Biomedical subjects

Thomas Similowski

Publications and source records attributed to Thomas Similowski.

At least 19 recordsLinked to original sources

PubMed2007

Effects of hypercapnia and hypocapnia on ventilatory variability and the chaotic dynamics of ventilatory flow in humans.

In humans, lung ventilation exhibits breath-to-breath variability and dynamics that are nonlinear, complex, sensitive to initial conditions, unpredictable in the long-term, and chaotic. Hypercapnia, as produced by the inhalation of a CO(2)-enriched gas mixture, stimulates ventilation. Hypocapnia, as produced by mechanical hyperventilation, depresses ventilation in animals and in humans during sleep, but it does not induce apnea in awake humans. This emphasizes the suprapontine influences on ventilatory control. How cortical and subcortical commands interfere thus depend on the prevailing CO(2) levels. However, CO(2) also influences the variability and complexity of ventilation. This study was designed to describe how this occurs and to test the hypothesis that CO(2) chemoreceptors are important determinants of ventilatory dynamics. Spontaneous ventilatory flow was recorded in eight healthy subjects. Breath-by-breath variability was studied through the coefficient of variation of several ventilatory variables. Chaos was assessed with the noise titration method (noise limit) and characterized with numerical indexes [largest Lyapunov exponent (LLE), sensitivity to initial conditions; Kolmogorov-Sinai entropy (KSE), unpredictability; and correlation dimension (CD), irregularity]. In all subjects, under all conditions, a positive noise limit confirmed chaos. Hypercapnia reduced breathing variability, increased LLE (P = 0.0338 vs. normocapnia; P = 0.0018 vs. hypocapnia), increased KSE, and slightly reduced CD. Hypocapnia increased variability, decreased LLE and KSE, and reduced CD. These results suggest that chemoreceptors exert a strong influence on ventilatory variability and complexity. However, complexity persists in the quasi-absence of automatic drive. Ventilatory variability and complexity could be determined by the interaction between the respiratory central pattern generator and suprapontine structures.

Adult↗

PubMed2006

Electroencephalographic evidence for pre-motor cortex activation during inspiratory loading in humans.

Faced with mechanical inspiratory loading, awake animals and anaesthetized humans develop alveolar hypoventilation, whereas awake humans do defend ventilation. This points to a suprapontine compensatory mechanism instead of or in addition to the 'traditional' brainstem respiratory regulation. This study assesses the role of the cortical pre-motor representation of inspiratory muscles in this behaviour. Ten healthy subjects (age 19-34 years, three men) were studied during quiet breathing, CO2-stimulated breathing, inspiratory resistive loading, inspiratory threshold loading, and during self-paced voluntary sniffs. Pre-triggered ensemble averaging of Cz EEG epochs starting 2.5 s before the onset of inspiration was used to look for pre-motor activity. Pre-motor potentials were present during voluntary sniffs in all subjects (average latency (+/-s.d.): 1325 +/- 521 ms), but also during inspiratory threshold loading (1427 +/- 537 ms) and during inspiratory resistive loading (1109 +/- 465 ms). Pre-motor potentials were systematically followed by motor potentials during inspiratory loading. Pre-motor potentials were lacking during quiet breathing (except in one case) and during CO2-stimulated breathing (except in two cases). The same pattern was observed during repeated experiments at an interval of several weeks in a subset of three subjects. The behavioural component of inspiratory loading compensation in awake humans could thus depend on higher cortical motor areas. Demonstrating a similar role of the cerebral cortex in the compensation of disease-related inspiratory loads (e.g. asthma attacks) would have important pathophysiological implications: it could for example contribute to explain why sleep is both altered and deleterious in such situations.

Adult↗

PubMed2006

Control of tracheal cuff pressure: a pilot study using a pneumatic device.

OBJECTIVE: To evaluate the efficacy of a simple mechanical device to maintain constant endotracheal cuff pressure (Pcuff) during mechanical ventilation (large encased inflatable cuff connected to the endotracheal cuff and receiving constant pressure from a heavy mass attached to an articulated arm). DESIGN AND SETTING: Single-center, prospective, randomized, crossover, pilot study in a medical intensive care unit. PATIENTS AND PARTICIPANTS: Nine consecutive mechanically ventilated patients (age 62+/-20 years, SAPS II score 39+/-15). INTERVENTIONS: Control day: Pcuff monitored and adjusted with a manometer (Hi-Lo, Tyco Healthcare) according to current recommendations (twice a day and after each intervention on the tracheal tube); initial target Pcuff 22-28 cmH20. Prototype day: test device connected to the endotracheal cuff; same initial target. Continuous Pcuff recording during both days. Control and prototype days in random order. RESULTS: Pcuff values over 50 cmH20 were recorded in six patients during the control day (178+/-159min), never during the prototype day. During the control day, Pcuff was between 30 and 50 cmH20 for 29+/-25% of the time, vs 0.3+/-0.3% during the prototype day (p<0.01). Pcuff was between 15 and 30 cmH20 for 56+/-36% of the time during the control day, vs 95+/-14% during the prototype day p<0.01). During the control day, Pcuff was below 15 cmH20 for 15+/-17% of the time, vs 4.7+/-15% during the prototype day (p<0.05). CONCLUSIONS: The tested device successfully controlled Pcuff with minimal human resource consumption. Prospective studies are required to assess its clinical impact.

Cross-Over Studies↗

PubMed2006

Inspiratory resistances facilitate the diaphragm response to transcranial stimulation in humans.

BACKGROUND: Breathing in humans is dually controlled for metabolic (brainstem commands) and behavioral purposes (suprapontine commands) with reciprocal modulation through spinal integration. Whereas the ventilatory response to chemical stimuli arises from the brainstem, the compensation of mechanical loads in awake humans is thought to involve suprapontine mechanisms. The aim of this study was to test this hypothesis by examining the effects of inspiratory resistive loading on the response of the diaphragm to transcranial magnetic stimulation. RESULTS: Six healthy volunteers breathed room air without load (R0) and then against inspiratory resistances (5 and 20 cmH2O/L/s, R5 and R20). Ventilatory variables were recorded. Transcranial magnetic stimulation (TMS) was performed during early inspiration (I) or late expiration (E), giving rise to motor evoked potentials (MEPs) in the diaphragm (Di) and abductor pollicis brevis (APB). Breathing frequency significantly decreased during R20 without any other change. Resistive breathing had no effect on the amplitude of Di MEPs, but shortened their latency (R20: -0.903 ms, p = 0.03) when TMS was superimposed on inspiration. There was no change in APB MEPs. CONCLUSION: Inspiratory resistive breathing facilitates the diaphragm response to TMS while it does not increase the automatic drive to breathe. We interpret these findings as a neurophysiological substratum of the suprapontine nature of inspiratory load compensation in awake humans.

Airway Resistance↗

PubMed2006

Dyspnea as a noxious sensation: inspiratory threshold loading may trigger diffuse noxious inhibitory controls in humans.

Dyspnea, a leading respiratory symptom, shares many clinical, physiological, and psychological features with pain. Both activate similar brain areas. The neural mechanisms of dyspnea are less well described than those of pain. The present research tested the hypothesis of common pathways between the two sensations. Six healthy men (age 30-40 yr) were studied. The spinal nociceptive flexion reflex (RIII) was first established in response to electrical sural stimulation. Dyspnea was then induced through inspiratory threshold loading, forcing the subjects to develop 70% of their maximal inspiratory pressure to inhale. This led to progressive inhibition of the RIII reflex that reached 50 +/- 12% during the fifth minute of loading (P < 0.001), was correlated to the intensity of the self-evaluated respiratory discomfort, and had recovered 5 min after removal of the load. The myotatic H-reflex was not inhibited by inspiratory loading, arguing against postsynaptic alpha motoneuron inhibition. Dyspnea, like pain, thus induced counterirritation, possibly indicating a C-fiber stimulation and activation of diffuse noxious inhibitory descending controls known to project onto spinal dorsal horn wide dynamic range neurons. This confirms the noxious nature of certain types of breathlessness, thus opening new physiological and perhaps therapeutic perspectives.

Adult↗

PubMed2006

Effects of mandibular advancement on upper airway dynamics in awake normal subjects: a pilot study with phrenic nerve stimulation.

BACKGROUND: Mandibular advancement (MA) can be an alternative to continuous positive airway pressure (CPAP) in the treatment of obstructive sleep apnea syndrome. This study was designed to describe its effects on upper airway mechanics. METHODS: Six awake healthy subjects (four men, 31+/-8 years, body mass index (BMI) 25+/-2kg/m2) were studied supine. Phrenic stimulation was used to measure pressure-flow relationships in response to diaphragm contractions without pre-activation of upper airway dilators during progressive MA. RESULTS: Phrenic stimulation consistently induced flow limitation (initial peak V'-t1, corresponding to the limiting pressure; reduction to a nadir V'-t2; reincrease to a second peak V'-t3). The upper airway resistances were negatively correlated with MA. Their values at 4mm were significantly lower than at baseline. Further MA reduced resistances. CONCLUSIONS: As with CPAP, MA in normal subjects decreases the propensity of the upper airway to collapse in response to a negative pressure pulse.

Adult↗

PubMed2006

Optimized analysis of surface electromyograms of the scalenes during quiet breathing in humans.

Studying the inspiratory recruitment of the scalenes is clinically relevant, but the interpretation of surface electromyographic (EMG) recordings is difficult. The aim of this study was to optimize an averaging method to analyze the surface EMG activity of the scalenes. Ten healthy subjects were studied. Nasal flow and surface EMG of the right scalene were recorded during 15 min epochs of quiet breathing. In four subjects, needle scalene EMG was also recorded. The flow signal was used to trigger the ensemble averaging of the ventilatory wave forms from 80 consecutive breaths. In eight cases, this evidenced a phasic inspiratory activation of the scalenes and permitted the determination of the electromechanical inspiratory delay (134+/-55 ms) and post-inspiratory activity (811+/-233 ms). When simultaneously available, surface and intramuscular recordings provided identical results. An averaging method triggered from a respiratory flow signal can identify and characterize a low phasic inspiratory activity of the scalenes within a noisy surface signal.

Adult↗

PubMed2006

Responses of the diaphragm to transcranial magnetic stimulation during wake and sleep in humans.

UNLABELLED: The human ventilation depends on bulbospinal and corticospinal commands. This study assessed their interactions in five healthy volunteers (two men, age 25-35) through the description of diaphragm and abductor pollicis brevis (APB) motor potentials (DiMEPs, abpMEPs) evoked by transcranial magnetic stimulation (TMS) during relaxed expiration and tidal inspiration and during wake and sleep. NREM decreased corticospinal excitability and REM further did so, for both the diaphragm and the APB. During wake, inspiration shortened supine DiMEPs latencies (expiration 18.56+/-1.90ms; inspiration 17.37+/-1.48ms, P<0.001). This persisted during sleep in an augmented manner (expiration: 21.05+/-1.39ms; inspiration 18.69+/-1.17ms, P=0.002). Inspiration had no effect on apbMEPs during wake and sleep. IN CONCLUSION: (1) the tidal bulbospinal input to phrenic motoneurones is sufficient to modulate the throughput of the corticospinal pathway to these neurones; (2) this modulation is best seen after the sleep related removal of corticospinal and/or afferent inputs.

Adult↗

PubMed2006

Reduced breathing variability as a predictor of unsuccessful patient separation from mechanical ventilation.

OBJECTIVES: To compare descriptors of the breath-to-breath respiratory variability during a 60-min spontaneous breathing trial in patients successfully and unsuccessfully separated from the ventilator and the endotracheal tube and to assess the usefulness of these predictors in discriminating these two categories of patients. DESIGN: Prospective observational study. SETTING: Four general intensive care units in university hospitals. PATIENTS: A total of 51 consecutive patients mechanically ventilated for >24 hrs. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Tidal volume, respiratory period, inspiratory time, expiratory time, mean inspiratory flow (tidal volume/inspiratory time), and duty cycle (inspiratory time/respiratory period) were obtained from the flow signal. Breath-by-breath variability was expressed in terms of their coefficients of variation (CV), the number of breaths among which a significant correlation was found (lag), and the autocorrelation coefficient between one breath and the following one. Five patients were excluded because of nonstationarity of the data, leaving 46 cases for analysis. Between-group comparison was conducted with the Mann-Whitney test, and a nonparametric classification and regression tree was used to identify variables discriminating "success" (n = 32) and "failure" patients (n = 14). All coefficients of variation were significantly higher in success patients, who also exhibited significantly less respiratory autocorrelation (shorter "short memory"). The classification and regression tree analysis allocated all success patients to a group defined by a coefficient of variation of tidal volume/inspiratory time of > or =19% and a coefficient of variation of inspiratory time/respiratory period of > or =10% that did not contain any failure patient. All failure patients belonged to a group with coefficient of variation of tidal volume/inspiratory time of <19%, a lag tidal volume of > or =11, and that contained no success patient. CONCLUSIONS: In intensive care unit patients undergoing a spontaneous breathing trial, breathing variability is greater in patients successfully separated from the ventilator and the endotracheal tube. Variability indices are sufficient to separate success from failure cases.

