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Chapter 20: Physics

Which of the following most closely represents the equivalent of 10 cm H2O?

(A) 1.47 psi

(B) 97 mbar

(C) 7.4 mm Hg

(D) 0.1 kPa

(E) 0.5 atm

The answer is C. Pressure is the ratio of force to the area over which it is applied. While the SI unit for force is the pascal (Pa), which is equal to 1 newton per square meter, other traditional non-SI units are commonly used, especially in the United States. For example, in anesthetic practice, pounds per square inch (psi) is used to describe cylinder and pipeline pressures. Pressure is commonly described by its ability to displace a column of fluid; for instance, millimeters of mercury (mm Hg) are used for pressures within body systems (e.g., arterial blood pressure, intracranial pressure, tissue compartment pressure). The use of centimeters of water (cm H2O) is typically confined to measurement of airway pressure. However, the reporting of central venous pressures using water manometry in cm H2O is still relatively common practice and a frequent source of confusion for trainees. The use of electronic pressure transducers is now almost universal for CVP monitoring, and these values are reported in mm Hg. Millibar and atmosphere units are not typically used in clinical medicine. A conversion table is presented in Table 20-1.

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Table 20-1. Approximate conversions between SI (kPa) and other common units of pressure measurement.


cm H2O

mm Hg










Ref: Middleton B, Phillips J, Thomas R, Stacey S. Physics in Anesthesia. 1st ed. Banbury, UK: Scion Publishing; 2012.

Which of the following best prevents the inadvertent connection of the wrong gas cylinder to the anesthesia machine?

(A) the Diameter Index Safety System

(B) the Pin Index Safety System

(C) yokes that are specifically sized for each type of gas

(D) Department of Transport markings on cylinders

(E) color-coding of cylinders

The answer is B. Connection of the wrong cylinder of gas to the oxygen yoke of an anesthesia machine presents a serious and avoidable hazard. Color-coding of cylinders was an early strategy to prevent this, although it did not prevent human error. Moreover, the US convention for color-coding oxygen and air differs from the international convention (Table 20-2...

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