Lung Mechanics in Diaphragmatic Paralysis1 G. J. GIBSON2 and N. B. PRIDE
In patients with mild respiratory muscle weakness, there may be a definite decrease in maximal inspiratory pressure before lung volumes are decreased below the normal range. This would be anticipated from the shape of the pressure-volume (PV) relationships; indeed, if the static PV curves of the lungs and chest wall remained unchanged, quite marked weakness should have little effect on lung volumes. However, the results of several studies suggest that the relationship between loss of inspiratory pressure and loss of lung volume is not always predictable. The decrease in the inspiratory capacity is often disproportionate to the loss of distending pressure, both in patients with respiratory muscle weakness and in normal subjects in whom a normal maximal transpulmonary pressure cannot be
generated, either because of partial curarization or because full expansion is prevented by strapping of the chest wall. In view of the known effects of chest wall strapping on the elastic behavior of the lungs, we investigated the static PV curves of 7 patients with severe respiratory muscle weakness (1). Of our 7 patients, 4 had been demonstrated to have complete bilateral paralysis of the diaphragm, and in the others the diaphragm was grossly weak. The patients, however, had generalized muscle diseases, so that weakness of other respiratory muscles might also have contributed to the changes we found. The PV curves of the lungs showed decreases in both the maximal transpulmonary pressure and the static compliance. We speculated that the decreased lung compliance might be the result of widespread microatelectasis or some alteration in surface elastic properties in the lungs. It might also be that in the presence of diaphragmatic paralysis, it is not possible to develop the transpulmonary pressure across all parts of the lungs.
i From the Department of Medicine, Royal Postgraduate Medical School, Hammersmith Hospital, London W12, England. 2 Present address: Regional Cardiothoracic Centre, Freeman Hospital, Newcastle-Upon-Tyne, England.
so
BO TLC °/o predicted 60
TLC °/o predicted 60
20
Pst m max
15 20 cmH20
Ci
1 2 3 °/o predicted TLC
4 cm HO"1
Fig. .1. Relation of total lung capacity (TLC) to maximal transpulmonary pressure, Pst(i) max, and to static lung compliance (Q) in 7 patients. Both TLC and Ci are expressed in relation to predicted TLC to allow for effects of body size. 119
120
GIBSON AND PRIDE
Whatever the reason, the lungs were in effect "stiff," and we suggested that this might offer a mechanical explanation for the pattern of breathing that these patients adopt: rapid frequency and small tidal volume. The decreased compliance also inevitably implies that the lung volumes were smaller than would have resulted from simple "truncation" of the static PV relationships with no change in compliance. When we related the value of total lung capacity (TLC) (expressed as a percentage of the predicted value) to the maximal transpulmonary pressure, there was no correlation (figure 1).
However, there was a clear relationship between TLC and the compliance at functional residual capacity, suggesting that the secondary change in lung distensibility in these patients may be a more important determinant of the degree of volume restriction than the loss of distending pressure per se. Reference 1. Gibson, G. J., Pride, N. B., Newsom Davis, J., and Loh, L. C.: Pulmonary mechanics in patients with respiratory muscle weakness, Am Rev Respir Dis, 1977,115, 389.
In patients with mild respiratory muscle weakness, there may be a definite decrease in maximal inspiratory pressure before lung volumes are decreased below the normal range. This would be anticipated from the shape of the pressure-volume (PV) relationships; indeed, if the static PV curves of the lungs and chest wall remained unchanged, quite marked weakness should have little effect on lung volumes. However, the results of several studies suggest that the relationship between loss of inspiratory pressure and loss of lung volume is not always predictable. The decrease in the inspiratory capacity is often disproportionate to the loss of distending pressure, both in patients with respiratory muscle weakness and in normal subjects in whom a normal maximal transpulmonary pressure cannot be
generated, either because of partial curarization or because full expansion is prevented by strapping of the chest wall. In view of the known effects of chest wall strapping on the elastic behavior of the lungs, we investigated the static PV curves of 7 patients with severe respiratory muscle weakness (1). Of our 7 patients, 4 had been demonstrated to have complete bilateral paralysis of the diaphragm, and in the others the diaphragm was grossly weak. The patients, however, had generalized muscle diseases, so that weakness of other respiratory muscles might also have contributed to the changes we found. The PV curves of the lungs showed decreases in both the maximal transpulmonary pressure and the static compliance. We speculated that the decreased lung compliance might be the result of widespread microatelectasis or some alteration in surface elastic properties in the lungs. It might also be that in the presence of diaphragmatic paralysis, it is not possible to develop the transpulmonary pressure across all parts of the lungs.
i From the Department of Medicine, Royal Postgraduate Medical School, Hammersmith Hospital, London W12, England. 2 Present address: Regional Cardiothoracic Centre, Freeman Hospital, Newcastle-Upon-Tyne, England.
so
BO TLC °/o predicted 60
TLC °/o predicted 60
20
Pst m max
15 20 cmH20
Ci
1 2 3 °/o predicted TLC
4 cm HO"1
Fig. .1. Relation of total lung capacity (TLC) to maximal transpulmonary pressure, Pst(i) max, and to static lung compliance (Q) in 7 patients. Both TLC and Ci are expressed in relation to predicted TLC to allow for effects of body size. 119
120
GIBSON AND PRIDE
Whatever the reason, the lungs were in effect "stiff," and we suggested that this might offer a mechanical explanation for the pattern of breathing that these patients adopt: rapid frequency and small tidal volume. The decreased compliance also inevitably implies that the lung volumes were smaller than would have resulted from simple "truncation" of the static PV relationships with no change in compliance. When we related the value of total lung capacity (TLC) (expressed as a percentage of the predicted value) to the maximal transpulmonary pressure, there was no correlation (figure 1).
However, there was a clear relationship between TLC and the compliance at functional residual capacity, suggesting that the secondary change in lung distensibility in these patients may be a more important determinant of the degree of volume restriction than the loss of distending pressure per se. Reference 1. Gibson, G. J., Pride, N. B., Newsom Davis, J., and Loh, L. C.: Pulmonary mechanics in patients with respiratory muscle weakness, Am Rev Respir Dis, 1977,115, 389.
