Changes induced in the lower- and upper-limb pulse transit-time ratio during inspiratory resistive breathing.

The ankle brachial index (ABI) has been widely used to monitor the pathogenesis of peripheral arterial diseases such as ischemia of the extremities. Owing to the occluding nature of ABI measurement, this may not be appealing to less cooperative patients when multiple prolonged screening is required. Recently, a simple non-occluding technique termed pulse transit-time ratio (PTTR) has shown potential as a surrogate ABI marker. It is also known that abrupt changes in inspiratory efforts can lead to increased blood pressure (BP) and heart rate. Since transit-time measurements can be confounded by these parameters, it is important to understand their effects on PTTR normality. We recruited 12 healthy adults (8 males, aged 27.0+/-3.1 years) to perform three inspiratory activities. Friedman and Wilcoxon statistical results both showed that significant changes in transit-time oscillations were observed for higher inspiratory loads (p<0.05). These results were verified by a corresponding air-pressure difference measurement, for which a similar significant increase was also registered (p<0.05). However, limited changes were observed in the derived PTTR parameter (p>0.05). These findings suggest that, similar to ABI, PTTR is only confounded by abnormal local changes in either of the peripheral BPs measured.     

Magnitude estimation of inspiratory resistive loads by double-lung transplant recipients.

The purpose of this study was to investigate the role of afferent input from the lung and lower airways in magnitude estimation of inspiratory resistive loads (R). To assess the role of lung vagal afferents in respiratory sensation, sensations related to inspiratory R, reflected by subjects' percentage of handgrip responses (HG%), were compared between double-lung transplant (DLT) recipients with normal lung function and healthy control (Nor) subjects. Perceptual sensitivity to the external load was measured as the slope of HG% as a function of peak mouth pressure (Pm), and the slope of HG% as a function of R, after a log-log transformation. The results showed that the DLT group had a similar HG% response, as well as the slopes of log HG%-log Pm and log HG%-log R, compared with the Nor group. Furthermore, the ventilatory responses to external loads were also similar between the two groups. These results suggest that lung vagal afferents do not play a significant role in magnitude estimation of inspiratory resistive loads in humans.     

Movement of the human upper airway during inspiration with and without inspiratory resistive loading

The electromyographic (EMG) activity of human upper airway muscles, particularly the genioglossus, has been widely measured, but the relationship between EMG activity and physical movement of the airway muscles remains unclear. We aimed to measure the motion of the soft tissues surrounding the airway during normal and loaded inspiration on the basis of the hypothesis that this motion would be affected by the addition of resistance to breathing during inspiration. Tagged MR imaging of seven healthy subjects was performed in a 3-T scanner. Tagged 8.6-mm-spaced grids were used, and complementary spatial modulation of magnetization images were acquired beginning ～200 ms before inspiratory airflow. Deformation of tag line intersections was measured. The genioglossus moved anteriorly during normal and loaded inspiration, with less movement during loaded inspiration. The motion of tissues at the anterior border of the upper airway was nonuniform, with larger motions inferiorly. At the level of the soft palate, the lateral dimension of the airway decreased significantly during loaded inspiration (-0.15 ± 0.09 and -0.48 ± 0.09 mm during unloaded and loaded inspiration, respectively, P < 0.05). When resistance to inspiratory flow was added, genioglossus motion and lateral dimensions of the airway at the level of the soft palate decreased. Our results suggest that genioglossus motion begins early to dilate the airway prior to airflow and that inspiratory loading reduces the anterior motion of the genioglossus and increases the collapse of the lateral airway walls at the level of the soft palate.     

Detection of inspiratory resistive loads in double-lung transplant recipients.

The afferent pathways mediating respiratory load perception are still largely unknown. To assess the role of lung vagal afferents in respiratory sensation, detection of inspiratory resistive loads was compared between 10 double-lung transplant (DLT) recipients with normal lung function and 12 healthy control (Nor) subjects. Despite a similar unloaded and loaded breathing pattern, the DLT group had a significantly higher detection threshold (2.91 +/- 0.5 vs. 1.55 +/- 0.3 cmH(2)O. l(-1). s) and Weber fraction (0.50 +/- 0.1 vs. 0.30 +/- 0.1) compared with the Nor group. These results suggest that inspiratory resistive load detection occurs in the absence of vagal afferent feedback from the lung but that lung vagal afferents contribute to inspiratory resistive load detection response in humans. Lung vagal afferents are not essential to the regulation of resting breathing and load compensation responses.     

