooth or cancel out some of these pitfalls, allows us to detect early compensation sooner and to intervene sooner.”

Human Medicine Literature

Shock index papers from 1980s and indeed earlier
Typically evaluating use of shock index in specific scenarios, e.g. haemorrhagic hypovolaemic shock, especially post-traumatic; obstetric patients; acute coronary syndrome, etc.
Has been evaluated:

  • To see whether it correlates with higher mortality and injury severity after trauma
  • As a predictor for length of hospital stay, number of ventilator days and likelihood of ICU admission
  • In healthy human adult blood donors

Suggests it is necessary to account for patient-specific circumstances that might influence ability to show compensatory heart rate and/or blood pressure responses; e.g. age, concurrent drug therapies such as calcium channel or beta-blockers, diseases that tend to cause hypertension, etc.

These sorts of confounding factors essentially raise two questions:

  • Firstly is there a role for the shock index across all human patients?
  • But secondly, how does it need to be modified for the different patient populations? Do you need a slightly different index or threshold depending on the specific patient population? In other words is a ‘one size fits all’ shock index actually appropriate?

“Shock Index for prediction of critical bleeding post-trauma: A systematic review” from Emergency Medicine Australasia in 2014:

“Early diagnosis of haemorrhagic shock (HS) might be difficult because of compensatory mechanisms. Clinical scoring systems aimed at predicting transfusion needs might assist in early identification of patients with HS. The Shock Index (SI) – defined as heart rate divided by systolic BP – has been proposed as a simple tool to identify patients with HS. This systematic review discusses the SI's utility post-trauma in predicting critical bleeding (CB).”

Their focus was largely on how they could use the shock index to help predict transfusion requirements.

“The SI being simple and repeatable, appears to be useful in predicting CB. Recommendations for the ideal cut-off were varied, with most studies using a cut-off of ≥0.9. However, the cut-off of ≥1.0 was observed to have higher specificity”.

“The Shock Index revisited – a fast guide to transfusion requirement? A retrospective analysis on 21,853 patients derived from the TraumaRegister DGU®” from Critical Care 2013:

Trauma registry of German Trauma Society
Aims: “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”.
Four groups:

  • No shock; SI < 0.6
  • Mild shock; SI ≥ 0.6 - < 1.0
  • Moderate shock; SI ≥ 1.0 - < 1.4
  • Severe shock; SI ≥ 1.4

“The SI upon ED arrival may be considered a clinical indicator of hypovolemic shock with respect to transfusion requirements, hemostatic resuscitation and mortality.” And they quite liked their four-group classification system!

“Correlation of Shock Index and Modified Shock Index with the Outcome of Adult Trauma Patients: A Prospective Study of 9860 Patients” from the North American Journal of Medical Sciences in 2014:

“Triage at emergency department is performed to identify those patients who are relatively more serious and require immediate attention and treatment. Despite current methods of triage, trauma continues to be a leading cause of morbidity and mortality…This study was to evaluate the predictive value of shock index (SI) and modified shock index (MSI) for hospital mortality among adult trauma patients.”

Modified shock index (MSI):

Less attention to date
= Heart Rate divided by Mean Arterial Blood Pressure
Using MAP incorporates diastolic blood pressure – some suggestion this may be better than just using systolic

“the present prospective study results show that MSI, as a potential marker for predicting the mortality rate is significantly better than HR, SBP, DBP, and SI alone. Thus, MSI emerges as a better and improved predictor for prediction of hospital mortality in adult trauma patients in the emergency room.”

“Utility of the shock index in patients with sepsis” from the American Journal of Medical Sciences 2015:

“have reviewed and summarized studies that have correlated the SI with other parameters of disease severity and outcomes in patients with sepsis to determine if it has utility in the management of these patients or the prediction of outcomes.”

“The SI provides an integrated assessment of cardiovascular responses in patients with critical illness; its predictive value and simplicity are important considerations that should promote its use in the field, EDs and ICUs. The authors offer a flow diagram for its use in patients with possible sepsis.”

