Introduction to a new inspiratory threshold loading device

Inspiratory threshold loading is an important tool in assessing inspiratory muscle function and has been widely used in studies of respiratory mechanics. A simple system was designed to provide a pure and precise inspiratory threshold load. The inspiratory threshold loading system comprises a flow generator attached via a pressure chamber, to the inspiratory port of a Hans-Rudolph valve. By limiting the circulation of air generated by the flow generator between the room and the pressure chamber, different levels of negative pressure are produced in the pressure chamber and applied to the inspiratory valve. An inspiratory pressure equal to the negative pressure in the chamber has to be generated to open the inspiratory valve for flow. The system was tested in four subjects by measuring mouth pressure (Pm), threshold pressure (Pth) and valve resistance (R). The Pm at the beginning of inspiratory flow was defined as the opening pressure (Pop). The Pth was constant throughout inspiration. Pop was linearly related to Pth (p<0.0001), with slopes within 2% of identity and intercepts within +/-0.2 cmH2O. R was between 0.7-1.1 cmH2O x L(-1) x s(-1) under all conditions and was independent of both Pth (p=0.76) and inspiratory flow (p=0.24). The results demonstrate that this system can provide a constant inspiratory threshold load throughout inspiration with little flow resistance, and is therefore ideal for respiratory studies.

Autonomic reaction to vestibular damage

The vestibular system provides inputs to many neurons in the brain stem that participate in autonomic control. This multiplicity of vestibular-autonomic connections plays a variety of roles. Whereas it has been known for decades that unilateral vestibular lesions can result in motion sickness, recent data suggest that the vestibular system participates in making adjustments in blood pressure and respiration that are necessary to maintain homeostasis during movement and changes in posture. Animals with bilateral vestibular lesions are more susceptible to posturally related hypotension than vestibularly intact animals, and it is also possible that orthostatic hypotension after space flight is caused in part by microgravity-related changes in otolith function. Patients with vestibular lesions could also be more vulnerable to respiratory disturbances related to posture, such as obstructive apnea. Vestibular dysfunction has additionally been linked with anxiety disorders, such as agoraphobia, which may result from alteration of vestibular inputs to brain stem monoaminergic neurons (which are known to process these signals). Even sleep disturbances might be connected with vestibular disorders because neurons in the pontine reticular formation that are critical in switching between sleep states may be influenced by labyrinthine inputs. Thus it is likely that vestibular damage will result in a number of parallel disturbances in autonomic function.

Rib cage muscle interaction in airway pressure generation

We have previously demonstrated in dogs that the change in airway opening pressure (DeltaPao) produced by isolated maximum activation of the parasternal intercostal or triangularis sterni muscle in a single interspace, the sternomastoids, and the scalenes is proportional to the product of muscle mass and the fractional change in muscle length per unit volume increase of the relaxed chest wall. In the present study, we have assessed the interactions between these muscles by comparing the DeltaPao obtained during simultaneous activation of a pair of muscles (measured DeltaPao) to the sum of the DeltaPao values obtained during their separate activation (predicted DeltaPao). Measured and predicted DeltaPao values were compared for the following pairs of muscles: the parasternal intercostals in two interspaces, the parasternal intercostals in one interspace and either the sternomastoids or the scalenes, two segments of the triangularis sterni, and the interosseous intercostals in two contiguous interspaces. For all these pairs, the measured DeltaPao was within approximately 10% of the predicted value. We therefore conclude that 1) the pressure changes generated by the rib cage muscles are essentially additive; and 2) measurements of the mass of a particular muscle and of its fractional change in length during passive inflation can be used to estimate the potential pressure-generating ability of the muscle during coordinated activity as well as during isolated activation.

Respiratory muscle fitness and exercise endurance in healthy humans

New evidence exists that the respiratory muscles may limit exercise performance in healthy humans. Four weeks of isolated respiratory training (30 min normocapnic hyperpnea, 5 d.wk-1 significantly increased the endurance time of respiratory muscles and the endurance time of constant-load bicycle tests in sedentary as well as physically active subjects once respiratory muscles had recovered from the training. Minute ventilation and blood lactate concentration were reduced during post-training exercise. Furthermore, respiratory trained subjects had lost the sensation of breathlessness. Maximal oxygen consumption was not affected by respiratory training. The mechanism by which respiratory training improves overall physical performance is as yet unknown.

