To the Editor:
We read with great interest the recent article by Fontana et al.  on oxygen consumption and cardiovascular function in children during profound intraoperative normovolemic hemodilution and wish to offer the following comment. As noted in Table 1(formulas used for calculated variables), page 221, line 9, the symbol g centered dot m centered dot beat-1 centered dot m-2 was used by the authors to represent work index. Strictly speaking, the units gram times meter or mass times distance do not accurately represent work. In absolute terms, work is equal to force times distance or pressure times volume and is expressed as an erg (dyne centered dot centimeter) or as a joule (newton centered dot meter). Historically, units based on weight came into existence because physical weights were used as simple sources of force in early mechanical devices. Here, weight displaced in proportion to an applied force was used to assess work. Gram is used as the weight and not as a unit of mass. The universal contribution of gravity in the definition of force equal to mass times acceleration is not incorporated into the calculation due to Earth's gravitational field. Thus, in absolute terms, the value obtained would be approximately 9.81 times the corresponding SI unit, which can be referred to as the gravitational unit. In modern terms, the units based on weight have the word force (rather than weight) in their names and represent the force exerted on a mass by the acceleration due to gravity (9.80665 m centered dot s-2). The following equations demonstrate the relation between force and gram-force: Equation 1 Because this error is still seen in the medical literature, we believe it should be reemphasized to the Anesthesiology and critical care physicians who use these units in their daily practice.
Arjang Khorasani, MD
Department of Anesthesiology and Pain Management, Cook County Hospital, Chicago, IL 60612
Samuel K. Appavu, MD
Department of Surgery, University of Illinois College of Medicine at Chicago
Division of Surgical Critical Care, Cook County Hospital, Chicago, IL 60612
1. Fontana JL, Welborn L, Mongan PD, Sturm P, et al. Oxygen consumption and cardiovascular function in children during profound intraoperative normovolemic hemodilution. Anesth Analg 1995;80:219-25.