UBC Theses and Dissertations

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UBC Theses and Dissertations

Sex differences in left ventricular mechanics in response to acute physiological stress Williams, Alexandra Mackenzie


Left ventricular (LV) mechanics, including rotation, twist and strain, specifically describe the patterns of myocardial deformation across the cardiac cycle. The twisting motion of the ventricle results from the contraction of helically oriented LV myofibres, and contributes to efficient systolic and diastolic function. Left ventricular mechanics are highly sensitive to alterations in preload, afterload and contractility, and therefore are profoundly altered when the heart is acutely stressed. While sex-related differences in LV structure and hemodynamics have been established, there is a paucity of data comparing LV mechanics between males and females. This thesis therefore investigated sex differences in LV mechanics in response to acute physiological stress. In Chapter 3, LV preload was reduced during progressive lower body negative pressure (LBNP). LV twist increased in both sexes during LBNP, but females had greater LV apical rotation and twist during -60 mmHg, and these sex differences may have resulted from differences in adrenergic stimulation, vagal withdrawal or LV geometry. In Chapter 4, LV mechanics were compared between the sexes, during increases and reductions to adrenergic stimulation with post exercise ischemia (PEI) and ß₁-adrenergic receptor (ß₁-AR) blockade, respectively. LV twist was not altered in either of the sexes during PEI, likely due to concomitant increases in LV afterload. However, during ß₁-AR blockade, LV apical rotation and twist were reduced in males, but not females, suggesting that LV twist mechanics in males are more sensitive to altered adrenergic stimulation. Chapter 5 then assessed the potential contribution of vagal autonomic control to sex differences in LV mechanics during LBNP and submaximal exercise. During -60 mmHg LBNP, females had greater LV apical rotation and twist than males, and these differences persisted during vagal blockade with glycopyrrolate. LV rotation and twist increased in both sexes during exercise up to 50% peak power, but were not different between the sexes in the control or blocked conditions. While sex differences in LV mechanics may not result from differences in cardiac autonomic control, it is possible that variations in other key regulators of LV mechanics, including LV myofibre configuration and chamber structure, determine sex differences in LV twist mechanics.

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