The effect of lung inflation on the inspiratory action of the canine parasternal intercostals

Association between thoracic musculoskeletal abnormalities and lung function in preterm infants

INTRODUCTION

In view of the difficulties and risks of performing lung function tests in infants and the hypothesis that children with abnormal pulmonary test may exhibit thoracic musculoskeletal alterations.

OBJECTIVES

This study aimed to determine the frequency of abnormal lung function and their relationship.

MATERIALS AND METHODS

This was a cross-sectional study with children from 6 to 12 months of corrected age, born at a gestational age of <37 weeks and with a birthweight ≤1500 g, who were subjected to a lung function test and photogrammetry--an objective and non-invasive procedure. To verify the association between the thoracic musculoskeletal abnormalities and measure changes in lung function, univariate linear regression was used. The level of statistical significance was setted at P < 0.05.

RESULTS

Of the 38 infants, 12 (31.6%) exhibited abnormal lung function, including 9 (23.7%) with obstructive function and 3 (7.9%) with restrictive function. A significant association was noted between forced expiratory volume at 0.5 second <-2 z score and the acromion/xiphoid process/acromion angle (β = 4.935); forced vital capacity <-2 z score and the angle of the manubrium/left acromion/trapezium (β = 0.033) and forced expiratory volume at 0.5 second and forced vital capacity ratio <-2 z score and the inframammillary point/xiphoid process/inframammillary point angle (β = 0.043).

CONCLUSION

Preterm infants with very low birthweight presented a high frequency of abnormal lung function, particularly obstructive type and thoracic musculoskeletal abnormalities were associated with changes in lung function.

Acute effects of Expiratory Positive Airway Pressure (EPAP) on different levels in ventilation and electrical activity of sternocleidomastoid and parasternal muscles in Chronic Obstructive Pulmonary Disease (COPD) patients: a randomized controlled trial

Objective

To investigate the acute effects of EPAP on the activity of sternocleidomastoid (SCM), parasternal muscles and ventilatory parameters in COPD patients.

Method

Twenty-four patients with COPD were studied using surface electromyography (sEMG) and a ventilometer. Patients were randomly assigned to EPAP 10 cmH₂O-EPAP₁₀ or 15 cmH₂O-EPAP₁₅ for 20 minutes.

Results

The parasternal muscle sEMG activity increased during EPAP₁₀ and EPAP₁₅; however, a greater and significant increase was observed with EPAP₁₀ (mean between-group difference: 12.5% RMS, 95% CI: 9.5 to 15.4, p<0.001). In relation to the baseline, at 10 and 20 minutes and upon recovery, respectively parasternal activity increased by 23.9%, 28.9% and 19.1% during EPAP₁₀ and by 10.7% at 10 and 20 minutes and upon recovery, respectively, 11.4% and 6.9% during EPAP₁₅ at 10 and 20 minutes and upon recovery, respectively. The sEMG activity of SCM muscle showed an opposite pattern, increasing with EPAP₁₅ and decreasing with EPAP₁₀ (mean between-group difference: 15.5% RMS, 95% CI: 12.6 to 18.4, p<0.001). SCM muscle activity during EPAP₁₅, increased by 4.8% and 6.1% at 10 and 20 minutes and decreased by -4.0% upon recovery compared to decreases of –5.6%, –20.6% and –21.3% during EPAP₁₀ at 10, 20 minutes, and recovery. Ventilation at both EPAP intensities promoted significant reductions in respiratory rate (RR) and dyspnea, more pronounced in EPAP₁₅: RR (mean between-group difference: –3,8bpm, 95%CI: –7,5 to –0,2, p=0,015) and dyspnea (mean between-group difference: –1.01, 95%CI: –1.4 to –0.53, p=0.028) .

Conclusion

In COPD patients, the use of EPAP₁₀ was more effective in reducing accessory inspiratory activity and increasing parasternal activity, which was accompanied by an improvement in ventilation and a reduction in dyspnea.

Mechanics of the canine diaphragm in pleural effusion

Pleural effusion is a complicating feature of many diseases of the lung and pleura, but its effects on the mechanics of the diaphragm have not been assessed. In the present study, radiopaque markers were attached along muscle bundles in the midcostal region of the diaphragm in anesthetized dogs, and the three-dimensional location of the markers during relaxation before and after the stepwise introduction of liquid into the left or right pleural space and during phrenic nerve stimulation in the same conditions was determined using computed tomography. From these data, accurate measurements of diaphragm muscle length and displacement were obtained, and the changes in pleural and abdominal pressure were analyzed as functions of these parameters. The effect of liquid instillation on the axial position of rib 5 was also measured. The data showed that 1) liquid leaked through the dorsal mediastinal sheet behind the pericardium so that effusion was bilateral; 2) effusion caused a caudal displacement of the relaxed diaphragm; 3) this displacement was, compared with passive lung inflation, much larger than the cranial displacement of the ribs; and 4) the capacity of the diaphragm to generate pressure, in particular pleural pressure, decreased markedly as effusion increased, and this decrease was well explained by the decrease in active muscle length. It is concluded that pleural effusion has a major adverse effect on the pressure-generating capacity of the diaphragm and that this is the result of the action of hydrostatic forces on the muscle.

