Chronic resistive loading induces diaphragm injury and ventilatory failure in the hamster

Hiccup-like Contractions in Mechanically Ventilated Patients: Individualized Treatment Guided by Transpulmonary Pressure

Hiccups-like contractions, including hiccups, respiratory myoclonus, and diaphragmatic tremor, refer to involuntary, spasmodic, and inspiratory muscle contractions. They have been repeatedly described in mechanically ventilated patients, especially those with central nervous damage. Nevertheless, their effects on patient-ventilator interaction are largely unknown, and even more overlooked is their contribution to lung and diaphragm injury. We describe, for the first time, how the management of hiccup-like contractions was individualized based on esophageal and transpulmonary pressure measurements in three mechanically ventilated patients. The necessity or not of intervention was determined by the effects of these contractions on arterial blood gases, patient-ventilator synchrony, and lung stress. In addition, esophageal pressure permitted the titration of ventilator settings in a patient with hypoxemia and atelectasis secondary to hiccups and in whom sedatives failed to eliminate the contractions and muscle relaxants were contraindicated. This report highlights the importance of esophageal pressure monitoring in the clinical decision making of hiccup-like contractions in mechanically ventilated patients.

Impact of Reverse Triggering Dyssynchrony During Lung-Protective Ventilation on Diaphragm Function: An Experimental Model

RATIONALE

Reverse triggering is a patient-ventilator interaction where a respiratory muscle contraction is triggered by a passive mechanical insufflation. Its impact on diaphragm structure and function is unknown.

OBJECTIVE

To establish an animal model of reverse triggering with lung injury receiving lung-protective ventilation and to assess its impact on structure and function of the diaphragm.

METHODS

Lung injury was induced by surfactant depletion and high stress ventilation in 32 ventilated pigs. Animals were allocated to receive passive mechanical ventilation or a lung-protective strategy with adjustments facilitating the occurrence of reverse triggering for 3 hours. Diaphragm function (transdiaphragmatic pressure (Pdi) during phrenic nerve stimulation [Force/frequency curve]) and structure (biopsies) were assessed. The impact of reverse triggering on diaphragm function was analyzed according to the breathing effort.

RESULTS

Compared to passive ventilation, the protective ventilation group with reverse triggering received significantly lower tidal volume (7 vs 10 ml/kg) and higher respiratory rate (45 vs 31 bpm). An entrainment pattern of 1:1 was frequent. Breathing effort induced by reverse triggering was highly variable across animals. Reverse triggering with the lowest tercile of breathing effort was associated with 23% higher twitch Pdi compared to passive ventilation, whereas reverse triggering with high breathing effort was associated with a 10% lower twitch Pdi and a higher proportion of abnormal muscle fibers.

CONCLUSION

In a reproducible animal model of reverse triggering with variable levels of breathing effort and entrainment patterns, reverse triggering with high effort is associated with impaired diaphragm function whereas reverse triggering with low effort is associated with preserved diaphragm force.

High flow nasal cannula (HFNC) with Heliox decreases diaphragmatic injury in a newborn porcine lung injury model

BACKGROUND

High flow nasal cannula (HFNC) improves ventilation by washing out nasopharyngeal dead space while delivering oxygen. Heliox (helium-oxygen gas mixture), a low-density gas mixture, decreases resistance to airflow, reduces the work of breathing, and facilitates distribution of inspired gas. Excessive lung work and potential injury increases the workload on the immature diaphragm predisposing the muscle to fatigue, and can lead to inflammatory and oxidative stress, thereby contributing to impaired diaphragmatic function. We tested the hypothesis that HFNC with Heliox will decrease the work of breathing thereby unloading the neonatal diaphragm, and potentially reducing diaphragmatic injury.

METHODS

Spontaneously breathing neonatal pigs were randomized to Nitrox (nitrogen-oxygen gas mixture) or Heliox, and studied over 4 hr following oleic acid injury. Gas exchange, pulmonary mechanics indices, and systemic markers of inflammation were measured serially. Diaphragm inflammation biomarkers and histology for muscle injury were assessed at termination.

