Impedance Cardiography Accurately Measures Cardiac Output During Exercise in Children With Cystic Fibrosis ( Discussion)
This study has demonstrated that thoracic electrical bioimpedance using the ICG-M401 provides simple, accurate, reliable measurements of Q during exercise in children with CF over a wide range of severity of airflow limitation. Edmunds et al studied ICG in healthy subjects breathing with external resistive loads during exercise and concluded that the method was not influenced by the presumed greater pleural pressure swings and chest wall excursions caused by loaded breathing. Results of the present study support and extend their observations. Chest wall configuration, another factor that could conceivably alter thoracic electrical bioimpedance measurements, likewise did not seem to alter the accuracy or reliability of impedance determination of Q. Although not formally tested in pediatric patients with other diseases characterized by airway obstruction such as asthma or bronchopulmonary dysplasia, one should have no reservations in generalizing findings in the present study to children with these disorders.
Technical difficulty arose in obtaining signals sufficiently free of artifact to yield consistent results. Stroke volume appeared to fall in a few patients with minimal spirometric abnormalities in heavy, compared with light, exercise. Although the proprietary software did not “ensemble-average” the impedance waveforms, this was not necessary and, indeed, was less desirable than a feature that allowed exclusion of noise-ridden data rather than averaging such signals with relatively noise-free results. Ensemble averaging of such signals would have likely resulted in acceptance of such an apparent fall in stroke volume in these patients as a true measurement, when there was no physiologic basis for this finding. The ICG-M401 allows determination of Q based on cardiac cycles over 8- or 16-s intervals and stores raw data collected over the chosen interval. The frequency of measurements per minute is largely dependent on the capability of the personal computer to which the ICG-M401 is connected, but up to two determinations per minute are possible. Based on the initial experience with healthy control children, the 8-s interval once per minute was found to give very acceptable results for comparison purposes. Qicg values obtained each minute during the interval over which Qrb was being measured in duplicate were averaged. However, problems were encountered with the indirect Fick (C02) method and resulted in unsuccessful measurement on a few occasions, related to blood sampling and to obtaining acceptable rebreathing maneuvers.
The potential applications of this technique open new doors to investigating exercise pathophysiology in children with obstructive lung disease. Q can now be measured rapidly without disturbing the subject, allowing minute-by-minute measurements during progressive exercise, obviating the need for blood sampling. Thus, one will be able to characterize the
stroke volume response to exercise more readily and draw conclusions on what factors limit exercise performance in patients with CF. Faster computers permit calculation of Q up to four or five times per minute, permitting one to study the kinetics of the Q response to exercise in children with lung disease and compare this with healthy subjects. The effects of loaded or unloaded breathing on Q can now be studied to assess the mechanical effect of airway obstruction on cardiac performance, without requiring the lungs to measure Q. The technique is even simpler to use at rest, and though no comparisons of impedance and indirect Fick (C02) measurements at rest were done, there should be little hesitation in extrapolating the conclusions of the present study to resting conditions. ICG can also measure other valuable parameters besides Q, such as systolic time intervals and indexes of diastolic function that may provide clues to putative changes in myocardial function in CF.
In conclusion, this study has demonstrated that ICG employing the ICG-M401 gives accurate and reproducible measurements of Q during exercise in children with CF, extending our previous findings in healthy children. The rapid, noninvasive method has the added advantage of providing accurate measurements at rest, and thus readily lends itself to a variety of clinical and research applications in the pediatric population.