Letter by Tomczak and Haykowsky Regarding Article, “Discrepancy Between Cardiac and Physical Functional Reserves in Stroke”
To the Editor:
Jakovljevic et al1 recently reported that peak cardiac output was not significantly different between patients with stroke and healthy control subjects despite a 31% lower peak oxygen uptake (V̇O2) in patients with stroke. The authors1 attributed the latter finding to a reduction in muscle function and further suggested that rehabilitation interventions should target such dysfunction. A reduction in peak V̇O2 is typical in chronic stroke, and thus improving V̇O2 should notably be of interest for stroke rehabilitation.
First, contrary to the authors' interpretation of our previous publication,2 we assessed peak cardiac output in 7 of 10 subjects, not 4 as the authors stated. The authors question our finding that cardiac output truly was different at peak exercise between patients with stroke and control subjects. Further corroborating our finding that cardiac output was lower in patients with stroke, we found that a lower peak V̇O2 was associated with slower postexercise V̇O2 recovery kinetics (R=−0.72, P<0.001), which supports the likelihood that an O2 availability limitation existed in our patient group. Moreover, heart rate reserve was lower by 31 beats/min in our patient group and likely reflects some degree of β-receptor downregulation that is typical in cardiac deconditioning/dysfunction.3 Based on the Fick principle, heart rate contributes substantially to cardiac output. Thus, even if stroke volume reserve was preserved in patients with stroke (which we found it was not), our finding of a significantly lower peak and reserve heart rate2 would conceivably still cause a substantial reduction in peak and reserve cardiac output.
Lastly, our finding of a 41% reduction in minute ventilation reserve that was attributable to impaired tidal volume reserve in our patient group should not be ignored2 and further highlights that both cardiac and ventilatory mechanisms contribute to the reduction in peak V̇O2 reported in patients with stroke.
Although the authors indicated their study limitations related to different exercise modalities and cardiac output estimation methods used between groups,1 the effect that these differences have on the strength of the authors' data interpretation and subsequent conclusions cannot be understated.
In the authors' study,1 patients with stroke and control subjects completed peak exercise differently (ie, cycle ergometry versus treadmill). Furthermore, data that could provide additional insight about how “maximal” the exercise actually was for control subjects were not provided. We found a greater reduction (43% lower) in peak V̇O2 for patients with stroke2 than the authors report (31%)1; does this reflect a difference in exercise capacity for participants between studies or is it an effect of the different exercise protocols used by Jakovljevic et al?1 We suspect it is the latter.
The authors estimated cardiac output with different methods between the groups (ie, bioreactance versus CO2 rebreathe).1 Despite the inherent error of each method, the study design “benefit” of systematic error from using only one method that would be present in both groups is lost. The authors ' findings and conclusions are confounded by using different methods for cardiac output estimation. Subsequently, the authors' recommendation for rehabilitation strategies “targeting muscle oxygen uptake” in light of their statement that “cardiac function and pumping capability are maintained”1 warrants caution, because we have found that peak and reserve cardiovascular function may indeed be impaired in patients with chronic stroke.2
We strongly suggest that stroke rehabilitation should integrate exercise therapy that targets cardiorespiratory, peripheral vascular, and skeletal muscle function (not just skeletal muscle function as the authors' suggest1) to improve peak V̇O2 and functional ability.
Corey R. Tomczak, PhD
Department of Speech Pathology and Audiology
University of Alberta
Edmonton, Alberta, Canada
Mark J. Haykowsky, PhD
Faculty of Rehabilitation Medicine
University of Alberta
Edmonton, Alberta, Canada
Stroke welcomes Letters to the Editor and will publish them, if suitable, as space permits. Letters must reference a Stroke published-ahead-of-print article or an article printed within the past 3 weeks. The maximum length is 750 words including no more than 5 references and 3 authors. Please submit letters typed double-spaced. Letters may be shortened or edited. Include a completed copyright transfer agreement form (available online at http://stroke.ahajournals.org and http://submit-stroke.ahajournals.org).
- © 2012 American Heart Association, Inc.
- Jakovljevic DG,
- Moore SA,
- Tan LB,
- Rochester L,
- Ford GA,
- Trenell MI
- Tomczak CR,
- Jelani A,
- Haennel RG,
- Haykowsky MJ,
- Welsh R,
- Manns PJ
- Brubaker PH,
- Kitzman DW