Atomic force microscopy ͑AFM͒ allows the acquisition of high-resolution images and the measurement of mechanical properties of living cells under physiological conditions. AFM cantilevers with blunted pyramidal tips are commonly used to obtain images of living cells. Measurement of mechanical properties with these tips requires a contact model that takes into account their blunted geometry. The aim of this work was to develop a contact model of a blunted pyramidal tip and to assess the suitability of pyramidal tips for probing mechanical properties of soft gels and living cells. We developed a contact model of a blunted pyramidal tip indenting an elastic half-space. We measured Young's modulus ͑E͒ and the complex shear modulus ͑G * = GЈ +iGЉ͒ of agarose gels and A549 alveolar epithelial cells with pyramidal tips and compared them with those obtained with spherical tips. The gels exhibited an elastic behavior with almost coincident loading and unloading force curves and negligible values of GЉ. E fell sharply with indentation up to ϳ300 nm, showing a linear regime for deeper indentations. A similar indentation dependence of E with twofold lower values at the linear regime was obtained with the spherical tip fitted with Hertz's model. The dependence of E on indentation in cells paralleled that found in gels. Cells exhibited viscoelastic behavior with GЉ / GЈ ϳ 1 / 4. Pyramidal tips commonly used for AFM imaging are suitable for probing mechanical properties of soft gels and living cells.

aaBreathing airflow is a physiological variable commonly monitored for assessing respiratory patterns and nocturnal events in studies aimed at diagnosing sleep disorders [1][2][3]. Given that in sleep studies the use of a pneumotachograph, which is the reference transducer for measuring flow, is rather cumbersome, airflow is routinely assessed by means of thermally sensitive devices such as thermistors and thermocouples. The main advantage of these devices is that they are small and light and, consequently, their use causes a minimum amount of disturbance to the patient during sleep. However, thermistors and thermocouples are semiquantitative devices since the flow signal they provide is not a direct measure of actual flow [4][5][6]. This drawback, which does not pose major problems for detecting apnoeas, is however relevant to scoring hypopnoeas [7,8], given that the definition of these events is made in terms of quantitative reduction in airflow.Precise characterization of the performance of thermistor/thermocouples when used for measuring airflow in sleep studies requires a laboratory study under controlled conditions. Indeed, contrary to what happens when testing linear transducers, the analysis of a slow-response nonlinear system such as a thermistor/thermocouple [6] requires subjecting it to a variety of input airflows covering the ranges of amplitudes, frequencies and waveforms found in the particular application. Such a systematic study cannot be carried out in patients during sleep since it is not possible to modify the pattern of the breathing airflow in a selective and controlled way. Moreover, pneumo-tachographs, which may modify the thermal conditions in the thermistor/thermocouple, or thoracoabdominal bands, which may not be adequate during padoxical breathing, are not perfect reference flow transducers in this application. The aim of this study was to characterize the accuracy of thermistor/thermocouples as flow-measuring devices for detecting hypopnoeas. To this end, we set up a respiratory model to reproduce the measuring conditions of nasal airflow by means of thermally sensitive devices. Materials and methodsThe response of thermistors and thermocouples was studied with the respiratory system model shown in figure 1. The model was based on a 60 L methacrylate chamber which was used as a buffer of heated air at 37°C. The air temperature in the chamber was measured with a mercury-bulb thermometer and was maintained (±0.2°C) by means of a 120 W heating resistance supplied with the required voltage. Homogeneous distribution of air temperature in the chamber was achieved by means of mixing airflow generated by an internal fan. A flow generator based on a servocontrolled linear motor attached to rubber bellows [9,10] We concluded that thermistor/thermocouples are inaccurate flow-measuring devices when used at the airflow conditions typical of sleep studies. Their use for quantifying hypopnoeas may lead to considerable underdetection of these respiratory events.

Assessment of upper airway mechanics in patients with obstructive sleep apnea/hypopnea (OSA) can be carried out qualitatively from indirect signals (flow pattern, snoring, strain gauges, inductance plethysmography) or quantitatively by means of invasive estimation of esophageal pressure. The forced oscillation technique (FOT) is a noninvasive method of potential interest for quantitatively assessing airway obstruction in the sleeping patient. The aim of this work was to ascertain in a model study whether FOT could provide an index of airway obstruction when applied at the conditions of total and partial occlusions similar to the ones found in patients with OSA. An airway analog closely mimicking upper airway collapsibility was constructed and mechanically characterized by the relationship between its flow, upstream and downstream pressures as well as by means of FOT. We simulated total collapse (apnea), different levels of partial collapse with flow limitation (hypopnea), and release of airway obstruction when the collapsible analog was used as an artificial upper airway in a spontaneously breathing subject submitted to continuous positive airway pressure (CPAP) up to 14 cm H2O.s/L. The results showed that the amplitude of airway impedance measured by FOT was a suitable index to detect obstruction in collapsible segments. We concluded from this realistic model study that FOT could be a valuable tool for quantitatively assessing airway obstruction in patients with OSA treated with CPAP. This noninvasive technique is potentially useful both in studying upper airway mechanics in detail and in automatically monitoring airway obstruction in routine studies.

The forced oscillation technique (FOT) is a noninvasive method to measure respiratory resistance (Rrs) potentially useful for monitoring upper airway obstruction in patients with obstructive sleep apnea/hypopnea syndrome (SAHS). The aim of this work was to test the clinical suitability of FOT in assessing dynamic changes in airflow obstruction in patients with SAHS during continuous positive airway pressure (CPAP) and to investigate the CPAP dependence of Rrs. Forced oscillation (5 Hz) was applied to six male patients with SAHS submitted to CPAP titration procedure. Esophageal pressure was measured with a balloon-tipped catheter. Mid-inspiratory resistance (Rrs,i), mid-expiratory resistance (Rrs,e), and esophageal pressure swings (deltaPes) were computed for the respiratory events recorded at each CPAP level. Rrs,i decreased markedly and significantly from 36.0 +/- 4.0 cm H2O x s/L (mean +/- SEM) at baseline CPAP (4 cm H2O) to 13.1 +/- 2.8 cm H2O x s/L at optimal CPAP (11.3 +/- 0.4 cm H2O). Rrs,e showed a faster decrease with increasing CPAP reaching normal values at approximately 8 cm H2O. Rrs,i was strongly correlated (r2 = 0.94) with deltaPes. Our results suggest that FOT can be used as an alternative to the esophageal balloon for assessing airflow obstruction in patients with SAHS and for CPAP titration. Moreover, FOT allows us to detect phasic changes in resistance within the breathing cycle.