Hemodynamic effects of spontaneous breathing in the post-operative period

Backround: During mechanical ventilatory support, spontaneous breathing has been linked to improved hemodynamics. These findings may be explained by a decrease in intrathoracic pressure which may improve venous return to the heart. Such a mechanism should result in a dose-response relation between the amount of spontaneous breathing and an increase in the global end-diastolic volume (GEDV) and cardiac output (Q(t)). Methods: To test this hypothesis, 15 patients were studied after major elective surgery during weaning from mechanical ventilation using bilevel positive airway pressure (BIPAP). BIPAP allows unrestricted spontaneous breathing during every phase of the respiratory cycle. Thus, ventilatory support was modified by changing the mechanical respiratory rate only, whereas inspiratory airway pressure and PEEP were kept constant. GEDV and Q(t) were measured by transpulmonary thermodilution. Results: GEDV (P = 0.055), stroke volume (P = 0.027) and subsequently also Q(t) (P < 0.001) increased when spontaneous breathing increased. In contrast, no difference was observed for central venous pressure (P = 0.19). Conclusion: The beneficial hemodynamic effects of spontaneous breathing during mechanical ventilatory support can partially be explained by improved venous return to the heart which increases stroke volume and Q(t).