Treatment with leuprolide acetate decreases the threshold of the ventilatory response to carbon dioxide in healthy males.

This investigation was designed to determine if suppression of testosterone alters the ventilatory response to carbon dioxide in the presence of high and low levels of oxygen. Eleven healthy male subjects completed a series of rebreathing trials during wakefulness, before and after treatment with a long-acting gonadotropin-releasing hormone agonist. Five subjects also completed studies during non-rapid eye movement (NREM) sleep. During wakefulness, subjects initially hyperventilated to reduce the partial pressure of carbon dioxide (P(ET,CO2)) below 25 Torr. Subjects then rebreathed from a bag containing a normocapnic (42 Torr), low (50 Torr) or high oxygen (140 Torr) gas mixture. During each trial P(ET,CO2) increased while oxygen was maintained at a constant level. The threshold of the ventilatory response to carbon dioxide was considered to be the point at which minute ventilation began to rise in a linear fashion as P(ET,CO2) increased. The slope of the ventilatory response above the threshold was used as a measure of sensitivity to carbon dioxide. During NREM sleep, hypocapnia was induced via nasal mechanical ventilation. Several trials were completed until the cessation of mechanical ventilation resulted in a central apnoea which demarcated the threshold of the ventilatory response to carbon dioxide. In response to treatment with leuprolide acetate, the threshold measured in wakefulness decreased during carbon dioxide rebreathing in the presence of low (41.05 +/- 0.77 versus 39.40 +/- 0.83 Torr; P = 0.01) and high (46.32 +/- 0.56 versus 44.78 +/- 0.83 Torr; P = 0.01) oxygen levels. An increase in sensitivity (4.82 +/- 0.61 versus 7.17 +/- 1.20 l min(-1) Torr(-1); P = 0.02) was also observed during rebreathing in the presence of high but not low oxygen levels. The increase in sensitivity was accompanied by an increase in carbon dioxide production. The findings observed during NREM sleep were similar to those observed during wakefulness, since the P(ET,CO2) that demarcated the threshold was decreased after leuprolide treatment (42.1 +/- 0.6 versus 39.6 +/- 0.6 Torr; P = 0.002). Additionally, the decrease in P(ET,CO2) required to induce an apnoea was greater after treatment with leuprolide (2.56 +/- 0.25 versus 4.06 +/- 0.29 Torr; P = 0.004). We conclude that suppression of testosterone decreases the threshold of the ventilatory response to carbon dioxide during both wakefulness and sleep.

Effect of testosterone on the apneic threshold in women during NREM sleep.

The hypocapnic apneic threshold (AT) is lower in women relative to men. To test the hypothesis that the gender difference in AT was due to testosterone, we determined the AT during non-rapid eye movement sleep in eight healthy, nonsnoring, premenopausal women before and after 10-12 days of transdermal testosterone. Hypocapnia was induced via nasal mechanical ventilation (MV) for 3 min with tidal volumes ranging from 175 to 215% above eupneic tidal volume and respiratory frequency matched to eupneic frequency. Cessation of MV resulted in hypocapnic central apnea or hypopnea depending on the magnitude of hypocapnia. Nadir minute ventilation as a percentage of control (%Ve) was plotted against the change in end-tidal CO(2) (Pet(CO(2))); %Ve was given a value of zero during central apnea. The AT was defined as the Pet(CO(2)) at which the apnea closest to the last hypopnea occurred; hypocapnic ventilatory response (HPVR) was defined as the slope of the linear regression Ve vs. Pet(CO(2)). Both the AT (39.5 +/- 2.9 vs. 42.1 +/- 3.0 Torr; P = 0.002) and HPVR (0.20 +/- 0.05 vs. 0.33 +/- 0.11%Ve/Torr; P = 0.016) increased with testosterone administration. We conclude that testosterone administration increases AT in premenopausal women, suggesting that the increased breathing instability during sleep in men is related to the presence of testosterone.

Influence of gender on upper airway mechanics: upper airway resistance and Pcrit.

It has been proposed that the difference in sleep apnea prevalence is related to gender differences in upper airway anatomy and physiology. To explain the prevalence difference, we hypothesized that men would have an increased upper airway resistance and increased critical closing pressure (Pcrit) compared with women. In protocol 1, resistance at two points, fixed flow of 0.2 l/s (RL) and peak flow (Rpk), was measured in 33 men and 27 women without significant sleep-disordered breathing. We found no difference in either RL (-6.9 +/- 5.9 vs. -8.6 +/- 8.2 cmH2O) or Rpk (-9.3 +/- 6.8 vs. -10.0 +/- 11.9 cmH2O) between the men and women. A multiple linear regression to correct for the effects of age and body mass index confirmed that gender had no effect on resistance. In protocol 2, Pcrit was measured in eight men and eight women without sleep-disordered breathing. We found no difference in Pcrit (-10.4 +/- 3.1 vs. -8.8 +/- 2.7 cmH2O) between men and women. We conclude that there are no significant differences in collapsibility between men and women. We present an unifying hypothesis to explain the divergent findings of gender differences in upper airway physiology.

