Shock Index: A Predictor Of Morbidity And Mortality?

Emergent airway management and severe sepsis are both high-risk situations that are commonly encountered by emergency physicians. It is well known that complications can be high in both situations, which in turn can lead to increased morbidity and mortality. For instance, about 1/4 of patients who are hemodynamically stable prior to intubation get post-intubation hypotension (PIH) after rapid sequence intubation. Also septic patients may not be reliably identified by systemic inflammatory response syndrome (SIRS) markers early in their disease course. The Shock Index (SI) may be an adjunct that is easy to calculate and could predict both PIH and severe sepsis.

What is Shock Index (SI), and what is a normal value?

• Heart Rate (bpm)/ Systolic Blood Pressure (mmHg)
• Normal Range = 0.5 – 0.7 in healthy adults1

What is the best predictor of PIH during emergency airway management?2

What they did:

• Retrospective cohort study
• 300 patients
• SBP > 90 mmHg, 30 minutes prior to intubation (inclusion criteria)
• 2 groups (PIH vs No PIH)
• PIH defined as SBP < 90 mmHg within 60 minutes of intubation

Primary Outcome: Predictors of PIH after emergency intubation

Results: 

• PIH occurred in 66/300 (22%) of patients
• Post-intubation cardiac arrest occurred in 8/300 (3%) of patients
• PIH experienced higher in-hospital mortality (35% vs 20%)
• Strongest indicator of PIH: pre-intubation Shock Index (OR 55)
• Shock index ≥ 0.8 predicted PIH with sensitivity 67% and specificity 80%

Variable	OR	95% CI
Preintubation SI	55.1	13 – 232
End-Stage Renal Disease (ESRD)	3.7	1.1 – 13.1
Chronic Renal Insufficiency	3.4	1.2 – 9.6
Intubation for Respiratory Failure	2.1	1.0 – 4.5
Age	1.03	1.01 – 1.04

Limitations:

• Retrospective study
• Single center, urban tertiary care hospital
• Monitoring was done with non-invasive blood pressure assessment (intermittent monitoring)

Conclusion: Pre-intubation SI ≥ 0.8, strongly and independently predicts PIH after emergency intubation

Can SI predict which sepsis patients have severe sepsis (i.e. elevated lactate) and 28-day mortality?

What they did:

• Retrospective cohort study
• 2,524 patients
• Screened patients with suspected infection for severe sepsis

Primary Outcomes

• Hyperlactatemia (marker for morbidity)
• 28-day mortality

Results:

• 290/2524 (11.5%) had hyperlactatemia
• 361/2524 (14%) died within 28 days 
• Shock index (SI) ≥ 0.7 (15.8% of patients) were three times more likely to present with hyperlactatemia vs patients with normal SI (4.9%)
• NPV of SI ≥ 0.7 was 95% (identical to NPV of SIRS)

SI ≥ 1.0	SI ≥ 0.7	SIRS
PPV	0.24	0.16	0.18
NPV	0.92	0.95	0.95
Sensitivity	0.48	0.83	0.78
Specificity	0.81	0.42	0.52

SI ≥ 1.0	SI ≥ 0.7	SIRS
PPV	0.23	0.17	0.18
NPV	0.88	0.89	0.89
Sensitivity	0.37	0.71	0.64
Specificity	0.8	0.41	0.51

Limitations:

• No external validation of study
• Retrospective chart review
• Medication information (i.e. beta blockers) not available
• Elderly cohort with mean age of 73 years

Conclusion: 

• SI ≥ 0.7 performed as well as SIRS criteria in NPV and was the more sensitive screening test for hyperlactatemia and 28-day mortality
• SI ≥ 1.0 is the most specific predictor of both outcomes

My Final Thoughts

Shock Index (SI) is a quick, easy, and cheap way to predict post intubation hypotension (PIH), hyperlactatemia in sepsis, and 28-day mortality in sepsis, but requires further prospective trials before it is ready for primetime use.

1.Berger T, Green J, Horeczko T, et al. Shock index and early recognition of sepsis in the emergency department: pilot study. West J Emerg Med. 2013;14(2):168-174. [PubMed]2.Heffner A, Swords D, Nussbaum M, Kline J, Jones A. Predictors of the complication of postintubation hypotension during emergency airway management. J Crit Care. 2012;27(6):587-593. [PubMed]

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Salim Rezaie, MD

ALiEM Associate Editor Clinical Assistant Professor of EM and IM University of Texas Health Science Center at San Antonio Founder, Editor, Author of R.E.B.E.L. EM and REBEL Reviews

@srrezaie

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