Left ventricular pressure - Volume and Frank-Starling relations in endurance athletes. Implications for orthostatic tolerance and exercise performance

Background. Endurance athletes have a high incidence of orthostatic intolerance. We hypothesized that this is related to an abnormally large decrease in left ventricular enddiastolic volume (LVEDV) and stroke volume (SV) for any given decrease in filling pressure. Methods and Results. We measured pulmonary capillary wedge (PCW) pressure (Swan-Ganz catheter), LVEDV (two-dimensional echocardiography), and cardiac output (C₂H₂ rebreathing) during lower body negative pressure (LBNP, -15 and -30 mm Hg) and rapid saline infusion (15 and 30 ml/kg) in seven athletes and six controls (V̇O₂max, 68 ± 7 and 41 ± 4 ml/kg/min). Orthostatic tolerance was determined by progressive LBNP to presyncope. Athletes had steeper slopes of their SV/PCW pressure curves than nonathletes (5.5 ± 2.7 versus 2.7 ± 1.5 ml/mm Hg, p<0.05). The slope of the steep, linear portion of this curve correlated significantly with the duration of LBNP tolerance (r = 0.58, p = 0.04). The athletes also had reduced chamber stiffness (increased chamber compliance) expressed as the slope (k) of the dP/dV versus P relation (chamber stiffness, k = 0.008 ± 0.004 versus 0.031 ± 0.004, p<0.005; chamber compliance, 1/k = 449.8 ± 283.8 versus 35.3 ± 4.3). This resulted in larger absolute and relative changes in end-diastolic volume over an equivalent range of filling pressures. Conclusions. Endurance athletes have greater ventricular diastolic chamber compliance and distensibility than nonathletes and thus operate on the steep portion of their Starling curve. This may be a mechanical, nonautonomic cause of orthostatic intolerance.