Intra-arterial pressure

Abstract

The force of cardiac contraction is transmitted to the arterial system as pressure which maintains patency of blood vessels and perfusion of capillaries. The term pressure is derived from the Latin verb, premere, meaning to act on with steady force or weight. It is defined as the static force per unit area exerted by a column of fluid according to the equation: P = h × d × a, where h and d are the height and density of the column of fluid and a is the acceleration of gravity. In physics or engineering, the units of pressure are expressed in absolute units of the cgs (centimetergram-second) system as cm × g/cm³ × cm/sec². This expression reduces to g/sec²-cm or g-cm/sec²-cm² or dynes/cm², where the dyne is the unit of force. However, for biological systems, these units are simplified to the height in millimeters of a standard reference fluid, mercury by convention, with the density of mercury, d, and the acceleration of gravity, a, omitted since they are constants. In physiology, the units of pressure are then expressed as mm Hg. Mercury is used as a standard fluid because of its high density, thereby permitting measurement of arterial pressure with a relatively short tube. The relation between mm Hg and cgs units can be calculated as follows: 1 mm Hg = (0.1 cm) (13.6 g/cm³) (980 cm/sec²) where 0.1 cm is 1 mm height of the column, 13.6 g/cm³ is the density of mercury, and 980 cm/sec² is the acceleration due to gravity.