RESULTS

Heliox breathing animals demonstrated decreased respiratory load and work of breathing with lower pressure-rate product, lower labored breathing index, and lower levels of diaphragmatic inflammatory markers, and muscle injury score as compared to Nitrox.

CONCLUSION

These results suggest that HFNC with Heliox is a useful adjunct to attenuate diaphragmatic fatigue in the presence of lung injury by unloading the diaphragm, resulting in a more efficient breathing pattern, and decreased diaphragm injury.

Chronic upper airway obstruction induces abnormal sleep/wake dynamics in juvenile rats

OBJECTIVES

Conventional scoring of sleep provides little information about the process of transitioning between vigilance-states. We used the state space technique to explore whether rats with chronic upper airway obstruction (UAO) have abnormal sleep/wake states, faster movements between states, or abnormal transitions between states.

DESIGN

The tracheae of 22-day-old Sprague-Dawley rats were surgically narrowed to increase upper airway resistance with no evidence for frank obstructed apneas or hypopneas; 24-h electroencephalography of sleep/wake recordings of UAO and sham-control animals was analyzed using state space technique. This non-categorical approach allows quantitative and unbiased examination of vigilance-states and state transitions. Measurements were performed 2 weeks post-surgery at baseline and following administration of ritanserin (5-HT2 receptor antagonist) the next day to stimulate sleep.

MEASUREMENTS AND RESULTS

UAO rats spent less time in deep (delta-rich) slow wave sleep (SWS) and near transition zones between states. State transitions from light SWS to wake and vice versa and microarousals were more frequent and rapid in UAO rats, indicating that obstructed animals have more regions where vigilance-states are unstable. Ritanserin consolidated sleep in both groups by decreasing the number of microarousals and trajectories between wake and light SWS, and increasing deep SWS in UAO.

CONCLUSIONS

State space technique enables visualization of vigilance-state transitions and velocities that were not evident by traditional scoring methods. This analysis provides new quantitative assessment of abnormal vigilance-state dynamics in UAO in the absence of frank obstructed apneas or hypopneas.

Chronic intrinsic transient tracheal occlusion elicits diaphragmatic muscle fiber remodeling in conscious rodents

BACKGROUND

Although the prevalence of inspiratory muscle strength training has increased in clinical medicine, its effect on diaphragm fiber remodeling is not well-understood and no relevant animal respiratory muscle strength training-rehabilitation experimental models exist. We tested the postulate that intrinsic transient tracheal occlusion (ITTO) conditioning in conscious animals would provide a novel experimental model of respiratory muscle strength training, and used significant increases in diaphragmatic fiber cross-sectional area (CSA) as the primary outcome measure. We hypothesized that ITTO would increase costal diaphragm fiber CSA and further hypothesized a greater duration and magnitude of occlusions would amplify remodeling.

METHODOLOGY/PRINCIPAL FINDINGS

Sprague-Dawley rats underwent surgical placement of a tracheal cuff and were randomly assigned to receive daily either 10-minute sessions of ITTO, extended-duration, 20-minute ITTO (ITTO-20), partial obstruction with 50% of cuff inflation pressure (ITTO-PAR) or observation (SHAM) over two weeks. After the interventions, fiber morphology, myosin heavy chain composition and CSA were examined in the crural and ventral, medial, and dorsal costal regions. In the medial costal diaphragm, with ITTO, type IIx/b fibers were 26% larger in the medial costal diaphragm (p<0.01) and 24% larger in the crural diaphragm (p<0.05). No significant changes in fiber composition or morphology were detected. ITTO-20 sessions also yielded significant increases in medial costal fiber cross-sectional area, but the effects were not greater than those elicited by 10-minute sessions. On the other hand, ITTO-PAR resulted in partial airway obstruction and did not generate fiber hypertrophy.

