Affiliations: Leeds General Infirmary, Leeds, UK,
Royal Oak, MI
Correspondence to: Lip-Bun Tan, DPhil, Department of Cardiology, Leeds General Infirmary, Leeds, LS1 3EX, UK
We read with interest the article by Gallagher et al (June 2005)1 comparing cardiorespiratory fitness in patients with morbid obesity vs those with established heart failure. The authors found that obese subjects had maximum oxygen uptake (V̇o2max) values very similar to those of nonobese heart failure subjects. We were surprised to find the published V̇o2max values were expressed in “milliliters per kilogram per minute,” after correction for body weight, rather than the absolute uncorrected V̇o2max (milliliters per minute).
For the same absolute value of V̇o2max (milliliters per minute), subjects with greater body mass would have smaller corrected V̇o2max (milliliters per minute per kilogram). In the absence of absolute V̇o2max (milliliters per minute) or body weight data in the published article, it is possible to estimate the surrogate V̇o2max by assuming height was not significantly dissimilar between the groups; thus, multiplying by body mass index can give a rough indication of the V̇o2max of each group (Table 1
These data suggest that the obese group may have the highest V̇o2max, the opposite of the impression given by V̇o2max per kilogram. We propose that uncorrected V̇o2max (milliliters per minute) and weight data should be presented to avoid misleading information and conclusions.
Maximal oxygen consumption when measured absolutely (milliliters per minute) is a reflection of total body energy expenditure. In those individuals with a larger body habitus, higher absolute values are obtained based solely on having larger muscle mass.1Clinicians will generally divide this absolute value by body weight in kilograms to allow for a more equitable comparison between individuals of variable sizes. Thus, when we express oxygen consumption in milliliters of oxygen per kilogram body weight (milliliters per kilogram per minute), we utilize this variable in an effort to compare the three groups, using the best single index of physical work capacity or cardiorespiratory fitness.2 Using the uncorrected oxygen consumption in milliliters per minute would have not allowed these important inferences on cardiorespiratory fitness.
As previous studies3–4 have identified a low level or aerobic fitness as an independent risk factor for all-cause and cardiovascular mortality, we believed our data reflected aerobic conditioning best when considered in this fashion. Additionally, by using oxygen consumption in milliliters per kilogram per minute, we were better able to compare our data with those standards previously reported for healthy individuals and for those with heart failure. In general, it has been reported that a maximum oxygen consumption of < 10.0 mL/kg/min signifies a poor prognosis in heart failure, with only a 50% 1-year survival rate. In addition, a maximum oxygen consumption of < 14.0 mL/kg/min is often used as a signal to consider cardiac transplantation.5 There are no such heuristics with uncorrected oxygen consumption.
Lastly, by reporting our data in milliliters per kilogram per minute, we are better able to relate the functional capacity to daily activities and/or exercise tolerance. Thus, when comparing our data to the metabolic costs of traditional activities of daily living (ie, walking or gardening), one is better able understand the significant aerobic impairment we observed in the obese.
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