Calculators · Obstetrics & Gynaecology · Formula

Obstetric shock index

Also called shock index in PPH, SI postpartum haemorrhage, heart rate / systolic BP

Heart rate ÷ systolic BP in obstetric haemorrhage, with the 0.9 / 1.4 / 1.7 alert thresholds validated in pregnant and postpartum women.

/min
mmHg

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Formula and scoring

Shock index = heart rate (beats/min) ÷ systolic BP (mmHg). Thresholds: <0.9 reassuring; ≥0.9 referral / close monitoring; ≥1.4 urgent intervention; ≥1.7 high risk of adverse outcome.

Exam and ward pearls

  • In Nathan 2015, SI had the highest AUROC of the vital signs for predicting ICU admission after PPH (0.75 vs 0.64 for systolic BP).
  • After PPH ≥1500 mL, SI ≥0.9 had 100% sensitivity and SI ≥1.7 had 97.7% specificity for ICU admission.

When not to rely on it

  • Beta-blockers, pain, anxiety, fever and sepsis all alter the heart rate; interpret with estimated blood loss and the clinical picture.
  • Thresholds come from cohorts in the UK, South Africa and low-resource settings and were proposed for referral/alert decisions.

Source

  1. Nathan HL, El Ayadi A, Hezelgrave NL, et al. Shock index: an effective predictor of outcome in postpartum haemorrhage? BJOG. 2015;122(2):268-75. doi:10.1111/1471-0528.13206 · PMID 25546050
  2. El Ayadi AM, Nathan HL, Seed PT, et al. Vital sign prediction of adverse maternal outcomes in women with hypovolemic shock: the role of shock index. PLoS One. 2016;11(2):e0148729. doi:10.1371/journal.pone.0148729 · PMID 26901161
  3. Nathan HL, Seed PT, Hezelgrave NL, et al. Shock index thresholds to predict adverse outcomes in maternal hemorrhage and sepsis: a prospective cohort study. Acta Obstet Gynecol Scand. 2019;98(9):1178-86. doi:10.1111/aogs.13626 · PMID 31001814

Formula and cut-offs checked against these sources on 8 Nov 2026. Educational tool for clinicians; it does not replace clinical judgement.

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