MAP, Shock Index and Pulse Pressure: The Evidence Behind the Tool

A 28-year-old man is brought to casualty after his motorcycle skidded on the highway near Udupi. He is talking. His left thigh is swollen. The nurse reads out the vitals: pulse 118, blood pressure 112/86. The intern looks relieved. “Sir, the BP is normal.” This is an illustrative case, not a real patient.

It is not, and the three numbers this tool computes show why. By the end of this article you will know why the organs care about the mean pressure rather than the systolic. You will know how the shock index finds the patient who is bleeding before his pressure falls, and what the pulse pressure tells you about the heart’s output. All three come from the same set of vitals you already have.

1. Why the organs follow the mean pressure

Perfusion of the kidney, the gut and the brain is driven by the mean pressure across the whole cardiac cycle, not by the peak. At a normal rate the heart spends roughly a third of the cycle in systole and two thirds in diastole. That is where the approximation comes from: the diastolic plus a third of the pulse pressure, or equivalently the systolic plus twice the diastolic divided by three. Both forms give the same number.

“Why a third?” Because the heart spends about a third of each beat in systole and two thirds in diastole, so the pressure spends most of its time near the diastolic. The average of the whole cycle therefore sits a third of the way up from the diastolic, not halfway. For our young man: 86 plus a third of (112 minus 86), which is 86 plus about 9, a mean of about 95 mmHg.

The tool keeps the two places where this simple arithmetic misleads. In marked tachycardia, diastole shortens far more than systole, so the true mean sits above the calculated one. And an oscillometric cuff estimates the mean directly and derives the systolic and diastolic from it, so in a vasoconstricted, hypotensive or arrhythmic patient the mean the machine reports is usually more trustworthy than the systolic beside it.

The target most residents know, 65 mmHg, is a septic shock target. The Surviving Sepsis Campaign guideline of 2026 recommends an initial 65 rather than a higher one, and for adults aged 65 and above an initial range of 60 to 65. It is not a universal floor: a patient with long-standing untreated hypertension may become oliguric at a mean that would be adequate for someone else, and bleeding before surgical control runs the other way, towards permissive hypotension.

2. The shock index finds the patient whose pressure has not fallen yet

A fit young adult defends his blood pressure fiercely. As he bleeds, the heart rate climbs first, and the systolic holds in the normal range until a large share of the blood volume, about 30 per cent, has gone. The systolic is the last thing to change, and it is the number most often relied on. The shock index puts the early sign and the late sign in one figure: the heart rate divided by the systolic pressure.

For our young man: 118 divided by 112 is 1.05. The normal range is 0.5 to 0.7. How much does a value of 1 matter? A meta-analysis of adult trauma found an initial index of 1 or above carried a fourfold increase in in-hospital mortality, pooled risk ratio 4.15.

In other words, a trauma patient who arrives with an index of 1 or more is about four times as likely to die in hospital as one who does not. Above 0.9 is the more widely accepted threshold for activating the massive transfusion protocol. His “normal” blood pressure has an index that calls for blood.

“So is anything above 0.7 shock?” No, and the tool is careful here. Pain, fever, anxiety, dehydration and a bronchodilator all put a well ward patient above 0.7. Treat 0.7 to 0.9 as the band that should make you look again, not the band that reassures. In a general adult it asks for a second look and a repeat set of vitals. In trauma or sepsis, where a cause of loss or vasodilatation is already known, it is read as compensated shock.

The index has blind spots, and the commonest one is on every medical ward. It depends entirely on the heart rate rising. A beta blocker or a pacemaker stops the rate from rising, and the index stays normal while the patient bleeds. Age blunts the heart rate response too, which is why the age shock index exists. For why beta blockade hides the response, see the author’s article Beta-Blockers: Clinical Pharmacology and Practice Pearls. After delivery the normal range shifts to 0.7 to 0.9; the queue’s Postpartum Haemorrhage article carries how the obstetric shock index is used there.

3. The pulse pressure stands in for the stroke volume

The pulse pressure is the systolic minus the diastolic, conventionally about 40 mmHg in a healthy adult. It reflects the stroke volume pushed into the aorta, so a falling pulse pressure in a patient who is otherwise compensating is one of the earliest usable signs that the stroke volume is dropping. It narrows in hypovolaemia, tamponade and aortic stenosis.

Our young man’s pulse pressure is 112 minus 86, only 26 mmHg. The proportional pulse pressure is the more useful figure, because it does not depend on how high the pressure is overall: the pulse pressure divided by the systolic, as a percentage. His is 26 divided by 112, about 23 per cent.

“Where does that percentage come from?” From heart failure, not trauma. Stevenson and Perloff established the bedside haemodynamic profiles in heart failure. In 50 patients with known chronic heart failure a proportional pulse pressure below 25 per cent identified a cardiac index below 2.2 L/min/m², sensitivity 91 per cent and specificity 83 per cent. A later series using a cut-off of 27.4 per cent reported 95.5 and 90.1 per cent.

In plain words: in a patient with heart failure, a proportional pulse pressure below 25 per cent picked out nine in ten of those whose cardiac output was low. In a breathless patient with a low output state, that single division is the closest thing to a cardiac index available without a catheter. In a bleeding patient the same narrowing means the same thing, a small stroke volume, for a different reason.

4. Read the three together

No one number decides. The tool reads them as patterns, and this is the one that fits our young man.

This is where residents most often go wrong, and it is easy to see why: a blood pressure of 112/86 looks normal, and the systolic is what we were taught to watch. Here the mean is about 95, the index is 1.05 and the pulse pressure is 26 mmHg. That is compensated hypovolaemia. His swollen thigh is the likely source. He needs blood and the source controlled now, not after his pressure falls.

And repeat the vitals. Two readings twenty minutes apart, read side by side, will tell you more than any index computed from one.

Try this

A 70-year-old man on metoprolol has fallen at home and fractured his hip. His pulse is 72 and his blood pressure 96/70. Work out his shock index, his age shock index and his pulse pressure. What do they tell you? Illustrative case.

Answer. Shock index: 72 divided by 96 is 0.75, only a little above the normal range. Age shock index: 70 multiplied by 0.75 is 52.5, and 50 and above is treated as haemodynamically unstable in geriatric trauma. Pulse pressure: 26 mmHg, narrow. Remember the metoprolol: his heart rate cannot rise, so his shock index is falsely reassuring. Treat him as a bleeding patient until proved otherwise.

Take-home points

• The organs are perfused at the mean pressure; the systolic is the last number to fall in a young adult who is bleeding.
• A shock index of 1 or above carries a fourfold mortality in trauma; above 0.9 is the usual trigger for massive transfusion.
• Read 0.7 to 0.9 as a reason to look again, not as reassurance.
• A beta blocker, a pacemaker, age and chronic hypertension all make the index read falsely normal.
• A proportional pulse pressure below 25 per cent points to a low cardiac output.

Declaration: the author built and maintains the tool. It is free to use. The figures in this article are those the tool carried in its Evidence tab on 5 November 2026, each from the source the tool names. The illustrative cases follow the tool’s rules; check the tool’s result for a real patient.

References

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