Sharing the load: compensatory pressure generation in the mdx mouse model

In Duchenne muscular dystrophy (DMD) patients, respiratory infections throughout life are common due to inadequate airway clearance and respiratory failure is the primary cause of death (Ishikawa et al., 2011; LoMauro et al., 2015). Evaluations of respiratory muscle function across a range of motor behaviours can provide important information about the underlying neural control of the life-sustaining task of breathing as well as other essential motor functions in healthy and pathological states. Burns and colleagues (2019) recently published a paper in The Journal of Physiology that presents a comprehensive assessment of respiratory muscle function in the mdx mouse model of DMD at 8 weeks of age. The authors’ primary outcome measure was oesophageal pressure, which represents the combined efforts of primarily the diaphragm and, to a lesser extent, other chest wall muscles. In multiple models of neuromuscular impairment, pressure generation is preserved during ventilatory behaviours but impaired during near-maximal expulsive airway clearance behaviours (Greising et al., 2015; Khurram et al., 2018; Khurram et al., 2019). Thus, Burns et al. (2019) hypothesized that respiratory muscle weakness is associated with impaired peak pressure generating capacity in mdx mice. One of the major strengths of this paper is their comprehensive analysis of respiratory muscle function; in addition to measurements of oesophageal pressure, they performed diaphragm and external intercostal (EIC) electromyography, whole body plethysmography, and measurements of diaphragm muscle ex vivo isometric force. However, a few important points regarding the interpretation of their data warrant attention.

Considering that pressure generation during augmented breaths is preserved even after unilateral diaphragm paralysis (Khurram et al., 2017), it is not surprising that a ~50% reduction in diaphragm muscle isometric force in mdx mice was not associated with a decrease in oesophageal pressure during augmented breaths. What is surprising, however, is that the oesophageal pressures generated during augmented breaths in mdx mice were greater than those recorded in the control mice, despite there being no evidence for a difference in diaphragm or EIC EMG amplitude. This non-intuitive result may in fact be a function of the measurement of oesophageal pressure itself. As the diaphragm muscle contracts, the flattening of its dome shape and the concurrent increase in lung volume increase its the radius of curvature. The transduction of muscle force into pressure is governed by the Laplace relationship, which suggests that pressure is inversely proportional to the radius of curvature. If there is indeed increased compensation by other respiratory muscles in mdx mice, as the authors suggest, it is possible that the change in the radius of curvature of the diaphragm of mdx mice may have been less than the change in control mice. This may explain why the change in oesophageal pressure was less in control mice compared to mdx mice during augmented breaths. In this regard, it is worth noting that when the airway was occluded, the control mice generated oesophageal pressures equal to or possibly greater than the mdx mice.

As a percent of maximum, it seems that the pressures generated during airway occlusion in mice (Greising et al., 2015; Burns et al., 2019) are greater than those generated by rats (Fogarty et al., 2018; Khurram et al., 2018; Khurram et al., 2019). However, the pressure generated by the simultaneous contraction of all respiratory muscles excluding the diaphragm is less than 25% of the maximal pressure generating capacity of the diaphragm (see Discussion in Khurram et. al., 2017) and likely further reduced in DMD due to respiratory muscle weakness (Khirani et al., 2014; Mead et al., 2014; LoMauro et al., 2015). An improvement to the study by Burns and colleagues (2019) would be the inclusion of bilateral phrenic nerve stimulation during the pressure measurements. In light of the diaphragm muscle isometric force data, they may have expected a ~50% reduction in the oesophageal pressure generated during isolated maximal diaphragm muscle activation elicited via bilateral phrenic nerve stimulation. If airway occlusion necessitates near-maximal activation of respiratory muscles, bilateral phrenic nerve stimulation data could allow for estimations of the approximate contribution of the diaphragm muscle to the pressure generation. Additionally, contraction of the diaphragm muscle leads to an increase in abdominal pressure. Comparisons of gastric and oesophageal pressures, similar to those performed in previous DMD studies (e.g. Mead et al., 2014), can be quite informative and the ratio of the gastric and oesophageal pressures may even provide some capacity to assess increased accessory respiratory muscle involvement (LoMauro et al., 2015).

Interestingly, the respiratory frequency at baseline in conscious mdx mice is lower than during anaesthesia (Figure 2 in Burns et al., 2019). The respiratory frequency in conscious mdx mice was not different from control mice, while the respiratory frequency in anesthetized mice at baseline is significantly greater by ~20% (Table 2 in Burns et al., 2019). The authors report another baseline respiratory frequency in anesthetized animals that is not different between groups (Figure 3 in Burns et al., 2019). Potential systematic differences in anaesthetic plane due to differences in lean body mass could explain the discrepancies in respiratory rate as well as the greater oesophageal pressure generation during spontaneous augmented breaths in mdx mice compared to control mice. Although the authors report no difference in body mass between groups, the lean body mass in mdx mice may still have been reduced.

Despite these points, Burns et al. (2019) should be commended for their presentation of a well-designed and comprehensive study into the neuromotor control of respiratory muscles in the mdx mouse model. The evidence presented by the authors highlights the remarkable level of compensation for diaphragm muscle weakness in the early-stage mdx mouse. Considering previous work in this field (Khirani et al., 2014; Mead et al., 2014), it seems that the authors rightly conclude that this compensation is a result of auxiliary respiratory muscles. Their results provide insights about respiratory compensation in mdx mice and lay the groundwork for future studies evaluating respiratory muscle function throughout disease progression. These types of studies will help identify therapeutic windows and determine auspicious rehabilitation strategies for combatting muscular dystrophies.