Early Repolarization and Arrhythmia Death

We appreciate the acknowledgement that the foreign gas rebreathing method is highly accurate for measuring the effective cardiac output (the blood flow to ventilated lung). This technique and its development are reviewed in great detail in an outstanding paper by Laszlo (5). Investigators at our institution, UT Southwestern, developed the most common modification of this technique, which made it easy to use in exercising humans (6). We have further enhanced the technique by allowing the participant to determine his/her own inspiratory volume. The inert and soluble gases are inhaled directly from an upstream regulator through a turbine flow meter that directly measures inspiratory volume. This modification is described in great detail in the publication by Jarvis et al. (7), where it was validated against invasive techniques, including both direct Fick and thermodilution. During exercise, the normal increase in tidal volume (and respiratory rate) that occurred in these and most individuals allowed them to inspire an adequate tidal volume and mix the air in the bag with the air in the lung within 2 or 3 breaths. The typical error for repeat measurements of cardiac output during exercise in our laboratory is 5%. Thus, we feel that any change in venous return solely as a result of the rebreathing technique is likely to be small. Moreover, this technique was identical over all conditions, thus making the comparisons between conditions valid. Of note, our technique is similar although not identical to the technique used to differentiate APC versus TCPC Fontan mentioned earlier (3). However, as stated in our paper, this method of cardiac output measurement should be studied further in patients with congenital heart disease. In our analysis of the data, we performed both the Bonferroni and Scheffe methods for multiple comparisons. With regards to minute ventilation, the results were identical using both methods. Given our study size, we believe that it was most appropriate to use conservative methods for testing, and we would need a larger study to more fully assess additional changes in respiratory patterns. We appreciate the opportunity to further discuss our study, and it will serve as a springboard for additional studies evaluating the complex physiology of the Fontan circulation both in our laboratory as well as those of others.