Anaes · Measurement & monitoring physics
SI units, dimensions and measurement in anaesthesia
Also known as SI units · International System of Units · Dimensions · Dimensional analysis · Unit conversion · Measurement in anaesthesia
Every quantity the anaesthetist measures — a blood pressure, a gas concentration, a flow, a dose — is a number carrying a unit, and the unit is as load-bearing as the number. This topic rebuilds the foundations of measurement for the fellowship examination. First, the International System of Units (SI) rests on SEVEN base units, each now defined by a fixed constant of nature (post the 2019 revision): the metre, kilogram, second, ampere, kelvin, mole and candela. Second, the DERIVED units — newton, pascal, joule, watt, hertz, volt, ohm, coulomb, farad — are coherent combinations of the base units. Third, every quantity has DIMENSIONS (M, L, T, I, theta, N, J) and dimensional analysis (both sides of any physical equation must share identical dimensions) is the single most powerful check on a derived formula. Fourth, PREFIXES scale units by powers of ten, and a misplaced prefix — milli mistaken for micro — is a thousand-fold dosing error. Fifth, medicine runs on a MIX of SI, legacy and pragmatic units: pressure in kPa, mmHg and cmH2O; flow in L per min; dose in mg per kg and mcg per kg per min; temperature in Celsius; concentration in mmol per L and per cent; and the anaesthetist must convert fluently between them (1 atmosphere equals 101.325 kPa equals 760 mmHg equals 1033 cmH2O). Sixth, SIGNIFICANT FIGURES and scientific notation govern how a measurement is reported and propagated through calculation, and ACCURACY (closeness to the true value) is independent of PRECISION (reproducibility). Anchored in the AHA scientific statement on blood-pressure measurement (Pickering 2005), the clinician review of oscillometric pressure (Alpert 2014), the SI revision review (Saito 2021), the anaesthesia drug-safety review (Orser 2013), the paediatric anaesthesia medication-error study (Leahy 2018), the perioperative medication-error reduction review (Bekes 2021) and the drug-name-confusion evidence (Lambert 2016).
Checked against its sources on 3 Sept 2026
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- The seven SI BASE units are metre (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount), candela (luminous intensity); since 20 May 2019 the kilogram, ampere, kelvin and mole are defined by fixed constants (Planck h, elementary charge e, Boltzmann k, Avogadro NA).
- Key DERIVED units: newton (force, kg m per s squared), pascal (pressure, kg per m per s squared), joule (energy, kg m squared per s squared), watt (power, kg m squared per s cubed), hertz (frequency, per s), volt (W per A), ohm (V per A), coulomb (A s), farad (C per V).
- Pressure conversion to memorise: 1 atmosphere equals 101.325 kPa equals 760 mmHg equals 1033 cmH2O equals 1.013 bar equals 14.7 psi. Blood pressure in mmHg, airway and venous pressure in cmH2O, cylinders in bar.
- A misplaced PREFIX is a common fatal dosing error: milli (10 to the minus 3) versus micro (10 to the minus 6) is a thousand-fold difference. Never abbreviate microgram as mcg AND milligram as mg without an independent double-check on high-alert infusions.
- ACCURACY (closeness to the true value) is not PRECISION (reproducibility). A precise but biased instrument gives a reproducibly wrong answer; calibration removes bias, averaging removes random error.
- DIMENSIONAL ANALYSIS: both sides of any physical equation must share identical dimensions (M, L, T, I, theta, N, J). Use it to verify every derived formula (for example, Poiseuille flow has dimensions of volume per time).
- Dose conversions: 1 mcg per kg per min equals 0.06 mg per kg per h. Cardiac output in L per min equals mL per min divided by 1000. PaCO2 5.3 kPa equals 40 mmHg (multiply kPa by 7.5).
Meet the patient
A 70 kg patient needs noradrenaline at 0.1 mcg per kg per min. You draw up 4 mg in 50 mL (that is 80 mcg per mL) and programme the pump. What is the rate, and what happens if the registrar reads "mcg" as "mg"?[4]
The rate is 5.25 mL per h — pure dimensional analysis. But if mcg becomes mg, the pump delivers a thousand-fold overdose and the patient arrests before you reach the door. Every number in anaesthesia carries a unit, and the unit is as load-bearing as the number.[4][5]
References7ShowHide
- [1]Pickering TG, Hall JE, Appel LJ, Falkner BE, Graves J, Hill MN, Jones DW, Kurtz T, Sheps SG, Roccella EJ Recommendations for blood pressure measurement in humans and experimental animals: part 1: blood pressure measurement in humans: a statement for professionals from the Subcommittee of Professional and Public Education of the American Heart Association Council on High Blood Pressure Research Circulation, 2005.PMID 15699287
- [2]Alpert BS, Quinn D, Gallick D Oscillometric blood pressure: a review for clinicians J Am Soc Hypertens, 2014.PMID 25492837
- [3]Saito N [Redefinition of the International System of Units (SI) and Related Quantities in the Field of Ionizing Radiation]. Igaku Butsuri, 2021.PMID 33853980
- [4]Orser BA, Hyland S, U D, Sheppard I, Wilson CR Review article: improving drug safety for patients undergoing anesthesia and surgery Can J Anaesth, 2013.PMID 23264011
- [5]Leahy IC, Lavoie M, Zurakowski D, Baier AW, Brustowicz RM Medication errors in a pediatric anesthesia setting: Incidence, etiologies, and error reduction strategies J Clin Anesth, 2018.PMID 29913393
- [6]Bekes JL, Sackash CR, Voss AL, Gill CJ Pediatric Medication Errors and Reduction Strategies in the Perioperative Period AANA J, 2021.PMID 34342569
- [7]Lambert BL, Schroeder SR, Galanter WL Does Tall Man lettering prevent drug name confusion errors? Incomplete and conflicting evidence suggest need for definitive study BMJ Qual Saf, 2016.PMID 26700541