[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Nursing](https://durgical.com/en/nursing/) › [Critical care](https://durgical.com/en/nursing/areas/critical-care/) › ICU bedside formulas: mean arterial pressure, cerebral perfusion pressure, cardiac output and index, shock index # ICU bedside formulas: mean arterial pressure, cerebral perfusion pressure, cardiac output and index, shock index - Calculation Five bedside formulas turn monitor numbers into a picture of perfusion: MAP, cerebral perfusion pressure, cardiac output and index, and shock index. Updated Nov 13, 2026 ## Key points - Educational example, not a clinical order. - MAP ≈ diastolic + (systolic − diastolic) ÷ 3; the estimate is less reliable with arrhythmias or very fast heart rates. - CPP = MAP − ICP; zero both transducers at the same reference level the protocol sets. - Cardiac output (L/min) = heart rate × stroke volume; cardiac index = cardiac output ÷ body surface area. - Shock index = heart rate ÷ systolic pressure; about 0.5 to 0.7 is usual in healthy adults, and 1 or more is a warning sign. ## What it is Monitors calculate most of these values, but the nurse must know where they come from, check that they make sense and recognise a dangerous result. Mean arterial pressure (MAP) can be estimated from the systolic and diastolic pressures. Cerebral perfusion pressure (CPP) is the MAP minus the intracranial pressure. Cardiac output is heart rate times stroke volume, and cardiac index adjusts it for body size. Shock index is heart rate divided by systolic pressure. All the numbers in the examples are made up for practice; the targets for a real patient are set by the medical team. ## Formulas - MAP ≈ DBP + (SBP − DBP) ÷ 3, in mmHg. - CPP = MAP − ICP, in mmHg. - Cardiac output (L/min) = heart rate (beats/min) × stroke volume (in litres per beat). - Cardiac index (L/min/m²) = cardiac output ÷ body surface area (m²); about 2.5 to 3.5 is generally considered normal. - Shock index = heart rate ÷ systolic pressure; modified shock index = heart rate ÷ MAP. ## How to calculate it - Write down the values with their units and the time they were taken — a formula only works with matching, current values. - For MAP, subtract the diastolic from the systolic, divide by 3 and add the diastolic — the heart spends longer in diastole. - For CPP, subtract the ICP from the MAP — check both transducers are at the reference level the protocol sets. - For cardiac output, convert stroke volume to litres and multiply by the heart rate — the result is in L/min. - For cardiac index, divide the cardiac output by the body surface area — it makes patients of different size comparable. - For shock index, divide the heart rate by the systolic pressure — round to one or two decimals. - Compare the result with the monitor and with the patient — a value that does not fit the bedside picture is checked again. ## Worked examples (made-up numbers, practice only) | Question | Calculation | Result | | --- | --- | --- | | MAP with a blood pressure of 120/60 mmHg | 60 + (120 − 60) ÷ 3 = 60 + 20 | 80 mmHg | | MAP with a blood pressure of 90/45 mmHg | 45 + (90 − 45) ÷ 3 = 45 + 15 | 60 mmHg: below a goal of 65, so it is reported | | CPP with MAP 80 mmHg and ICP 20 mmHg | 80 − 20 | 60 mmHg | | Cardiac output and index: heart rate 80/min, stroke volume 70 mL (0.07 L), body surface area 2.0 m² | 80 × 0.07 = 5.6; 5.6 ÷ 2.0 | Cardiac output 5.6 L/min; cardiac index 2.8 L/min/m² | | Shock index with heart rate 120/min and systolic pressure 100 mmHg | 120 ÷ 100 | 1.2: a warning sign, report it | ## Common mistakes - Using a diastolic and a systolic pressure taken at different times. - Forgetting to convert stroke volume to litres, so the cardiac output comes out a thousand times too high. - Calculating CPP with transducers zeroed at different reference levels. - Trusting the shock index in someone on heart-rate-lowering medicines, in older adults or in compensated shock, where the vital signs can still look normal. - Treating a calculated number without looking at the patient. ## Double-checking - Repeat the calculation and compare it with the value the monitor shows. - Check that the units match (mmHg, beats/min, litres, m²) before you calculate. - Report any result outside the goals the team set, and say which values you used. ## Test yourself Think of the answer, then tap the question to see it. ## What is the estimated MAP for a blood pressure of 150/90 mmHg? 90 + (150 − 90) ÷ 3 = 90 + 20 = 110 mmHg (made-up example). ## How is cerebral perfusion pressure calculated? CPP = MAP − ICP. With a MAP of 85 mmHg and an ICP of 15 mmHg, CPP is 70 mmHg (made-up example). ## What is the shock index with a heart rate of 90/min and a systolic pressure of 120 mmHg? 90 ÷ 120 = 0.75 (made-up example); values of 1 or more are a warning sign. ## Why is cardiac index used instead of cardiac output alone? Cardiac index divides cardiac output by body surface area, so patients of different size can be compared. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/nursing/icu-bedside-formulas-mean-arterial-pressure-cerebral-perfusion-pressure-cardiac/) ## Elsewhere in DurgiPedia - [Cardiac output and what sets it](https://durgical.com/en/physiology/cardiac-output-and-what-sets-it/) - [Cerebral blood flow, brain metabolism and intracranial pressure](https://durgical.com/en/physiology/cerebral-blood-flow-brain-metabolism-and-intracranial-pressure/) ## Connections ## Related 2 - [Haemodynamic monitoring in general terms: MAP, CVP, cardiac output, preload, afterload and perfusion signs](https://durgical.com/en/nursing/haemodynamic-monitoring-in-general-terms-map-cvp-cardiac-output-preload-afterloa/) - [Intracranial pressure monitoring and the external ventricular drain: levelling, drainage, waveforms and infection](https://durgical.com/en/nursing/intracranial-pressure-monitoring-and-the-external-ventricular-drain-levelling-dr/) [← All of nursing](https://durgical.com/en/nursing/) Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order. Sources: [Foundations and Advancements in Hemodynamic Monitoring: Part I - Elements of Hemodynamics (Demir ZA et al., Turk J Anaesthesiol Reanim 2025;53(3):87-97)](https://pmc.ncbi.nlm.nih.gov/articles/PMC12123657/) (Turkish Journal of Anaesthesiology and Reanimation, open access in PubMed Central, 2025); [Hemodynamic monitoring: basic principles in operation room and intensive care unit (Mirus M, Saugel B, Spieth PM, J Clin Monit Comput 2026;40(2):375-389)](https://pmc.ncbi.nlm.nih.gov/articles/PMC13053478/) (Journal of Clinical Monitoring and Computing, open access in PubMed Central, 2026); [Monitoring the brain (Elwishi M, Dinsmore J, BJA Education 2019;19(2):54-59)](https://pmc.ncbi.nlm.nih.gov/articles/PMC7807842/) (BJA Education, open access in PubMed Central, 2019); [Shock index in the emergency department: utility and limitations (Koch E et al., Open Access Emerg Med 2019;11:179-199)](https://pmc.ncbi.nlm.nih.gov/articles/PMC6698590/) (Open Access Emergency Medicine, open access in PubMed Central, 2019). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/nursing/icu-bedside-formulas-mean-arterial-pressure-cerebral-perfusion-pressure-cardiac/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/nursing/icu-bedside-formulas-mean-arterial-pressure-cerebral-perfusion-pressure-cardiac/