Discussion
The motivation for undertaking this study was the striking similarities between the shape and magnitude of the simultaneously recorded Pdi or Pes and M-mode diaphragmatic displacement waveforms (figure 1). A question arises why the tracings obtained with the pressure transducer and the tracing of the diaphragmatic displacement, obtained with the ECHO, show a similar configuration. Pdi and Pes are the consequence of the respiratory muscles’ contraction and relaxation. As we can see in figure 1A,C, Pes and Pdi start, peak and end simultaneously with diaphragmatic displacement. Furthermore, in figure 1B we can see that Pes and diaphragmatic displacement are identical in shape even in the smallest details! Additionally, we can easily understand that the higher the respiratory muscles’ diaphragmatic contraction documented by a higher diaphragmatic displacement, the higher the increase in Pdi and decrease in Pes. In fact, our study provides and compares the values of these three respiratory variables during normal breathing and sniff-like manoeuvres. Diaphragmatic displacement decreased significantly during sniff (from 1.64 and 1.7cm to 1.4 cm), while Pdi increased (from 21±5.8 to 35±8.9) and Pes decreased (from −10.6±3.4 to -31.4±15). The large decrease in Pes during sniff, despite a decrease in diaphragmatic displacement, took place because the respiratory muscles -including the diaphragm- exert a vacuum effect on the thoracic cavity consequent to airway occlusion. Conversely, as the diaphragmatic displacement decreased during sniff, thus not contributing to the increase in gastric pressure, the increase in Pdi may be exclusively attributed to Pes contribution.
As we can see in figures 3 and 4, while breathing unobstructed, each cm of diaphragmatic displacement produces 5.5 cmH2O of pressure in Pdi and −4.5 cmH2O in Pes. Additionally, performing a sniff, each cm of diaphragmatic displacement produces much more Pdi and Pes (14.5 cmH2O and −22 cmH2O, respectively). However, the Δ pressure of Pdi (9 cmH2O) from breathing unobstructed to sniff, is less compared with Δ Pes (−17.5 cmH2O), indicating the absence of diaphragmatic displacement contribution, as a pressure generator to the Pdi. It is interesting to notice that the significant relationship between diaphragmatic displacement, Pes and Pdi is further improved during the sniff. This improvement is probably due to the increased values of pressure produced during a sniff. When we perform statistics with small values, like those of Pes and Pdi pressure during quiet breathing, it is difficult to obtain statistical significance, risking a type one or alpha-statistical error (the statistical significance, although it exists, cannot be found).
A strong correlation has been found between the slope of the relaxation-descending part of the M-mode diaphragmatic displacement waveform and the slope of the Pdi curve during relaxation, over a wide range of Pdi and during two different respiratory conditions, spontaneous breathing, and sniff (figure 5). Diaphragmatic relaxation is of extreme physiological importance since it is associated with rapid adaptation to changes in respiratory load and breathing frequency.17 Earlier studies using simultaneous diaphragmatic EMG and Pdi recordings have shown that EMG activity of the diaphragm ceases at peak Pdi,18 therefore, the initial rapid change in the slope of the Pdi curve is thought to represent the maximal rate of diaphragmatic relaxation. A slowing of the relaxation rate of the inspiratory muscles has been shown to occur well before the development of fatigue, which implies that a formal analysis of diaphragmatic relaxation may predict the fate of an ongoing weaning trial before an endpoint of respiratory failure is reached.
In this study, we assumed that diaphragmatic muscle relaxation rate could be also measured from the initial steepest part of the diaphragmatic displacement waveform, and a good relationship with the MRR derived from the Pdi waveform is determined. With regard to the M-mode diaphragmatic displacement waveform, previous studies that used spirometry concomitantly with diaphragmatic M-mode sonography, have shown no differences in expiratory time measured with either method,19 implying that the peak diaphragmatic excursion coincides with the beginning of the diaphragmatic relaxation phase; sonography provides a direct assessment of the actual velocity of motion of the diaphragmatic muscle, whereas in all previous studies20 it was assumed that changes in pressure curves demonstrated changes in relaxation rate. However, there is evidence to support that the decay of pleural (or Pes) pressure ends before the crural and costal diaphragmatic parts return to their initial conditions of length and tension20 21; in such case, these pressure swings recorded during relaxation may not totally coincide with diaphragmatic relaxation time, whereas the sonographic method will correctly demonstrate the whole duration of relaxation time and, subsequently, the right period of time for the relaxation rate to be determined.
Although diaphragmatic MRR appears in many studies as an attractive method to detect diaphragmatic fatigue and predict weaning failure, it has not won widespread acceptance as a way of monitoring patients during weaning in clinical practice. This may be due to the practical difficulties in recording easily, accurately and repeatedly, bedside, the slope of the MRR from the Pdi curve. Moreover, the reproducibility of the Pdi curve has never been evaluated, possibly for ethical reasons. Therefore, we believe that it is worthy to evaluate the sonographic equivalent of MRR in larger studies.
There are some limitations to our study. A limited number of patients were studied, however, a significant number of breaths per patient and conditions were analysed, more than 200 hundred breaths in pairs in total, that yielded a statistical significant correlation at p<0.05 level. Moreover, interobserver and intraobserver variability was not measured, however, the high reproducibility and the low interobserver and intraobserver variability of the M-mode diaphragmatic displacement was demonstrated in many studies.6–8 Furthermore, diaphragmatic motion can be affected by respiratory mechanics, abdominal compliance, rib cage or abdominal muscle activity, possibly influencing the relationship between diaphragm displacement and Pes or Pdi in a different set of patients.