Title: Sex differences in the human thorax; mechanical cardiopulmonary interactions and exercise capacity
Thesis Supervisor: Dr. A. William Sheel
Committee Member: Dr. Eric Stöhr, Dr. Robert Boushel
Chair:Dr. David Wright
Abstract:
There is evidence that the thorax of the human female has different characteristics vs. males. When matched for torso length females have smaller lungs and the extent of the chest wall is more prismatic than that of males. The smaller diameter of conducting airways in females relative to lung size, termed dysanapsis, is of consequence. A relatively narrower airway diameter causes a disproportionately higher work of breathing (WB) in females for a given ventilation during exercise, after a so-called breakpoint of approximately 60 L.min-1.
The heart lies alongside the lungs within the thoracic cavity. Intrathoracic pressure (ITP) changes are caused by the expansion and recoil of the lungs, powered respiratory muscles that act upon the chest wall. These ITP swings have direct influence on cardiac output (Q̇) via the augmentation of preload. It is unknown if females rely disproportionately on ITP swings for Q̇ production during exercise. Artificially reducing these naturally occurring ITP swings via direct inspiratory pressure assist or administration of a less dense inert backing gas (helium), may reveal any sex differences present the ITP swing reliance of Q̇. It is also important to analyse in detail how respiratory mechanics themselves change during unloaded conditions vs. spontaneous breathing, independent of the effects on cardiac mechanics.
The disproportionate WB in females after the breakpoint, may also impact exercise capacity. Unloaded breathing may reveal a disproportionate improvement in WB and time to exhaustion on a constant load exercise test, in females vs. males. This may be underlined by the disappearance of expiratory flow limitation and exercise induced arterial hypoxemia during unloaded conditions.