Before discussing the obtained results, two points should be clarified.
First, despite the reported linear relationship between diaphragmatic excursion and inspired volumes , it was also suggested that diaphragm movements, measured by ultrasonography, poorly reflect the pulmonary function . The explanation that various inspiratory volumes are measured for the same diaphragmatic excursion, is not evidence-based.
Second, the point of the radiographically determined normal position of the right hemidiaphragm at the level of the anterior sixth rib, appears to originate from a single study [9, 10]. An obstacle for such a way of referencing a diaphragm position is a poor visibility of costal portions of the anterior ribs. Thoracic spine has also been used as a reference point, but without validation in population studies [11, 12].
Having in mind these limitations, our method, based on the diaphragm apex as a determinant of the diaphragm position, could also be put into question because of use of the postero-anterior projection only, without analysis of movements in the lateral projection. However, studies that analysed both diaphragm apex and costophrenic angle movements, showed that both movements were synchronous and followed a linear relationship , thus justifying our method of measurement.
Finally, related to the method of the postoperative lung function prediction, although it was demonstrated that both perfusion scintigraphy and Juhl-Frost formula may correlate well with the observed postoperative FVC and FEV1, the superiority of calculation by using a perfusion scintigraphy was clearly demonstrated . We used the Juhl-Frost method because Nakahara formula is not suitable for pneumonectomy and because the primary study end point was a diaphragm motion, not the prediction method itself. We routinely use perfusion lung scintigraphy in patients with moderate and severe COPD, that was not a case in a subset of patients with a pneumonectomy in the present study.
In the present study, preoperative diaphragm amplitudes determined by ultrasound are in the range of those determined in other studies being 6 to 7 cm, 6 ± 0.7 cm or 6.8 ± 0.8 cm . However, preoperative diaphragm amplitudes differed both depending on the measurement method and diaphragm side. Ultrasonographically measured amplitudes were significantly higher vs. radiographically determined ones (4.28 ± 2.13 cm and 6.51 ± 2.28 cm) only on the diseased side. There are no literature data to compare these results. The probable explanation of the obtained differences is the fact that the reference points for registering diaphragm movements were different, as described in the methods section. In fact, the current study design did not anticipate amplitudes to closely correspond to each other, but to assess their eventual influence to the lung function prediction.
Similarly, it can only be speculated why these differences were smaller on the contralateral side (4.58 ± 1.99 cm vs. 3.67 ± 1.52 cm).
On the other hand, differences in side-to-side diaphragmatic motion are more analysed – ultrasonographically measured values outside the range of 0.5 to 1.6 for the right-to-left ratio of maximal excursion on deap breathing should be considered as abnormal . It can explain our ultrasonographically measured amplitudes on the diseased and contralateral side being 6.51 ± 2.28 cm vs. 3.67 ± 1.52 cm. Difficulties in left hemidiaphragm visualisation are usually regarded as possible cause of these side-to-side differences. So, in one study, the diaphragmatic motion of the left hemidiaphragm was recorded in only 45/210 (21%) subjects . Another study failed to record left hemidiaphragm excursion in 15/23(65%) volunteers . This because the left hemidiaphragm may be obscured by the expanding lung during deep breathing and the position of the probe may not be readily adjusted as the spleen window is small.
The ultrasonographic side-to-side amplitude differences were more pronounced compared with those measured radiographically - 4.28 ± 2.13 cm vs. 4.58 ± 1.99 cm. Although the relevance of the side-to-side diaphragmatic motion comparison has been noted in fluoroscopy studies , there are no available literature data trying to explain it.
As expected, after the lung resection, both radiographically and ultrasonographically measured diaphragm amplitudes of the diseased side decreased. On the opposite side, the same trend existed only when amplitudes were determined radiographically. When assessed by ultra sound, postoperative amplitudes on the non-tumour bearing side were higher compared with preoperative ones.
Concerning postoperative percent change of preoperative and postoperative amplitudes, in relation to preoperative values, both methods followed the same trend of postoperative amplitude decrease on the diseased side - 54.3 ± 16.4% and 23.3 ± 28.9% decrease respectively when assessed radiographically and ultrasonographically. As expected, on the non-tumour bearing side, there was no difference between preoperative and postoperative diaphragm amplitudes, independently on the used method.
Concerning the primary end point of this study - influence of the diaphragm movements to discrepancy between the predicted and actual postoperative lung function, it is evident that some aspects of the diaphragm motility may significantly contribute to the lung function prediction, but not as independent factor.
Absence of significant influence of the extent of resection to the lung function prediction is important for practice. A lung function prediction may be more delicate if a lobectomy is anticipated, with several different methods of the lung function prediction being in use, as opposed to a very simple calculation before pneumonectomy by using a perfusion lung scintigraphy . However, the small number of patients with pneumonectomy in the analysed group does not allow firm conclusions about the influence of the extent of resection.
As presented, different ways of expressing diaphragm amplitudes were used in attempt to assess eventual correlation with the lung function prediction. In our study, a significant correlation between diaphragm movements and lung function prediction existed only if the diaphragm movements were presented as 1) diaphragm amplitude as a percentage of the preoperative apex-base distance in inspiration, or 2) as a difference between preoperative and postoperative ipsilateral diaphragm amplitudes. These facts, together with a significant influence of a patients’ height to the lung function prediction, support the need to express the amplitudes of the diaphragm movements in relation to some fixed distance or to take into consideration the loss in diaphragm amplitudes, rather than to correlate absolute values of amplitudes. The exception are emphysema patients, in whom magnetic resonance revealed smaller mean excursions than in control subjects and movements of the ventral portion of the diaphragm in paradox to the change in lung area . Such a bias did not exist in the present study.
Of practical benefit could be our finding that accurate registering of diaphragm movements may predict whether the difference between ppo FEV1 and actual FEV1 will exceed 550 ml.
Beside the limited patient number, one additional point should be clarified. Almost identical results were obtained by the two methods in relation to the mean difference between preoperative and postoperative diaphragm amplitudes of the diseased side, (2.42 ± 1.25 cm measured radiographically, vs 2.11 ± 2.04 cm by ultrasound (p > 0.05). It may be confusing, having in mind significant differences in both preoperative and postoperative diaphragm amplitudes, depending on the used method. In our opinion, the key point is not related to the measurement method, but to the real change in preoperative vs. postoperative diaphragm movement. Such a statement is supported by our results showing a clear difference between preoperative and postoperative amplitudes on diseased side, but only if they were expressed as a percent of the preoperative amplitudes, (54.3 ±16.4% measured radiographically vs 23.3 ± 28.9% measured by ultrasound). Although these differences did not reach the level of statistical significance, they are evident and it is an important achievement of a pilot study, giving a direction for further research.
The identical trend of the obtained results means that the measurement method is not essential.
We are convinced that a limited patient number, together with some methodological inconsistencies that may be attributed to both methods, may influence these results.