Right ventricular function in paced patients : a study using pulsed Doppler ultrasound
There is increasing interest in right ventricular function as an important determinant of cardiac output. However, the chamber is difficult to study, because of its shape and relationship to the left ventricle. Invasive studies, radionuclide studies and two-dimensional echocardiography are all useful approaches, but all have serious limitations. Systolic time intervals, best measured by pulsed Doppler ultrasound in the proximal pulmonary artery, offer one method of assessing right ventricular systolic function. Previous "normal" ranges, however, could be criticised on many counts. I present data from carefully checked normal controls and compare to previous publications, and explore variability and relationships between the various systolic time intervals. Most variables have skewed frequency distributions; the ranges are somewhat wider than previously described; most heart rate corrections are found to have serious limitations; and the effect of age is explored. Complete heart block offers a model to study the the effects of varying atrioventricular intervals whilst the ventricular rate is held unphysiologically steady by an artificial pacemaker. Given the current controversy about the merits of single- versus dual-chamber pacing, the issue is of topical interest also. The effect of varying the "P-R" interval within the physiological range is explored, and "optimal" ranges identified. A curious "nadir" effect, previously unknown, was discovered. When P waves followed paced QRS complexes at about 50-100ms, forward flow into the pulmonary artery (as judged from systolic time intervals) fell in most patients, and in some subjects virtually ceased. As a small included invasive part of the study showed, this was accompanied by falls in RV systolic pressure and rises in right atrial pressure. This study demonstrates that right ventricular systolic time intervals can be used to study right ventricular function in pacing situations, and is further evidence of the unsatisfactory nature of single-chamber ventricular pacing.