Effect of REM sleep on retroglossal cross-sectional area and compliance in normal subjects.

It has been proposed that the upper airway compliance should be highest during rapid eye movement (REM) sleep. Evidence suggests that the increased compliance is secondary to an increased retroglossal compliance. To test this hypothesis, we examined the effect of sleep stage on the relationship of retroglossal cross-sectional area (CSA; visualized with a fiber-optic scope) to pharyngeal pressure measured at the level of the oropharynx during eupneic breathing in subjects without significant sleep-disordered breathing. Breaths during REM sleep were divided into phasic (associated with eye movement, PREM) and tonic (not associated with eye movements, TREM). Retroglossal CSA decreased with non-REM (NREM) sleep and decreased further in PREM [wake 156.8 +/- 48.6 mm(2), NREM 104.6 +/- 65.0 mm(2) (P < 0.05 wake vs. NREM), TREM 83.1 +/- 46.4 mm(2) (P = not significant NREM vs. TREM), PREM 73.9 + 39.2 mm(2) (P < 0.05 TREM vs. PREM)]. Retroglossal compliance, defined as the slope of the regression CSA vs. pharyngeal pressure, was the same between all four conditions (wake -0.7 + 2.1 mm(2)/cmH(2)O, NREM 0.6 +/- 3.0 mm(2)/cmH(2)O, TREM -0.2 +/- 3.3 mm(2)/cmH(2)O, PREM -0.6 +/- 5.1 mm(2)/cmH(2)O, P = not significant). We conclude that the intrinsic properties of the airway wall determine retroglossal compliance independent of changes in the neuromuscular activity associated with changes in sleep state.

Injection molding of chondrocyte/alginate constructs in the shape of facial implants.

Over one million patients per year undergo some type of procedure involving cartilage reconstruction. Polymer hydrogels, such as alginate, have been shown to be effective carriers for chondrocytes in subcutaneous cartilage formation. The goal of our current study was to develop a method to create complex structures (nose bridge, chin, etc.) with good dimensional tolerance to form cartilage in specific shapes. Molds of facial implants were prepared using Silastic ERTV. Suspensions of chondrocytes in 2% alginate were gelled by mixing with CaSO(4) (0.2 g/mL) and injected into the molds. Constructs of various cell concentrations (10, 25, and 50 million/mL) were implanted in the dorsal aspect of nude mice and harvested at times up to 30 weeks. Analysis of implanted constructs indicated progressive cartilage formation with time. Proteoglycan and collagen constructs increased with time to approximately 60% that of native tissue. Equilibrium modulus likewise increased with time to 15% that of normal tissue, whereas hydraulic permeability decreased to 20 times that of native tissue. Implants seeded with greater concentrations of cells increased proteoglycan content and collagen content and equilibrium and decreased permeability. Production of shaped cartilage implants by this technique presents several advantages, including good dimensional tolerance, high sample-to-sample reproducibility, and high cell viability. This system may be useful in the large-scale production of precisely shaped cartilage implants.

The use of clinical prediction formulas in the evaluation of obstructive sleep apnea.

STUDY OBJECTIVES: To prospectively study the utility of four clinical prediction models for either predicting the presence of obstructive sleep apnea (OSA, apnea-hypopnea index [AHI] > or = 10/hour), or prioritizing patients for a split-night protocol (AHI(3)20/hour). DESIGN: All patients presenting for OSA evaluation completed a research questionnaire that included questions from previously developed clinical prediction models. The probability of sleep apnea for each patient for each model was calculated based upon the equation used in the model. Based upon two cutoffs of apnea-hypopnea index, 10 and 20, the sensitivity, specificity, and positive predictive value were calculated. For the cutoffs AHI > or =10 and > or =20, receiver operating characteristic curves were generated and the areas under the curves calculated. Comparisons of demographic information and symptom response were compared between patients with and without OSA, and men vs. women. SETTING: Urban, accredited sleep disorders center. PATIENTS OR PARTICIPANTS: All patients referred for evaluation of OSA who underwent polysomnography. INTERVENTIONS: N/A. RESULTS: 370 patients (191 men, 179 women) completed the study. 248 of the 370 (67%) patients had an AHI(3)10; 180 of the 370 (49%) had an AHI> or =20. For AHI > or =10, the sensitivities ranged from 76 to 96%, specificities from 13%-54%, positive predictive values from 69%-77% using the probability cutoff of the original investigators; the areas under the curve from 0.669 to 0.736. For AHI(3)20, the areas under the ROC curves ranged from 0.700 to 0.757; using cutoffs to maximized specificity, the sensitivities ranged from 33%-39%, specificities from 87%-93%, and positive predictive values from 72%-85%. All the models performed better for men. CONCLUSIONS: The clinical prediction models tested are not be sufficiently accurate to discriminate between patients with or without OSA but could be useful in prioritizing patients for split-night polysomnography.

The effect of tensor veli palatini stimulation on upper airway patency.

