Effects of single-lung inflation on inspiratory muscle function in dogs

Abstract

After single-lung transplantation (SLT) for emphysema, a hyperinflated (native) lung operates in parallel with a normal (transplanted) lung. The interpulmonary distribution of the changes in pleural pressure (DeltaP(pl)) during breathing, however, is unknown. To approach the problem, two endotracheal tubes were inserted in the right and left main stem bronchi of anaesthetized dogs, one lung was passively inflated, and the values of inspiratory DeltaP(pl) over the two lungs were assessed by measuring the changes in airway opening pressure (DeltaP(ao)) in the two tubes during occluded breaths. With single-lung inflation, DeltaP(ao) decreased in both lungs, but the decrease in the inflated lung was invariably larger than in the non-inflated lung; when transrespiratory pressure in the inflated lung was set at 30 cmH(2)O, DeltaP(ao) in this lung was 27.7 +/- 2.0% of the value of functional residual capacity (FRC), whereas DeltaP(ao) in the non-inflated lung was 74.4 +/- 4.5% (P < 0.001). This difference was abolished after the ventral mediastinal pleura was severed. The ribs in both hemithoraces were displaced cranially with inflation, such that the displacement in the contralateral hemithorax was 75% of that in the ipsilateral hemithorax, and parasternal intercostal activity remained unchanged. These observations suggest that in patients with SLT for emphysema (1) the inspiratory DeltaP(pl) over the transplanted lung are greater than those over the native lung and (2) this difference results primarily from the greater pressure-generating ability of the inspiratory muscles, in particular the diaphragm, on the transplanted side.

Figure 1. Effects of single-lung inflation on the pressure changes in the individual lungs in a representative animal

A, shows the traces of airway opening pressure (P_ao) in the right and the left lung during an unimpeded breath and an occluded breath with both lungs at resting end-expiration (FRC); the traces of EMG activity in the parasternal intercostals (3rd space) and external intercostals (2nd space) in the right and left hemithoraces are also shown. B, also shows an unimpeded breath at FRC followed by an occluded breath; between the two breaths, however, the right lung was passively inflated to a transrespiratory pressure of 24 cmH₂O. Note that during the occluded breath after right lung inflation, the change in P_ao (ΔP_ao) in the right lung is much smaller than that during the occluded breath at FRC; ΔP_ao in the left lung is greater than that in the right lung but smaller than that at FRC. Note also the decrease in external intercostal activity on both sides of the chest during the occluded breath after inflation.

Figure 2. Effect of gradual single-lung inflation on airway opening pressure in the individual lungs

Values of ΔP_ao (mean ± s.e.m) in the inflated (○) and non-inflated (•) lungs obtained from 12 animals during single-lung inflation. The right lung was inflated in seven animals, and the left lung was inflated in five animals The values of transrespiratory pressure along the abscissa refer only to the inflated lung.

Figure 3. Effect of single-lung inflation on external intercostal EMG activity in the two hemithoraces

Values of external Intercostal EMG activity (mean ± s.e.m) in the Ipsilateral (○) and contralateral (•) hemithoraces obtained from 12 animals during single-lung inflation. These values are expressed as percentages of the activity recorded at FRC.

Figure 4. Effects of external intercostal muscle section on airway opening pressure during single-lung inflation

Values of ΔP_ao (mean ± s.e.m) in the inflated (○) and non-inflated (•) lungs obtained from four animals during gradual single-lung inflation before (A) and after (B) section of the external intercostal muscles in interspaces 1–7. Note that the interpulmonary difference in ΔP_ao was unchanged after section of the muscles.

Figure 5. Effect of mediastinal excision on airway opening pressure during single-lung inflation

The data shown are the ΔP_ao values recorded in the left (○) and right (•) lungs during inflation of the left lung in a representative animal before (A) and after (B) excision of the ventral mediastinal pleura. Note that the difference in ΔP_ao between the two lungs is eliminated after excision.

Figure 6. Effect of single-lung inflation on rib cage expansion

Values of cranial rib displacement (mean ± s.e.m) obtained from six animals during passive Inflation of the ipsilateral (○) and contralateral lung (•). The cranial rib displacement observed during bilateral lung inflation is shown for comparison ().

Figure 7. Graphical analysis of the force developed by the diaphragm during single-lung inflation

The thin continuous lines are the force–length relationships for the maximally active and the passive diaphragm, the thick continuous line is the force–length relationship for the submaximally active diaphragm, and the dashed lines are the load curves describing the load imposed by the lung and chest wall on the two hemidiaphragms. Force is expressed as a fraction of maximum, and muscle length is expressed as a fraction of optimal length (L_o). The force generated by each hemidiaphragm during inspiration is given by the intersection of the load curve with the submaximally active length–tension curve (•). If after single-lung inflation, the ipsilateral hemidiaphragm (inflated) before contraction were shorter than the contralateral hemidiaphragm (non-inflated), it would generate less force during inspiration.