“Is the shock index a universal predictor in the emergency department? A cohort study”

Poster presentation at the 35th International Symposium on Intensive Care and Emergency Medicine
“The shock index…is a widely reported tool to identify acutely ill patients at risk for circulatory collapse in the emergency department (ED). Because old age, diabetes, essential hypertension, and β-/Ca2+ channel-blockers might reduce the compensatory increase in heart rate and mask blood pressure reductions in shock or pre-shock states, we hypothesized that these factors weaken the association between SI and mortality, reducing the utility of SI to identify patients at risk.”

Cohort study from Odense University Hospital of all first-time visits to the ED between 1995 and 2011 (n = 111,019)
Outcome was 30-day mortality

“SI is independently associated with 30-day mortality in a broad population of ED patients. Old age, hypertension and β-/Ca2+ channel-blockers weaken this association, but the association remains prognostic. SI ≥1 suggests substantial risk of 30-day mortality in all ED patients.”

Veterinary Literature

Two studies identified in clinical canine patients; both from September/October 2013 issue of the Journal of Veterinary Emergency and Critical Care.

“Evaluation of the shock index in dogs presenting as emergencies” by Porter, Rozanski, Sharp et al.

Aims of prospective study:

  • To determine a normal range for shock index (SI) in simulated patients – these healthy controls were staff and student dogs
  • To investigate whether SI is increased in dogs deemed to be in moderate to severe shock via assessment of plasma lactate – defined as venous lactate > 5 mmol/l. Exclusion criteria included:
    • Inability to obtain systolic blood pressure
    • Diagnosis of a disease condition that could result in hyperlactataemia in the absence of shock (e.g. increased oxygen demand or type-B lactic acidosis)
  • To compare SI in shock group to that of healthy dogs and dogs not judged to be in shock on presentation to the emergency room – defined as venous lactate ≤ 1.5 mmol/l on presentation.

Why did they focus on moderate to severe shock which is not something that is typically challenging to identify?
Why did they use a biochemical parameter, lactate, to define this instead of physical examination parameters?

“These data provide a pilot evaluation of SI in shock patients, but our study did not evaluate shock in occult hypoperfusion, which is an important distinction. In human studies, the proposed use and proven value of the SI is in identification of early hypovolemia or occult hypoperfusion, as well as in sustained occult shock during resuscitation…This study was designed to introduce the SI to veterinary medicine; further studies evaluating dogs with early, developing shock are warranted.”

“While defining shock solely on a biochemical marker such as lactate is not conventional nor advised in a clinical setting, shock was defined in this manner for several reasons. The first, and most relevant, is that, if selection were based upon heart rate and presence of hypotension, there would be a clear selection for dogs with a high SI. By instead selecting a biochemical marker consistently linked with shock…this study was attempting to avoid this bias. Importantly, assessment of HR and blood pressure are clinically relevant, and should be performed in a clinical setting. Secondly, classic objective parameters used to identify shock in a clinical setting vary drastically between breeds and even individuals within a breed. Setting an inclusion criteria for tachycardia (ie, 160/min) may exclude large breed dogs in shock while including small, anxious dogs that are not in shock. Clinical evaluation of shock status of an individual dog requires the synthesis of a number of parameters, but for the purpose of population analysis use of a biochemical marker of increased plasma lactate to define shock allowed for a more objective inclusion criteria.”

Blood pressure measurement:

Non-invasive oscillometric method preferred technique; used Doppler if this failed
In accordance with the American College of Veterinary Internal Medicine (ACVIM) guidelines:

  • Cuff size was chosen based on the width of the cuff approximating 40% of the circumference of the measured limb
  • Series of 3 BP measurements were taken, with the average SBP reported

Shock index:

Classified a priori as a binary variable: > 1 versus ≤ 1

“A cut off of 1 was considered clinically relevant and higher than what is used in people since dogs generally have more rapid heart rates than people, despite having similar systolic blood pressure. Thus, a normal dog would be expected to have a higher SI than a normal person. The sensitivity and specificity, along with area under the receiver operator characteristic (ROC) curve, were calculated to determine the discrimination of the shock index in healthy dogs versus shock dogs, and, separately, to determine the discrimination of the SI for [emergency] dogs not in shock versus [emergency] dogs in shock. The area under the ROC curve (AUCROC) investigates the predictive ability of shock index to predict a diagnosis of shock.”

