Compartment Differential Pressure Calculator
Compartment Differential Pressure Calculator
Diastolic blood pressure minus intracompartmental pressure, with the mean-arterial variant alongside it — and the published evidence that this number rules acute compartment syndrome OUT far better than it rules it in.
Differential pressure, both published definitions
Diastolic and mean arterial variantsDiastolic blood pressure 75 mmHg, measured intracompartmental pressure 38 mmHg, mean arterial pressure 95 mmHg
Two published differentials
differential pressure = mean arterial pressure − intracompartmental pressure (the older variant; published threshold 45 mmHg)
- a difference, not a pressure
- the quantity is a subtraction, so it carries the uncertainty of both measurements. A 10 mmHg error in either term is a 10 mmHg error in the answer, and the published thresholds are 30 and 45 mmHg
- transposing the two terms negates the answer
- compartment pressure minus diastolic pressure is the same magnitude with the opposite sign. At diastolic equal to compartment pressure both orderings give 0 and every wrong version agrees, so this page’s proof asserts the sign change at unequal values rather than at that point. The same trap caught a beam formula elsewhere in this project, 2.1% low in the unsafe direction, because the only test was the symmetric case
- which direction the threshold works in
- the AAOS 2018 guideline supports a differential above 30 mmHg “to assist in ruling out acute compartment syndrome”, graded moderate. That is a rule-out statement. The same guideline’s rule-in statement is weaker: monitoring “assists in diagnosing” the syndrome, also moderate, with no threshold attached
- how often a low differential means nothing
- 19 isolated lower-limb fractures without the syndrome: 11 (58%) within 20 mmHg of the diastolic, 8 (42%) within 30 mmHg of the mean arterial pressure, none developing compartment syndrome. And in a randomised trial of 200 tibia fractures, 18 monitored patients had a differential under 30 mmHg and none developed the syndrome
- the anaesthesia artefact
- “∆P may be artifactually low due to anesthesia – induced hypotension”, so the OTA advises using the preoperative diastolic pressure for intraoperative decisions. A hypotensive anaesthetised patient can cross either threshold with nothing happening in the limb
- derivation
- McQueen MM and Court-Brown CM, “Compartment monitoring in tibial fractures: the pressure threshold for decompression”, J Bone Joint Surg Br 1996;78-B(1):99–104 — 116 patients with tibial diaphyseal fractures, 3 (2.6%) with acute compartment syndrome and none with sequelae. A tibial diaphyseal fracture cohort is the population this threshold was derived in
Worked example
Diastolic blood pressure 75 mmHg, measured intracompartmental pressure 38 mmHg, mean arterial pressure 95 mmHg
Differential pressure = 75 − 38 = 37 mmHg, by McQueen and Court-Brown's definition
The mean-arterial variant = 95 − 38 = 57 mmHg, against that definition's published threshold of 45 mmHg
Both differentials sit above their published thresholds, and what the AAOS guideline supports for that is a rule-OUT: moderate evidence that a differential above 30 mmHg assists in ruling out acute compartment syndrome. It supports no corresponding rule-in threshold
Drop the diastolic to 55 mmHg with the same compartment pressure: 55 − 38 = 17 mmHg. That is below both thresholds — and in a series of 19 fractures with no compartment syndrome at all, 11 of 19 had a reading within 20 mmHg of the diastolic pressure. A low differential on its own is weak evidence
Raise the compartment pressure instead, to 60 mmHg with the original 75 mmHg diastolic: 75 − 60 = 15 mmHg. Note that an absolute compartment pressure of 60 mmHg and a differential of 15 mmHg are the same finding described two ways; the differential is preferred because the absolute number ignores the perfusion pressure driving blood into the limb
Transpose the two terms deliberately: 38 − 75 = −37 mmHg, the same magnitude with the opposite sign. Set the diastolic equal to the compartment pressure and both orderings give 0, which is why the proof for this page checks the transposition at unequal values
The two published differentials and their thresholds
| Definition | Threshold | Source and cohort |
|---|---|---|
| Diastolic blood pressure − intracompartmental pressure | under 30 mmHg in the original recommendation; above 30 mmHg “to assist in ruling out” in the AAOS guideline | McQueen and Court-Brown 1996, 116 patients with tibial diaphyseal fractures, 3 (2.6%) with acute compartment syndrome; threshold restated by the AAOS 2018 guideline as moderate evidence |
| Mean arterial pressure − intracompartmental pressure | under 45 mmHg | Printed by the OTA teaching set alongside the diastolic definition; no cohort is given for it there |
| Absolute intracompartmental pressure | no threshold used here | Superseded by the differentials, because an absolute pressure ignores the perfusion pressure driving blood into the limb |
How often a low differential occurs without the syndrome
| Series | Cohort | Finding |
|---|---|---|
| Prospective baseline measurements, J Trauma 2006 | 19 isolated lower-extremity fractures, none with clinical compartment syndrome | 11 (58%) had at least one reading within 20 mmHg of the diastolic pressure; 8 (42%) within 30 mmHg of the mean arterial pressure; 10 (53%) within 40 mmHg of it. None developed the syndrome |
| Randomised trial of continuous monitoring, J Trauma 2006 | 200 consecutive acute extra-articular tibia fractures | “In the monitored group, there were 18 patients with DeltaP less than 30 mm Hg” and none developed compartment syndrome or late sequelae. Conclusion: monitoring “is not indicated in alert patients who are adequately observed” |