Acidosis, Respiratory↗

PubMed2005

Successful reimplantation of a phrenic nerve stimulator after traumatic exteriorisation.

A quadriplegic patient experienced post-traumatic exteriorisation of the subcutaneous receiver of a phrenic pacemaker. In the absence of infection and stimulation dysfunction in the patient, the device was reimplanted with full success at 1 year. This strategy can, exceptionally, be considered if removal and subsequent implantation is impossible or refused.

Diaphragm↗

PubMed2005

Chaotic dynamics of resting ventilatory flow in humans assessed through noise titration.

The mammalian ventilatory behaviour exhibits nonlinear dynamics as reflected by certain nonlinearity or complexity indicators (e.g. correlation dimension, approximate entropy, Lyapunov exponents, etc.) but this is not sufficient to determine its possible chaotic nature. To address this, we applied the noise titration technique, previously shown to discern and quantify chaos in short and noisy time series, to ventilatory flow recordings obtained in quietly breathing normal humans. Nine subjects (8 men and 1 woman, 24-42 years) were studied during 15-min epochs of ventilatory steady-state (10.1+/-3.0 breaths/min, tidal volume 0.63+/-0.2 L). Noise titration applied to the unfiltered signals subsampled at 5 Hz detected nonlinearity in all cases (noise limit 20.2+/-12.5%). Noise limit values were weakly correlated to the correlation dimension and the largest Lyapunov exponent of the signals. This study shows that the noise titration approach evidences a chaotic dimension to the behavior of ventilatory flow over time in normal humans during tidal breathing.

Adult↗

PubMed2005

Diaphragmatic dysfunction in patients with idiopathic inflammatory myopathies.

Polymyositis, dermatopolymyositis, and inclusion body myositis imply chronic inflammation of skeletal muscles. Pulmonary complications include aspiration pneumonia, interstitial pneumonitis, or respiratory muscle myositis. This study aims at better describing their impact on respiratory muscle. Twenty-three consecutive patients (12 PM, 5 DM, 6 IBM) were studied (static inspiratory and expiratory pressures; diaphragm function in terms of the mouth and transdiaphragmatic pressure responses to bilateral phrenic stimulation). Pulmonary parenchymatous abnormalities were mild (6 cases) or absent. The mouth pressure produced by phrenic stimulation was 6.83+/-3.01 cm H2O, with 18 patients (78%) diagnosed with diaphragm weakness (<10 cm H2O) and lower values in DM (4.35+/-1.48 cm H2O) than in IBM and in PM (P<0.05). Diaphragm weakness is frequent and probably overlooked in inflammatory myopathies. Further studies are needed to delineate the clinical relevance of these results.

Aged↗

PubMed2005

Patients aged 90 years or older in the intensive care unit.

BACKGROUND: Age is an important prognostic factor in patients admitted to intensive care units (ICUs), but it is not as important as illness severity. However, age seems to remain an important independent triage criterion for ICU admission, and 90 years of age seems to represent a psychological barrier for many ICU physicians. The aim of this preliminary study is to compare the management and outcome of patients aged 90 years or older admitted to a respiratory ICU with those of patients aged 70 years or younger. METHODS: In our matched case-control study over a 6-year period, 36 patients aged 90 years or older (case patients) were selected and matched according to sex with 72 controls chosen in the 20- to 69-year age range. The Simplified Acute Physiology Score (SAPS) II was then computed without using age as a variable. RESULTS: Pre-existing comorbidities were significantly less frequent in cases than in controls (5.1% vs 30.5%, p <.01). Compared to controls, cases were more frequently admitted for cardiac failure (22% vs 7%, p <.05) and less frequently for neurological diseases (0% vs 11%, p <.05). The use of advanced life-support measures in the ICU such as mechanical ventilation, central venous or arterial catheterization, and vasoactive and/or inotropic drugs was not significantly different between case patients and controls. This was also the case for ICU mortality and for the mean duration of ICU and hospital stay. Although there was a trend toward a higher hospital mortality among case patients than among controls, it did not reach statistical significance (47% vs 27%, p =.07). CONCLUSION: Our results reinforce the idea that age alone is not a relevant criterion for ICU admission.

