Energy requirements and body surface area of cats and dogs.

E nergy requirements, the size of some organs (eg, brain and kidneys), metabolic processes (eg, glomerular filtration rate), and the doses of some drugs (eg, digitalis and chemotherapeutic agents) do not necessarily vary directly with body weight (BW). Some of these factors vary exponentially with BW (eg, BW 0.75 , referred to as metabolic BW) or with body surface area (BSA). Understanding whether energy requirements and other metabolic functions vary with BW, metabolic BW, or BSA is crucial when carefully managing patients that vary in body size (eg, dogs and cats). For example, use of BW or an exponent of BW that is too high will typically underestimate the energy requirements of smaller animals and overestimate energy requirements in larger animals. Overfeeding can be especially life-threatening for mal-nourished patients and will promote obesity in well-nourished patients. The relationship between size and function may also yield insight into metabolic processes. The idea that heat loss from a warm-bodied animal should be roughly proportional to BSA was first suggested as early as 1839. 1 To compensate for heat loss, heat production (similar to the situation for BSA) should be relatively greater in smaller animals than in larger ones. Empirical support for this principle was provided for dogs in 1883. 2 In that study, it was reported that the fasting metabolic rate decreased relative to BW as size increased, but it remained comparatively constant relative to BSA (1,000 to 1,200 kcal/m 2 daily [90 to 110 kcal/ft 2 daily]). Subsequently, it was suggested that this principle could be generalized to all species of birds and mammals. 3 Since that time, few investigators have measured BSA directly in dogs and cats because it is difficult to measure. Dead animals can be skinned and the area of skin measured, but it is difficult to determine how much the skin should be stretched once it has been removed. A paper mold can be made to cover the skin surface, but it is not easy to conform such a mold to the cracks and crannies of an animal' s body. Alternatively, the body can be divided into standard geometric shapes (eg, cylinders); the surface area can be calculated for each of the geometric shapes and then combined to obtain a total BSA. All of these measurements are extremely time consuming and likely to be accurate only to within 10% to 20%. 1 Body surface area may be …