Asymmetry in reflex responses of nasal muscles in anesthetized guinea pigs

Nasal reflexes elicited by mechanical or electrical stimulation of nasal afferents were studied in anesthetized guinea pigs. Probing the nasal cavity of one side evoked a greater activation of the contralateral than the ipsilateral nasal muscles and, occasionally, sneezing. Similarly, electrical stimulation of the ethmoidal nerve often caused sneezing, with a greater activation of the nasal muscles and a greater increase in resistance on the contralateral side. Asymmetrical activation of the nasal muscles in response to mechanical stimuli induces asymmetrical airflows, especially during sneezing, between the two sides of the nasal cavity. Most of the expired air is forcibly blown out through the ipsilateral nostril, thus improving the elimination of irritants from the nose.

Pontine cholinergic respiratory depression in neonatal and young rats

We postulated that activation of pontine cholinergic mechanisms would cause respiratory depression in neonatal and young rats. Phrenic activity was recorded in decerebrate, paralyzed, ventilated and vagotomized rats of 4 to 22 days after birth. Small volumes (10-60 nl) of carbachol (44-88 mM) were injected into the medial portion of the rostral pons. The injection of carbachol, but not saline, decreased phrenic peak activity (83 +/- 6% of control) and respiratory frequency (64 +/- 9.5% of control) within 2 min following the injection in neonates and the depression lasted for less than 10 min. The site of injection in the pontine reticular formation was confirmed by histology. Results suggest that cholinergic mechanisms in the medial pons depress respiratory activity in the neonate.

[Preliminary study on determination methods of respiratory muscle functions]

Attention has been paid to the research of respiratory muscle functions recently. In this paper some lung function indices concerning respiratory muscle functions, such as maximal inspiratory and expiratory pressure at mouth, inspiratory and expiratory tolerance time, transdiaphragmatic pressure and maximal transdiaphragmatic pressure and diaphragmatic electromyogram etc, were determined in 24 healthy young men and the normal values were obtained. They were important in the study of respiratory muscle functions in hard physical labours, especially in divers, pilots and sportsmen etc. These indices were also useful for evaluating convalescent therapy chronic lung diseases, respiratory muscles training programs or the drug effects on respiratory muscle fatigue etc.

Effect of alae nasi activation on maximal nasal inspiratory airflow in humans

The upper airway is a complicated structure that is usually widely patent during inspiration. However, on inspiration during certain physiological and pathophysiological states, the nares, pharynx, and larynx may collapse. Collapse at these locations occurs when the transmural pressure (Ptm) at a flow-limiting site (FLS) falls below a critical level (Ptm'). On airway collapse, inspiratory airflow is limited to a maximal level (VImax) determined by (-Ptm')/Rus, where Rus is the resistance upstream to the FLS. The airflow dynamics of the upper airway are affected by the activity of its associated muscles. In this study, we examine the modulation of VImax by muscle activity in the nasal airway under conditions of inspiratory airflow limitation. Each of six subjects performed sniffs through one patent nostril (pretreated with an alpha agonist) while flaring the nostril at varying levels of dilator muscle (alae nasi) EMG activity (EMGan). For each sniff, we located the nasal FLS with an airway catheter and determined VImax, Ptm', and Rus. Activation of the alae nasi from the lowest to the highest values of EMGan increased VImax from 422 +/- 156 to 753 +/- 291 ml/s (P < 0.01) and decreased Ptm' from -3.6 +/- 3.0 to -6.0 +/- 4.7 cmH2O (P < 0.05). Activation of the alae nasi had no consistent effect on Rus. VImax was positively correlated with EMGan, and Ptm' was negatively correlated with EMGan in all subjects. Our findings demonstrate that alae nasi activation increases VImax through the nasal airway by decreasing airway collapsibility.

Functional properties of the ryanodine receptor type 3 (RyR3) Ca2+ release channel

Single-channel analysis of sarcoplasmic reticulum vesicles prepared from diaphragm muscle, which contains both RyR1 and RyR3 isoforms, revealed the presence of two functionally distinct ryanodine receptor calcium release channels. In addition to channels with properties typical of RyR1 channels, a second population of ryanodine-sensitive channels with properties distinct from those of RyR1 channels was observed. The novel channels displayed close-to-zero open-probability at nanomolar Ca2+ concentrations in the presence of 1 mM ATP, but were shifted to the open conformation by increasing Ca2+ to micromolar levels and were not inhibited at higher Ca2+ concentrations. These novel channels were sensitive to the stimulatory effects of cyclic adenosine 5'-diphosphoribose (cADPR). Detection of this second population of RyR channels in lipid bilayers was always associated with the presence of the RyR3 isoform in muscle preparations used for single-channel measurements and was abrogated by the knockout of the RyR3 gene in mice. Based on the above, we associated the novel population of channels with the RyR3 isoform of Ca2+ release channels. The functional properties of the RyR3 channels are in agreement with a potential qualitative contribution of this channel to Ca2+ release in skeletal muscle and in other tissues.

The diaphragm and respiratory muscles

This article describes the functional anatomy of the muscles that contribute to the generation of ventilatory pressures. The physiologic principles that guide their function are analyzed, and their composition and metabolism are described, inasmuch as this affects their adaptation and capacity to face acute and chronic loads. Special emphasis is placed on the clinical and laboratory evaluation of their function. In addition, the concept of respiratory muscle fatigue is developed so the reader can place all these concepts in the context of their clinical applications.

Breathing efficiency during inspiratory threshold loading in patients with chronic obstructive pulmonary disease

Patients with chronic obstructive pulmonary disease (COPD) demonstrate an increased oxygen cost of breathing. It is as yet unclear whether this is related to a decreased breathing efficiency. The aim of the present study was to compare breathing efficiency in 16 patients with COPD (11 men, five women) and 16 healthy elderly subjects (seven men, nine women), and to investigate a possible relationship between breathing efficiency and resting energy expenditure (REE). REE was measured using a ventilated hood system. Breathing efficiency was assessed by measuring oxygen consumption (V'O2), mean inspiratory mouth pressure (MIP) and flow during breathing at rest and subsequently during breathing against an inspiratory threshold (40% of maximal inspiratory pressure). During loaded breathing there was a significant increase in V'O2, MIP, and external work of breathing compared with unloaded breathing in both groups. As intended, ventilation did not increase significantly during the breathing efficiency test in the patients with COPD. The breathing efficiency (median, range) of the patients with COPD was similar (3.7%, 1.4-8.7%) to that of the healthy elderly subjects (3.2%, 1.7-8.3%). Breathing efficiency was not correlated with REE in either group. In the present study, in which dynamic hyperinflation was probably prevented, no difference in breathing efficiency was found between healthy elderly subjects and COPD patients when breathing against an external inspiratory threshold. Furthermore, breathing efficiency was not related to REE in both groups.

Assessment of diaphragm function

Assessment of diaphragm function begins with the physical examination, but neither the physical examination nor radiography is sensitive enough to detect subtle abnormalities of diaphragm function. Maximal static transdiaphragmatic pressure and maximal static inspiratory mouth pressure have been widely used as measures of diaphragm and inspiratory muscle strength, respectively. Both are useful as rough indications of muscle strength, but, because they are highly effort dependent, they should not be relied upon for absolute accuracy. Objective measurement of diaphragm function requires phrenic nerve stimulation.

Prophylactic inspiratory muscle training in patients undergoing coronary artery bypass graft

Pulmonary complications after cardiac surgery are a leading cause of postoperative morbidity and mortality. Respiratory muscle weakness may contribute to the postoperative pulmonary abnormalities. We hypothesized that: (1) there is a decrease in inspiratory muscle strength (PImax at residual volume) and endurance (Pmpeak/PImax) following coronary artery bypass graft (CABG); (2) this weakness is associated with reduced pulmonary function tests (PFTs), impaired gas exchange, and a higher rate of pulmonary complications; and (3) prophylactic inspiratory muscle training (IMT) can prevent those changes. Eighty-four candidates for CABG, with ages ranging from 33 to 82 years, were evaluated prior to operation and randomized into two groups: 42 patients underwent IMT using a threshold trainer for 30 min/day for 2 weeks, 1 month before operation (group A); 42 patients served as a control group and underwent sham training (group B). There was a significant decrease in respiratory muscle function, PFTs, and gas exchange in the control group following CABG, whereas these parameters remained similar to those before entering the study in the training group. The differences between the groups were statistically significant. In addition 11 (26%) patients in the control group but only 2 (5%) in the training group needed postsurgical mechanical ventilation longer than 24 hours, CABGs have a significant deteriorating effect on inspiratory muscle function, PFTs, and arterial blood gases. The decrease in these parameters can be prevented by prophylactic inspiratory muscle training, which may also prevent postsurgical pulmonary complications.

Ventilatory assistance and respiratory muscle activity. 1: Interaction in healthy volunteers

We have investigated the response of 12 normal, healthy subjects to resistance loading and ventilator assistance of spontaneous breathing. Three ventilators, the Hamilton Veolar, Engström Erica and Puritan Bennett 7200, were used to provide synchronized intermittent mandatory ventilation and two levels of pressure assistance. Total respiratory elastance and resistance were measured. The equivalent (negative) pressure of respiratory muscle activity (pmus) was then calculated from measurement of flow and pressure at the mouth. With ventilatory assistance, subjects maintained frequency, decreased inspiratory time and the magnitude of pmus, but increased tidal volume, thus not taking full advantage of ventilatory assistance. The waveform of pmus varied in detail within and between subjects and conditions, but the all-subject mean waveforms showed for all conditions a consistency of trajectory. Increasing the level of assistance decreased the duration and hence the (negative) peak value of pmus. The results suggest that some waveforms of flow or pressure from the ventilators may be more acceptable to patients than others, and that different patients may prefer different waveforms.

Ventilatory assistance and respiratory muscle activity. 2: Simulation with an adaptive active ("aa" or "a-squared") model lung

The aim of this study was to develop a lung model which adapted its active simulation of spontaneous breathing to the ventilatory assistance it received--an "aa" or "a-squared" lung model. The active element required was the waveform of negative pressure (pmus), which is equivalent to respiratory muscle activity. This had been determined previously in 12 healthy volunteers and comprised a contraction phase, relaxation phase and expiratory pause. Ventilatory assistance had shortened the contraction and relaxation phases without changing their shape, and lengthened the pause phase to compensate. In this study, the contraction and relaxation phases could be adequately represented by two quadratic equations, in addition to a third to provide a smooth transition. Therefore, the adaptive element required was the prediction of the duration of the contraction phase. The best predictive variables were flow at the end of contraction or peak mouth pressure. Determination of either of these allowed adjustment of the "standard" waveform to the level of assistance produced by an "average" ventilator, in a manner that matched the mean response of 12 healthy conscious subjects.

Reproducibility of twitch mouth pressure, sniff nasal inspiratory pressure, and maximal inspiratory pressure

Twitch mouth pressure (Pmo,tw) during magnetic phrenic nerve stimulation and sniff nasal inspiratory pressure (SNIP) were recently proposed as alternative noninvasive methods for assessing inspiratory muscle strength. This study aimed to compare their reproducibility with maximal inspiratory pressure (MIP) in normal subjects. Ten healthy subjects were studied at functional residual capacity in semirecumbent position. Cervical magnetic phrenic nerve stimulation was performed during gentle expiration against an occlusion incorporating a small leak. Constancy of stimulation was controlled by recording diaphragmatic electromyogram. Within and between-session reproducibility of pressure were studied for Pmo,tw, SNIP, and MIP. The subjects were studied during a session of 10 manoeuvres repeated after 1 day and 1 month. The mean values were 16 cmH2O for Pmo,tw, 118 cmH2O for SNIP, and 115 cmH2O for MIP. For the three tests, the within subject variation was small in relation to between-subject variation, with the intraclass correlation coefficient ranging 0.79-0.90 for Pmo,tw, 0.85-0.92 for SNIP, and 0.88-0.92 for MIP. At 1 day interval, the coefficient of repeatability (2 SD of differences) was 3.6 cmH2O for Pmo,tw, 32 cmH2O for SNIP and 28 cmH2O for MIP. At 1 month interval, the coefficient of repeatability was 5.8 cmH2O for Pmo,tw, 23 cmH2O for SNIP and 21 cmH2O for MIP. We conclude that the within session reproducibility of the new tests twitch mouth pressure and sniff nasal inspiratory pressure is sufficient to be clinically useful. For sniff nasal inspiratory pressure, the between session reproducibility established after 1 day was maintained after 1 month. For twitch mouth pressure, the between session reproducibility declined slightly after 1 month. These characteristics should be considered when using these methods to follow an individual patient over time.

Structure and function relationships of the respiratory muscles

Potential relationships between the structure of the diaphragm and external intercostals and several indices of respiratory muscle function, lung function and nutrition in 27 patients (61+/-10 yrs of age) subjected to thoracotomy as a result of a lung neoplasm have been investigated. Prior to surgery the nutritional status of the patients was assessed and lung function (spirometry, lung volumes, transfer factor of the lungs for carbon monoxide, arterial blood gases) and respiratory muscle function (maximal inspiratory pressure (MIP) and diaphragmatic function were measured). Biopsies of the diaphragm (and external intercostals) were obtained during surgery. On average, patients showed mild airflow limitation (forced expiratory volume in one second (FEV1), 70+/-14% of predicted value, FEV1/forced vital capacity (FVC), 70+/-9%) with some air trapping (residual volume (RV), 139+/-50% pred) and normal gas exchange (arterial oxygen tension (Pa,O2), 11.3+/-1.33 kPa (85+/-10 mmHg)) and arterial carbon dioxide tension (Pa,CO2) 5.4+/-0.5 kPa (40.6+/-4 mmHg). MIP was 77+/-25% pred; maximal transdiaphragmatic pressure was 90+/-27 cmH2O. Most morphometric measurements of the diaphragm and external intercostals were within the range of values reported previously in other skeletal muscles. The size of the fibres of these two respiratory muscles was positively related (p<0.05) to MIP (% pred). There were no significant relationships between the structure of both muscles and nutritional status or any index of lung function. In conclusion, in the population studied, the fibre size of the diaphragm and external intercostals appears to relate to their ability to generate force.

Effect of aerobic training on ventilatory muscle endurance of spinal cord injured men

The functional consequences of ventilatory muscle impairment of spinal cord injured (SCI) subjects has been evaluated through spirometric and maximal respiratory pressure tests. Nevertheless, underlying functional abnormalities may be evident only under dynamic conditions, such as with a ventilatory muscle endurance test (VME). In order to evaluate the VME of thoracic SCI men and the effect of physical training on it we evaluated 12 SCI subjects (Group I) and 12 able-bodied controls (Group II). The subjects were submitted to clinical evaluation, spirometry, maximum voluntary ventilation in 12 s (MVV-12 sec) and a test of VME-the highest time of sustained ventilation at 70% of the maximum voluntary ventilation in isocapnic conditions (MVV-70% time). Gr. II was evaluated before and after an arm cranking aerobic training program (30 min/session, three times/week, 6 weeks) with training target heart rate corresponding to ventilatory anaerobic threshold. On the initial evaluation, Gr. I subjects presented a significantly reduced forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1) and MVV-12 sec when compared to controls (P < 0.05). Also, the VME was severely reduced in Group I (median, ranges; 1.15, 0.61-12.22) when compared to Group II (14.60, 1.20-15.00) - P < 0.001. When Gr. I subjects were separated by the level of lesion, the VME was lower in high injured (T1-T7) than intermediate (T8-T10) and low injured patients (T11-T12)-P < 0.05. After aerobic training, Group I subjects incremented significantly the FVC (P < 0.05) and the VME (P < 0.001), so that MVV-70% time values post-training were not different from the initial values of the Gr. II. In conclusion, (i) the VME of thoracic SCI men was severely reduced when compared to able-bodied controls; (ii) a 6-weeks arm cranking aerobic training program was efficient to normalize the VME of SCI subjects.

Diaphragm function during sighs in awake dogs after laparotomy

Pulmonary complications after upper abdominal surgery are usually ascribed to temporary postoperative impairment of diaphragm function, which may not originate from intrinsic, structural injury but from reflex inhibition of diaphragm contractility. Spontaneous breathing is interrupted periodically by sighs, even after upper abdominal surgery. If postoperative dysfunction of the diaphragm arises from a reflexic inhibition, then the sigh should temporarily override the inhibition and restore normal diaphragm function. We implanted sonomicrometer and electromyogram transducers chronically in six dogs by laparotomy, then directly measured length, shortening, and electromyogram activity of costal and crural diaphragm segments, parasternal intercostal, and transversus abdominis muscles an average of 8.7 (range, 1-16) d later during resting tidal breathing and sighs. In each animal we analyzed a sequence of breaths, including a sigh, when costal or crural diaphragm contractility was abnormal. With each sigh, the shape and amplitude of costal and crural diaphragm segmental shortening improved abruptly, from 0.9 and 1.4% of baseline length (% LBL) during resting breathing to 12.1 and 11.1% LBL, respectively, during sighs. The sighs were compared to CO2-stimulated breaths of equivalent tidal volume, which did not show either pattern or amplitude of shortening equivalent to sighs. We conclude that diaphragm dysfunction after laparotomy arises from a reflex inhibition, which is overridden abruptly to return diaphragm function briefly to normal during each spontaneous sigh.

[Comparative analysis of responses of the thoracic and abdominal respiration components to hypercapnia and muscular work]

The ratio of thoracic and abdominal contribution to ventilatory responses to hypercapnia in humans was found to depend on the initial ratio of thoracic and abdominal breathing reserves as determined by the body position in space. In muscular work, participation of the respiratory muscles in locomotor loads becomes a factor affecting the ratio of thoracic and abdominal components of ventilatory response.

Respiratory function in children during recovery from neuromuscular blockade

Residual neuromuscular blockade is a major risk factor for respiratory insufficiency. We examined the relationship between neuromuscular and respiratory function in 18 ASA I or II children aged 2-4 years. Lung function was measured by pneumotachography and transpulmonary pressure, neuromuscular transmission by first twitch response ratio (T1:T1) and train-of-four ratio (TOFR), before and at specific points in recovery from vecuronium paralysis. The tidal volume was directly related to maximal inspiratory pressure at occlusion (PIOCC), P < 0.001, whereas the minute ventilation (VE) was related to the respiratory drive (P0.1), P < 0.001. The best predictors of minute ventilation were the P0.1 (r = 0.57), and the TOFR (r = 0.62). PIOCC and P0.1 correlated closely (r = 0.889, P = 0.002) but TOFR and T1:T1 did not correlate with either. Our results show that the occlusion pressure measurements, P0.1 and PIOCC, were good predictors of both VE.kg-1 and respiratory work.

Lung volume reduction surgery and airflow limitation

Interest has recently been renewed in lung volume reduction surgery (LVRS) for end-stage emphysema. However, numerous questions about its role in the treatment of emphysema remain, including the clinical characteristics of optimal candidates and its mechanism of improvement in pulmonary function. In this report, we develop a mathematical analysis and graphic depiction of the mechanism of improvement in expiratory airflow and vital capacity. This analysis is based on consideration of the interaction between lung function and respiratory muscle function. We also reexamine previously published pulmonary mechanics in patients with alpha1-antitrypsin deficiency, chronic obstructive pulmonary disease, and asthma. We find a major determinant of airflow limitation common to these diseases is the ratio of residual volume to total lung capacity (RV/TLC). Moreover, RV/TLC is found to be the single most important determinant of the improvement in pulmonary function after LVRS. Regardless of the specific underlying lung disease, the impairment of airflow is due primarily to mismatch between the sizes of the lung and the chest wall, and the effects of LVRS are almost exclusively due to improvement of that match. This analysis can be used to develop testable hypotheses to guide patient selection for this procedure.

Relationships between hippocampal activity and breathing patterns

Single cell discharge, EEG activity, and optical changes accompanying alterations in breathing patterns, as well as the knowledge that respiratory musculature is heavily involved in movement and other behavioral acts, implicate hippocampal regions in some aspects of breathing control. The control is unlikely to reside in oscillatory breathing movements, because such patterns emerge in preparations retaining only the medulla (and perhaps only the spinal cord). However, momentary changes in breathing patterns induced by affect, startle, whole-body movement changes, or compensatory ventilatory changes mediated by rostral brain regions likely depend on hippocampal action in aspects of control. Hippocampal activity was enhanced prior to sighs, and this enhancement was accompanied by increased slow theta activity. Theta frequency increased during apnea, prior to return of breathing. Consideration of hippocampal contributions to breathing control should be viewed in the context that significant interactions exist between blood pressure changes and ventilation, and that modest breathing challenges, such as exposure to hypercapnia or to increased resistive loads, bring into action a vast array of brain regions involving nearly every level of the neuraxis.