CONCLUSIONS/SIGNIFICANCE

The results suggest that the magnitude of the load was more influential in altering fiber cross-sectional area than extended-duration conditioning sessions. The results also indicated that ITTO was associated with type II fiber hypertrophy in the medial costal region of the diaphragm and may be an advantageous experimental model of clinical respiratory muscle strength training.

Tension-time index as a predictor of extubation outcome in ventilated children

RATIONALE

Indices that assess the load on the respiratory muscles, such as the tension-time index (TTI), may predict extubation outcome.

OBJECTIVES

To evaluate the performance of a noninvasive assessment of TTI, the respiratory muscle tension time index (TTmus), by comparison to that of the diaphragm tension time index (TTdi) and other predictors of extubation outcome in ventilated children.

METHODS

Eighty children (median [range] age 2.1 yr [0.15-16]) admitted to pediatric intensive care units at King's College and St Mary's Hospitals who required mechanical ventilation for more than 24 hours were studied.

MEASUREMENTS AND MAIN RESULTS

TTmus, maximal inspiratory pressure, respiratory drive, respiratory system mechanics, and functional residual capacity using a helium dilution technique, the rapid shallow breathing and CROP indices (compliance, rate, oxygenation, and pressure) indexed for body weight were measured and standard clinical data recorded in all patients. TTdi was measured in 28 of the 80 children using balloon catheters. Eight children (three in the TTdi group) failed extubation. TTmus (0.199 vs. 0.09) and TTdi (0.157 vs. 0.07) were significantly higher in children who failed extubation. TTmus greater than 0.18 (n = 80) and TTdi greater than 0.15 (n = 28) had sensitivities and specificities of 100% in predicting extubation failure. The other predictors performed less well.

CONCLUSIONS

Invasive and noninvasive measurements of TTI may provide accurate prediction of extubation outcome in mechanically ventilated children.

Upper airway loading induces growth retardation and change in local chondrocyte IGF-I expression is reversed by stimulation of GH release in juvenile rats

Chronic resistive airway loading (CAL) impairs growth in juvenile rats. The effects of CAL on epiphyseal growth plate (EGP) structure and insulin-like growth factor (IGF)-I gene expression have not been explored. Little is known about whether stimulants of endogenous growth hormone (GH) secretion can normalize this growth impairment. This study explored the effect of CAL on circulating and EGP GH/IGF-I pathway GH and the effect of ritanserin (endogenous GH stimulant) on somatic growth and the GH/IGF-I axis. We hypothesized that CAL would lead to a decrease in body temperature (Tb) and alterations of GH/IGF-I pathways, consequently leading to growth retardation. The tracheae of 22-day-old male rats were obstructed by tracheal banding (38 sham-operated control, 42 CAL). Tibial EGP morphometry, liver and EGP IGF mRNA, and serum GH and IGF-I levels were analyzed with quantitative real-time PCR and ELISA. Tb and locomotion activity (MA) were measured with telemetric transmitters inserted into the abdominal cavity. CAL animals had lower Tb and MA despite preserved food consumption. CAL impaired both tibial and tail length gains. Tail and tibial length gains inversely correlated with tracheal resistance. Circulating GH and IGF-I, liver and EGP IGF-I mRNA, and EGP width were decreased in the CAL group. Ritanserin administration to CAL animals normalized circulating and local EGP GH and IGF-I levels and minimized the longitudinal growth impairment. We conclude that CAL causes growth delay associated with alterations in the GH/IGF-I axis. Stimulation of GH release by ritanserin restored both global and local GH/IGF-I pathways, yet growth parameters were only partially restored.

