Pressure is force per unit area. Both gases and liquids exert pressure. In the British fps, pressure is measured in pounds per square inch (lb/in2) or psi. The SI unit of pressure is the N/m2, or pascal (Pa). Pressure in the cgs system is measured in dynes/cm2. We also can measure pressure indirectly as the height of column of liquid, such as water or mercury. Thus mm Hg or cm H2O are also valid pressure measures.
Atmospheric pressure is the absolute force per unit area exerted by the weight of the atmosphere above us. Measures of atmospheric pressure are used in blood gas and pulmonary function laboratories to calibrate equipment and derive correct partial pressure measurements. Although most modern equipment that requires this information automatically measures atmospheric pressure, the gold standard is still the mercury barometer.
As show in the figure to the left, a mercury barometer is a simple evacuated glass tube about a meter long. The top of the tube is closed off, while the lower end of the tube is immersed in a mercury reservoir. The pressure exerted on the mercury reservoir by the atmosphere forces the mercury up the vacuum tube. The measured atmospheric pressure (P) equals the height of the column (h) times the density of the mercury (d). For example, in British fps units, a column height of 29.9 inches would equate to a pressure of 14.7 psi:
P (fps) = 29.9 in (height) x 0.491 lb/in2 (Hg density) = 14.7 lb/in2
Usually only the height of the column is reported. At sea level, the normal height of the mercury column (normal atmospheric pressure) is 760 mm Hg, (76 cm Hg) or 29.9 inches. In hyperbaric medicine, this standard pressure is also called one atmosphere absolute or 1 ATA. (in inches, mm or cm)
Alternatively, you may see the term torr used in pressure readings. Torr is short for Torricelli, the 17th century inventor of the mercury barometer. At sea level, one torr equals one mm Hg. Thus a pressure reading of 760 torr is the same as 760 mm Hg.
At the bedside, we usually measure relative pressures, not absolute pressures. Relative pressure measurement represent the difference in pressure above or below atmospheric. Thus a relative pressure measurement of 0 (any units) equals 760 mm Hg, 76 cm Hg, 29.9 inches, 14.7 psi or 1 ATA.
There are three ways to measure relative pressures: fluid columns, aneroid manometers and electronic pressure transducers.
Fluid Columns. A fluid column is similar to a barometer, but the tube is open to the atmosphere. If the pressure being measured is the same as atmospheric pressure, the fluid level remains at zero. If the pressure being measured exceeds atmospheric pressure, the fluid in the column rises to a height equivalent to the force exerted. The most common example of this kind of pressure measuring tool is the mercury sphygmomanometer (figure to the right). Because of its high density (13.6 g/cm3), mercury assumes a height that is easy to read for the pressures seen in the arterial side of the circulation.
Measuring small pressure differences with a mercury fluid column is difficult, because the changes in column height is so small. Instead, when we need to measure low pressures with a fluid column, we us water as the liquid, instead of mercury. Whereas a 1 mm column of Hg would be hard to read, the same pressure measured with a water column would be almost 14 mm or 1.4 cm high. This is because water is 13.6 times less dense than mercury.
The best example of water column pressure measurement at the bedside is the traditional CVP water manometer (left). The CVP water manometer consists of a simple water-filled plastic tube and 3-way stopcock connected in-line with an IV system and central venous catheter. To measure the CVP, the stopcock is briefly turned to close off the IV delivery system and the CVP is recorded as the height of the water column, in cm H2O. With this type measurement system, great care must be taken to ensure that the base of the fluid column is at a level equal to the point of measurement (right atrium).
A variation of the simple CVP water column is the U-tube water manometer (right). The U-tube manometer is used to measure small gas pressure differences. Its most common application is a calibration standard for mechanical manometers or electronic pressure transducers (see below). Reading a U-tube manometer is simple. At zero pressure (relative to atmospheric), the water levels on the two sides of the U-tube are equal. When pressure is applied to one side of the manometer, the water column is displace down on that side, and up on the opposite side. The difference in height between the two water levels equals the pressure, in cm H2O.
Aneroid Manometers. Fluid manometers are awkward to set up, can be messy, and are not very good at displaying rapidly changing pressures. To overcome these problems, we often use aneroid (non-liquid) manometers. An aneroid manometer is a relatively simple mechanical device consisting of a flexible metal chamber that expands and contracts with applied pressure changes. The chamber motion activates a geared pointer, which provides a scale reading analogous to pressure.
Respiratory therapists use many types of aneroid manometers. Aneroid manometers can be used on sphygmomanometers (instead of a mercury column). These devices also are used as the analog pressure gauges on essentially all ventilators. You also use aneroid manometers to measure maximum inspiratory or expiratory airway pressures (figure) and to obtain cuff pressures on endotracheal or tracheostomy tubes. The pressure gauges on high pressure gas cylinders are aneroid manometers, as are the vacuum pressure indicators on wall and portable suction units.
Depending on their application, aneroid manometers may be calibrated in cm H2O (ventilators, airway pressure, cuff pressure) mm Hg (wall suction, sphygmomanometers), in Hg (most portable suction systems) or psi (cylinder regulators). Some manometers even use SI units (pascals or kilopascals). Respiratory therapists must be able to convert among these various units.
Electronic Pressure Transducers. Although we still use both fluid columns and aneroid manometers to measure relative pressures, these traditional tools are rapidly being replaced by electronic pressure-measuring devices called pressure transducers. A pressure transducer converts the mechanical energy of pressure into an electronic signal or varying voltage or current. This electronic signal can be continuously displayed in either analog or digital form. If digitized, the pressure signal can also be stored and undergo sophisticated computer analyses.
There are many variations in design of electronic pressure transducers. Today, many of these devices are miniaturized solid-state single-patient-use system that come preassembled with disposable vascular pressure kits. The most common nondisposable design is the strain-gauge pressure transducers (figure below). In this device, pressure changes expand and contract a flexible metal diaphragm connected to electrical wires. The physical strain on the diaphragm changes the amount of electricity flowing through the wires. By measuring this change in electrical flow, we are indirectly measuring changes in pressure.
Strain gauge pressure transducer. A, no pressure is applied. B, pressure is applied to the transducer. An ammeter shows a change in electrical current, proportional to the magnitude of pressure applied. Figure adapted from Scanlan, CL, Wilkins, RL & Stoller, JK (Eds.). Egans Fundamentals of Respiratory Care (7th Ed.). St. Louis, MO: Mosby, 1999.
The figure below show the typical signal output obtained from a strain gauge pressure transducer connected to an arterial line. The resulting display is the arterial pulse pressure waveform.
Which pressure measurement device do you use? Most of the time you have no choice the device is already connected to the system being checked or monitored. However, a few key pointers can be helpful when given a specific task involving pressure measurement: