Breathing responses to small inspiratory threshold loads in humans.
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To investiage the effect of inspiratory threshold load (ITL) on breathing, all previous work studied loads that were much greater than would be encountered under pathophysiological conditions. We hypothesized that mild ITL from 2.5 to 20 cmH2O is sufficient to modify control and sensation of breathing. The study was performed in healthy subjects. The results demonstrated that with mild ITL 1) inspiratory difficulty sensation could be perceived at an ITL of 2.5 cmH2O; 2) tidal volume increased without change in breathing frequency, resulting in hyperpnea; and 3) although additional time was required for inspiratory pressure to attain the threshold before inspiratory flow was initiated, the total inspiratory muscle contraction time remained constant. This resulted in shortening of the available time for inspiratory flow, so that the tidal volume was maintained or increased by significant increase in mean inspiratory flow. On the basis of computer simulation, we conclude that the mild ITL is sufficient to increase breathing sensation and alter breathing control, presumably aiming at maintaining a certain level of ventilation but minimizing the energy consumption of the inspiratory muscles. (+info)
Compliance and stability of the bronchial wall in a model of allergen-induced lung inflammation.
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Airway wall remodeling in response to inflammation might alter load on airway smooth muscle and/or change airway wall stability. We therefore determined airway wall compliance and closing pressures in an animal model. Weanling pigs were sensitized to ovalbumin (OVA; ip and sc, n = 6) and were subsequently challenged three times with OVA aerosol. Control pigs received 0.9% NaCl (n = 4) in place of OVA aerosol. Bronchoconstriction in vivo was assessed from lung resistance and dynamic compliance. Semistatic airway compliance was recorded ex vivo in isolated segments of bronchus, after the final OVA aerosol or 0.9% NaCl challenge. Internally or externally applied pressure needed to close bronchial segments was determined in the absence or presence of carbachol (1 microM). Sensitized pig lungs exhibited immediate bronchoconstriction to OVA aerosol and also peribronchial accumulations of monocytes and granulocytes. Compliance was reduced in sensitized bronchi in vitro (P < 0.01), and closing pressures were increased (P < 0.05). In the presence of carbachol, closing pressures of control and sensitized bronchi were not different. We conclude that sensitization and/or inflammation increases airway load and airway stability. (+info)
Barometric pressures on Mt. Everest: new data and physiological significance.
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Barometric pressures (PB) near the summit of Mt. Everest (altitude 8, 848 m) are of great physiological interest because the partial pressure of oxygen is very near the limit for human survival. Until recently, the only direct measurement on the summit was 253 Torr, which was obtained in October 1981, but, despite being only one data point, this value has been used by several investigators. Recently, two new studies were carried out. In May 1997, another direct measurement on the summit was within approximately 1 Torr of 253 Torr, and meteorologic data recorded at the same time from weather balloons also agreed closely. In the summer of 1998, over 2,000 measurements were transmitted from a barometer placed on the South Col (altitude 7,986 m). The mean PB values during May, June, July, and August were 284, 285, 286, and 287 Torr, respectively, and there was close agreement with the PB-altitude (h) relationship determined from the 1981 data. The PB values are well predicted from the equation PB = exp (6.63268 - 0.1112 h - 0.00149 h2), where h is in kilometers. The conclusion is that on days when the mountain is usually climbed, during May and October, the summit pressure is 251-253 Torr. (+info)
Influence of gas density on simulated snoring.
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According to a recent theoretical model, snoring is related to instability of the upper airway (UA). Factors promoting UA instability include increased gas density. The aim of this study was to test the influence of gas density on simulated snoring production and supraglottic resistance. Supraglottic pressure and flow rate (V') were measured in 10 healthy seated subjects during simulated snoring. Subjects breathed three different gas mixtures: Helium-oxygen, He 79%-O2 21% (He-O2); air; and sulphur hexafluoride-oxygen, F6S 79%-O2 21% (F6S-O2) administered in a random order. Supraglottic resistance (Rsg) was measured on its linear range during quiet breathing and V' was measured at the onset and middle of snoring. Linear Rsg increased and V' conversely decreased with gas density. These data are in agreement with predictions of a mathematical model of the upper airway showing that snoring occurs at lower flow rates when gas density is increased. (+info)
Acute mountain sickness is not related to cerebral blood flow: a decompression chamber study.
