Control of breathing during acute change in cardiac preload in a patient with partial cardiopulmonary bypass

We recently had the opportunity to investigate the ventilatory effects of changing the rate of venous return to the heart (and thus pulmonary gas exchange) in a patient equipped with a venous-arterial oxygenated shunt (extracorporeal membrane oxygenation (ECMO) support). The presence of the ECMO support provided a condition wherein venous return to the right heart could be increased or decreased while maintaining total aortic blood flow and arterial blood pressure (ABP) constant. The patient, who had received a heart transplant 12 years ago, was admitted for acute cardiac failure related to graft rejection. The clinical symptomatology was that of right heart failure. We studied the patient on the 4th day of ECMO support, while she was breathing spontaneously. The blood flow diverted through the ECMO system represented 2/3 of the total aortic flow (4 l min⁻¹). With these ECMO settings, the baseline level of ventilation was low (3.89 ± 0.99 l min⁻¹), but PET_CO2$\text{PE}{\text{T}}_{\text{C}{\text{O}}_2}$ was not elevated (37 ± 2 mmHg). When Pa_CO2$\text{P}{\text{a}}_{\text{C}{\text{O}}_2}$ in the blood coming from the ECMO was increased, no stimulatory effect on ventilation was observed. However, when the diversion of the venous return to the ECMO was stopped then restored, minute ventilation respectively increased then decreased by more than twofold with opposite changes in PET_CO2$\text{PE}{\text{T}}_{\text{C}{\text{O}}_2}$. These maneuvers were associated with large changes in the size of the right atrium and ventricle and of the left atrium. This observation suggests that the change in venous return affects breathing by encoding some of the consequences of the changes in cardiac preload. The possible sites of mediation are discussed.