The Cardiopulmonary Vascular System and the Ventilatory Reflex; Scientific Merits and Clinical Implications

Introduction: The “ventilatory baroreflex” is not well explained phenomenon, where an increase in heart rate and decrease in blood pressure are associated with an increased tidal volume. We hypothesize that changes in lung blood pool and capillary pressure directly affect lung compliance and play a key role in mediating this “reflex”. The study investigated this hypothesis.

Methods: The pulmonary blood pool was modulated by inducing slowly progressing pneumothorax in mechanically ventilated rabbits (n=7), by continuous air injection into the pleural space. Hemodynamic parameters, tidal pressures and flows, EtCO₂ and SpO₂ were recorded. Tidal volume and respiratory system compliance were calculated.

Results & Discussion: The slowly progressing pneumothorax was associated with immediate progressive decline in the BP and compensatory increase in HR. A counterintuitive decrease in EtCO₂ was observed at the initial phase, concurrent with a gradual increase in the tidal volume (+14.6±5.3%) and respiratory compliance (13.7±5.2%). The respiratory rate and the inspiratory pressure were constant. Therefore, the increase in tidal volume resulted from a gradual increase in lung compliance. Only after 28 min the respiratory indices exhibited the reverse responses, when tension pneumothorax developed.

The initial phase mimics the ventilatory reflex. However, the counterintuitive increase in tidal volume and decrease in EtCO₂ were not due to involvement of the central nervous system, as the rabbits were mechanically ventilated at constant inspiratory pressure. The effect appears with mild pneumothorax, demonstrating the high sensitivity of lung compliance to changes in lung circulation. The opposite occurs in heart failure where the pulmonary capillary blood pool increases, leading to smaller lung compliance and dyspnea.

Conclusions: The “ventilatory reflex” was observed in ventilated animal (without nervous pathway) and it is determined by a direct effect of the pulmonary circulation on lung compliance. Lung blood pool and capillary pressure are important determinants of the cardio-pulmonary "baroreflex".