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To evaluate the pathogenetic role of cerebral blood flow (CBF) changes occurring before and during the development of acute mountain sickness (AMS), peak mean middle cerebral artery flow velocities () were assessed by transcranial Doppler sonography in 10 subjects at 490-m altitude, and during three 12-min periods immediately (SA1), 3 (SA2), and 6 (SA3) h after decompression to a simulated altitude of 4,559 m. AMS cerebral scores increased from 0. 16 +/- 0.14 at baseline to 0.44 +/- 0.31 at SA1, 1.11 +/- 0.88 at SA2 (P < 0.05), and 1.43 +/- 1.03 at SA3 (P < 0.01); correspondingly, three, seven, and eight subjects had AMS. Absolute and relative at simulated altitude, expressed as percentages of low-altitude values (%), did not correlate with AMS cerebral scores. Average % remained unchanged, because % increased in three and remained unchanged or decreased in seven subjects at SA2 and SA3. These results suggest that CBF is not important in the pathogenesis of AMS and shows substantial interindividual differences during the first hours at simulated altitude. (+info)
Preliminary evaluation of a new prototype laryngeal mask in children.
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We have assessed a prototype laryngeal mask airway (pLMA) in 50 anaesthetized children for ease of insertion, oropharyngeal leak pressures, gastric insufflation and fibreoptic position. The pLMA has a second smaller mask, which rests against the upper oesophageal sphincter, and a second cuff to increase the seal pressure of the glottic mask. All insertions were graded as easy and an effective airway was achieved in all patients. Oropharyngeal leak pressure was > 40 cm H2O in 49 of 50 patients. Gastric insufflation was not detected by epigastric auscultation. In 46 of 50 patients, the vocal cords were seen via a fibreoptic laryngoscope. One patient regurgitated clear fluid, but aspiration did not occur. On removal, blood staining was detected in three of 50 children. We conclude that the pLMA was easy to insert, facilitated high airway pressure ventilation and may provide some protection against gastric insufflation. (+info)
Small tidal volume ventilation using a zero deadspace tracheal tube.
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The zero deadspace tracheal tube (ZEDS-TT) is a double-lumen endobronchial tube with a truncated bronchial limb. Functionally it is unrelated to the familiar endobronchial tube used in lung isolation surgery. It is placed in the same position as a regular tracheal tube and, by means of special connectors, one limb is used for inspiration and the other for expiration, thereby greatly reducing anatomical and apparatus deadspace. In this study, we have compared respiratory and ventilatory effects of reduction of tidal volume (VT) via a single-lumen tracheal tube and the ZEDS-TT during controlled ventilation with a Siemens Elema 900C Servo ventilator. Eleven consenting adult patients (ASA I and II) undergoing elective peripheral surgery were studied. Starting at a VT value of 10 ml kg-1, data were recorded for each tube type. VT was reduced by 2.5 ml kg-1 every 10 min and stabilized data recorded. Minute volume was kept constant by increasing ventilatory frequency at each reduction in VT. We found that the ZEDS-TT produced a significant reduction in PaCO2 and airway pressure for any VT used, while maintaining oxygenation. (+info)
Comparison of four methods for assessing airway sealing pressure with the laryngeal mask airway in adult patients.
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We have compared four tests for assessing airway sealing pressure with the laryngeal mask airway (LMA) to test the hypothesis that airway sealing pressure and inter-observer reliability differ between tests. We studied 80 paralysed, anaesthetized adult patients. Four different airway sealing pressure tests were performed in random order on each patient by two observers blinded to each other's measurements: test 1 involved detection of an audible noise; test 2 was detection of end-tidal carbon dioxide in the oral cavity; test 3 was observation of the aneroid manometer dial as the pressure increased to note the airway pressure at which the dial reached stability; and test 4 was detection of an audible noise by neck auscultation. Mean airway sealing pressure ranged from 19.5 to 21.3 cm H2O and intra-class correlation coefficient was 0.95-0.99. Inter-observer reliability of all tests was classed as excellent. The manometric stability test had a higher mean airway sealing pressure (P < 0.0001) and better inter-observer reliability (P < 0.0001) compared with the three other tests. We conclude that for clinical purposes all four tests are excellent, but that the manometric stability test may be more appropriate for researchers comparing airway sealing pressures. (+info)