OBJECTIVE: To evaluate the effect of selective electrical stimulation of the tensor veli palatini muscle on upper airway patency. METHODS: Pressure-flow relationships were evaluated, in a feline isolated upper airway preparation, to determine the role of the soft palate musculature on airflow dynamics. The tensor veli palatini muscles were selectively stimulated while monitoring upper airway collapsibility (critical pressure), maximal inspiratory airflow, and the nasal resistance upstream to the flow-limiting site. RESULTS: Tensor veli palatini stimulation resulted (mean +/- SEM) in an increase in maximal inspiratory airflow from 74 +/- 13 mL/s to 93 +/- 18 mL/s (P= .04). The increase in maximal inspiratory airflow was associated with a decrease in critical pressure from -2.3 +/- 1.7 cm H2O to -4.7 +/- 2.7 cm H2O (P= .01) and an increase in nasal resistance from 32.4 +/- 24.3 cm H2O x L(-1) s(-1) to 50.8 +/- 29.7 cm H2O x L(-1) s(-1) (P= .02). CONCLUSIONS: Tensor veli palatini stimulation decreases upper airway collapsibility and is likely an integral component in maintaining airway patency. However, the effects of the isolated tensor veli palatini muscles are less significant than those seen previously with physiologic stimuli such as hypercapnia. These findings suggest that upper airway patency, although contributed to by the tensor veli palatini, requires the coordinated activation of palatopharyngeal muscles to adequately influence upper airway collapsibility.

Alginate hydrogels as synthetic extracellular matrix materials.

Alginate hydrogels are used extensively in cell encapsulation, cell transplantation, and tissue engineering applications. Alginates possess many favorable properties required in biomaterials, but are unable to specifically interact with mammalian cells. We have therefore covalently modified alginate polysaccharides with RGD-containing cell adhesion ligands utilizing aqueous carbodiimide chemistry. The chemistry has been optimized and quantified with reaction efficiencies reaching 80% or greater. The concentration of peptide available for reaction was then varied to create hydrogels with a range of ligand densities. Mouse skeletal myoblasts were cultured on alginate hydrogel surfaces coupled with GRGDY peptides to illustrate achievement of cellular interaction with the otherwise non-adhesive hydrogel substrate. Myoblasts adhere to GRGDY-modified alginate surfaces, proliferate, fuse into multinucleated myofibrils, and express heavy-chain myosin which is a differentiation marker for skeletal muscle. Myoblast adhesion and spreading on these GRGDY-modified hydrogels was inhibited with soluble ligand added to the seeding medium, illustrating the specificity of adhesion to these materials. Alginate may prove to be an ideal material with which to confer specific cellular interactive properties, potentially allowing for the control of long-term gene expression of cells within these matrices.

Release from alginate enhances the biological activity of vascular endothelial growth factor.

A primary factor which limits engineering tissues of substantial size is the lack of nutrients readily available to transplanted cells. One potential solution to this nutrient limitation is to encourage the rapid development of a vascular network within three-dimensional tissue engineering matrices. Vascular endothelial growth factor (VEGF) has been identified as a potent stimulator of angiogenesis in vivo. Though effective at stimulating endothelial cells to form blood vessels VEGF degrades rapidly. Spherical alginate beads (3.3+/-0.1 mm diameter) were examined as a means of delivering biologically functional VEGF at a controlled rate over extended times. The alginate beads demonstrated the ability to incorporate VEGF with an efficiency between 30 and 67%, depending on the processing conditions, and release it at a constant rate (5%/day) for up to 14 days in vitro. The released VEGF, when assayed for its ability to stimulate endothelial cells in culture, was found not only to be functional but more potent (three to five times) than the same mass of VEGF added directly to the culture medium. The release kinetics of freeze dried VEGF containing alginate beads were also examined and found to be comparable to non-freeze dried samples.

The effect of rapid eye movement (REM) sleep on upper airway mechanics in normal human subjects.

1. It has been proposed that the upper airway is more compliant during rapid eye movement (REM) sleep than during non-rapid eye movement (NREM) sleep. The purpose of this study was to test this hypothesis in a group of subjects without sleep-disordered breathing. 2. On the first night, the effect of sleep stage on the relationship of retropalatal cross-sectional area (CSA; visualized with a fibre-optic scope) to pharyngeal pressure (PPH) measured at the soft palate during eupnoeic breathing was studied. Breaths during REM sleep were divided into phasic (associated with eye movements) and tonic (not associated with eye movements). There was a significant decrease in pharyngeal CSA during NREM sleep compared with wakefulness. There was no further decrease observed during either tonic or phasic REM sleep. Pharyngeal compliance, defined as the slope of the regression CSA versus PPH, was significantly increased during NREM sleep compared with wakefulness and REM sleep, with the compliance during both tonic and phasic REM sleep being similar to that observed in wakefulness. 3. On the second night, the effect of sleep stage on pressure-flow relationships of the upper airway was investigated. There was a trend towards the upper airway resistance being highest in NREM sleep compared with wakefulness and REM sleep. 4. We conclude that the upper airway is stiffer and less compliant during REM sleep than during NREM sleep. We postulate that this difference is secondary to differences in upper airway vascular perfusion between REM and NREM sleep.