Physical quantities and units of measurement

International system of units

A clean transition from the old to the new metric units failed to occur. The old system was based on the centimetre-gram-second (CGS), and was supplemented with many noncoherent derived units such as the millimetre of mercury (mmHg) for pressure and the calorie for work, which could not be related to the basic units by factors, which were powers of ten. The new system, the Système Internationale (SI), is based on the metre-kilogram-second (MKS) and comprises base and derived units which are obtained simply by multiplication or division without the introduction of numbers, not even powers of ten.

Base units are metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (thermodynamic temperature), mole (amount of substance) and candela (luminous intensity).

Derived units include newton (force: kilograms metre second ^–2 ), pascal (pressure: newton metre ^–2 ), joule (work: newton metre) and hertz (periodic frequency: second ^–1 ).

Special non-SI units are recognized as having sufficient practical importance to warrant retention for general or specialized use. These include litre, day, hour, minute and the standard atmosphere.

Nonrecommended units include the dyne, bar and calorie and gravity-dependent units such as the kilogram-force, centimetre of water (cmH ₂ O) and mmHg, but many remain in use today.

The use of SI units in respiratory physiology and clinical practice remains incomplete. The kilopascal has replaced the mmHg for blood gas partial pressures in Europe, but the old units continue to be used in the US and Australasia. The introduction of the kilopascal for fluid pressures in the medical field has failed to occur. In the clinical setting we continue to record arterial pressure in mmHg and venous pressure in cmH ₂ O.

As in previous editions of this book, it has proved necessary to make text and figures bilingual, with both SI and CGS units for the benefit of readers who are unfamiliar with one or other of the systems. Some useful conversion factors are listed in Table A.1 . There are still some areas of physiology and medicine where non-SI units continue to be extensively used, such as mmHg for most vascular pressures and cmH ₂ O for airway pressure, so these units are retained throughout this book to aid clarity.

Physical quantities relevant to respiratory physiology are defined below, together with their mass/length/time (MLT) units. These units provide a most useful check of the validity of equations and other expressions which are derived in the course of studies of respiratory function. Only quantities with identical MLT units can be added or subtracted, and the units must be the same on both sides of an equation.