The effects of inspiratory intrathoracic pressure production on the cardiovascular response to submaximal exercise in health and chronic heart failure

We sought to determine whether the normal inspiratory intrathoracic pressures (P_ITP) produced during exercise contribute to the blunted cardiac output and locomotor limb blood flow responses observed in chronic heart failure (CHF). Five chronically instrumented dogs exercised on a treadmill at 2.5 mile/h at 5% grade while healthy or after the induction of tachycardia-induced CHF. We observed several key differences in the cardiovascular responses to changes in the inspiratory P_ITP excursion between health and CHF; namely, 1) removing ∼70% of the normally produced inspiratory P_ITP excursion during exercise (with 15 cmH₂O inspiratory positive pressure ventilation) significantly reduced stroke volume (SV) in healthy animals by 5 ± 2% (P < 0.05) but significantly increased SV and cardiac output (Q_TOT) in animals with CHF by 5 ± 1% (P < 0.05); 2) doubling the magnitude of the inspiratory P _ITP excursion had no effect on SV or Q_TOT in healthy animals but significantly reduced steady-state Q_TOT and SV in animals with CHF by -4 ± 3% and -10 ± 3%, respectively; 3) removing the majority of the normally produced inspiratory P_ITP excursion had no effect on blood flow distribution in healthy animals but increased hindlimb blood flow (9 ± 3%, P < 0.05) out of proportion to the increases in Q_TOT; and 4) the only similarity between healthy and CHF animals was that increasing the inspiratory P_ITP excursion significantly reduced steady-state locomotor limb blood flow by 5 ± 2% and 6 ± 3%, respectively (P < 0.05 for both). We conclude that 1) the normally produced inspiratory P_ITP excursions are required for a maximal SV response to submaximal exercise in healthy animals but detrimental to the SV and Q _TOT responses to submaximal exercise in CHF, 2) the respiratory muscle ergoreflex tonically restrains locomotor limb blood flow during submaximal exercise in CHF, and 3) excessive inspiratory muscle work further compromises cardiac function and blood flow distribution in both health and CHF.