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EFFECT OF ALTERATIONS IN BLOOD VOLUME ON CARDIAC FUNCTIONING DURING MAXIMAL EXERCISE: RESPONSE

Warburton, Darren E. R.

Medicine & Science in Sports & Exercise: June 1998 - Volume 30 - Issue 6 - p 971-973
Letters to the Editor-In-Chief
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Faculty of Physical Education and the Division of Cardiology; University of Alberta

Dear Editor-in-Chief:

Thank you for inviting us to respond to Dr. Kanstrup's letter to the editor in which our recent paper was criticized (9). We would like to state emphatically that, contrary to Dr. Kanstrup's interpretation, the major findings in our paper are consistent with both the findings of Kanstrup and Ekblom (8) and the findings of several other investigators in the past ten years which support the importance blood volume independent of Hb in maximal aerobic capacity and endurance performance (2,3,10).

In our paper, we stated that Kanstrup and Ekblom (8) reported a strong relationship between changes in blood volume and maximal aerobic capacity. We did not make any further conclusions with regards to their paper; this reference merely served to address the importance of blood volume on maximal aerobic capacity. As Dr. Kanstrup wrote in the letter to the editor, the major conclusion of their 1982 paper was that "acute induced hypervolemia by plasma volume expansion may increase stroke volume and cardiac output during submaximal and maximal exercise, thus compensating for reduced arterial oxygen content and keeping maximum aerobic power unchanged." The authors went on to say that differences between blood volume expansion and blood letting were related to the "higher preload after volume expansion, which may be regarded as a primary Starling effect on the central circulation during exercise up to maximum." This conclusion directly supports the findings of our investigation and further highlights the importance of the Frank-Starling mechanism, which is why the article was referenced.

In no way did we imply that haemoglobin concentration and oxygen-carrying capacity of blood are not important determinants of maximal aerobic capacity. In fact, several articles have been published from our laboratory (1,4,5) that highlight the importance of oxygen-carrying capacity on maximal aerobic capacity and endurance performance. However, this current publication addresses the recent findings that blood volume expansion, independent of increasing oxygen-carrying capacity, can augment maximal aerobic capacity in untrained participants. It is also important to point out, that unlike Kanstrup and Ekblom's paper (8), the majority of the investigations in which there was an increase in maximal aerobic capacity after plasma volume expansion were conducted using untrained participants. Similar to our study (9), these investigators have shown that stroke volume and cardiac output increase sufficiently to offset the hemodilution effects of plasma volume expansion, allowing maximal aerobic capacity to be increased (2,3). Thus, an increased cardiac output compensates for the reduced arterial oxygen content after plasma volume expansion in untrained individuals-a finding that directly supports that of Kanstrup and Ekblom (8), who stated that "acute plasma volume expansion led to increased cardiac output and stroke volume at rest and during exercise, including maximal cardiac output, which compensates for reduced arterial oxygen content...." The difference between our paper and that of Kanstrup and Ekblom (8) is that maximal oxygen consumption was not increased after plasma volume expansion in their study. However, both findings are consistent with the literature. In endurance-trained athletes, the increase in stroke volume and cardiac output are generally insufficient to offset the proportional dilution in haemoglobin (3) and therefore fail to result in a further increase in maximal aerobic capacity as found by Kanstrup and Ekblom (8).

A careful review of our paper will reveal that we did not state that maximal oxygen consumption is not modified by the oxygen-carrying capacity of the blood. We did state that maximal oxygen consumption in untrained subjects can be enhanced simply by increasing plasma volume-a finding which has significant support in the literature. In fact, a recent investigation from our laboratory ((11) abstract, manuscript under review) revealed that plasma volume expansion has no ergogenic effect on endurance athletes who already possess a very large blood volume. Thus, the ergogenic properties of Macrodex seen in untrained participants do not appear to apply to highly trained endurance athletes (2,6).

Dr. Kanstrup seems to take issue with the relative importance of blood volume and oxygen-carrying capacity in maximal oxygen consumption. Dr. Kanstrup and coworkers have clearly shown the importance of the total amount of haemoglobin to maximal aerobic capacity. However, their work also highlights the importance of blood volume to maximal oxygen consumption. In Kanstrup and Ekblom's 1984 (7) article, the authors evaluated the relative importance of haemoglobin concentration and blood volume for the determination of maximal oxygen consumption. They reported that "a reduced [Hb] occurring concomitantly with an elevated plasma volume may result in an unchanged O2max. This can be attributed to an increased cardiac output, compensating the reduced arterial oxygen content." This finding also supports our observations. In fact, Kanstrup and Ekblom (7) revealed that there is a "very close relationship between O2max and [Hb] as well as BV." In this investigation, the authors stated that "It could not be confirmed which of the two variables, [Hb] or BV, was of greater importance in determining O2max; however, O2max was highly correlated to the product, i.e., to total Hb." Also, in their discussion of the same paper they stated, "The present study confirms that both the size of the blood volume and the haemoglobin concentration are of importance for the O2max and physical performance."

The major difference between the our work and that of Kanstrup et al. is that they found that plasma volume expansion did not result in a further increase in their participants' maximal oxygen consumption. We believe that these differences are simply related to the fitness level of the participants involved. Kanstrup and coworkers used athletes with maximal oxygen consumptions of approximately 59.6 and 63.8 mlβkg−1βmin−1, respectively, in their investigations (7,8), whereas our participants were sedentary with a maximal oxygen consumption of 41.5 mlβkg−1βmin−1. We (11) and other investigators (2) have recently found no ergogenic effect in endurance trained athletes after acute plasma volume expansion. However, significant improvements in maximal aerobic capacity can be seen in untrained participants as a result of acute plasma volume expansion (9). We postulated this may be in large part due to the sub-optimal blood volume of the untrained participants. Endurance-trained athletes may therefore be at an optimal blood volume, which perhaps approaches the limits of its diastolic reserve capacity (6). Therefore, it is quite understandable to believe that the benefits of blood doping (1) in this group are primarily dependent on the increase in total haemoglobin content. However, to argue that blood volume has little influence on maximal oxygen consumption is erroneous and is contrary to the existing literature including that of Dr. Kanstrup.

Darren E. R. Warburton

Faculty of Physical Education and the Division of Cardiology University of Alberta

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REFERENCES

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