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

The “ventilatory baroreflex” phenomenon describes the association between hemodynamic and respiratory changes, as the association of an increase in heart rate (HR) and a decrease in blood pressure (BP) with an increase in the tidal volume (V_T). We investigated the hypothesis that changes in lung blood pool directly affect lung compliance (C_L) and mediate this “reflex”. The pulmonary blood pool was modulated by inducing a slowly progressing pneumothorax (PTX) in mechanically ventilated rabbits (n=7). The progressing PTX was associated with immediate progressive decline in BP and increase in HR. A decrease in EtCO₂ was observed at the initial phase of PTX, concurrent with a gradual increase in V_T, increase in the HR and decrease in the BP, mimicking the ventilatory reflex. This counterintuitive "improvement" in ventilation resulted from an increase in C_L, as the ventilation pressure was constant. The study strongly supports the suggested hypothesis. C_L is highly sensitive to changes in lung blood pool, and this phenomenon explains the tight dependence of the respiratory effort on changes in the cardio-vascular system.