This study has shown that a predictable change in CWD occurs during the recovery from ventilatory failure in infants with severe LTB. The pattern of CWD consisted of asynchronous displacement of the chest wall and abdominal compartments during inspiration, resulting in a decrease in the Ve, and a decrease in Vt, principally through the loss of the chest wall excursion during inspiration. The chest wall motion, measured as Irc, was paradoxic to the abdomen in the most severe state, but with resolution of the clinical illness, the motion became temporally displaced rather than paradoxic. In this study, the progression of the CWD was quantitatively associated with an increasing phase angle, thus indicating that both the increase in asyn-chrony of chest wall motion (phase angle) and the decrease in chest wall displacement (Irc) contributed to alveolar hypoventilation. This is in agreement with previous observations and clinical scores where deterioration in the clinical condition was associated with more obvious inward movement of the lower part of the chest wall. The loss of the chest wall contribution led to a fall in Vt, a fall in Ve, and thus alveolar hypoventilation with elevated tcPco2.
The two previous studies reported have not found a direct relationship between the phase angle and the tcPco2. The different results in this study may be due, in part, to the narrow age range studied (7 to 13 months), leading to a very homogeneous group of previously healthy infants with a single disease process. The observations were homogeneous with the six infants demonstrating a similar response in phase angle and rib cage inductance to a fall in the alveolar ventilation (Fig 5). These results were in contrast to the study of Sivan et al who studied 30 infants, only 8 of whom had LTB, and were examined short-term following the administration of racemic epinephrine (Vaponefrin). Similarly, the group of infants studied by Allen et al were between 2 and 13 months, had chronic lung disease, and were assessed within 15 to 20 min following administration of a bronchodilator.
Furthermore, under conditions of respiratory failure, the intercostal muscles may well be fatigued, such that the compliance of the chest wall (Cw) will be principally determined by the passive characteristics. In turn, the passive Cw is more likely to be an age-determined factor, although evidence to support this conjecture is minimal. In this manner, the close relationship between phase angle and tcPco2 is most likely a (unction of the group selection rather than a generally applicable statement.
The analysis of relative motion of the two compartments using a Lissajous figure allows a rapid means of measuring the degree of asynchrony. Because the motion included in the Lissajous figure is dependent on the time relationship between two sine waves, it is an appropriate means of rapidly assessing the timing of chest wall movement and the vector. When there is inward displacement of the chest wall throughout the whole of the inspiratory phase, then the loop changes quadrant, the phase angle is between 90° and 180°, and this change can quickly be recognized using either an oscilloscope or X-Y plot. Previous authors have noted that, despite the limitation that the Irc and Iabd displacement curves do not fit as perfect sine waves, the error in using the Lissajous loop for analysis is small. Our own work confirms this observation in that the comparison of the two different methods of analysis, only one of which is dependent on the assumption of a sine wave, demonstrates there is no significant error associated with the phase angle by measurement.
Figure 5. The separate analysis, by patient, of the changes in the inductance of the rib cage (upper panel), and phase angle (lower panel), with changes in the alveolar ventilation (transcutaneous carbon dioxide tension [tcPcoj). Each infant is represented by a different symbol.