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ARTICLES: Ellen A. Dawson, Mark A. Black, Jennifer Pybis, N. Timothy Cable, and Daniel J. Green The impact of exercise on derived measures of central pressure and augmentation index obtained from the SphygmoCor device J Appl Physiol 2009; 106: 1896-1901 [Abstract] [Full text] [PDF]

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The impact of methodology on derived measures of central pressure during exercise

James E Sharman, James H Hull, Alberto Avolio.   (2 June 2009)

The impact of methodology on derived measures of central pressure during exercise 2 June 2009
James E Sharman, Senior Research Fellow Menzies Research Institute, University of Tasmania, James H Hull, Alberto Avolio. Send letter to journal: Re: The impact of methodology on derived measures of central pressure during exercise James.Sharman{at}menzies.utas.edu.au James E Sharman, et al.	In 8 men, Dawson and colleagues[1] recorded radial artery pressure waveforms non-invasively in both arms after one handed hand-grip exercises designed to change local (ie forearm) vascular tone, but without altering heart rate or brachial blood pressure. They reported that central hemodynamic indices (augmentation index, AIx, and systolic blood pressure, SBP) derived from the radial pressure waveforms using a commercial transfer function (SphygmoCor) were significantly different when estimated from the exercising and non-exercising arms. From this it is concluded that exercise may compromise the validity of the transfer function to derive central hemodynamics from the radial pulse. In order to substantiate this conclusion it would be necessary to have a study design in which mean arterial pressure (the major confounding variable of AIx)[2] was not different between the exercising and non- exercising arms, when the radial waveforms were acquired. This is not the case in the data presented.[1] Post-exercise mean arterial pressure was significantly higher for the exercising arm compared with the non- exercising arm. Indeed, the difference of 2-3 mmHg is close to the difference of around 3 mmHg found for central SBP. It is therefore not surprising that the derived AIx (and central SBP) in the exercising arm is higher than the non-exercising arm. Importantly, radial AIx (which does not require a transfer function) follows the same within-group pattern as the derived central AIx and, also as expected, is significantly different between the exercising and non-exercising arms. So the reason why central AIx is greater when assessed from the exercising arm is not as unclear as the authors suggest. The higher blood flow in the exercising arm suggests peripheral dilatation and reduction of wave reflection. This has the effect of reducing the initial peak value of the radial pulse while maintaining the same value of the second shoulder, hence producing a calculated higher peripheral AIx. This suggests some difference in the measured radial pulse waveform in the exercising and non-exercising arms. To attribute these differences to problems with the transfer function in this context is not entirely justified. Even so, the difference in central SBP was <3%, despite substantial vasodilation in the exercising arm. Heart rate (another confounder of AIx)[3] was also significantly higher for the exercising arm, although not to such a level as to offset the concomitant rise in AIx. Altogether the data suggest that there is a time differential between the point at which hand-grip exercises were ceased and the point at which radial waveforms were recorded in the exercising arm compared with the non-exercising arm. Higher mean arterial pressures and heart rates in the exercising arm imply that radial waveforms in the non-exercising arm were acquired at a later time. The protocol of only measuring blood pressure in the non-exercising arm immediately after exercise may be the source of the problem – how could radial tonometry be measured at the same time point? The capability of generalized transfer functions to accurately estimate central hemodynamic variables from the peripheral pulse may, indeed, be questionable at higher intensity exercise.[4] Certainly, the rationale for the study by Dawson et al[1] is interesting and novel. However, we agree with their concession that simultaneous recording of radial waveforms in both arms would have been a better experimental design. Dr James E Sharman Menzies Research Institute University of Tasmania, Hobart, 7000 Australia. Dr James H Hull Faculty of Health and Social Care Sciences, Kingston University and St George’s, London, SW17 0RE United Kingdom. Professor Alberto Avolio Australian School of Advanced Medicine Macquarie University, Sydney, 2109 Australia. References 1. Dawson EA, Black MA, Pybis J, Cable NT, Green DJ. The impact of exercise on derived measures of central pressure and augmentation index obtained from the SphygmoCor device. J Appl Physiol. 2009;91564.92008. 2. Nichols WW, O'Rourke MF. McDonald's blood flow in arteries: Theoretical, experimental and clinical principles. 2005; London: Hodder Arnold. 3. Wilkinson IB, MacCallum H, Flint L, Cockcroft JR, Newby DE, Webb DJ. The influence of heart rate on augmentation index and central arterial pressure in humans. J Physiol. 2000;525:263-270. 4. Sharman JE. New insights into cardiovascular risk from the exercise central waveform. Artery Research. 2009;2:132-137. Conflict of interest: Dr Sharman has research collaborations with AtCor Medical.