| McQueen and Court-Brown 1996 | 116 patients with tibial diaphyseal fractures | 3 (2.6%) had acute compartment syndrome; none of the 116 had any sequelae of it at latest review |
| Clinical examination, Ulmer meta-analysis as quoted by the OTA | not stated in the source read | Sensitivity 13 to 19%, positive predictive value 11 to 15%, specificity 97% — the examination has the same asymmetry as the differential |
| The OTA teaching set’s own rendering of the 19-fracture series | same 19 fractures | States that “84% met the ∆P < 30 mm Hg fasciotomy criterion”. No 84% appears at any of the three thresholds in the primary abstract (42%, 53% and 58%). The primary figures are what this page prints |
A number that excludes better than it diagnoses
The differential pressure is a subtraction: the diastolic blood pressure minus the measured intracompartmental pressure. McQueen and Court-Brown proposed it from a prospective study of 116 patients with tibial diaphyseal fractures and recommended decompression if it dropped below 30 mmHg. The reasoning is that an absolute compartment pressure of 40 mmHg means something different in a patient with a diastolic of 90 and in one with a diastolic of 50, because what perfuses muscle is the gradient and not the compartment pressure alone. An older variant uses the mean arterial pressure with a threshold of 45 mmHg; the OTA prints both side by side, and because the mean is higher than the diastolic the two differentials are not the same quantity and cannot share a threshold.
The direction the number works in is the useful part, and it is routinely stated backwards. The AAOS 2018 clinical practice guideline supports a differential above 30 mmHg, on moderate evidence, to assist in ruling out acute compartment syndrome. It supports intracompartmental monitoring as an aid to diagnosis, also on moderate evidence, but attaches no threshold to that. The asymmetry is real. In a prospective series of 19 isolated lower-limb fractures, none of which had compartment syndrome, 11 had at least one reading within 20 mmHg of the diastolic pressure and 8 within 30 mmHg of the mean arterial pressure. In a randomised trial of 200 tibia fractures, 18 monitored patients had a differential under 30 mmHg and none developed the syndrome. A low differential is common in limbs that are going to be fine.
Two measurement traps are worth stating. The first is anaesthesia: the OTA’s teaching material warns that the differential may be artefactually low because of anaesthesia-induced hypotension, and advises using the preoperative diastolic pressure for intraoperative decisions. A hypotensive anaesthetised patient can cross either threshold with nothing happening in the limb. The second is timing: in late or missed presentation, the AAOS guideline states that compartment pressure monitoring does not provide useful information to guide decision making — that is graded as the work group’s opinion, in the absence of reliable evidence.
This page carries no coloured band, and that is deliberate. A bar under this number would read as an instruction, and decompressing a limb is a surgical decision made on a whole patient by a surgeon who can examine it. The clinical examination is no substitute either: the figures the OTA quotes for clinical findings are 13 to 19% sensitivity and 11 to 15% positive predictive value against 97% specificity, which is the same asymmetry as the differential and a reason to use both rather than either. Where the compartment syndrome follows an open fracture, the Gustilo-Anderson grade describes the injury and the Injury Severity Score the whole patient.
This page renders no clinical or surgical decision; it computes the published quantity and states what the literature reports for it, with every threshold and performance figure given with the cohort it came from. A measurement is not a diagnosis, and a figure from a published series is a property of that series rather than a fact about one patient. Agreement between observers is part of what a classification means. A category two experienced readers assign differently on the same film is not a measurement of the fracture, and the published kappa values for these systems run from almost nothing to almost perfect. Each page here prints them with the number of observers and the number of radiographs they came from.
Frequently asked questions
What is the compartment differential pressure?
The diastolic blood pressure minus the measured intracompartmental pressure. McQueen and Court-Brown defined it that way in a prospective study of 116 patients with tibial diaphyseal fractures and recommended decompression if it fell below 30 mmHg. An older variant subtracts the compartment pressure from the mean arterial pressure, with a threshold of 45 mmHg.
Does a differential below 30 mmHg mean compartment syndrome?
No, and the evidence on this is fairly clear. The AAOS 2018 guideline attaches its threshold to ruling the syndrome OUT — moderate evidence that a differential above 30 mmHg assists in ruling it out — and attaches no threshold to ruling it in. In a series of 19 isolated lower-limb fractures without compartment syndrome, 11 had a reading within 20 mmHg of the diastolic pressure, and in a randomised trial of 200 tibia fractures, 18 monitored patients had a differential under 30 mmHg and none developed the syndrome.
Why use a differential rather than the absolute compartment pressure?
Because what perfuses muscle is the gradient between the arterial pressure and the pressure in the compartment. The same absolute compartment pressure means something different in a patient with a diastolic pressure of 90 and in one with a diastolic of 50. Both published differentials exist for that reason.
Why is there no coloured band on this calculator?
