Breathing Responses to Imposed Mechanical Loads

Diagnosis of amyotrophic lateral sclerosis by respiratory function test

The diagnostic criterion for amyotrophic lateral sclerosis (ALS) based on the findings of concomitant clinical and electrophysiological evidence of upper and lower motor neuron involvement may remain unsatisfied for months and in some patients, even for years in the early stage of the disease. Since respiratory involvement is an onset symptom of ALS in only 1-3% of patients, pulmonary assessment has never been considered useful in the early diagnosis of ALS. However, studies on pulmonary function are lacking, especially in those early stages where neurologic tests are also inconclusive. In contrast to the scarcity of data in the early stages, as the disease progresses, it is increasingly enriched by a rich set of symptoms and positive respiratory tests until respiratory failure occurs, which represents the main cause of death in ALS. Hereby we analyze the main pulmonary function tests (PFT) in the various stages of the disease, up to the recent evidence for the possibility of an early diagnosis.

Effects of fentanyl overdose-induced muscle rigidity and dexmedetomidine on respiratory mechanics and pulmonary gas exchange in sedated rats

The objective of our study was to establish in sedated rats the consequences of high-dose fentanyl-induced acute muscle rigidity on the mechanical properties of the respiratory system and on the metabolic rate. Doses of fentanyl that we have previously shown to produce persistent rigidity of the muscles of the limbs and trunk in the rat (150 -300 microg/kg iv), were administered in 23 volume-controlled mechanically ventilated and sedated rats. The effects of a low dose of the FDA approved central alpha-2 agonist, dexmedetomidine (3 microg/kg iv), which has been suggested to oppose fentanyl-induced muscle rigidity, were determined after fentanyl administration. Fentanyl produced a significant decrease in Crs in the 23 rats that were studied. In 13 rats, an abrupt response occurred within 90 seconds, consisting in rapid rhythmic contractions of most skeletal muscles, that were replaced by persistent tonic/tetanic contractions leading a significant decrease of Crs (from 0.51 ± 0.11 ml/cmH₂O to 0.36 ± 0.08 ml/cmH₂O, 3 minutes after fentanyl injection). In the other 10 animals, a Crs progressively decreased to 0.26 ± 0.06 ml/cmH₂O at 30 minutes. There was a significant rise in V̇O₂ during muscle tonic contractions (from 8.48 ± 4.31 to 11.29 ± 2.57 ml/min), which contributed to a significant hypoxemia, despite ventilation being held constant. Dexmedetomidine provoked a significant and rapid increase in Crs towards baseline levels, while decreasing the metabolic rate and restoring normoxemia. We propose that the changes in respiratory mechanics and metabolism produced by opioid-induced muscle rigidity contribute to fentanyl lethality.

Mechanical loads modulate tidal volume and lung washout during high-frequency percussive ventilation

High-frequency percussive ventilation (HFPV) has been proved useful in patients with acute respiratory distress syndrome. However, its physiological mechanisms are still poorly understood. The aim of this work is to evaluate the effects of mechanical loading on the tidal volume and lung washout during HFPV. For this purpose a single-compartment mechanical lung simulator, which allows the combination of three elastic and four resistive loads (E and R, respectively), underwent HFPV with constant ventilator settings. With increasing E and decreasing R the tidal volume/cumulative oscillated gas volume ratio fell, while the duration of end-inspiratory plateau/inspiratory time increased. Indeed, an inverse linear relationship was found between these two ratios. Peak and mean pressure in the model decreased linearly with increasing pulsatile volume, the latter to a lesser extent. In conclusion, elastic or resistive loading modulates the mechanical characteristics of the HFPV device but in such a way that washout volume and time allowed for diffusive ventilation vary agonistically.

Effects of mechanical load on flow, volume and pressure delivered by high-frequency percussive ventilation

High-frequency percussive ventilation (HFPV) has proved its unique efficacy in the treatment of acute respiratory distress, when conventional mechanical ventilation (CMV) has demonstrated a limited response. We analysed flow (V(dot)), volume (V) and airway pressure (Paw) during ventilation of a single-compartment mechanical lung simulator, in which resistance (R) and elastance (E) values were modified, while maintaining the selected ventilatory settings of the HFPV device. These signals reveal the physical effect of the imposed loads on the output of the ventilatory device, secondary to constant (millisecond by millisecond) alterations in pulmonary dynamics. V(dot), V and Paw values depended fundamentally on the value of R, but their shapes were modified by R and E. Although peak Paw increased 70.3% in relation to control value, mean Paw augmented solely 36.5% under the same circumstances (maximum of 9.4 cm H2O). Finally, a mechanism for washing gas out of the lung was suggested.