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The cardiology unit converter is an essential clinical tool for translating haemodynamic, pressure, resistance, flow, and biomarker measurements between the unit systems used in different clinical contexts, countries, and medical literature. Cardiology is uniquely rich in unit diversity: pressure is variously reported in mmHg (most clinical settings), kPa (European scientific literature), cmH2O (ventilator and CVP monitoring), and Pa (physics/engineering). Vascular resistance is reported in Wood units (WU) in catheterisation labs and as dyne.s.cm-5 in older literature — a factor of 80 separates the two. Cardiac flow is described in L/min at the bedside and mL/s in fluid dynamics research. Cholesterol and lipoproteins are reported in mmol/L in the UK, Europe, and Australasia but as mg/dL in the USA and much of Asia — a conversion factor of 38.67 applies to LDL and total cholesterol (molecular weight 386.7 g/mol). Troponin units have transformed with the introduction of high-sensitivity assays: older assays reported in ng/mL, newer assays in ng/L (= pg/mL), representing a 1000-fold shift. Misinterpreting a troponin of 52 ng/L as 52 ng/mL (the old unit) would be a catastrophic clinical error. This converter covers the full breadth of cardiology unit conversions encountered in clinical practice, catheterisation laboratories, echocardiography, cardiac intensive care, and cardiovascular pharmacology, serving clinicians working across international systems and interpreting literature from different healthcare environments.
Pressure: 1 mmHg = 0.133322 kPa = 1.3596 cmH2O = 133.322 Pa | Resistance: 1 Wood Unit = 80 dyne.s.cm-5 | Cholesterol: mg/dL = mmol/L x 38.67 (LDL/TC) | mmol/L x 88.57 (TG) | Troponin: 1 ng/mL = 1000 ng/L = 1000 pg/mL
- 1Identify the measurement category: pressure, vascular resistance, flow rate, cardiac biomarker, or lipid panel — each uses a distinct conversion factor set.
- 2Convert pressure: multiply mmHg by 0.1333 to obtain kPa; multiply by 1.3596 to obtain cmH2O; multiply by 133.322 to obtain Pa. To reverse: divide kPa by 0.1333 to get mmHg.
- 3Convert vascular resistance: multiply Wood Units (WU) by 80 to obtain dyne.s.cm-5; divide dyne.s.cm-5 by 80 to return to WU. Normal SVR is 800–1200 dyne.s.cm-5 = 10–15 WU; PVR >3 WU is significant pulmonary hypertension.
- 4Convert flow: multiply L/min by 16.667 to obtain mL/s; divide mL/s by 16.667 for L/min. Normal cardiac output 4–8 L/min = 66.7–133.3 mL/s.
- 5Convert cholesterol/lipids: multiply mmol/L by 38.67 for LDL or total cholesterol (mg/dL); divide mg/dL by 38.67 to return to mmol/L. For triglycerides, multiply mmol/L by 88.57. For HDL, use factor 38.67 (same molecular weight basis as LDL for approximate conversion).
- 6Convert troponin: multiply ng/mL by 1000 to get ng/L (= pg/mL); divide ng/L by 1000 to return to ng/mL. High-sensitivity troponin T (hsTnT) and I (hsTnI) assays report in ng/L; 99th percentile URL for hsTnT is typically 19 ng/L (= 0.019 ng/mL) — never confuse these units.
- 7Verify the clinical plausibility of the converted value: a PCWP of 15 mmHg = 1.999 kPa = 20.4 cmH2O; a PVR of 3 WU = 240 dyne.s.cm-5; a troponin of 45 ng/L = 0.045 pg/mL (not ng/mL).
Scandinavian and some European countries routinely report blood pressure in kPa; UK, US, and most international clinical practice use mmHg.
The conversion factor 1 kPa = 7.5006 mmHg (exact: 133.322 Pa/mmHg) is the key. Approximate rule: divide kPa by 0.133 or multiply by 7.5.
PVR > 3 WU (240 dyne.s.cm-5) defines significant pulmonary hypertension; PVR >5 WU may indicate irreversible pulmonary vascular disease in transplant evaluation.
Wood Units (named after Paul Dudley Wood) are directly calculated as (mPAP - PCWP) / CO in mmHg / (L/min). Multiplying by 80 converts to the CGS unit dyne.s.cm-5 used in older haemodynamic literature.
