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دانستن تفاوت میان این دو واحد اندازه گیری فشار در پزشکی بخصوص در بخش ویژه از اهمیت بسزایی برخوردار است و گاهی مورد بی دقتی واقع می شود:

Know the Difference Between mmHg and cm H2O

Anthony D. Slonim MD, DrPH

Pressure is defined as the force per unit area. It can be applied either to a solid or a liquid interface at a point perpendicular to the surface and is represented by the height of the column that exerts a pressure at its base. Depending on the fluid used, the height of a column of fluid will differ because its density and therefore the pressure it exerts will differ.

Mercury (Hg) is a very dense fluid (13.5951 g/cm3) and is often used to calibrate pressure gradients and differentials. Normal atmospheric pressure can support 760mmHg. One millimeter of Hg therefore represents approximately 1/760 of an atmosphere and is a useful measure for comparing pressures. This measure is often referred to as a Torr, after the Italian physicist Torricelli. The unit mmHg is very useful in measurements for biologic systems. It is the conventional unit used to measure blood pressure.

Water has been used as a measure of pressure for numerous physiologic parameters and remains in use. Water is less dense than mercury, and the conversion from measurements using mercury to those using water follows the relationship 1 mmHg = 1.36 cm H2O. This unit is still used for measurements of central venous pressure, intracranial pressure, pressures in mechanical ventilation, and pulmonary capillary wedge pressures. It is important for the intensive care unit clinician to recognize which fluid scales are being used to correctly interpret the measurements and provide therapeutic interventions based on the numeric values. Confusion about the scales used can lead to inappropriate actions and compromise the patient's care.

 

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