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LETTER TO THE EDITOR
First, the relation between fitness and venous compliance has been well documented in humans using strain-gauge plethysmography by several groups. In addition to the cited article by Louisy et al. (5), important observations on the relation between fitness and venous function have been published by Luft et al. (4), Monahan et al. (7), and Pawelczyk et al. (8). Luft et al. (4) found a high correlation between venous compliance and maximal oxygen consumption in runners and nonrunners that was associated with decreased tolerance to lower body negative pressure. Pawelczyk et al. (8) found venous compliance was increased by a 7-wk endurance training program. Monahan et al. (7) showed that venous compliance is reduced with age in sedentary and endurance-trained men but that habitual exercise was associated with increased venous compliance in both young and older men. Furthermore, a series of studies by Convertino and colleagues [for example, see Convertino et al. (2)] explored the relationship between changes in muscle mass and venous compliance. Although there remain important questions regarding the relation between fitness and venous function, the presence of this link appears to be a resolved issue.
Second, the application of plethysmography to study venous function in humans has been described extensively. Admittedly, several different methods currently see widespread use by investigators. However, the critical assumptions and underlying physiology have been well characterized for these methods [for example, see Halliwill et al. (3)]. In light of what has been published previously on measuring venous compliance, the technical considerations raised by Alomari et al. (1) appear to conflict with known venous physiology. For example, the observation that limb volume continues to increase during venous compliance plethysmography is a well-known result of venous wall creep and increased capillary filtration rates. Thus the "increased venous capacitance" reported by the authors after a 10-min venous collection period (vs. a 5-min period) represents the time-dependent effects of creep and filtration. In addition, the pressure-dependent nature of capacitance is clearly established and newer methods of estimating venous compliance incorporate this complexity (3). Finally, the measure of "venous outflow" does not recognize that venous emptying follows an exponential decay pattern that is best described by a time constant, as detailed by Magder (6) and others. As such, the analysis of "outflow" in the recent study by Alomari et al. (1) merely reflects the volume stored in the veins before cuff release and is not an appropriate index of venous function. Thus we see clear errors in interpretation of the data provided by Alomari et al. (1) and it would be unfortunate if this unsophisticated and erroneous methodology were to be repeated by others, unaware of the rich history of venous physiology with which it conflicts.
In summary, based on prior publications and known venous physiology, it appears that the methods suggested by Alomari et al. (1) for the study of venous function have serious shortcomings. As such, the conclusions drawn regarding the link between fitness and venous function are doubtful. However, as mentioned above, considerable and consistent information regarding this link is available from earlier studies.
REFERENCES
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