US target for high-risk patients: LDL <70 mg/dL (1.81 mmol/L); UK target for very high risk: LDL <1.4 mmol/L (54 mg/dL).
The conversion factor 38.67 is derived from the molecular weight of cholesterol (386.65 g/mol) divided by 10. The same factor applies to LDL-C, HDL-C, and total cholesterol. Triglycerides use a different factor (88.57) because triglyceride is a larger molecule (MW 885.4 g/mol).
99th percentile URL for hsTnT (Roche Elecsys) is 19 ng/L; 52 ng/L is approximately 2.7x the URL — elevated. Serial measurement at 0h and 1h/2h required per ESC NSTEMI pathway.
High-sensitivity troponin assays detect concentrations 10–100 times lower than conventional assays. A result of 52 ng/L would have been reported as 0.052 ng/mL on an older assay — below the old reporting threshold of 0.1 ng/mL but clinically significant on hs-assay. Always note which assay and which unit is in use.
Catheterisation laboratory: converting PVR from Wood Units to dyne.s.cm-5 for publication or cross-centre comparison in pulmonary hypertension research, enabling practitioners to make well-informed quantitative decisions based on validated computational methods and industry-standard approaches
International patient care: interpreting cholesterol results from overseas labs reported in mg/dL for a UK or European clinician accustomed to mmol/L, helping analysts produce accurate results that support strategic planning, resource allocation, and performance benchmarking across organizations
Emergency department: correctly interpreting high-sensitivity troponin results and avoiding dangerous misinterpretation due to ng/L vs. ng/mL confusion, allowing professionals to quantify outcomes systematically and compare scenarios using reliable mathematical frameworks and established formulas
Cardiology training and exam preparation: understanding unit relationships for MRCP, USMLE, and board exam questions involving haemodynamic calculations, supporting data-driven evaluation processes where numerical precision is essential for compliance, reporting, and optimization objectives
Cardiac ICU: reconciling ventilator cmH2O and haemodynamic mmHg readings when assessing PEEP-induced haemodynamic effects, which requires precise quantitative analysis to support evidence-based decisions, strategic resource allocation, and performance optimization across diverse organizational contexts and professional disciplines
Troponin Assay Generations and Unit Chaos
Three generations of troponin assays co-exist in clinical use: (1) Conventional assays (CV >10% at 99th percentile URL) reported in ng/mL; (2) Contemporary sensitive assays; (3) High-sensitivity (hs) assays (CV <10%, >50% of healthy individuals detectable) reported in ng/L or pg/mL. The 99th percentile URL differs by assay manufacturer: hsTnT (Roche Elecsys) 19 ng/L; hsTnI (Abbott Architect) 26 ng/L; hsTnI (Siemens Atellica) 53 ng/L. Always refer to your institution's assay-specific reference range rather than a generic threshold.
SVR vs. PVR Calculation and Units
SVR (systemic vascular resistance) = (MAP - CVP) / CO in Wood units x 80 for dyne.s.cm-5. PVR (pulmonary vascular resistance) = (mPAP - PCWP) / CO in Wood units x 80 for dyne.s.cm-5. The indexing of SVR/PVR to BSA (SVRI, PVRI) allows comparison between patients of different sizes. Normal SVRI = 1970–2390 dyne.s.cm-5.m2; normal PVRI = 255–285 dyne.s.cm-5.m2.
BNP vs. NT-proBNP Units and Interpretation
B-type natriuretic peptide (BNP) and its inactive fragment NT-proBNP are both reported in pg/mL (= ng/L). They are NOT interchangeable: BNP and NT-proBNP have different molecular weights, half-lives, and reference ranges. BNP <100 pg/mL makes acute heart failure unlikely; NT-proBNP thresholds are age-adjusted (< 450 pg/mL age <50; <900 age 50–75; <1800 age >75). A conversion formula between BNP and NT-proBNP does not exist — never attempt to substitute one for the other.
mmHg vs. cmH2O in Respiratory/Cardiac Interface
Ventilator parameters (PEEP, plateau pressure, driving pressure) are routinely displayed in cmH2O, while haemodynamic parameters (CVP, PAP, PCWP) are displayed in mmHg. When assessing the haemodynamic impact of positive pressure ventilation — for example, PEEP-induced reduction in venous return — both sets of numbers must be in the same unit. A PEEP of 10 cmH2O = 7.35 mmHg, which is a significant fraction of a normal CVP of 8 mmHg and can meaningfully reduce cardiac preload.