Because a coloured bar under this number would read as a verdict, and the decision to decompress a limb is a surgical one made on the whole patient. This page gives both published differentials, both published thresholds, the cohorts they came from and the evidence that a low value is common without the syndrome. It renders no decision.
Can anaesthesia affect the differential?
Yes, and this is a documented trap. The OTA’s teaching material warns that the differential may be artefactually low because of anaesthesia-induced hypotension, and advises using the preoperative diastolic pressure when making intraoperative decisions. A fall in diastolic pressure lowers the differential without anything changing inside the compartment.
Why does this page ask for the mean arterial pressure instead of calculating it?
Because another record already owns that arithmetic, including its selectable weighting of the systolic and diastolic pressures, and two copies of one formula drift apart over time. Use the mean arterial pressure calculator or your monitor’s own figure, and enter it here.
Related calculators
References
- McQueen MM, Court-Brown CM. Compartment monitoring in tibial fractures: the pressure threshold for decompression. J Bone Joint Surg Br. 1996;78-B(1):99–104. Abstract read at boneandjoint.org.uk. “a prospective study of 116 patients with tibial diaphyseal fractures”. The quantity is defined there as “differential pressure (diastolic minus compartment pressure)”, and the authors “recommended that decompression should be performed if the differential pressure level drops to under 30 mmHg”. “Three patients had acute compartment syndrome (2.6%).” “None of our 116 patients had any sequelae of the compartment syndrome” at latest review.
- American Academy of Orthopaedic Surgeons. Management of Acute Compartment Syndrome, clinical practice guideline, 7 December 2018, endorsed by the American Orthopaedic Foot and Ankle Society; read at guidelinecentral.com/guideline/9783. “Moderate evidence supports that intracompartmental pressure monitoring assists in diagnosing acute compartment syndrome. (M)” And, crucially for the direction this quantity is used in: “Moderate evidence supports the use of repeated/continuous intracompartmental pressure monitoring” with “a threshold of diastolic blood pressure minus intracompartmental pressure >30 mmHg” “to assist in ruling out acute compartment syndrome. (M)”. In late or missed presentation, “compartment pressure monitoring does not provide useful information to guide decision making”, graded (C) — the work group’s opinion in the absence of reliable evidence.
- Baseline compartment pressure measurements in isolated lower extremity fractures without clinical compartment syndrome. J Trauma. May 2006. Named as Prayson et al 2006 in the OTA slide set below; abstract read at qxmd.com/r/16688067 and the full text was not reachable, so the author list is not reproduced here. “A prospective analysis of compartment measurements was performed in 19 isolated lower extremity fractures”, none with clinical compartment syndrome. “eight patients (42%) had a reading within 30 mm Hg of the mean arterial pressure”; “Ten patients (53%) had a reading within 40 mm Hg of their mean arterial pressure (delta P)”; “Eleven patients (58%) had at least one compartment reading within 20 mm Hg of their diastolic pressure”. The authors conclude that “use of direct compartment measurements with existing thresholds and formulations” “may not accurately reflect a true existence of the syndrome” and that “A search for other quantitative measures to more accurately reflect the presence of compartment syndrome is warranted”.
- Continuous compartment pressure monitoring for tibia fractures: does it influence outcome? J Trauma. June 2006. Abstract read at qxmd.com/r/16766979. “We randomized 200 consecutive acute extra-articular tibia fractures” to monitoring or no monitoring, with assessment for late sequelae at six months. “In the monitored group, there were 18 patients with DeltaP less than 30 mm Hg”, none of whom developed compartment syndrome or late sequelae. “Continuous compartment pressure monitoring is not indicated in alert patients who are adequately observed.”
- Orthopaedic Trauma Association. Compartment Syndrome, teaching slide set G04, ota.org/sites/files/2018-06. Gives both published differentials side by side — “MAP- IMP < 45 mm Hg; DBP – IMP < 30 mm Hg” — and warns that “∆P may be artifactually low due to anesthesia – induced hypotension”, advising “Use preoperative DBP when making intra-operative decisions”. For the clinical examination it quotes the Ulmer meta-analysis figures of 13–19% sensitivity, 11–15% positive predictive value and 97% specificity. The same slide set renders Prayson’s series as “84% met the ∆P < 30 mm Hg fasciotomy criterion”, which the primary abstract above does not support at any of its three thresholds; the primary figures are what this page prints. Two successive versions of the same slide also frame the quantity differently, once as “the most reliable indicator of when fasciotomy is not necessary” and once as “the most reliable indicator of compartment syndrome” — opposite directions of inference from one number.
- Institute for Work & Health. Information about DASH and QuickDASH Outcome Measure Licences, dash.iwh.on.ca/licences. Recorded here because it set this batch’s line on patient-reported outcome measures: licence category B, “Commercial software and/or web-based software for repeated use”, is “$1,250.00” for one year with “$650.00” annual renewal “plus applicable user fees”, the user-fee scale running to “$7,600” for up to 10,000 users, and “The licencee is not permitted to use the DASH/QuickDASH Instruments in their project prior to the issue” of a licence. No patient-reported outcome measure is built in this category.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/