Detection threshold for inspiratory resistive loads and respiratory-related evoked potentials.

The relationship between detection threshold of inspiratory resistive loads and the peaks of the respiratory-related evoked potential (RREP) is unknown. It was hypothesized that the short-latency and long-latency peaks of the RREP would only be elicited by inspiratory loads that exceeded the detection threshold. The detection threshold for inspiratory resistive loads was measured in healthy subjects with inspiratory-interruption or onset load presentations. In a separate protocol, the RREPs were recorded with resistive loads that spanned the detection threshold. The loads were presented in stimulus attend and ignore sessions. Onset and interruption load presentations had the same resistive load detection threshold. The P(1), N(f), and N(1) peaks of the RREP were observed with loads that exceeded the detection threshold in both attend and ignore conditions. The P(300) was present with loads that exceeded the detection threshold only in the attend condition. No RREP components were elicited with subthreshold loads. The P(1), N(f), and P(300) amplitudes varied with resistive load magnitude. The results support the hypothesis that there is a resistive load threshold for eliciting the RREPs. The amplitude of the RREP peaks vary as a function of load magnitude. The cognitive P(300) RREP peak is present only for detectable loads and when the subject attends to the stimulus. The absence of the RREP with loads below the detection threshold and the presence of the RREP elicited by suprathreshold loads are consistent with the gating of these neural measures of respiratory mechanosensory information processing.     

Effect of resistive loading on ventilatory response to hypoxia.

Ventilatory responses to hypoxia, with and without an inspiratory resistive load, were measured in eight normal subjects, using a rebreathing technique. During the studies, the end-tidal P-CO2 was kept constant at mixed venous level (Pv-CO2) by drawing expired gas through a variable CO2-absorbing bypass. The initial bag O2 concentration was 24% and rebreathing was continued until the O2 concentration in the bag fell to 6% or the subject's arterial oxygen saturation (Sa-O2), monitored continuously by ear oximetry, fell to 70%. Studies with and without the load were performed in a formally randomized order for each subject. Linear regressions for rise in ventilation against fall in Sa-O2 were calculated. The range of unloaded responses was 0.78-3.59 1/min per 1% fall in Sa-O2 and loaded responses 0.37-1.68 1/min per 1% fall in Sa-O2. In each subject, the slope of the response curve during loading fell by an almost constant fraction of the unloaded response, such that the ratio of loaded to unloaded slope in all subjects ranged from 0.41 to 0.48. However, the extrapolated intercept of the response curve on the Sa-O2 axis did not alter significantly indicating that the P-CO2 did not alter between experiments. These results suggest that the change in ventilatory response to hypoxia during inspiratory resistive loading is related to the mechanical load applied, with the loaded slope being directly proportional to the unloaded one.     

Influence of acute inspiratory loading upon diaphragm motor-evoked potentials in healthy humans.

Acute prior activity of the inspiratory muscles can enhance inspiratory muscle strength and reduce effort perception during subsequent inspiratory efforts. However, the mechanisms subserving these changes are poorly understood. Responses to magnetic stimulation in 10 subjects were studied after an acute bout of nonfatiguing inspiratory muscle loading (IML), corresponding to 40% of subjects' initial maximal inspiratory pressure (MIP), and after an acute bout of nonloaded, forced inspiration (NLF). Motor-evoked potentials elicited by cortical stimulation (MEP(c)) and by phrenic nerve stimulation (MEP(p)) were recorded transcutaneously from the diaphragm before, immediately after, and 15 min after two sets of 30 inspiratory efforts, at rest and during an MIP effort. After IML, MIP increased to 113 +/- 3% (SE) of baseline and diaphragm MEP(p) (during MIP) significantly increased (129 +/- 10% of baseline). Diaphragmatic MEP(c) (during MIP), expressed as a percentage of maximal MEP(p), decreased after IML (from 29 +/- 9% to 20 +/- 6%; P = 0.017) and after NLF (from 43 +/- 5% to 31 +/- 5%; P = 0.032). Observations from the biceps brachi demonstrated that changes after IML and NLF were specific to the inspiratory muscle, since no significant changes were observed in biceps force generation or in MEP(p) or MEP(c) amplitudes. These data indicate that after IML increased global inspiratory strength is accompanied by increased peripheral excitability and by a dampening of corticospinal excitability of the diaphragm.     

