Influence of breathing frequency and the tidal volume on cardiac output

The aim of our experiment was to investigate the influence of increasing either breathing frequency or tidal volume on cardiac output (Q̇), in normocapnia. We measured Q̇ with a CO2 rebreathing method in 6 men and 6 women in the sitting and the supine position, imposing different breathing patterns: in one set of experimental tidal volume was kept constant at 1 L while breathing frequency was randomly changed between 20, 30 and 40 breaths/min; in another breathing frequency was kept constant at 30 breaths/min while tidal volume was randomly altered between 1, 1.5 and 2 L. Switching from open circuit breathing to rebreathing (for measurement of Q̇) required no change in breathing pattern. From the beginning, CO2 was added to the inspired gas to maintain end-tidal FCO2 at 0.054, so as to obtain steady state conditions throughout the measurements. Q̇ rose significantly when tidal volume was increased (938 ml/L rise in tidal volume when sitting, and 743 ml/L when supine). Breathing frequency had an insignificant effect (213 ml/10 breaths frequency increase when sitting and 142 ml/10 breaths when supine). The greater influence of ventilation on Q̇ when sitting than when supine is best explained by the fact in the latter position venous return is already high. There are no demonstrable differences in this effect between males and females.