Chronic upper airway resistive loading induces growth retardation via the GH/IGF-I axis in prepubescent rats

The effect of upper airway loading on longitudinal bone growth and various components of the growth hormone (GH)/insulin-like growth factor I (IGF-I) axis has not been fully elucidated. In the present study, the effect of chronic resistive airway loading (CAL) in a prepubescent rat model on linear bone growth and weight gain was investigated. We hypothesize that CAL induced in prepubescent rats will lead to impaired longitudinal growth due to impairment in circulating and liver GH/IGF-I parameters. The tracheae of 22-day-old rats were obstructed by tracheal banding to increase inspiratory esophageal pressure. The GH/IGF-I markers were analyzed using ELISA, RT-PCR, and Western immunoblot analysis 14 days after surgery. Animals exhibited impaired longitudinal growth as demonstrated by reduction of tibia and tail length gains by 40% ( P < 0.0001) and body weight gain by 24% ( P < 0.0001). No differences were seen in total body energy balance, i.e., oxygen consumption, daily food intake, or arterial blood gases. Circulating GH, IGF-I, and IGF binding protein-3 (IGFBP-3) levels were reduced by 40% ( P = 0.037), 30% ( P < 0.006), and 27% ( P = 0.02), respectively, in the CAL group. Liver IGF-I mRNA level decreased by 20% ( P < 0.0002), whereas GH receptor mRNA and protein expression were unchanged. We conclude that impaired longitudinal growth in prepubescent CAL rats is related to a decrease in GH, IGF-I, and IGFBP-3 levels.

Chronic resistive airway loading reduces weight due to low serum IGF-1 in rats

One of the consequences of chronic resistive airway loading in rats is malfunction in body weight gain post-surgery. The lower body weight of the obstructed animals was not related to lower caloric intake or to the oxygen consumption/food intake ratio. In the current study, we determined whether the retardation in body weight gain was related to impairment of serum insulin-like growth factor-1 (IGF-1) level or due to activation of inflammatory factors 21 weeks post-surgery. During the observation period, the airway-loaded animals (n=8) gained 44% less body weight (P<0.001) compared with controls (n=8) with no apparent effect on skeletal growth, i.e., body, tail and tibia length. Chronic airway-loaded animals had 32.5% lower serum IGF-1 levels (P<0.001) compared to the controls. Interleukin-6 and tumor necrosis factor-alpha levels were below 30 pg/ml in both groups. These data suggest that the weight loss in the chronic airway-loading rats is associated with a decreased IGF-1 level and not to activation of the inflammatory response.

Differential Cytokine Gene Expression in the Diaphragm in Response to Strenuous Resistive Breathing

Strenuous resistive breathing induces plasma cytokines that do not originate from circulating monocytes. We hypothesized that cytokine production is induced inside the diaphragm in response to resistive loading. Anesthetized, tracheostomized, spontaneously breathing Sprague-Dawley rats were subjected to 1, 3, or 6 hours of inspiratory resistive loading, corresponding to 45-50% of the maximum inspiratory pressure. Unloaded sham-operated rats breathing spontaneously served as control animals. The diaphragm and the gastrocnemius muscles were excised at the end of the loading period, and messenger ribonucleic acid expression of tumor necrosis factor-alpha, tumor necrosis factor-beta, interleukin (IL)-1alpha, IL-1beta, IL-2, IL-3, IL-4, IL-5, IL-6, IL-10, IFN-gamma, and two housekeeping genes was analyzed using multiprobe RNase protection assay. IL-6, IL-1beta, and, to lesser extents, tumor necrosis factor-alpha, IL-10, IFN-gamma, and IL-4 were significantly increased in a time-dependent fashion in the diaphragms but not the gastrocnemius of loaded animals or in the diaphragm of control animals. Elevation of protein levels of IL-6 and IL-1beta in the diaphragm of loaded animals was confirmed with immunoblotting. Immunostaining revealed IL-6 protein localization inside diaphragmatic muscle fibers. We conclude that increased ventilatory muscle activity during resistive loading induces differential elevation of proinflammatory and antiinflammatory cytokine gene expression in the ventilatory muscles.