International Lipid Reporting: Whom to Ask
If a patient presents with lab results from a different country without clear unit labels: USA lab results almost certainly use mg/dL (LDL typically listed as 80–200 in US patients on statin therapy); UK/Australia/Europe use mmol/L (LDL typically 1.5–4.5). A total cholesterol of 5.2 makes clinical sense as mmol/L (200 mg/dL) but not as mg/dL (0.134 mmol/L — incompatible with life). Always sanity-check: an LDL of 130 without a unit is almost certainly mg/dL = 3.36 mmol/L.
| Measurement | From | To | Multiply By | Clinical Normal Range |
|---|---|---|---|---|
| Pressure | mmHg | kPa | 0.1333 | MAP 70–100 mmHg / 9.3–13.3 kPa |
| Pressure | kPa | mmHg | 7.5006 | SBP 100–140 mmHg / 13.3–18.7 kPa |
| Pressure | mmHg | cmH2O | 1.3596 | CVP 2–8 mmHg / 2.7–10.9 cmH2O |
| Pressure | cmH2O | mmHg | 0.7355 | PCWP <12 mmHg / <16.3 cmH2O |
| Pressure | mmHg | Pa | 133.322 | Normal MAP 9333 Pa |
| Resistance | Wood Units | dyne.s.cm-5 | 80 | SVR 10–15 WU / 800–1200 dyne.s.cm-5 |
| Resistance | dyne.s.cm-5 | Wood Units | 0.0125 | PVR <2.5 WU / <200 dyne.s.cm-5 |
| Flow | L/min | mL/s | 16.667 | CO 4–8 L/min / 66.7–133 mL/s |
| Flow | mL/s | L/min | 0.06 | CI 2.5–4.0 L/min/m2 |
| Cholesterol (LDL/TC/HDL) | mmol/L | mg/dL | 38.67 | LDL <3 mmol/L / <116 mg/dL (general); <1.4 mmol/L / <54 mg/dL (very high risk) |
| Triglycerides | mmol/L | mg/dL | 88.57 | TG <1.7 mmol/L / <150 mg/dL |
| Troponin | ng/mL | ng/L (= pg/mL) | 1000 | hsTnT URL 19 ng/L = 0.019 ng/mL |
| Troponin | ng/L | ng/mL | 0.001 | hsTnI URL varies by assay: ~26 ng/L (Abbott) |
Why are there so many different units for the same measurements in cardiology?
Cardiology evolved through contributions from multiple disciplines — classical physiology used CGS units (dyne, cm, s); clinical medicine adopted mmHg for pressure (atmospheric convention); European scientific literature adopted SI units (kPa, Pa); the catheterisation lab adopted Wood Units for convenience (they are directly computed from measured haemodynamic variables without unit conversion); biochemistry labs in different countries standardised to different measurement frameworks. Globalisation of medical literature and digital health records is gradually increasing pressure to standardise, but the diversity remains clinically important for the foreseeable future.
What is the exact conversion between mmHg and kPa?
Exactly: 1 mmHg = 133.3224 Pa = 0.1333224 kPa. For clinical purposes: 1 kPa = 7.5006 mmHg (≈ 7.5 mmHg). Practical rule: multiply mmHg by 0.133 to get kPa; multiply kPa by 7.5 to get mmHg. For blood pressure, the maximum rounding error using these approximations is less than 1 mmHg at clinically relevant values.
What are Wood Units and why do we use them?
Wood Units (WU or Harvey Cushing Units in older literature) are a pulmonary vascular resistance unit defined as (mPAP - PCWP) / CO where mPAP is mean pulmonary artery pressure in mmHg, PCWP is pulmonary capillary wedge pressure in mmHg, and CO is cardiac output in L/min. Because CO is already measured in L/min and pressures in mmHg, WU requires no unit conversion — making it convenient in the catheterisation laboratory. Normal PVR is 0.5–2.5 WU; PVR > 3 WU is significant; PVR > 5 WU with poor vasoreactivity may preclude cardiac transplantation.
How do I convert triglycerides between mmol/L and mg/dL?
