Response to letter regarding article, "The effect of selective heart rate slowing in heart failure with preserved ejection fraction" by Pal, N, Sivaswamy, N, Mahmod, M, Yavari, A, Rudd, A, Singh, S, Dawson, D, Francis, J, Dwight, J, Watkins, H, Neubauer, S, Frenneaux, M, Ashrafian, H