Respiratory muscle injury, fatigue and serum skeletal troponin I in rat

To evaluate injury to respiratory muscles of rats breathing against an inspiratory resistive load, we measured the release into blood of a myofilament protein, skeletal troponin I (sTnI), and related this release to the time course of changes in arterial blood gases, respiratory drive (phrenic activity), and pressure generation. After approximately 1.5 h of loading, hypercapnic ventilatory failure occurred, coincident with a decrease in the ratio of transdiaphragmatic pressure to integrated phrenic activity (P(di)/ integral Phr) during sighs. This was followed at approximately 1.9 h by a decrease in the P(di)/ integral Phr ratio during normal loaded breaths (diaphragmatic fatigue). Loading was terminated at pump failure (a decline of P(di) to half of steady-state loaded values), approximately 2.4 h after load onset. During 30 s occlusions post loading, rats generated pressure profiles similar to those during occlusions before loading, with comparable blood gases, but at a higher neural drive. In a second series of rats, we tested for sTnI release using Western blot-direct serum analysis of blood samples taken before and during loading to pump failure. We detected only the fast isoform of sTnI, release beginning midway through loading. Differential detection with various monoclonal antibodies indicated the presence of modified forms of fast sTnI. The release of fast sTnI is consistent with load-induced injury of fast glycolytic fibres of inspiratory muscles, probably the diaphragm. Characterization of released fast sTnI may provide insights into the molecular basis of respiratory muscle dysfunction; fast sTnI may also prove useful as a marker of impending respiratory muscle fatigue.

Morphological and functional recovery from diaphragm injury: an in vivo rat diaphragm injury model

Our objective was to develop an in vivo model to study the timing and mechanisms underlying diaphragm injury and repair. Diaphragm injury was induced in anesthetized rats by the application of a 100 mM caffeine solution for a 10-min period to the right abdominal diaphragm surface. Diaphragms were removed 1, 4, 6, 12, 24, 48, 72, and 96 h and 10 days after the injury, with contractile function being assessed in strips in vitro by force-frequency curves. The extent of caffeine-induced membrane injury was indicated by the percentage of fibers with a fluorescent cytoplasm revealed by inward leakage of the procion orange dye. One hour after caffeine exposure, 32.9 ± 3.1 (SE) % of fibers showed membrane injury that resulted in 70% loss of muscle force. Within 72–96 h, the percentage of fluorescent cells decreased to control values. Muscle force, however, was still reduced by 30%. Complete muscle strength recovery was observed 10 days after the injury. Whereas diaphragmatic fiber repair occurred within 4 days after injury induction, force recovery took up to 10 days. We suggest that the caffeine-damaged rat diaphragm is a useful model to study the timing and mechanisms of muscle injury and repair.

Time course of diaphragm injury and calpain activity during resistive loading

The purpose of this study was to determine the time course of arterial blood gas (ABG) deterioration, increased calpain activity, and diaphragm injury during 4 d of resistive loading. Adult Sprague- Dawley rats were divided into control (C) animals and groups that were tracheally banded (TB) for 1 d (TB1), 2 d (TB2), 3 d (TB3), and 4 d (TB4). In TB rats, the carotid artery was cannulated and the trachea was banded during anesthesia. TB groups (TB1, TB2, TB3, and TB4) had a 67% smaller internal cross-sectional area of the trachea than did C animals. ABG samples from awake rats showed a decreased arterial oxygen tension (Pa(O(2))) and a respiratory acidosis in the TB1, TB2, and TB3 groups. Calpain activity was higher in the diaphragm of TB than of C rats; calpainlike activities in soluble fractions of diaphragm tissue were greater in all TB groups than in C rats, whereas those in bound fractions were greater in the TB2 and TB3 groups. Point counting of hematoxylin and eosin-stained cross-sections showed that the area fraction (A(A)) of normal diaphragm was lower and the A(A) of abnormal muscle and connective tissue was higher in TB3 than in C rats. Increased resistive loading induced by tracheal banding was associated with hypercapnic ventilatory failure, increased calpain activity, and diaphragm injury. Ventilatory failure in response to resistive loading may be due to diaphragm injury and/or to decreased minute ventilation.