Ventilatory responses to cardiac output changes in patients with pacemakers

Cardiac output changes were induced by step changes of heart rate (HR) in six patients with cardiac pacemakers during monitoring of ventilation and gas exchange, breath-by-breath. Mean low HR was 48 beats/min; mean high HR was 82 beats/min. The change of oxygen uptake immediately after the HR change was used as an index of altered cardiac output. After HR increase, oxygen uptake (V02) rose by 34 +/- 20% (SD), and after HR decrease, Vo2 fell by 24 +/- 11%. There was no change in arterial blood pressure. After HR increase, ventilation increased, after a mean delay of 19 +/- 4 s; after HR reduction, ventilation fell, after a mean delay of 29 +/- 7 s. In the period between HR increase and the resulting increase in ventilation, end-tidal PCO2 (PETCO2) rose by 2.6 +/- 2.0 Torr, and in the period between HR decreases and the fall in ventilation, PETCO2 dropped by 2.9 +/- 2.2 Torr. The response time and end-tidal gas tension changes implicate the chemoreceptors in the reflex correction of blood gas disturbances that may result from imbalances between cardiac output and ventilation.