Main results:

  • 68 healthy dogs; median shock index 0.78 (range 0.37-1.30)
  • 19 dogs assessed as not being in shock (venous lactate ≤ 1.5 mmol/l): median shock index 0.73 (range 0.56-1.20)
  • 18 dogs assessed as being in shock (venous lactate > 5.0 mmol/l): median shock index 1.37 (range 0.87-3.12). SI statistically significant difference to other two groups but the lower end of the range for these shock dogs overlaps with the ranges for the healthy and the ‘no shock’ group. 

Underlying disease conditions for shock group included pericardial effusion with cardiac tamponade (6), gastric dilatation-volvulus (3), haemoabdomen (2), and a single case of various others. Median plasma lactate 7.1 mmol/l (range 5.0-12.9 mmol/l).

“sensitivity (Sn), specificity (Sp) and ROC area were calculated using a cut off of SI > 1 defined a priori as a clinically relevant cutpoint:

  • In healthy dogs compared to those dogs in shock, an area under the receiver operator characteristic (AUROC) of 0.89 (CI 0.81–0.98) was seen, with a Sn of 89% and Sp of 90%.
  • In [emergency] dogs not deemed in shock compared to those deemed in shock, an AUROC of 0.92 (CI 0.83–1.00) was seen, with a Sn of 89% and Sp of 95%.”

[Area under an ROC curve (AUROC) approaching 1.0 would be considered excellent.]

Authors’ conclusions:

  • This study documented that the SI may be determined in dogs and that SI is significantly higher in dogs with shock compared to both healthy dogs and dogs presenting to the emergency room but not deemed to be in shock.
  • Specifically, an SI of > 1.0 is a highly sensitive and specific indicator to distinguish ER dogs not in shock and healthy dogs from dogs with biochemical evidence of moderate to severe shock.
  • Our findings support that SI has value as an indicator of shock in sick dogs presenting to the ER, and may serve as part of an initial evaluation.
  • In addition, the SI has not previously been evaluated in a veterinary population, so this study serves to introduce the SI and establish a reference interval for shock index in dogs (0.37–1.30).

Remember, did not look at dogs with lactate between 1.5-5.0 mmol/l so potentially that category in which the shock index could have the most value.

“Assessment of shock index in healthy dogs and dogs in hemorrhagic shock” by Peterson, Hardy and Hall.

Aims of retrospective study:

  • To establish a normal reference interval for canine SI
  • To compare SI in normal healthy dogs to dogs with known haemorrhage

Hypothesis: SI would differentiate a population of dogs with haemorrhagic shock from healthy controls.

Retrospectively analysed data collected prospectively for two previous studies
Blood pressur measurement: either non-invasive oscillometric or Doppler techniques

Control group (healthy dogs): 78 client-, student-, and staff-owned dogs
Haemorrhage group consisted: 38 dogs diagnosed with acute haemorrhagic shock, which presented to the Emergency Service. Variety of causes; bleeding intra-abdominal mass most common.

Bleeding dogs retrospectively classified by three board-certified ECC clinicians into 1 of 4 categories of shock based on heart rate, blood pressure, base excess and comorbidities:

  • All classified as having at least mild haemorrhagic shock
  • Used a combination of physical examination, base excess instead and comorbidities

Statistically significant difference in shock index between haemorrhage group and healthy group:

  • Haemorrhage group: median SI 1.37 (range 0.78–4.35)
  • Healthy group: median SI 0.91 (range 0.57–1.53)
  • Noteworthy overlap in ranges

Other results:

  • Statistical correlation between shock index and lactate
  • No correlation between SI and length of hospital stay in haemorrhage group
  • No increased risk of mortality (death or euthanasia) with increasing SI in dogs with haemorrhage

Evaluated sensitivity and specificity for different shock index cut-offs
Using a shock index cut-off of 1.0 (as was used a priori in the other canine study) performed more poorly here
Also analysed how well heart rate and systolic blood pressure performed in differentiating haemorrhagic shock dogs from healthy dogs

“Our study does not suggest that SI is a superior tool to SBP or HR, but the data support its ability to differentiate between a normal population of dogs from a population of dogs with hemorrhagic shock. Although there is some overlap of SI between normal dogs and dogs in hemorrhagic shock, the calculation could be used along with clinical assessment as an additional triage tool for emergency clinicians and may prompt further investigation for hemorrhage if the value is above 0.9.”

