Open letter to the editors of my favorite exercise physiology textbooks

Dear editors of my favorite exercise physiology textbooks,

I really enjoy reading your textbooks. The information is clear and concise, the chapters are usually well-written with abundant beautiful figures.

This being said, two things really annoy me. First, the section related to cerebrovascular physiology is usually really small microscopic. When there is one, the usual sentence appears: “there is no change in brain blood flow during exercise”. Dude, this.is.not.true!!!1!! There is now plenty of evidence in the literature describing changes in brain blood flow during exercise.

Please, you need to update my favorite exercise physiology textbooks so I will stop hyperventilating when reading the section related to cerebrovascular physiology, which will preclude a reduction in PaCO2 and the subsequent lowering in brain blood flow…

I hope that you will consider my request,

Regards,

p

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Manipulation of LVAD pump speed during exercise in patients with heart failure: what are the consequences on cardiac output and peripheral blood flow?

The left ventricular assist device (LVAD) is utilized as a bridge to recovery or transplantation and is now increasingly used as destination therapy in patients with advanced heart failure. Even if cardiac output and the capacity to perform exercise are comparable between pulsatile and continuous-flow LVADs, the latter is preferred because of its longer life span and its implantation is associated with lower incidence of stroke and comparable, or even improved, quality of life vs. pulsatile LVADs.

Since continuous-flow LVADs do not work in an automatic mode as pulsatile LVADs, it cannot modulate cardiac output during exercise. This could become an obstacle for patients who would like to exercise (even if it is just climbing stairs). So, we decided to manipulate the continuous-flow LVAD pump speed during exercise in order to see if it could elevate cardiac output, the perfusion of the legs and the brain, and eventually enhance the capacity to perform exercise.

Eight patients with end-stage heart failure provided with a continuous-flow LVAD participated to this study. They randomly performed two incremental exercise protocols (one exercise protocol was performed with the usual fixed LVAD pump speed set at rest (range: 9400 to 10 200 rpm) and the other exercise protocol was performed with increasing pump speed (400 rpm per exercise stage) on a modified cycle ergometer). During these two exercise protocols, we measured numerous variables such as cardiac output, leg blood flow (in 3 patients) and cerebral blood flow and oxygenation (well, we measured a lot more variables that I won’t refer to in this post). Of note, we did not have access to breath-by-breath gas exchange analysis so exercise capacity was measured by maximal workload and exercise time.

The figure below shows the main results of the study:

Even if the LVAD pump speed was kept constant (open circles in the figure), cardiac output and perfusion of the legs increased significantly during strenuous cycling exercise. Cerebral perfusion, however, was rather low at rest (80% of the perfusion observed in normal individuals) and it did not increase with exercise (Brain blood flow velocity measured in the middle cerebral artery usually increases up to moderate intensity exercise.  At higher exercise intensities, it levels off, at around 50-60% of maximal exercise, and then decreases during heavy exercise).

A moderate increase in LVAD pump speed during exercise did not improve exercise tolerance. However, the increase in pump speed (filled circles in the figure) was associated with higher cardiac output and improved cerebral perfusion during light exercise (i.e. at an exercise intensity close to the intensity of everyday life activities). Consequently, the rather low cerebral perfusion observed in these patients increased to a small extent during exercise.

A lot of research will be necessary to better understand the influence of a change in LVAD pump speed on cardiac output and organ blood flow during exercise. In light of these pilot results, our research team considers that it could be advantageous during exercise to increase the pump speed of continuous-flow LVADs or at the very least, these results support maintaining interest in improving automaticity of continous-flow LVADs!

Reference:

Brassard, P., Jensen, A. S., Nordsborg, N., Gustafsson, F., Møller, J. E., Hassager, C., Boesgaard, S., et al. (2011). Central and peripheral blood flow during exercise with a continuous-flow left ventricular assist device: constant versus increasing pump speed: a pilot study. Circulation. Heart failure, 4(5), 554–560. doi:10.1161/CIRCHEARTFAILURE.110.958041