Putative protective effect of inspiratory threshold loading against exercise-induced supraspinal diaphragm fatigue.

The present investigation was intended to assess the consequences of an inspiratory load on the diaphragm central component of fatigue during exercise. We recorded the motor potential evoked (MEP) by transcranial magnetic stimulation of the motor cortex in 10 subjects. The diaphragm and rectus femoris were studied before and 10, 20, and 40 min after two 16-min cycling exercise (E) trials requiring 55% of maximal oxygen uptake: 1) one with an inspiratory threshold load (E + ITL), corresponding to 10% of maximal inspiratory pressure; and 2) the other without the load (E). Dyspnea, heart rate, electromyographic activity of the sternocleidomastoid, and diaphragm work were significantly higher in E + ITL than in E. Neither trial affected the response to phrenic magnetic stimulation, which was performed 15 and 25 min postexercise, or the maximal inspiratory pressure (116 and 120 cm H(2)O before E and E + ITL, respectively, and 110 and 114 cm H(2)O at 30 min postexercise). Whereas the amplitude of the diaphragm MEP was unaffected by E + ITL (+2.1 +/- 29.4%), a significant decrease was observed 10 min after E compared with baseline (-37.1 +/- 22.3%) and compared with E + ITL. The MEP amplitude of rectus femoris remained unchanged with E and E + ITL. The recruitment of synergistic agonists during E + ITL may have normalized the major ventilatory stress and reset up the excitability of the diaphragm pathway.     

The role of Src &amp; ERK1/2 kinases in inspiratory resistive breathing induced acute lung injury and inflammation

Background

Inspiratory resistive breathing (IRB), a hallmark of obstructive airway diseases, is associated with large negative intrathoracic pressures, due to strenuous contractions of the inspiratory muscles. IRB is shown to induce lung injury in previously healthy animals. Src is a multifunctional kinase that is activated in the lung by mechanical stress. ERK1/2 kinase is a downstream target of Src. We hypothesized that Src is activated in the lung during IRB, mediates ERK1/2 activation and IRB-induced lung injury.

Methods

Anaesthetized, tracheostomized adult rats breathed spontaneously through a 2-way non-rebreathing valve. Resistance was added to the inspiratory port to provide a peak tidal inspiratory pressure of 50% of maximum (inspiratory resistive breathing). Activation of Src and ERK1/2 in the lung was estimated during IRB. Following 6 h of IRB, respiratory system mechanics were measured by the forced oscillation technique and bronchoalveolar lavage (BAL) was performed to measure total and differential cell count and total protein levels. IL-1b and MIP-2a protein levels were measured in lung tissue samples. Wet lung weight to total body weight was measured and Evans blue dye extravasation was estimated to measure lung permeability. Lung injury was evaluated by histology. The Src inhibitor, PP-2 or the inhibitor of ERK1/2 activation, PD98059 was administrated 30 min prior to IRB.

Results

Src kinase was activated 30 min after the initiation of IRB. Src inhibition ameliorated the increase in BAL cellularity after 6 h IRB, but not the increase of IL-1β and MIP-2a in the lung. The increase in BAL total protein and lung injury score were not affected. The increase in tissue elasticity was partly inhibited. Src inhibition blocked ERK1/2 activation at 3 but not at 6 h of IRB. ERK1/2 inhibition ameliorated the increase in BAL cellularity after 6 h of IRB, blocked the increase of IL-1β and returned Evans blue extravasation and wet lung weight to control values. BAL total protein and the increase in elasticity were partially affected. ERK1/2 inhibition did not significantly change total lung injury score compared to 6 h IRB.

