Pressure, Transducers, and Manometers – a basic review Thus: Absolute pressure = Atmospheric pressure + Gauge pressure
Example of a pressure above atmospheric pressure: If the systolic pressure is 130 mm Hg and atmospheric pressure is 760 mm Hg, the absolute systolic blood pressure is 890 mm Hg. By convention, we measure blood pressure as a gauge pressure, i.e., the pressure difference above atmospheric pressure.
Example of a pressure below atmospheric pressure:
Jonathan Roth, MD Background: Transducers and manometers are devices for measuring pressure. Pressure is the sum of all the forces of a fluid (gas or liquid) applied against a surface divided by the area of the applied force (P = F / A). Absolute pressure is zero-referenced against a perfect vacuum. An example would be atmospheric pressure as used by meteorologists. Just as there is no such thing as a negative absolute temperature, there is no such thing as a negative absolute pressure. Pressure is the result of the sum of all the impacts of a fluid (resulting in a force) divided by the area of impacts. The lowest possible value is 0, i.e., no impacts. In common usage, the term “negative pressure” means sub-atmospheric pressure. Gauge pressure is zero-referenced against ambient air pressure (i.e., atmospheric pressure). Examples include blood pressure, tire pressure, or pressure in a gas cylinder. When the cylinder pressure gauge reads “0”, there is still gas in the cylinder that is at atmospheric pressure. That gas is not accessible because there is no pressure gradient that would expel the remaining gas in the cylinder.
When a blood vessel is opened to the environment, if the pressure in the blood pressure is greater than atmospheric, it will bleed. If the pressure in the vessel is less than atmospheric, air will tend to enter the blood vessel. Fluids always flow from high pressure to low pressure. During spontaneous ventilation, the central venous pressure becomes sub-atmospheric (e.g., -5 mm Hg). If a central venous catheter is then open to the air, air will enter the circulation. Common usage says air is “sucked” in, which is technically not correct. “Vacuum” and “suction” refer to an area of lower pressure. More correctly, air is pushed into the circulation from the higher atmospheric pressure (760 mm Hg absolute or 0 mm Hg gauge) to the lower central venous pressure (755 mm Hg absolute or –5 mm Hg gauge). Air is not pulled, or sucked, by the source of lower pressure.
Converting from one unit of pressure measurement to another: There are multiple units of pressure measurement commonly used: 1 atmosphere = 760 mm Hg = 760 Torr = 14.7 psi (pounds per square inch) = 1,000 millibar = 1 Bar = 101.3 kPa (kilopascal) = 101,325 Pa (pascals) = 100% = 34 feet fresh water = 33 feet salt water. To convert from one unit of measurement to another measuring the same quantity, we employ the technique of unit conversion. This utilizes the multiplicative identity that if you multiply any quantity by 1, the value does not change. continued on page 13
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SENTINEL | The Pennsylvania Society of Anesthesiologists Newsletter
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