Respiratory muscle function and activation in chronic obstructive pulmonary disease

Inspiratory muscles are uniquely adapted for endurance, but their function is compromised in chronic obstructive pulmonary disease (COPD) due to increased loads, reduced mechanical advantage, and increased ventilatory requirements. The hyperinflation of COPD reduces the flow and pressure-generating capacity of the diaphragm. This is compensated by a threefold increase in neural drive, adaptations of the chest wall and diaphragm shape to accommodate the increased volume, and adaptations of muscle fibers to preserve strength and increase endurance. Paradoxical indrawing of the lower costal margin during inspiration in severe COPD (Hoover's sign) correlates with high inspiratory drive and severe airflow obstruction rather than contraction of radially oriented diaphragm fibers. The inspiratory muscles remain highly resistant to fatigue in patients with COPD, and the ultimate development of ventilatory failure is associated with insufficient central drive. Sleep is associated with reduced respiratory drive and impairments of lung and chest wall function, which are exaggerated in COPD patients. Profound hypoxemia and hypercapnia can occur in rapid eye movement sleep and contribute to the development of cor pulmonale. Inspiratory muscles adapt to chronic loading with an increased proportion of slow, fatigue-resistant fiber types, increased oxidative capacity, and reduced fiber cross-sectional area, but the capacity of the diaphragm to increase ventilation in exercise is compromised in COPD. In COPD, neural drive to the diaphragm increases to near maximal levels in exercise, but it does not develop peripheral muscle fatigue. The improvement in exercise capacity and dyspnea following lung volume reduction surgery is associated with a substantial reduction in neural drive to the inspiratory muscles.

Mechanism of increased inspiratory rib elevation in ascites

The detrimental effect of ascites on the lung-expanding action of the diaphragm is partly compensated for by an increase in the inspiratory elevation of the ribs, but the mechanism of this increase is uncertain. To identify this mechanism, the effect of ascites on the response of rib 4 to isolated phrenic nerve stimulation was first assessed in four dogs with bilateral pneumothoraces. Stimulation did not produce any axial displacement of the rib (X(r)) in the control condition and caused a cranial rib displacement in the presence of ascites. This displacement, however, was small. In a second experiment, the effects of ascites on the pleural pressure swing (DeltaP(pl)), intercostal activity, and X(r) during spontaneous inspiration were measured in eight animals. As the volume of ascites increased from 0 to 200 ml/kg body wt, X(r) increased from 3.5 +/- 0.5 to 7.5 +/- 0.9 mm (P < 0.001), DeltaP(pl) decreased from -6.4 +/- 0.4 to -3.6 +/- 0.3 cmH(2)0 (P < 0.001), and parasternal intercostal activity increased 61 +/- 19% (P < 0.001). The role of the decrease in DeltaP(pl) in causing the increase in X(r) was then separated from that of the increase in intercostal muscle force using the relation between X(r) and DeltaP(pl) during passive lung inflation. The loss in DeltaP(pl) accounted for two-thirds of the increase in X(r). These observations indicate that 1) the increased inspiratory elevation of the ribs in ascites is not the result of the increase in the rib cage-expanding action of the diaphragm and 2) it is due mostly to the decrease in DeltaP(pl) and partly to the increase in the force exerted by the parasternal intercostals on the ribs. These observations also suggest, however, that the rib cage expansion caused by ascites makes the parasternal intercostals less effective in pulling the ribs cranially.

Effect of acute inflation on the mechanics of the inspiratory muscles

When the lung is inflated acutely, the capacity of the diaphragm to generate pressure, in particular pleural pressure (Ppl), is impaired because the muscle during contraction is shorter and generates less force. At very high lung volumes, the pressure-generating capacity of the diaphragm may be further reduced by an increase in the muscle radius of curvature. Lung inflation similarly impairs the pressure-generating capacity of the inspiratory intercostal muscles, both the parasternal intercostals and the external intercostals. In contrast to the diaphragm, however, this adverse effect is largely related to the orientation and motion of the ribs, rather than the ability of the muscles to generate force. During combined activation of the two sets of muscles, the change in Ppl is larger than during isolated diaphragm activation, and this added load on the diaphragm reduces the shortening of the muscle and increases muscle force. In addition, activation of the diaphragm suppresses the cranial displacement of the passive diaphragm that occurs during isolated intercostal contraction and increases the respiratory effect of the intercostals. As a result, the change in Ppl generated during combined diaphragm-intercostal activation is greater than the sum of the pressures generated during separate muscle activation. Although this synergistic interaction becomes particularly prominent at high lung volumes, lung inflation, either bilateral or unilateral, places a substantial stress on the inspiratory muscle pump.

Sternomastoid, rib cage, and expiratory muscle activity during weaning failure

We hypothesized that patients who fail weaning from mechanical ventilation recruit their inspiratory rib cage muscles sooner than they recruit their expiratory muscles, and that rib cage muscle recruitment is accompanied by recruitment of sternomastoid muscles. Accordingly, we measured sternomastoid electrical activity and changes in esophageal (ΔPes) and gastric pressure (ΔPga) in 11 weaning-failure and 8 weaning-success patients. At the start of trial, failure patients exhibited a higher ΔPga-to-ΔPes ratio than did success patients ( P = 0.05), whereas expiratory rise in Pga was equivalent in the two groups. Between the start and end of the trial, failure patients developed additional increases in ΔPga-to-ΔPes ratio ( P < 0.0014) and the expiratory rise in Pga also increased ( P < 0.004). At the start of trial, sternomastoid activity was present in 8 of 11 failure patients contrasted with 1 of 8 success patients. Over the course of the trial, sternomastoid activity increased by 53.0 ± 9.3% in the failure patients ( P = 0.0005), whereas it did not change in the success patients. Failure patients recruited their respiratory muscles in a sequential manner. The sequence began with activity of diaphragm and greater-than-normal activity of inspiratory rib cage muscles; recruitment of sternomastoids and rib cage muscles approached near maximum within 4 min of trial commencement; expiratory muscles were recruited slowest of all. In conclusion, not only is activity of the inspiratory rib cage muscles increased during a failed weaning trial, but respiratory centers also recruit sternomastoid and expiratory muscles. Extradiaphragmatic muscle recruitment may be a mechanism for offsetting the effects of increased load on a weak diaphragm.

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

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.