Other Considerations/Implications

Practical implications of using the shock index?

•    Is it quick and easy to do?
•    Does it require any equipment or additional training or resources?
•    And does it end up costing the pet’s carers any extra money?

Heart rate typically quick and easy to do; ensure no pulse deficits if using pulse rate.

Blood pressure:

Not all practices have blood pressure devices
Doppler-based devices are fine as they approximate systolic blood pressure (some literature discussion about this, including cats vs. dogs and conscious vs. under anaesthesia, but this is the consensus position)
Important to adhere to best practice guidelines for measurement and to ensure readings are repeatable, reliable and trendable

Depending on how blood pressure measurement is charged in your practice, could using the shock index end up costing your clients more and if so, do you think it is worth it?

“I think after learning what I have through researching this episode I will in the future start to pay attention to what the shock index is in individual canine patients and just get a personal anecdotal sense of what I feel about it, how it performs. But of course we have to remember that if you use the shock index you don’t just forget about everything else. You should be seeing it as another tool to enhance your identification, assessment and management of dogs in shock rather than replacing what you currently do. So we use our physical perfusion parameters, of which heart rate is just one, and assess them together looking at the whole picture…We use lactate and blood pressure in addition and put all these findings together to assess and manage these patients. And as long as you do that then I can certainly see it doing no harm and potentially being helpful. So I am interested to see how it performs and especially to tease out how it performs in patients in which pain for example is a component of their initial tachycardia.

What we definitely can’t do at the moment in my opinion is to use the shock index in an overt way to predict progression or prognosis. We most definitely do not have anything like the evidence base we would need to start trying to use the shock index in this way and I am not sure if we ever will. So as a supplementary assessment and monitoring tool, sure, as anything more than that, then I would say no, at least not in 2015.”

If you would like a copy of any of the papers mentioned below then do get in touch

Do you use the shock index? If so, how do you find it?
Would you consider using the shock index after listening to this episode?


Veterinary literature:

Peterson KL, Hardy BT, Hall K. Assessment of shock index in healthy dogs and dogs in hemorrhagic shock. J Vet Emerg Crit Care 2013. 23(5):545-550.

Porter A, Rozanski E, Sharp C, et al. Evaluation of the shock index in dogs presenting as emergencies. J Vet Emerg Crit Care 2013. 23(5):538–544.

Human Medicine literature:

Allgöwer M, Burri C. Shock index. Dtsch Med Wochenschr 1967. 92:1947–1950.

Kristensen A, Holler J, Hallas J, Lassen A, Shapiro N. Is the shock index a universal predictor in the emergency department? A cohort study. Critical Care 2015, 19(Suppl 1):P148. Poster presentation.

Mutschler M, Nienaber U, Münzberg M, et al. The Shock Index revisited – a fast guide to transfusion requirement? A retrospective analysis on 21,853 patients derived from the TraumaRegister DGU®. Crit Care 2013. 17(4):R172.

Olaussen A, Blackburn T, Mitra B, Fitzgerald M. Review article: Shock Index for prediction of critical bleeding post-trauma: A systematic review. Emerg Med Austral 2014. 26(3):223-228.

Pandit V, Rhee P, Hashmi A, et al. Shock index predicts mortality in geriatric trauma patients: an analysis of the National Trauma Data Bank.  J Trauma Acute Care Surg 2014. 76(4):1111-1115.

Rady MY, Nightingale P, Little RA, et al. Shock index: a re-evaluation in circulatory failure. Resuscitation 1992. 23(3):237–234.

Singh A, Ali S, Agarwal A, Nath Srivastava R. Correlation of Shock Index and Modified Shock Index with the Outcome of Adult Trauma Patients: A Prospective Study of 9860 Patients. N Am J Med Sci 2014. 6(9):450–452.

Tseng J, Nugent K. Utility of the shock index in patients with sepsis. Am J Med Sci 2015. 349(6):531-5.

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