Conclusions

Src and ERK1/2 are activated in the lung following IRB and participate in IRB-induced lung injury.

     

Ferrylmyoglobin formation induced by acute magnesium deficiency in perfused rat heart causes cardiac failure

The oxidation states in intracellular myoglobin and cytochrome oxidase aa₃ were monitored by reflectatnce spectrophotometry in isoltaed perfused rat hearts subjected to an acutely magnesium deficient environment. After exposure to low extracellular [Mg²⁺]_o (i.e., 0.3 mM) for 30 min, more than 80% of the oxymyoglobin converted to its deoxygenated form. The level of reduced cytochrome oxidase aa₃ also increased about 80% in low [Mg²⁺]_o. the deoxymyoglobin was converted further to a species identified as ferrymyoglobin by its reaction with Na₂S to form ferrous sulfmyoglobin which was optically visible. This process, set into motion by acute Mg deficiency, resulted from a direct accessibility of the exogenous peroxide to the cytosolic protein. The results suggest that a pathway leading to cardiac tissue damage, induced magnesium deficiency, is probably involved in the generation of a ferrylmyoglobin radical which could be prevented by addition of ascorbate, which is known to be a one-electron reductant of this hypervalent form of myglobin. In further studies, we also investigated whether addition of different concentrations of ascorbic acid (AA) to the perfusate could enhance myocardial function after exposure to log [Mg²⁺]_o perfusion. Four concentrations of AA (0.5, 1, 5, 10 mM) were tested, indicate that they exert their effects in a concentration-dependent manner; 1 mM AA was the most effective dose in improving aortic output in a Mg-deficient heart. Ferrylmyoglobin formation was found to be formed considerably before intracellylar release of either creatine phosphokinase or lactic dehydrogenase. These studies may have wide implications as a new mechanisms by which extracellular Mg²⁺ can induce myocardial injury and subsequent cardiac failure.     

Phrenic activity and intercostal muscle emg during inspiratory loading in newborn kittens

The effects of airway occlusions at functional residual capacity (FRC) on both "integrated" phrenic activity (Phr) and intercostal muscle electromyogram (intEMG) were studied in intact and vagotomized spontaneously breathing kittens during the 1st wk of life. Animals were anesthetized im with a mixture of ketamine (30 mg/kg) and acepromazine (1.1 mg/kg). In the intact kittens, inspiratory loading led to a significant increase in peak amplitudes of both Phr and intEMG and prolongation of inspiratory (TI) and expiratory (TE) times. Mean values of rate of rise of Phr and intEMG measured at 200 ms (intEMG200) from the onset of inspiration were unaffected. The results indicated that in newborns the vagal component of the load compensation is of great importance. Following vagotomy, airway occlusion produced a significant increase in mean values of TI and intEMG only. These small but significant changes suggest that most of the load compensation reflex is dependent on prolongation of TI. Increased intEMG200 during loading in the vagotomized kittens, observed during several trials, implies that the intercostal fusion-alpha interaction may operate in newborns.     

Localization of the insulin-like growth factor system in a rat model of heart failure induced by myocardial infarction.

Although cardiac effects of growth hormone (GH) and insulin-like growth factor (IGF)-I have been reported in experimental models of heart failure and in human dilated cardiomyopathy, the IGF system has not been comprehensively assessed in the failing heart. We therefore localized the IGF system in the left ventricle during congestive heart failure after myocardial infarction (MI) in the rat. The left anterior descending coronary artery was ligated in adult female Sprague-Dawley rats and hearts were examined after 6 months when congestive heart failure had developed. In situ hybridization histochemistry was used to localize mRNA for the components of the IGF system in the left ventricle of sham and congestive heart failure animals. We were able to detect changes in the spatial distribution of mRNA for IGF-I and IGF binding proteins 3, 4, 5, and 6 in the left ventricle during congestive heart failure after MI. IGF-I and the binding proteins were predominantly increased in the infarct/peri-infarct area of the left ventricle. Other components of the IGF system were indistinguishable from the low to undetectable levels in sham-operated rats. These results demonstrate that the IGF system is altered in the failing heart and suggest that the IGF system plays an important role in the response of the heart to MI and consequent failure.