Triglycerides have a molecular weight of approximately 885.4 g/mol (based on a typical mixed triglyceride). The conversion factor is: mg/dL = mmol/L x 88.57; mmol/L = mg/dL / 88.57. Normal triglycerides < 1.7 mmol/L = <150 mg/dL; borderline high 1.7–5.65 mmol/L = 150–500 mg/dL; very high >5.65 mmol/L = >500 mg/dL (risk of pancreatitis). Do not use the cholesterol factor (38.67) for triglycerides.
What is the clinical importance of troponin unit confusion?
High-sensitivity troponin assays (introduced from approximately 2010) report in ng/L (nanograms per litre), which is numerically 1000 times smaller than the ng/mL values from conventional assays. A hsTnT of 52 ng/L = 0.052 ng/mL. If a clinician reads '52' and assumes ng/mL (the old unit), they will believe the troponin is 1000-fold higher than it is. Conversely, a conventional troponin of 0.5 ng/mL = 500 ng/L — far above the high-sensitivity assay upper reference limit. Always confirm units with your laboratory and verify against the assay-specific reference range.
What is PCWP and what are its normal values in different units?
Pulmonary capillary wedge pressure (PCWP, also called pulmonary artery occlusion pressure, PAOP) is measured by inflating the balloon of a pulmonary artery catheter, occluding a pulmonary artery branch and thus measuring back-pressure approximating left atrial pressure. Normal PCWP is 6–12 mmHg = 0.8–1.6 kPa = 8.2–16.3 cmH2O. PCWP >18 mmHg indicates elevated left atrial pressure (pulmonary oedema threshold); >25 mmHg is severe. PCWP is used in the haemodynamic classification of pulmonary hypertension: pre-capillary PH has PCWP ≤15 mmHg; post-capillary PH has PCWP >15 mmHg.
How do I convert between cmH2O and mmHg for CVP monitoring?
1 mmHg = 1.3596 cmH2O. Therefore: cmH2O / 1.36 = mmHg; mmHg x 1.36 = cmH2O. Normal CVP is 2–8 mmHg = 2.7–10.9 cmH2O. Older bedside CVP manometers read in cmH2O (water column); electronic transducers read in mmHg. A CVP of 10 cmH2O = 7.35 mmHg. This distinction matters when comparing readings from different monitoring systems or interpreting older literature.
What is the cholesterol molecular weight and why does it affect conversion?
Cholesterol has a molecular weight of 386.65 g/mol. The mmol/L to mg/dL conversion for any substance is: mg/dL = mmol/L x (molecular weight / 10). For cholesterol: mmol/L x 386.65/10 = mmol/L x 38.665 ≈ 38.67. This same factor applies to LDL-C, HDL-C, non-HDL-C, and total cholesterol because they are all measured as cholesterol mass. Lipoprotein particles themselves are much larger molecules, but the mass reported is the cholesterol content of those lipoproteins.
نصيحة احترافية
Create a laminated quick-reference card of the five most critical cardiology unit conversions for your clinical area: (1) mmHg to kPa (x0.133); (2) WU to dyne.s.cm-5 (x80); (3) cholesterol mmol/L to mg/dL (x38.67); (4) TG mmol/L to mg/dL (x88.57); (5) troponin ng/mL to ng/L (x1000). In an era of increasing international collaboration and cross-border patient records, these five conversions will cover 95% of real-world unit discrepancies encountered in cardiology practice.
هل تعلم؟
The unit 'mmHg' dates to 1896 when Riva-Rocci invented the mercury sphygmomanometer. It was formally defined as the pressure exerted by a 1 mm column of mercury at 0°C at standard gravity (9.80665 m/s2) — producing the exact value of 133.322 Pa. Mercury sphygmomanometers are now banned in hospitals across Europe due to mercury toxicity, but the unit named after the liquid metal persists as the dominant clinical pressure unit worldwide — a curious relic that now lives entirely in digital calibration tables.
المراجع
- ›Simonneau G et al. Updated clinical classification of pulmonary hypertension (Wood Unit definition). Eur Respir J 2019
- ›Collet JP et al. 2020 ESC Guidelines for the management of acute coronary syndromes in patients presenting without persistent ST-segment elevation (troponin thresholds and units). Eur Heart J 2021
- ›Mach F et al. 2019 ESC/EAS Guidelines for the management of dyslipidaemias (cholesterol units and targets). Eur Heart J 2020
- ›Joint British Societies' consensus recommendations for the prevention of cardiovascular disease (JBS3). Heart 2014
- ›White HD et al. Troponin assay terminology — a call for standardisation. Clin Chem 2020