Age Factors↗

PubMed2005

Prognostic value of the hematocrit in patients with severe COPD receiving long-term oxygen therapy.

BACKGROUND: Although traditionally associated with polycythemia, COPD has a systemic inflammatory component that could interfere with erythropoiesis. This study describes the distribution and prognostic value of the hematocrit in patients with severe COPD receiving long-term oxygen therapy (LTOT). METHODS: A total of 2,524 patients with COPD, FEV1/vital capacity (VC) < 70%, FEV1 < 80% of predicted, and Pa(O2) < 7.3 kPa in whom a hematocrit was available at entry was identified between 1980 and 1999 in the French Association Nationale pour le Traitement à Domicile de l'Insuffisance Respiratoire chronic respiratory insufficiency and home-care database (male/female ratio, 5/1; mean +/- SD age, 68 +/- 10 years for men, and 70 +/- 10 years for women). Correlations between hematocrit, demographic data, and pulmonary function data were examined. A multivariate Cox proportional hazard regression was performed to identify prognostic factors. RESULTS: Mean hematocrit was 45.9 +/- 7.0% in men and 43.9 +/- 6.0% in women (< 39% in 12.6% of men, and < 36% in 8.2% of women) according to the World Health Organization definition of anemia. Hematocrit was negatively correlated with age (r = - 0.245) and FEV1/VC (r = - 0.068) and was positively correlated with Pa(CO2) (r = 0.161) and body mass index (r = 0.127). Multivariate analysis found hematocrit to be an independent predictor of survival, hospital admission rate, and cumulative duration of hospitalization. The 3-year survival was 24% (95% confidence interval, 16 to 33%) when the hematocrit was < 35%, and 70% (63 to 76%) when the hematocrit was > or = 55%. CONCLUSIONS: A low hematocrit is not uncommon in LTOT/COPD patients. Hematocrit is negatively associated with mortality and morbidity. Whether the association is causative or not and whether or not corrective measures are warranted remain to be determined.

Aged↗

PubMed2004

[Diagnosis and management of exacerbations and acute respiratory failure in patients with chronic obstructive pulmonary disease].

Acute episode of chronic obstructive pulmonary disease occurs in almost all patients, during which cough, expectoration and dyspnea increase. When the underlying disease is not severe and the acute episode not life-threatening, the term "exacerbation" is appropriate, and the patients can be managed at home. When the underlying disease is advanced and the acute episode possibly life-threatening, the terms of "acute respiratory failure" or "decompensation" can be used. These patients are most often admitted to the hospital, and at times to the intensive care unit. Bronchodilators and respiratory physiotherapy form the basis of the management of acute episodes of COPD. In severe cases, oxygen must be administered, and the decision of an hospitalisation considered. Antibiotics and corticosteroids shoud not be prescribed in a systematic manner. In the most severe cases, non-invasive ventilation must be accessible. The prevention of acute episodes of COPD is best achieved through tobacco cessation and influenza vaccine. Finally, an acute episode may be an opportunity to make a diagnosis of COPD if this has not been done before.

Acute Disease↗

PubMed2004

Upper airway afferents are sufficient to evoke the early components of respiratory-related cortical potentials in humans.

Repeated inspiratory occlusions in humans elicit respiratory-related cortical potentials, the respiratory counterpart of somatosensory-evoked potentials. These potentials comprise early components (stimulus detection) and late components (cognitive processing). They are considered as the summation of several afferent activities from various part of the respiratory system. This study assesses the role of the upper airway as a determinant of the early and late components of the potentials, taking advantage of the presence of a tracheotomy in patients totally or partially deafferented. Eight patients who could breathe either through the mouth or through a tracheotomy orifice (whole upper airway bypassed) were studied (4 quadriplegic patients with phrenic pacing, 4 patients with various sources of inspiratory pump dysfunction). Respiratory-related evoked potentials were recorded in CZ-C3 and CZ-C4. They were consistently present after mouth occlusions, with a first positive P1 and a first negative N1 components of normal latencies (P1: 40.4 +/- 6.1 ms in CZ-C3 and 47.6 +/- 7.6 ms in CZ-C4; N1: 84.4 +/- 27.1 ms in CZ-C3 and 90.2 +/- 17.4 ms in CZ-C4) and amplitudes. Tracheal occlusions did not evoke any cortical activity. Therefore, in patients with inspiratory pump dysfunction, the activation of upper airway afferents is sufficient to produce the early components of the respiratory-related evoked cortical potentials. Per contra, in this setting, pulmonary afferents do not suffice to evoke these components.

Adult↗

PubMed2004

Iatrogenic-induced dysfunction of the neuromuscular respiratory system.

In the presence of respiratory symptoms that are associated with alveolar hypoventilation or a restrictive ventilatory defect and in the absence of parenchymal or pleural abnormalities on the chest radiograph, iatrogenic causes must be evoked, exactly as they are in the presence of interstitial lung disease. In most cases, the anamnestic and clinical contexts provide a strong diagnostic presumption. It is important to establish carefully the mechanism of the observed disorders, using the currently available arsenal of diagnostic tools for clinical and prognostic reasons and from a medicolegal standpoint. It is necessary to evaluate precisely the clinical repercussions of the respiratory neuromuscular abnormality to serve as a basis for follow-up and to discuss therapeutic options in certain cases (eg, nocturnal ventilation to correct nocturnal hypoventilation due to diaphragmatic dysfunction, diaphragm plication to alleviate dyspnea after complete phrenic nerve destruction, phrenic nerve pacing), again in the perspective of medicolegal actions.

Diaphragm↗

PubMed2004

Proportional-assist ventilation compared with pressure-support ventilation during exercise in volunteers with external thoracic restriction.

OBJECTIVES: Proportional-assist ventilation (PAV) is able to unload respiratory muscles in proportion to the subject's inspiratory effort. However, leak-related alterations in the flow signal, effort-induced modifications in respiratory mechanics, or approximate adjustment of PAV could jeopardize such a theory. The aim of this study was to compare noninvasive PAV and pressure-support ventilation (PSV) in healthy volunteers with external thoracic restriction at rest and during exercise. DESIGN: Prospective, crossover, randomized study. SETTING: Investigation unit in a nonteaching hospital. PATIENTS: Seven volunteers with external thoracic restriction. INTERVENTION: After external thoracic restriction to increase elastance (9.00 +/- 1.63 cm H2O/L estimated from the level of elastic assistance), PAV and PSV were compared at rest and during exercise (90 W for 10 mins). MEASUREMENTS AND MAIN RESULTS: Flow, airway pressure, and changes in esophageal pressure were measured, and the tidal volume (Vt) and inspiratory muscle effort indexes were calculated. At rest, all variables were comparable during PSV and PAV. Exercise produced a 200% increased in Vt with no change in the breathing frequency and a 400% increased in inspiratory muscle effort indexes. During exercise, peak inspiratory airway pressure was significantly higher with PAV than with PSV (24 +/- 5 vs. 10 +/- 2 cm H2O, p <.05). The Vt and breathing frequency (23 +/- 4 vs. 24 +/- 3 breaths/min) were similar, but the inspiratory muscle effort indexes were significantly lower with PAV than with PSV. A significant linear correlation was found between changes in esophageal pressure and the peak inspiratory airway pressure during PAV (r =.94, p =.0001), whereas, as expected, it was not the case during PSV (r =.27, p =.34). CONCLUSION: In volunteers with external thoracic restriction mimicking a patient with increased elastic work of breathing, the breathing pattern at rest and during exercise were comparable with PSV and PAV, whereas inspiratory muscle effort was lower with PAV during exercise because of the significant automatic increase in assistance with PAV.

Adult↗